Graves' disease

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Graves%27_disease 

 

Graves' disease
Other names	Toxic diffuse goiter, Flajani–Basedow–Graves disease
The classic finding of exophthalmos and lid retraction in Graves' disease
Specialty	Endocrinology
Symptoms	Enlarged thyroid, irritability, muscle weakness, sleeping problems, fast heartbeat, weight loss, poor tolerance of heat
Complications	Graves' ophthalmopathy
Causes	Unknown
Risk factors	Family history, other autoimmune diseases
Diagnostic method	Blood tests, radioiodine uptake
Treatment	Radioiodine therapy, medications, thyroid surgery
Frequency	0.5% (males), 3% (females)

Graves' disease, also known as toxic diffuse goiter, is an autoimmune disease that affects the thyroid. It frequently results in and is the most common cause of hyperthyroidism. It also often results in an enlarged thyroid. Signs and symptoms of hyperthyroidism may include irritability, muscle weakness, sleeping problems, a fast heartbeat, poor tolerance of heat, diarrhea and unintentional weight loss. Other symptoms may include thickening of the skin on the shins, known as pretibial myxedema, and eye bulging, a condition caused by Graves' ophthalmopathy. About 25 to 80% of people with the condition develop eye problems.

The exact cause of the disease is unclear; however, it is believed to involve a combination of genetic and environmental factors. A person is more likely to be affected if they have a family member with the disease. If one twin is affected, a 30% chance exists that the other twin will also have the disease. The onset of disease may be triggered by physical or emotional stress, infection or giving birth. Those with other autoimmune diseases such as type 1 diabetes and rheumatoid arthritis are more likely to be affected. Smoking increases the risk of disease and may worsen eye problems. The disorder results from an antibody, called thyroid-stimulating immunoglobulin (TSI), that has a similar effect to thyroid stimulating hormone (TSH). These TSI antibodies cause the thyroid gland to produce excess thyroid hormones. The diagnosis may be suspected based on symptoms and confirmed with blood tests and radioiodine uptake. Typically, blood tests show a raised T₃ and T₄, low TSH, increased radioiodine uptake in all areas of the thyroid and TSI antibodies.

The three treatment options are radioiodine therapy, medications, and thyroid surgery. Radioiodine therapy involves taking iodine-131 by mouth, which is then concentrated in the thyroid and destroys it over weeks to months. The resulting hypothyroidism is treated with synthetic thyroid hormones. Medications such as beta blockers may control some of the symptoms, and antithyroid medications such as methimazole may temporarily help people while other treatments are having effect. Surgery to remove the thyroid is another option. Eye problems may require additional treatments.

Graves' disease will develop in about 0.5% of males and 3% of females. It occurs about 7.5 times more often in women than in men. Often, it starts between the ages of 40 and 60 but can begin at any age. It is the most common cause of hyperthyroidism in the United States (about 50 to 80% of cases). The condition is named after Irish surgeon Robert Graves, who described it in 1835. A number of prior descriptions also exist.

Signs and symptoms

Main article: Signs and symptoms of Graves' disease

 

Graves' disease symptoms

The signs and symptoms of Graves' disease virtually all result from the direct and indirect effects of hyperthyroidism, with main exceptions being Graves' ophthalmopathy, goiter, and pretibial myxedema (which are caused by the autoimmune processes of the disease). Symptoms of the resultant hyperthyroidism are mainly insomnia, hand tremor, hyperactivity, hair loss, excessive sweating, oligomenorrhea, itching, heat intolerance, weight loss despite increased appetite, diarrhea, frequent defecation, palpitations, periodic partial muscle weakness or paralysis in those especially of Asian descent, and skin warmth and moistness. Further signs that may be seen on physical examination are most commonly a diffusely enlarged (usually symmetric), nontender thyroid, lid lag, excessive lacrimation due to Graves' ophthalmopathy, arrhythmias of the heart, such as sinus tachycardia, atrial fibrillation, and premature ventricular contractions, and hypertension. People with hyperthyroidism may experience behavioral and personality changes, including psychosis, mania, anxiety, agitation, and depression.

Cause

The exact cause is unclear; however, it is believed to involve a combination of genetic and environmental factors. While a theoretical mechanism occurs by which exposure to severe stressors and high levels of subsequent distress such as PTSD (Post traumatic stress disorder) could increase the risk of autoimmune disease and cause an aggravation of the autoimmune response that leads to Graves' disease, more robust clinical data are needed for a firm conclusion.

Genetics

A genetic predisposition for Graves' disease is seen, with some people more prone to develop TSH receptor activating antibodies due to a genetic cause. Human leukocyte antigen DR (especially DR3) appears to play a role. To date, no clear genetic defect has been found to point to a single-gene cause.

Genes believed to be involved include those for thyroglobulin, thyrotropin receptor, protein tyrosine phosphatase nonreceptor type 22 (PTPN22), and cytotoxic T-lymphocyte–associated antigen 4, among others.

Infectious trigger

Since Graves' disease is an autoimmune disease which appears suddenly, often later in life, a viral or bacterial infection may trigger antibodies which cross-react with the human TSH receptor, a phenomenon known as antigenic mimicry.

The bacterium Yersinia enterocolitica bears structural similarity with the human thyrotropin receptor and was hypothesized to contribute to the development of thyroid autoimmunity arising for other reasons in genetically susceptible individuals. In the 1990s, it was suggested that Y. enterocolitica may be associated with Graves' disease. More recently, the role for Y. enterocolitica has been disputed.

Epstein–Barr virus (EBV) is another potential trigger.

Mechanism

Thyroid-stimulating immunoglobulins recognize and bind to the thyrotropin receptor (TSH receptor) which stimulates the secretion of thyroxine (T4) and triiodothyronine (T3). Thyroxine receptors in the pituitary gland are activated by the surplus hormone, suppressing additional release of TSH in a negative feedback loop. The result is very high levels of circulating thyroid hormones and a low TSH level.

Pathophysiology

Histopathological image of diffuse hyperplasia of the thyroid gland (clinically presenting as hyperthyroidism)

Graves' disease is an autoimmune disorder, in which the body produces antibodies that are specific to a self-protein: the receptor for thyroid-stimulating hormone. (Antibodies to thyroglobulin and to the thyroid hormones T3 and T4 may also be produced.)

These antibodies cause hyperthyroidism because they bind to the TSHr and chronically stimulate it. The TSHr is expressed on the thyroid follicular cells of the thyroid gland (the cells that produce thyroid hormone), and the result of chronic stimulation is an abnormally high production of T3 and T4. This, in turn, causes the clinical symptoms of hyperthyroidism, and the enlargement of the thyroid gland visible as goiter.

The infiltrative exophthalmos frequently encountered has been explained by postulating that the thyroid gland and the extraocular muscles share a common antigen which is recognized by the antibodies. Antibodies binding to the extraocular muscles would cause swelling behind the eyeball.

The "orange peel" skin has been explained by the infiltration of antibodies under the skin, causing an inflammatory reaction and subsequent fibrous plaques.

The three types of autoantibodies to the TSH receptor currently recognized are:

1. Thyroid stimulating immunoglobulins: these antibodies (mainly IgG) act as long-acting thyroid stimulants, activating the cells through a slower and more drawn out process compared to TSH, leading to an elevated production of thyroid hormone.
2. Thyroid growth immunoglobulins: these antibodies bind directly to the TSH receptor and have been implicated in the growth of thyroid follicles.
3. Thyrotrophin binding-inhibiting immunoglobulins: these antibodies inhibit the normal union of TSH with its receptor.
   • Some actually act as if TSH itself is binding to its receptor, thus inducing thyroid function.
   • Other types may not stimulate the thyroid gland, but prevent TSI and TSH from binding to and stimulating the receptor.

Another effect of hyperthyroidism is bone loss from osteoporosis, caused by an increased excretion of calcium and phosphorus in the urine and stool. The effects can be minimized if the hyperthyroidism is treated early. Thyrotoxicosis can also augment calcium levels in the blood by as much as 25%. This can cause stomach upset, excessive urination, and impaired kidney function.

Diagnosis

Graves' disease may present clinically with one or more of these characteristic signs:

• Rapid heartbeat (80%)
• Diffuse palpable goiter with audible bruit (70%)
• Tremor (40%)
• Exophthalmos (protuberance of one or both eyes), periorbital edema (25%)
• Fatigue (70%), weight loss (60%) with increased appetite in young people and poor appetite in the elderly, and other symptoms of hyperthyroidism/thyrotoxicosis
• Heat intolerance (55%)
• Tremulousness (55%)
• Palpitations (50%)

Two signs are truly 'diagnostic' of Graves' disease (i.e., not seen in other hyperthyroid conditions): exophthalmos and nonpitting edema (pretibial myxedema). Goiter is an enlarged thyroid gland and is of the diffuse type (i.e., spread throughout the gland). Diffuse goiter may be seen with other causes of hyperthyroidism, although Graves' disease is the most common cause of diffuse goiter. A large goiter will be visible to the naked eye, but a small one (mild enlargement of the gland) may be detectable only by physical examination. Occasionally, goiter is not clinically detectable, but may be seen only with computed tomography or ultrasound examination of the thyroid.

Another sign of Graves' disease is hyperthyroidism; that is, overproduction of the thyroid hormones T3 and T4. Normal thyroid levels are also seen, and occasionally also hypothyroidism, which may assist in causing goiter (though it is not the cause of the Graves' disease). Hyperthyroidism in Graves' disease is confirmed, as with any other cause of hyperthyroidism, by measuring elevated blood levels of free (unbound) T3 and T4.

Other useful laboratory measurements in Graves' disease include thyroid-stimulating hormone (TSH, usually undetectable in Graves' disease due to negative feedback from the elevated T3 and T4), and protein-bound iodine (elevated). Serologically detected thyroid-stimulating antibodies, radioactive iodine (RAI) uptake, or thyroid ultrasound with Doppler all can independently confirm a diagnosis of Graves' disease.

Biopsy to obtain histiological testing is not normally required, but may be obtained if thyroidectomy is performed.

The goiter in Graves' disease is often not nodular, but thyroid nodules are also common. Differentiating common forms of hyperthyroidism such as Graves' disease, single thyroid adenoma, and toxic multinodular goiter is important to determine proper treatment. The differentiation among these entities has advanced, as imaging and biochemical tests have improved. Measuring TSH-receptor antibodies with the h-TBII assay has been proven efficient and was the most practical approach found in one study.

Eye disease

Further information: Graves' ophthalmopathy

Thyroid-associated ophthalmopathy (TAO), or thyroid eye disease (TED), is the most common extrathyroidal manifestation of Graves' disease. It is a form of idiopathic lymphocytic orbital inflammation, and although its pathogenesis is not completely understood, autoimmune activation of orbital fibroblasts, which in TAO express the TSH receptor, is thought to play a central role.

Hypertrophy of the extraocular muscles, adipogenesis, and deposition of nonsulfated glycoaminoglycans and hyaluronate, causes expansion of the orbital fat and muscle compartments, which within the confines of the bony orbit may lead to dysthyroid optic neuropathy, increased intraocular pressures, proptosis, venous congestion leading to chemosis and periorbital edema, and progressive remodeling of the orbital walls. Other distinctive features of TAO include lid retraction, restrictive myopathy, superior limbic keratoconjunctivitis, and exposure keratopathy.

Severity of eye disease may be classified by the mnemonic: "NO SPECS":

• Class 0: No signs or symptoms
• Class 1: Only signs (limited to upper lid retraction and stare, with or without lid lag)
• Class 2: Soft tissue involvement (oedema of conjunctivae and lids, conjunctival injection, etc.)
• Class 3: Proptosis
• Class 4: Extraocular muscle involvement (usually with diplopia)
• Class 5: Corneal involvement (primarily due to lagophthalmos)
• Class 6: Sight loss (due to optic nerve involvement)

Typically the natural history of TAO follows Rundle's curve, which describes a rapid worsening during an initial phase, up to a peak of maximum severity, and then improvement to a static plateau without, however, resolving back to a normal condition.

Management

Treatment of Graves' disease includes antithyroid drugs that reduce the production of thyroid hormone, radioiodine (radioactive iodine I-131) and thyroidectomy (surgical excision of the gland). As operating on a hyperthyroid patient is dangerous, prior to thyroidectomy, preoperative treatment with antithyroid drugs is given to render the patient euthyroid. Each of these treatments has advantages and disadvantages, and no single treatment approach is considered the best for everyone.

Treatment with antithyroid medications must be administered for six months to two years to be effective. Even then, upon cessation of the drugs, the hyperthyroid state may recur. The risk of recurrence is about 40–50%, and lifelong treatment with antithyroid drugs carries some side effects such as agranulocytosis and liver disease. Side effects of the antithyroid medications include a potentially fatal reduction in the level of white blood cells. Therapy with radioiodine is the most common treatment in the United States, while antithyroid drugs and/or thyroidectomy are used more often in Europe, Japan, and most of the rest of the world.

β-Blockers (such as propranolol) may be used to inhibit the sympathetic nervous system symptoms of tachycardia and nausea until antithyroid treatments start to take effect. Pure β-blockers do not inhibit lid retraction in the eyes, which is mediated by alpha adrenergic receptors.

Antithyroid drugs

The main antithyroid drugs are carbimazole (in the UK), methimazole (in the US), and propylthiouracil/PTU. These drugs block the binding of iodine and coupling of iodotyrosines. The most dangerous side effect is agranulocytosis (1/250, more in PTU). Others include granulocytopenia (dose-dependent, which improves on cessation of the drug) and aplastic anemia. Patients on these medications should see a doctor if they develop sore throat or fever. The most common side effects are rash and peripheral neuritis. These drugs also cross the placenta and are secreted in breast milk. Lugol's iodine may be used to block hormone synthesis before surgery.

A randomized control trial testing single-dose treatment for Graves' found methimazole achieved euthyroid state more effectively after 12 weeks than did propylthyouracil (77.1% on methimazole 15 mg vs 19.4% in the propylthiouracil 150 mg groups).

No difference in outcome was shown for adding thyroxine to antithyroid medication and continuing thyroxine versus placebo after antithyroid medication withdrawal. However, two markers were found that can help predict the risk of recurrence. These two markers are a positive TSHr antibody (TSHR-Ab) and smoking. A positive TSHR-Ab at the end of antithyroid drug treatment increases the risk of recurrence to 90% (sensitivity 39%, specificity 98%), and a negative TSHR-Ab at the end of antithyroid drug treatment is associated with a 78% chance of remaining in remission. Smoking was shown to have an impact independent to a positive TSHR-Ab.

Radioiodine

Scan of affected thyroid before (top) and after (bottom) radioiodine therapy

Radioiodine (radioactive iodine-131) was developed in the early 1940s at the Mallinckrodt General Clinical Research Center. This modality is suitable for most patients, although some prefer to use it mainly for older patients. Indications for radioiodine are failed medical therapy or surgery and where medical or surgical therapy are contraindicated. Hypothyroidism may be a complication of this therapy, but may be treated with thyroid hormones if it appears. The rationale for radioactive iodine is that it accumulates in the thyroid and irradiates the gland with its beta and gamma radiations, about 90% of the total radiation being emitted by the beta (electron) particles. The most common method of iodine-131 treatment is to administer a specified amount in microcuries per gram of thyroid gland based on palpation or radiodiagnostic imaging of the gland over 24 hours. Patients who receive the therapy must be monitored regularly with thyroid blood tests to ensure they are treated with thyroid hormone before they become symptomatically hypothyroid.

Contraindications to RAI are pregnancy (absolute), ophthalmopathy (relative; it can aggravate thyroid eye disease), or solitary nodules.

Disadvantages of this treatment are a high incidence of hypothyroidism (up to 80%) requiring eventual thyroid hormone supplementation in the form of a daily pill(s). The radioiodine treatment acts slowly (over months to years) to destroy the thyroid gland, and Graves' disease–associated hyperthyroidism is not cured in all persons by radioiodine, but has a relapse rate that depends on the dose of radioiodine which is administered. In rare cases, radiation induced thyroiditis has been linked to this treatment.

Surgery

Further information: Thyroidectomy

This modality is suitable for young and pregnant people. Indications for thyroidectomy can be separated into absolute indications or relative indications. These indications aid in deciding which people would benefit most from surgery. The absolute indications are a large goiter (especially when compressing the trachea), suspicious nodules or suspected cancer (to pathologically examine the thyroid), and people with ophthalmopathy and additionally if it is the person's preferred method of treatment or if refusing to undergo radioactive iodine treatment. Pregnancy is advised to be delayed for 6 months after radioactive iodine treatment.

Both bilateral subtotal thyroidectomy and the Hartley-Dunhill procedure (hemithyroidectomy on one side and partial lobectomy on other side) are possible.

Advantages are immediate cure and potential removal of carcinoma. Its risks are injury of the recurrent laryngeal nerve, hypoparathyroidism (due to removal of the parathyroid glands), hematoma (which can be life-threatening if it compresses the trachea), relapse following medical treatment, infections (less common), and scarring. The increase in the risk of nerve injury can be due to the increased vascularity of the thyroid parenchyma and the development of links between the thyroid capsule and the surrounding tissues. Reportedly, a 1% incidence exists of permanent recurrent laryngeal nerve paralysis after complete thyroidectomy. Removal of the gland enables complete biopsy to be performed to have definite evidence of cancer anywhere in the thyroid. (Needle biopsies are not so accurate at predicting a benign state of the thyroid). No further treatment of the thyroid is required, unless cancer is detected. Radioiodine uptake study may be done after surgery, to ensure all remaining (potentially cancerous) thyroid cells (i.e., near the nerves to the vocal cords) are destroyed. Besides this, the only remaining treatment will be levothyroxine, or thyroid replacement pills to be taken for the rest of the patient's life.

A 2013 review article concludes that surgery appears to be the most successful in the management of Graves' disease, with total thyroidectomy being the preferred surgical option.

Eyes

Mild cases are treated with lubricant eye drops or nonsteroidal anti-inflammatory drops. Severe cases threatening vision (corneal exposure or optic nerve compression) are treated with steroids or orbital decompression. In all cases, cessation of smoking is essential. Double vision can be corrected with prism glasses and surgery (the latter only when the process has been stable for a while).

Difficulty closing eyes can be treated with lubricant gel at night, or with tape on the eyes to enable full, deep sleep.

Orbital decompression can be performed to enable bulging eyes to retreat back into the head. Bone is removed from the skull behind the eyes, and space is made for the muscles and fatty tissue to fall back into the skull.

Eyelid surgery can be performed on upper and/or lower eyelids to reverse the effects of Graves' disease on the eyelids. Eyelid muscles can become tight with Graves' disease, making it impossible to close the eyes all the way. Eyelid surgery involves an incision along the natural crease of the eyelid, and a scraping away of the muscle that holds the eyelid open. This makes the muscle weaker, which allows the eyelid to extend over the eyeball more effectively. Eyelid surgery helps reduce or eliminate dry eye symptoms.

For management of clinically active Graves' disease, orbitopathy (clinical activity score >2) with at least mild to moderate severity, intravenous glucocorticoids are the treatment of choice, usually administered in the form of pulse intravenous methylprednisolone. Studies have consistently shown that pulse intravenous methylprednisolone is superior to oral glucocorticoids both in terms of efficacy and decreased side effects for managing Graves' orbitopathy.

Prognosis

If left untreated, more serious complications could result, including birth defects in pregnancy, increased risk of a miscarriage, bone mineral loss and, in extreme cases, death. Graves' disease is often accompanied by an increase in heart rate, which may lead to further heart complications, including loss of the normal heart rhythm (atrial fibrillation), which may lead to stroke. If the eyes are proptotic (bulging) enough that the lids do not close completely at night, dryness will occur – with the risk of a secondary corneal infection, which could lead to blindness. Pressure on the optic nerve behind the globe can lead to visual field defects and vision loss, as well. Prolonged untreated hyperthyroidism can lead to bone loss, which may resolve when treated.

Epidemiology

Most common causes of hyperthyroidism by age

Graves' disease occurs in about 0.5% of people. Graves' disease data has shown that the lifetime risk for women is around 3% and 0.5% for men. It occurs about 7.5 times more often in women than in men^[1] and often starts between the ages of 40 and 60. It is the most common cause of hyperthyroidism in the United States (about 50 to 80% of cases).

History

Graves' disease owes its name to the Irish doctor Robert James Graves, who described a case of goiter with exophthalmos in 1835. Medical eponyms are often styled nonpossessively; thus Graves' disease and Graves disease are variant stylings of the same term.

The German Karl Adolph von Basedow independently reported the same constellation of symptoms in 1840. As a result, on the European Continent, the terms Basedow syndrome, Basedow disease, or Morbus Basedow are more common than Graves' disease.

Graves' disease has also been called exophthalmic goiter.

Less commonly, it has been known as Parry disease, Begbie disease, Flajan disease, Flajani–Basedow syndrome, and Marsh disease. These names for the disease were derived from Caleb Hillier Parry, James Begbie, Giuseppe Flajani, and Henry Marsh. Early reports, not widely circulated, of cases of goiter with exophthalmos were published by the Italians Giuseppe Flajani and Antonio Giuseppe Testa, in 1802 and 1810, respectively. Prior to these, Caleb Hillier Parry, a notable provincial physician in England of the late 18th century (and a friend of Edward Miller-Gallus), described a case in 1786. This case was not published until 1825, which was still ten years ahead of Graves.

However, fair credit for the first description of Graves' disease goes to the 12th century Persian physician Sayyid Ismail al-Jurjani, who noted the association of goiter and exophthalmos in his Thesaurus of the Shah of Khwarazm, the major medical dictionary of its time.

Society and culture

Notable cases

Marty Feldman used his bulging eyes, caused by Graves' disease, for comedic effect.

 

Umm Kulthum in Life Magazine, 1962

• Ayaka, Japanese singer, was diagnosed with Graves' disease in 2007. After going public with her diagnosis in 2009, she took a two-year hiatus from music to focus on treatment.
• Susan Elizabeth Blow, American educator and founder of the first publicly funded kindergarten in the United States, was forced to retire and seek treatment for Graves' disease in 1884.
• George H. W. Bush, former U.S. president, developed new atrial fibrillation and was diagnosed in 1991 with hyperthyroidism due to the disease and treated with radioactive iodine. The president's wife, Barbara Bush, also developed the disease around the same time, which, in her case, produced severe infiltrative exophthalmos.
• Rodney Dangerfield, American comedian and actor
• Gail Devers, American sprinter: A doctor considered amputating her feet after she developed blistering and swelling following radiation treatment for Graves' disease, but she went on to recover and win Olympic medals.
• Missy Elliott, American hip-hop artist
• Marty Feldman, British comedy writer, comedian and actor
• Sia Furler, Australian singer and songwriter
• Sammy Gravano, Italian-American former underboss of the Gambino crime family.
• Jim Hamilton, Scottish rugby player, discovered he had Graves' disease shortly after retiring from the sport in 2017.
• Heino, German folk singer, whose dark sunglasses (worn to hide his symptoms) became part of his trademark look
• Herbert Howells, British composer; the first person to be treated with radium injections
• Yayoi Kusama, Japanese artist.
• Nadezhda Krupskaya, Russian Communist and wife of Vladimir Lenin
• Umm Kulthum was an Egyptian singer, songwriter, and film actress active from the 1920s to the 1970s.
• Barbara Leigh, an American former actress and fashion model, now spokeswoman for the National Graves' Disease Foundation
• Keiko Masuda, Japanese singer and one-half of the duo Pink Lady.
• Yūko Miyamura, Japanese voice actress
• Lord Monckton, former UKIP and Conservative politician and noted climate change skeptic.
• Sophia Parnok, Russian poet
• Sir Cecil Spring Rice, British ambassador to the United States during World War I, died suddenly of the disease in 1918.
• Christina Rossetti, English Victorian-era poet
• Dame Maggie Smith, British actress
• Mary Webb, British novelist and poet
• Wendy Williams, American TV show host
• Act Yasukawa, Japanese Professional wrestler.

Literature

In Italo Svevo's novel Zeno's Conscience, character Ada develops the disease.

Research

Agents that act as antagonists at thyroid stimulating hormone receptors are currently under investigation as a possible treatment for Graves' disease.

Wireless device radiation and health

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Wireless_device_radiation_and_health 

 

A man speaking on a mobile telephone

 

Part of a series on
Antennas

The antennas contained in mobile phones, including smartphones, emit radiofrequency (RF) radiation (non-ionizing "radio waves" such as microwaves); the parts of the head or body nearest to the antenna can absorb this energy and convert it to heat. Since at least the 1990s, scientists have researched whether the now-ubiquitous radiation associated with mobile phone antennas or cell phone towers is affecting human health. Mobile phone networks use various bands of RF radiation, some of which overlap with the microwave range. Other digital wireless systems, such as data communication networks, produce similar radiation.

In response to public concern, the World Health Organization established the International EMF (Electric and Magnetic Fields) Project in 1996 to assess the scientific evidence of possible health effects of EMF in the frequency range from 0 to 300 GHz. They have stated that although extensive research has been conducted into possible health effects of exposure to many parts of the frequency spectrum, all reviews conducted so far have indicated that, as long as exposures are below the limits recommended in the ICNIRP (1998) EMF guidelines, which cover the full frequency range from 0–300 GHz, such exposures do not produce any known adverse health effect. In 2011, International Agency for Research on Cancer (IARC), an agency of the World Health Organization, classified wireless radiation as Group 2B – possibly carcinogenic. That means that there "could be some risk" of carcinogenicity, so additional research into the long-term, heavy use of wireless devices needs to be conducted. The WHO states that "A large number of studies have been performed over the last two decades to assess whether mobile phones pose a potential health risk. To date, no adverse health effects have been established as being caused by mobile phone use."

International guidelines on exposure levels to microwave frequency EMFs such as ICNIRP limit the power levels of wireless devices and it is uncommon for wireless devices to exceed the guidelines. These guidelines only take into account thermal effects, as non-thermal effects have not been conclusively demonstrated. The official stance of the British Health Protection Agency (HPA) is that "[T]here is no consistent evidence to date that Wi-Fi and WLANs adversely affect the health of the general population", but also that "... it is a sensible precautionary approach ... to keep the situation under ongoing review ...". In a 2018 statement, the FDA said that "the current safety limits are set to include a 50-fold safety margin from observed effects of Radio-frequency energy exposure".

Exposure

Mobile phones

A mobile phone connects to the telephone network by radio waves exchanged with a local antenna and automated transceiver called a cellular base station (cell site or cell tower). The service area served by each provider is divided into small geographical areas called cells, and all the phones in a cell communicate with that cell's antenna. Both the phone and the tower have radio transmitters which communicate with each other. Since in a cellular network the same radio channels are reused every few cells, cellular networks use low power transmitters to avoid radio waves from one cell spilling over and interfering with a nearby cell using the same frequencies.

Mobile phones are limited to an effective isotropic radiated power (EIRP) output of 3 watts, and the network continuously adjusts the phone transmitter to the lowest power consistent with good signal quality, reducing it to as low as one milliwatt when near the cell tower. Tower channel transmitters usually have an EIRP power output of around 50 watts. Even when it is not being used, unless it is turned off, a mobile phone periodically emits radio signals on its control channel, to keep contact with its cell tower and for functions like handing off the phone to another tower if the user crosses into another cell. When the user is making a call, the phone transmits a signal on a second channel which carries the user's voice. Existing 2G, 3G, and 4G networks use frequencies in the UHF or low microwave bands, 600 MHz to 3.5 GHz. Many household wireless devices such as WiFi networks, garage door openers, and baby monitors use other frequencies in this same frequency range.

Radio waves decrease rapidly in intensity by the inverse square of distance as they spread out from a transmitting antenna. So the phone transmitter, which is held close to the user's face when talking, is a much greater source of human exposure than the tower transmitter, which is typically at least hundreds of metres away from the user. A user can reduce their exposure by using a headset and keeping the phone itself farther away from their body.

Next generation 5G cellular networks, which began deploying in 2019, use higher frequencies in or near the millimetre wave band, 24 to 52 GHz. Millimetre waves are absorbed by atmospheric gases so 5G networks will use smaller cells than previous cellular networks, about the size of a city block. Instead of a cell tower, each cell will use an array of multiple small antennas mounted on existing buildings and utility poles. In general, millimetre waves penetrate less deeply into biological tissue than microwaves, and are mainly absorbed within the first centimetres of the body surface.

Cordless phones

The HPA also says that due to the mobile phone's adaptive power ability, a DECT cordless phone's radiation could actually exceed the radiation of a mobile phone. The HPA explains that while the DECT cordless phone's radiation has an average output power of 10 mW, it is actually in the form of 100 bursts per second of 250 mW, a strength comparable to some mobile phones.

Wireless networking

Most wireless LAN equipment is designed to work within predefined standards. Wireless access points are also often close to people, but the drop off in power over distance is fast, following the inverse-square law. However, wireless laptops are typically used close to people. WiFi had been anecdotally linked to electromagnetic hypersensitivity but research into electromagnetic hypersensitivity has found no systematic evidence supporting claims made by sufferers.

Users of wireless networking devices are typically exposed for much longer periods than for mobile phones and the strength of wireless devices is not significantly less. Whereas a Universal Mobile Telecommunications System (UMTS) phone can range from 21 dBm (125 mW) for Power Class 4 to 33 dBm (2W) for Power class 1, a wireless router can range from a typical 15 dBm (30 mW) strength to 27 dBm (500 mW) on the high end.

However, wireless routers are typically located significantly farther away from users' heads than a phone the user is handling, resulting in far less exposure overall. The Health Protection Agency (HPA) says that if a person spends one year in a location with a WiFi hot spot, they will receive the same dose of radio waves as if they had made a 20-minute call on a mobile phone.

The HPA's position is that "... radio frequency (RF) exposures from WiFi are likely to be lower than those from mobile phones." It also saw "... no reason why schools and others should not use WiFi equipment." In October 2007, the HPA launched a new "systematic" study into the effects of WiFi networks on behalf of the UK government, in order to calm fears that had appeared in the media in a recent period up to that time. Michael Clark of the HPA says published research on mobile phones and masts does not add up to an indictment of WiFi.

Effects studied

See also: Electromagnetic radiation and health

Blood–brain barrier

A 2010 review stated that "The balance of experimental evidence does not support an effect of 'non-thermal' radio frequency fields" on the permeability of the blood-brain barrier, but noted that research on low frequency effects and effects in humans was sparse. A 2012 study of low-frequency radiation on humans found "no evidence for acute effects of short-term mobile phone radiation on cerebral blood flow".

Cancer

See also: Non-ionizing radiation

There is no known way in which radiofrequency radiation (in contrast to ionizing radiation) affects DNA and causes cancer. In 2011 the IARC, a World Health Organization working group, classified mobile phone use as "possibly carcinogenic to humans". The IARC summed up their conclusion with: "The human epidemiological evidence was mixed. Several small early case–control studies were considered to be largely uninformative. A large cohort study showed no increase in risk of relevant tumours, but it lacked information on level of mobile-phone use and there were several potential sources of misclassification of exposure. The bulk of evidence came from reports of the INTERPHONE study, a very large international, multicentre case–control study and a separate large case–control study from Sweden on gliomas and meningiomas of the brain and acoustic neuromas. While affected by selection bias and information bias to varying degrees, these studies showed an association between glioma and acoustic neuroma and mobile-phone use; specifically in people with highest cumulative use of mobile phones, in people who had used mobile phones on the same side of the head as that on which their tumour developed, and in people whose tumour was in the temporal lobe of the brain (the area of the brain that is most exposed to RF radiation when a wireless phone is used at the ear)". The CDC states that no scientific evidence definitively answers whether mobile phone use causes cancer.

In a 2018 statement, the US Food and Drug Administration said that "the current safety limits are set to include a 50-fold safety margin from observed effects of radiofrequency energy exposure".

On 1 November 2018, the US National Toxicology Program published the final version (after peer review that was performed through March 2018) of its "eagerly anticipated" study using rats and mice, conducted over some ten years. This report concludes after the review with an updated statement that "there is clear evidence that male rats exposed to high levels of radio frequency radiation (RFR) like that used in 2G and 3G cell phones developed cancerous heart tumors.... There was also some evidence of tumors in the brain and adrenal gland of exposed male rats. For female rats, and male and female mice, the evidence was equivocal as to whether cancers observed were associated with exposure to RFR". An analysis of preliminary results from the study argued that due to such issues as the inconsistent appearances of "signals for harm" within and across species and the increased chances of false positives due to the multiplicity of tests, the positive results seen are more likely due to random chance. The full results of the study were released for peer review in February 2018.

A 2021 review concluded 5G radio frequencies in the range of 450 MHz to 6,000 MHz are probably carcinogenic for humans, particularly for gliomas and acoustic neuromas. Conclusions could not be drawn for higher frequencies due to insufficient adequate studies.

Fertility and reproduction

A decline in male sperm quality has been observed over several decades. Studies on the impact of mobile radiation on male fertility are conflicting, and the effects of the radio frequency electromagnetic radiation (RF-EMR) emitted by these devices on the reproductive systems are currently under active debate. A 2012 review concluded that "together, the results of these studies have shown that RF-EMR decreases sperm count and motility and increases oxidative stress". A 2017 study of 153 men that attended an academic fertility clinic in Boston, Massachusetts found that self-reported mobile phone use was not related to semen quality, and that carrying a mobile phone in the pants pocket was not related to semen quality.

A 2021 review concluded 5G radio frequencies in the range of 450 MHz to 6,000 MHz affect male fertility, possibly affect female fertility, and may have adverse effects on the development of embryos, foetuses and newborns. Conclusions could not be drawn for higher frequencies due to insufficient adequate studies.

Electromagnetic hypersensitivity

Main article: Electromagnetic hypersensitivity

Some users of mobile phones and similar devices have reported feeling various non-specific symptoms during and after use. Studies have failed to link any of these symptoms to electromagnetic exposure. In addition, EHS is not a recognized medical diagnosis.

Glucose metabolism

According to the National Cancer Institute, two small studies exploring whether and how mobile phone radiation affects brain glucose metabolism showed inconsistent results.

Effects on children

A report from the Australian Government's Radiation Protection and Nuclear Safety Agency (ARPANSA) in June 2017 noted that:

The 2010 WHO Research Agenda identified a lack of sufficient evidence relating to children and this is still the case. ... Given that no long-term prospective study has looked at this issue to date this research need remains a high priority. For cancer in particular only one completed case-control study involving four European countries has investigated mobile phone use among children or adolescents and risk of brain tumour; showing no association between the two (Aydin et al. 2011). ... Given this paucity of information regarding children using mobile phones and cancer ... more epidemiological studies are needed.

Base stations

Cellular mobile and UHF antenna tower with multiple antennas

Experts consulted by France considered it was mandatory that the main antenna axis should not to be directly in front of a living place at a distance shorter than 100 metres. This recommendation was modified in 2003 to say that antennas located within a 100-metre radius of primary schools or childcare facilities should be better integrated into the city scape and was not included in a 2005 expert report. The Agence française de sécurité sanitaire environnementale as of 2009, says that there is no demonstrated short-term effect of electromagnetic fields on health, but that there are open questions for long-term effects, and that it is easy to reduce exposure via technological improvements. A 2020 study in Environmental Research found that "Although direct causation of negative human health effects from RFR from cellular phone base stations has not been finalized, there is already enough medical and scientific evidence to warrant long-term liability concerns for companies deploying cellular phone towers" and thus recommended voluntary setbacks from schools and hospitals.

Safety standards and licensing

To protect the population living around base stations and users of mobile handsets, governments and regulatory bodies adopt safety standards, which translate to limits on exposure levels below a certain value. There are many proposed national and international standards, but that of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) is the most respected one, and has been adopted so far by more than 80 countries. For radio stations, ICNIRP proposes two safety levels: one for occupational exposure, another one for the general population. Currently there are efforts underway to harmonize the different standards in existence.

Radio base licensing procedures have been established in the majority of urban spaces regulated either at municipal/county, provincial/state or national level. Mobile telephone service providers are, in many regions, required to obtain construction licenses, provide certification of antenna emission levels and assure compliance to ICNIRP standards and/or to other environmental legislation.

Many governmental bodies also require that competing telecommunication companies try to achieve sharing of towers so as to decrease environmental and cosmetic impact. This issue is an influential factor of rejection of installation of new antennas and towers in communities.

The safety standards in the US are set by the Federal Communications Commission (FCC). The FCC has based its standards primarily on those standards established by the National Council on Radiation Protection and Measurements (NCRP) a Congressionally chartered scientific organization located in the WDC area and the Institute of Electrical and Electronics Engineers (IEEE), specifically Subcommittee 4 of the "International Committee on Electromagnetic Safety".

Switzerland has set safety limits lower than the ICNIRP limits for certain "sensitive areas" (classrooms, for example).

In March 2020, for the first time since 1998, ICNIRP updated its guidelines for exposures to frequencies over 6 GHz, including the frequencies used for 5G that are over 6 GHz. The Commission added a restriction on acceptable levels of exposure to the whole body, added a restriction on acceptable levels for brief exposures to small regions of the body, and reduced the maximum amount of exposure permitted over a small region of the body.

Lawsuits

In the US, personal injury lawsuits have been filed by individuals against manufacturers (including Motorola,^[48] NEC, Siemens, and Nokia) on the basis of allegations of causation of brain cancer and death. In US federal courts, expert testimony relating to science must be first evaluated by a judge, in a Daubert hearing, to be relevant and valid before it is admissible as evidence. In a 2002 case against Motorola, the plaintiffs alleged that the use of wireless handheld telephones could cause brain cancer and that the use of Motorola phones caused one plaintiff's cancer. The judge ruled that no sufficiently reliable and relevant scientific evidence in support of either general or specific causation was proffered by the plaintiffs, accepted a motion to exclude the testimony of the plaintiffs' experts, and denied a motion to exclude the testimony of the defendants' experts.

Two separate cases in Italy, in 2009 and 2017, resulted in pensions being awarded to plaintiffs who had claimed their benign brain tumors were the result of prolonged mobile phone use in professional tasks, for 5–6 hours a day, which they ruled different from non-professional use.

In the UK Legal Action Against 5G sought a Judicial Review of the government's plan to deploy 5G. If successful, the group was to be represented by Michael Mansfield QC, a prominent British barrister. This application was denied on the basis that the government had demonstrated that 5G was as safe as 4G, and that the applicants had brought their action too late. 

Precautions

Precautionary principle

In 2000, the World Health Organization (WHO) recommended that the precautionary principle could be voluntarily adopted in this case. It follows the recommendations of the European Community for environmental risks.

According to the WHO, the "precautionary principle" is "a risk management policy applied in circumstances with a high degree of scientific uncertainty, reflecting the need to take action for a potentially serious risk without awaiting the results of scientific research." Other less stringent recommended approaches are prudent avoidance principle and as low as reasonably practicable. Although all of these are problematic in application, due to the widespread use and economic importance of wireless telecommunication systems in modern civilization, there is an increased popularity of such measures in the general public, though also evidence that such approaches may increase concern. They involve recommendations such as the minimization of usage, the limitation of use by at-risk population (e.g., children), the adoption of phones and microcells with as low as reasonably practicable levels of radiation, the wider use of hands-free and earphone technologies such as Bluetooth headsets, the adoption of maximal standards of exposure, RF field intensity and distance of base stations antennas from human habitations, and so forth. Overall, public information remains a challenge as various health consequences are evoked in the literature and by the media, putting populations under chronic exposure to potentially worrying information.

Precautionary measures and health advisories

In May 2011, the World Health Organization's International Agency for Research on Cancer announced it was classifying electromagnetic fields from mobile phones and other sources as "possibly carcinogenic to humans" and advised the public to adopt safety measures to reduce exposure, like use of hands-free devices or texting.

Some national radiation advisory authorities, including those of Austria, France, Germany, and Sweden, have recommended measures to minimize exposure to their citizens. Examples of the recommendations are:

• Use hands-free to decrease the radiation to the head.
• Keep the mobile phone away from the body.
• Do not use telephone in a car without an external antenna.

The use of "hands-free" was not recommended by the British Consumers' Association in a statement in November 2000, as they believed that exposure was increased. However, measurements for the (then) UK Department of Trade and Industry and others for the French Agence française de sécurité sanitaire environnementale [fr] showed substantial reductions. In 2005, Professor Lawrie Challis and others said clipping a ferrite bead onto hands-free kits stops the radio waves travelling up the wire and into the head.

Several nations have advised moderate use of mobile phones for children. A journal by Gandhi et al. in 2006 states that children receive higher levels of Specific Absorption Rate (SAR). When 5- and 10-year-olds are compared to adults, they receive about 153% higher SAR levels. Also, with the permittivity of the brain decreasing as one gets older and the higher relative volume of the exposed growing brain in children, radiation penetrates far beyond the mid-brain.

5G

Further information: 5G conspiracies

Further information: Misinformation related to 5G technology

See also: Misinformation related to the COVID-19 pandemic § 5G mobile phone networks

The FDA is quoted as saying that it "...continues to believe that the current safety limits for cellphone radiofrequency energy exposure remain acceptable for protecting the public health."

During the COVID-19 pandemic, misinformation circulated claiming that 5G networks contribute to the spread of COVID-19.

Bogus products

Products have been advertised that claim to shield people from EM radiation from mobile phones; in the US the Federal Trade Commission published a warning that "Scam artists follow the headlines to promote products that play off the news – and prey on concerned people."

According to the FTC, "there is no scientific proof that so-called shields significantly reduce exposure from electromagnetic emissions. Products that block only the earpiece – or another small portion of the phone – are totally ineffective because the entire phone emits electromagnetic waves." Such shields "may interfere with the phone's signal, cause it to draw even more power to communicate with the base station, and possibly emit more radiation." The FTC has enforced false advertising claims against companies that sell such products.

Cellular network

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cellular_network 

 

Top of a cellular radio tower

 

Indoor cell site in Germany

 

Part of a series on
Antennas

A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically three cell sites or base transceiver stations). These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell.

When joined together, these cells provide radio coverage over a wide geographic area. This enables numerous portable transceivers (e.g., mobile phones, tablets and laptops equipped with mobile broadband modems, pagers, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission.

Cellular networks offer a number of desirable features:

• More capacity than a single large transmitter, since the same frequency can be used for multiple links as long as they are in different cells
• Mobile devices use less power than with a single transmitter or satellite since the cell towers are closer
• Larger coverage area than a single terrestrial transmitter, since additional cell towers can be added indefinitely and are not limited by the horizon

Major telecommunications providers have deployed voice and data cellular networks over most of the inhabited land area of Earth. This allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet access. Private cellular networks can be used for research or for large organizations and fleets, such as dispatch for local public safety agencies or a taxicab company.

Concept

Example of frequency reuse factor or pattern 1/4

In a cellular radio system, a land area to be supplied with radio service is divided into cells in a pattern dependent on terrain and reception characteristics. These cell patterns roughly take the form of regular shapes, such as hexagons, squares, or circles although hexagonal cells are conventional. Each of these cells is assigned with multiple frequencies (f₁ – f₆) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent cells, which would cause co-channel interference.

The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the mobile communication switching system developed by Amos Joel of Bell Labs that permitted multiple callers in a given area to use the same frequency by switching calls to the nearest available cellular tower having that frequency available. This strategy is viable because a given radio frequency can be reused in a different area for an unrelated transmission. In contrast, a single transmitter can only handle one transmission for a given frequency. Inevitably, there is some level of interference from the signal from the other cells which use the same frequency. Consequently, there must be at least one cell gap between cells which reuse the same frequency in a standard frequency-division multiple access (FDMA) system.

Consider the case of a taxi company, where each radio has a manually operated channel selector knob to tune to different frequencies. As drivers move around, they change from channel to channel. The drivers are aware of which frequency approximately covers some area. When they do not receive a signal from the transmitter, they try other channels until finding one that works. The taxi drivers only speak one at a time when invited by the base station operator. This is a form of time-division multiple access (TDMA).

History

See also: History of mobile phones

The first commercial cellular network, the 1G generation, was launched in Japan by Nippon Telegraph and Telephone (NTT) in 1979, initially in the metropolitan area of Tokyo. Within five years, the NTT network had been expanded to cover the whole population of Japan and became the first nationwide 1G network. It was an analog wireless network. The Bell System had developed cellular technology since 1947, and had cellular networks in operation in Chicago and Dallas prior to 1979, but commercial service was delayed by the breakup of the Bell System, with cellular assets transferred to the Regional Bell Operating Companies.

The wireless revolution began in the early 1990s, leading to the transition from analog to digital networks. This was enabled by advances in MOSFET technology. The MOSFET, originally invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, was adapted for cellular networks by the early 1990s, with the wide adoption of power MOSFET, LDMOS (RF amplifier), and RF CMOS (RF circuit) devices leading to the development and proliferation of digital wireless mobile networks.

The first commercial digital cellular network, the 2G generation, was launched in 1991. This sparked competition in the sector as the new operators challenged the incumbent 1G analog network operators.

Cell signal encoding

To distinguish signals from several different transmitters, frequency-division multiple access (FDMA, used by analog and D-AMPS systems), time-division multiple access (TDMA, used by GSM) and code-division multiple access (CDMA, first used for PCS, and the basis of 3G) were developed.

With FDMA, the transmitting and receiving frequencies used by different users in each cell are different from each other. Each cellular call was assigned a pair of frequencies (one for base to mobile, the other for mobile to base) to provide full-duplex operation. The original AMPS systems had 666 channel pairs, 333 each for the CLEC "A" system and ILEC "B" system. The number of channels was expanded to 416 pairs per carrier, but ultimately the number of RF channels limits the number of calls that a cell site could handle. Note that FDMA is a familiar technology to telephone companies, that used frequency-division multiplexing to add channels to their point-to-point wireline plants before time-division multiplexing rendered FDM obsolete.

With TDMA, the transmitting and receiving time slots used by different users in each cell are different from each other. TDMA typically uses digital signaling to store and forward bursts of voice data that are fit into time slices for transmission, and expanded at the receiving end to produce a somewhat normal-sounding voice at the receiver. TDMA must introduce latency (time delay) into the audio signal. As long as the latency time is short enough that the delayed audio is not heard as an echo, it is not problematic. Note that TDMA is a familiar technology for telephone companies, that used time-division multiplexing to add channels to their point-to-point wireline plants before packet switching rendered FDM obsolete.

The principle of CDMA is based on spread spectrum technology developed for military use during World War II and improved during the Cold War into direct-sequence spread spectrum that was used for early CDMA cellular systems and Wi-Fi. DSSS allows multiple simultaneous phone conversations to take place on a single wideband RF channel, without needing to channelize them in time or frequency. Although more sophisticated than older multiple access schemes (and unfamiliar to legacy telephone companies because it was not developed by Bell Labs), CDMA has scaled well to become the basis for 3G cellular radio systems.

Other available methods of multiplexing such as MIMO, a more sophisticated version of antenna diversity, combined with active beamforming provides much greater spatial multiplexing ability compared to original AMPS cells, that typically only addressed one to three unique spaces. Massive MIMO deployment allows much greater channel re-use, thus increasing the number of subscribers per cell site, greater data throughput per user, or some combination thereof. Quadrature Amplitude Modulation (QAM) modems offer an increasing number of bits per symbol, allowing more users per megahertz of bandwidth (and decibels of SNR), greater data throughput per user, or some combination thereof.

Frequency reuse

The key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity. As described above, adjacent cells must use different frequencies, however, there is no problem with two cells sufficiently far apart operating on the same frequency, provided the masts and cellular network users' equipment do not transmit with too much power.

The elements that determine frequency reuse are the reuse distance and the reuse factor. The reuse distance, D is calculated as

${\displaystyle D=R{\sqrt {3N}}}$,

where R is the cell radius and N is the number of cells per cluster. Cells may vary in radius from 1 to 30 kilometres (0.62 to 18.64 mi). The boundaries of the cells can also overlap between adjacent cells and large cells can be divided into smaller cells.

The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).

In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).

If the total available bandwidth is B, each cell can only use a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.

Code-division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually.

Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with a frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinate resource allocation between different cell sites and to limit the inter-cell interference. There are various means of inter-cell interference coordination (ICIC) already defined in the standard. Coordinated scheduling, multi-site MIMO or multi-site beamforming are other examples for inter-cell radio resource management that might be standardized in the future.

Directional antennas

Cellular telephone frequency reuse pattern. See U.S. Patent 4,144,411

Cell towers frequently use a directional signal to improve reception in higher-traffic areas. In the United States, the Federal Communications Commission (FCC) limits omnidirectional cell tower signals to 100 watts of power. If the tower has directional antennas, the FCC allows the cell operator to emit up to 500 watts of effective radiated power (ERP).

Although the original cell towers created an even, omnidirectional signal, were at the centers of the cells and were omnidirectional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge. Each tower has three sets of directional antennas aimed in three different directions with 120 degrees for each cell (totaling 360 degrees) and receiving/transmitting into three different cells at different frequencies. This provides a minimum of three channels, and three towers for each cell and greatly increases the chances of receiving a usable signal from at least one direction.

The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas.

Cell phone companies also use this directional signal to improve reception along highways and inside buildings like stadiums and arenas.

Broadcast messages and paging

Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles. Commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one-to-one communication between the mobile transceiver and the base station. This is called paging. The three different paging procedures generally adopted are sequential, parallel and selective paging.

The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE, cells are grouped into Tracking Areas). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connections.

Movement from cell to cell and handing over

In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If communication was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency.

In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues.

The exact details of the mobile system's move from one base station to the other vary considerably from system to system (see the example below for how a mobile phone network manages handover).

Mobile phone network

WCDMA network architecture

The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station) or transmitting tower. Radio waves are used to transfer signals to and from the cell phone.

Modern mobile phone networks use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that the usually limited number of radio frequencies can be simultaneously used by many callers with less interference.

A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support a large number of active phones in that area. All of the cell sites are connected to telephone exchanges (or switches), which in turn connect to the public telephone network.

In cities, each cell site may have a range of up to approximately 1⁄2 mile (0.80 km), while in rural areas, the range could be as much as 5 miles (8.0 km). It is possible that in clear open areas, a user may receive signals from a cell site 25 miles (40 km) away.

Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term "cell phone" is in some regions, notably the US, used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower but may do so indirectly by way of a satellite.

There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne, CDMA2000, Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). The transition from existing analog to the digital standard followed a very different path in Europe and the US. As a consequence, multiple digital standards surfaced in the US, while Europe and many countries converged towards the GSM standard.

Structure of the mobile phone cellular network

A simple view of the cellular mobile-radio network consists of the following:

• A network of radio base stations forming the base station subsystem.
• The core circuit switched network for handling voice calls and text
• A packet switched network for handling mobile data
• The public switched telephone network to connect subscribers to the wider telephony network

This network is the foundation of the GSM system network. There are many functions that are performed by this network in order to make sure customers get the desired service including mobility management, registration, call set-up, and handover.

Any phone connects to the network via an RBS (Radio Base Station) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an uplink while the other way is termed downlink.

Radio channels effectively use the transmission medium through the use of the following multiplexing and access schemes: frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and space-division multiple access (SDMA).

Small cells

Main article: Small cell

Small cells, which have a smaller coverage area than base stations, are categorised as follows:

• Microcell -> less than 2 kilometres,
• Picocell -> less than 200 metres,
• Femtocell -> around 10 metres,
• Attocell -> 1–4 metres

Cellular handover in mobile phone networks

Main article: Handover

As the phone user moves from one cell area to another cell while a call is in progress, the mobile station will search for a new channel to attach to in order not to drop the call. Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel.

With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise code (PN code) that is specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.

In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test the target channel directly while communicating. In this case, other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel.

If there is no ongoing communication or the communication can be interrupted, it is possible for the mobile unit to spontaneously move from one cell to another and then notify the base station with the strongest signal.

Cellular frequency choice in mobile phone networks

Main article: Cellular frequencies

The effect of frequency on cell coverage means that different frequencies serve better for different uses. Low frequencies, such as 450  MHz NMT, serve very well for countryside coverage. GSM 900 (900 MHz) is suitable for light urban coverage. GSM 1800 (1.8  GHz) starts to be limited by structural walls. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800.

Higher frequencies are a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Picocells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbors.

Cell service area may also vary due to interference from transmitting systems, both within and around that cell. This is true especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio, and the transmitter should not send with too high transmission power in view to not cause interference with other transmitters. As the receiver moves away from the transmitter, the power received decreases, so the power control algorithm of the transmitter increases the power it transmits to restore the level of received power. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased anymore, the signal becomes corrupted and eventually unusable. In CDMA-based systems, the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, cell breathing.

One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites or by looking at independently crowdsourced maps such as Opensignal or CellMapper. In certain cases they may mark the site of the transmitter; in others, it can be calculated by working out the point of strongest coverage.

A cellular repeater is used to extend cell coverage into larger areas. They range from wideband repeaters for consumer use in homes and offices to smart or digital repeaters for industrial needs.

Cell size

The following table shows the dependency of the coverage area of one cell on the frequency of a CDMA2000 network:

Frequency (MHz)	Cell radius (km)	Cell area (km2)	Relative cell count
450	48.9	7521	1
950	26.9	2269	3.3
1800	14.0	618	12.2
2100	12.0	449	16.2