Editing Cellular network (section)

== Cell signal encoding ==
To distinguish signals from several different transmitters, a number of [[channel access method]]s have been developed, including [[frequency-division multiple access]] (FDMA, used by analog and [[D-AMPS]]{{citation needed|date=November 2019}} systems), [[time-division multiple access]] (TDMA, used by [[GSM]]) and [[code-division multiple access]] (CDMA, first used for [[Personal Communications Service|PCS]], and the basis of [[3G]]).<ref name=Zander/>

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 [[Advanced Mobile Phone Service|AMPS]] systems had 666 channel pairs, 333 each for the [[competitive local exchange carrier|CLEC]] "A" system and [[incumbent local exchange carrier|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. FDMA is a familiar technology to telephone companies, which 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 data|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 (audio)|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. TDMA is a familiar technology for telephone companies, which 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 reuse, 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.