Code-division multiple access
Many carriers (such as AT&T, UScellular and Verizon) shut down 3G CDMA-based networks in 2022 and 2024, rendering handsets supporting only those protocols unusable for calls, even to 911.
For space-based communication applications, CDMA has been used for many decades due to the large path loss and Doppler shift caused by satellite motion.
CDMA is often used with binary phase-shift keying (BPSK) in its simplest form, but can be combined with any modulation scheme like (in advanced cases) quadrature amplitude modulation (QAM) or orthogonal frequency-division multiplexing (OFDM), which typically makes it very robust and efficient (and equipping them with accurate ranging capabilities, which is difficult without CDMA).
[clarification needed] The technology of CDMA 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 base station.
"[11][12] In 1958, the USSR also started the development of the "Altai" national civil mobile phone service for cars, based on the Soviet MRT-1327 standard.
is called the spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.
[18][19] An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously.
In the analog case, a low-frequency data signal is time-multiplied with a high-frequency pure sine-wave carrier and transmitted.
Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings.
(Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.)
When mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, a different approach is required.
All forms of CDMA use the spread-spectrum spreading factor to allow receivers to partially discriminate against unwanted signals.
A CDM (synchronous CDMA), TDMA, or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems.
This leads to a general requirement in any asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver.
In CDMA cellular, the base station uses a fast closed-loop power-control scheme to tightly control each mobile's transmit power.
In 2019, schemes to precisely estimate the required length of the codes in dependence of Doppler and delay characteristics have been developed.
[20] Soon after, machine learning based techniques that generate sequences of a desired length and spreading properties have been published as well.
Similarly, FDMA systems must use a guard band between adjacent channels, due to the unpredictable Doppler shift of the signal spectrum because of user mobility.
The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.
There are a fixed number of orthogonal codes, time slots or frequency bands that can be allocated for CDM, TDMA, and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions.
In a bursty traffic environment like mobile telephony, the advantage afforded by asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization.
In other words, asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals.
CDM (synchronous CDMA), TDMA, and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters.
By comparison, asynchronous CDMA transmitters simply send when they have something to say and go off the air when they do not, keeping the same signature sequence as long as they are connected to the system.
Since narrow-band interference affects only a small portion of the spread-spectrum signal, it can easily be removed through notch filtering without much loss of information.
Since the spread-spectrum signal occupies a large bandwidth, only a small portion of this will undergo fading due to multipath at any given time.
The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.
Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system.
In a hard-hand-off situation, as the mobile telephone approaches a hand-off, signal strength may vary abruptly.
The authors show that it is possible to achieve this increase at a low complexity and high bit error rate performance in flat fading channels, which is a major research challenge for overloaded CDMA systems.
