Frequency reuseThe increased capacity in a cellular network, comparing to a network with a singletransmitter, comes from the fact that the same radio frequency can be reused in adifferent area for a completely different transmission. If there is a single plaintransmitter, only one transmission can be used on any given frequency. Unfortunately,there is inevitably some level of interference from the signal from the other cells whichuse the same frequency. This means that, in a standard FDMA system, there must beat least a one cell gap between cells which reuse the same frequency.The frequency reuse factor is the rate at which the same frequency can be used in thenetwork. It is 1/n where n is the number of cells which cannot use a frequency fortransmission.Code division multiple access based systems use a wider frequency band to achievethe same rate of transmission as FDMA, but this is compensated for by the ability touse a frequency reuse factor of 1. In other words, every cell uses the same frequencyand the different systems are separated by codes rather than frequencies.Depending on the size of the city, a taxi system may not have any frequency reuse inits own city, but certainly in other nearby cities, the same frequency can be used. In abig city, on the other hand, frequency reuse could certainly be in use.Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one orseveral frequency bands, or channels. It is particularly commonplace in satellite communication.FDMA, like other Multiple Access systems, coordinates access between multiple users. Alternativesinclude TDMA, CDMA, or SDMA. These protocols are utilized differently, at different levels of thetheoreticalOSI model.Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission. In FDMA all users share the satellite simultaneously but each user transmits at single frequency. FDMA can be used with both analog and digital signal. FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA. FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA. Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA. Each user transmits and receives at different frequencies as each user gets a unique frequency slot
FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple userssimultaneous access to a transmission system, FDD refers to how the radio channel is sharedbetween the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a mobile phone base station). Frequency-division multiplexing (FDM) is also distinct fromFDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channelsthrough a high-bandwidth channel. FDMA, on the other hand, is an access method in the data linklayer.FDMA also supports demand assignment in addition to fixed assignment. Demandassignment allows all users apparently continuous access of the radio spectrum by assigning carrierfrequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed byCOMSAT for use onthe Intelsat series IVA and V satellites.There are two main techniques: Multi-channel per-carrier (MCPC) Single-channel per-carrier (SCPC)
Time division multiple access (TDMA) is a channel access method for shared medium networks. Itallows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using its own time slot. Thisallows multiple stations to share the same transmission medium (e.g. radio frequency channel) whileusing only a part of its channel capacity. TDMA is used in the digital 2G cellular systems suchas Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC)and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portablephones. It is also used extensively in satellite systems, combat-net radio systems,and PON networks for upstream traffic from premises to the operator. For usage of Dynamic TDMApacket mode communication.TDMA is a type of Time-division multiplexing, with the special point that instead of havingone transmitter connected to one receiver, there are multiple transmitters. In the case ofthe uplink from a mobile phone to abase station this becomes particularly difficult because themobile phone can move around and vary the timing advance required to make its transmissionmatch the gap in transmission from its peers.TDMA in 2G systemsMost 2G cellular systems, with the notable exception of IS-95, are based on TDMA. GSM, D-AMPS,PDC, iDEN, and PHS are examples of TDMA cellular systems. GSM combines TDMAwith Frequency Hopping and wideband transmission to minimize common types of interference.In the GSM system, the synchronization of the mobile phones is achieved by sending timingadvance commands from the base station which instructs the mobile phone to transmit earlier andby how much. This compensates for the propagation delay resulting from the light speed velocity ofradio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guardinterval at the end of each time slot. As the transmission moves into the guard period, the mobilenetwork adjusts the timing advance to synchronize the transmission.Initial synchronization of a phone requires even more care. Before a mobile transmits there is noway to actually know the offset required. For this reason, an entire time slot has to be dedicated tomobiles attempting to contact the network (known as the RACH in GSM). The mobile attempts tobroadcast at the beginning of the time slot, as received from the network. If the mobile is locatednext to the base station, there will be no time delay and this will succeed. If, however, the mobilephone is at just less than 35 km from the base station, the time delay will mean the mobilesbroadcast arrives at the very end of the time slot. In that case, the mobile will be instructed tobroadcast its messages starting nearly a whole time slot earlier than would be expected otherwise.
Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in aneighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limitsthe range of a GSM cell to 35 km when no special extension techniques are used. By changing thesynchronization between the uplink and downlink at the base station, however, this limitation can beovercome.3G systemsAlthough most major 3G systems are primarily based upon CDMA, time divisionduplexing (TDD), packet scheduling (dynamic TDMA) and packet oriented multiple access schemesare available in 3G form, combined with CDMA to take advantage of the benefits of bothtechnologies.While the most popular form of the UMTS 3G system uses CDMA and frequency divisionduplexing (FDD) instead of TDMA, TDMA is combined with CDMA and Time Division Duplexing intwo standard UMTS UTRACode division multiple access (CDMA) is a channel access method used by various radiocommunication technologies. It should not be confused with the mobile phonestandards called cdmaOne, CDMA2000 (the 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carriers), which are often referred to as simply CDMA, and use CDMA as anunderlying channel access method.One of the basic concepts in data communication is the idea of allowing several transmitters to sendinformation simultaneously over a single communication channel. This allows several users to sharea band of frequencies (see bandwidth). This concept is called multiple access. CDMAemploys spread-spectrum technology and a special coding scheme (where each transmitter isassigned a code) to allow multiple users to be multiplexed over the same physical channel. Bycontrast, time division multiple access (TDMA) divides access bytime, while frequency-divisionmultiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling,since the modulated coded signal has a much higher data bandwidth than the data beingcommunicated.An analogy to the problem of multiple access is a room (channel) in which people wish to talk toeach other simultaneously. To avoid confusion, people could take turns speaking (time division),speak at different pitches (frequency division), or speak in different languages (code division). CDMAis analogous to the last example where people speaking the same language can understand eachother, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, eachgroup of users is given a shared code. Many codes occupy the same channel, but only usersassociated with a particular code can communicate. The technology of code division multiple access
channels has long been known. In the USSR, the first work devoted to this subject was published in1935 by professor D.V. Aggeev in the "CDMA". It was shown that through the use of linear methods,there are three types of signal separation: frequency, time and compensatory. The technology ofCDMA was used in 1957, when the young military radio engineer Leonid Kupriyanovich in Moscow,made an experimental model of a wearable automatic mobile phone, called LK-1 by him, with a basestation. LK-1 has a weight of 3 kg, 20-30 km operating distance, and 20-30 hours of battery life("Nauka i zhizn", 8, 1957, p. 49, "Yuniy technik", 7, 1957, p. 43-44). The base station, as describedby the author, could serve several customers. In 1958, Kupriyanovich made the new experimental"pocket" model of mobile phone. This phone weighs 0,5 kg. To serve more customers,Kupriyanovich proposed the device, named by him as correllator. ("Nauka i zhizn", 10, 1958, p.66,"Technika-molodezhi", 2, 1959, 18-19) In 1958, the USSR also started the development of the"Altay" national civil mobile phone service for cars, based on the Soviet MRT-1327 standard. Themain developers of the Altay system were VNIIS (Voronezh Science Research Institute ofCommunications)and GSPI (State Specialized Project Institute). In 1963 this service started inMoscow and in 1970 Altay service was used in 30 USSR cities.Space-Division Multiple Access (SDMA) is a channel access method based on creating parallelspatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it isable to offer superior performance in radio multiple access communication systems. In traditionalmobile cellular network systems, the base station has no information on the position of the mobileunits within the cell and radiates the signal in all directions within the cell in order to provide radiocoverage. This results in wasting power on transmissions when there are no mobile units to reach, inaddition to causing interference for adjacent cells using the same frequency, so calledco-channel cells. Likewise, in reception, the antenna receives signals coming from all directionsincluding noise and interference signals. By using smart antenna technology and differing spatiallocations of mobile units within the cell, space-division multiple access techniques offer attractiveperformance enhancements. The radiation pattern of the base station, both in transmission andreception, is adapted to each user to obtain highest gain in the direction of that user. This is oftendone using phased arraytechniques.In GSM cellular networks, the base station is aware of the mobile phones position by use of atechnique called "timing advance" (TA). The Base Transceiver Station (BTS) can determine howdistant the Mobile Station (MS) is by interpreting the reported TA. This information, along with otherparameters, can then be used to power down the BTS or MS, if a power control feature isimplemented in the network. The power control in either BTS or MS is implemented in most modernnetworks, especially on the MS, as this ensures a better battery life for the MS and thus a better userexperience (in that the need to charge the battery becomes less frequent). This is why it may
actually be safer to have a BTS close to you as your MS will be powered down as much as possible.For example, there is more power being transmitted from the MS than what you would receive fromthe BTS even if you are 6 m away from a mast. However, this estimation might not consider all theMSs that a particular BTS is supporting with EM radiation at any given time.Advanced Mobile Phone System (AMPS) was an analog mobile phone system standarddeveloped byBell Labs, and officially introduced in the Americas in 1983, Israel in 1986,and Australia in 1987. It was the primary analog mobile phone system in North America (and otherlocales) through the 1980s and into the 2000s. As of February 18, 2008, carriers in the United Stateswere no longer required to support AMPS and companies such as AT&T and Verizon havediscontinued this service permanently. AMPS was discontinued in Australia in September 2000