InterceptRadio.com Forums

Time Division Multiple Access (TDMA)

Time Division Multiple Access (TDMA) is a digital modulation technique used in digital cellular telephone and mobile radio communication. TDMA is one of two ways to divide the limited spectrum available over a radio frequency (RF) cellular channel. The other is known as frequency division multiple access (FDMA).

In simplest terms, TDMA enables multiple users to share the same frequency by dividing each cellular channel into different time slots. In effect, a single frequency supports multiple and simultaneous data channels. So, with a two-time slot TDMA, two users can share the same frequency. With a three-time slot TDMA, three users can share the same frequency and so on.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Frequency-division multiple access (FDMA)

is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user. Users can send data through a subchannel by modulating it on a carrier wave at the subchannel's frequency. It is used in satellite communication systems and telephone trunklines.

FDMA splits the total bandwidth into multiple channels. Each ground station on the earth is allocated a particular frequency group (or a range of frequencies). Within each group, the ground station can allocate different frequencies to individual channels, which are used by different stations connected to that ground station. Before the transmission begins, the transmitting ground station looks for an empty channel within the frequency range that is allocated to it and once it finds an empty channel, it allocates it to the particular transmitting station

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Code-division multiple access (CDMA)

is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code).

CDMA optimizes the use of available bandwidth as it transmits over the entire frequency range and does not limit the user's frequency range.

It is used as the access method in many mobile phone standards. IS-95, also called "cdmaOne", and its 3G evolution CDMA2000, are often simply referred to as "CDMA", but UMTS, the 3G standard used by GSM carriers, also uses "wideband CDMA", or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies. Many carriers (such as AT&T and Verizon) shut down 3G CDMA-based networks in 2022, rendering handsets supporting only those protocols unusable for calls, even to 911.

It can be also used as a channel or medium access technology, like ALOHA for example or as a permanent pilot/signalling channel to allow users to synchronize their local oscillators to a common system frequency, thereby also estimating the channel parameters permanently.

In these schemes, the message is modulated on a longer spreading sequence, consisting of several chips (0es and 1es). Due to their very advantageous auto- and crosscorrelation characteristics, these spreading sequences have also been used for radar applications for many decades, where they are called Barker codes (with a very short sequence length of typically 8 to 32).

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). Other schemes use subcarriers based on binary offset carrier modulation (BOC modulation), which is inspired by Manchester codes and enable a larger gap between the virtual center frequency and the subcarriers, which is not the case for OFDM subcarriers.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


What are the differences among TDMA, FDMA and CDMA?

Like TDMA, FDMA is also a channelization protocol. But, unlike TDMA, FDMA separates

cellular channels (bandwidth) by frequency, not time. Each station is assigned a specific frequency band to send data. So, if users want two channels, they get two separate frequency bands.

In FDMA, different stations with different frequency bands are separated by small bands of unused frequency bands (guard bands) to prevent interference between stations. Unlike TDMA, synchronization is not required in FDMA. However, the drawback of FDMA is that it delivers lower power efficiency. FDMA is mostly used in GSM and PDC systems.

An alternative multiplexing scheme to FDMA and TDMA is Code-Division Multiple Access (CDMA). CDMA takes the entire allocated frequency range for a given service and multiplexes information for all users across the spectrum range at the same time. To put it simply, CDMA makes it possible for all stations to transmit data simultaneously without the need for frequency- or time-based multiplexing.

The biggest advantages of CDMA are that it offers high data rates and is highly flexible. Further, unlike TDMA, CDMA does not require synchronization. A drawback of CDMA is that different stations share both bandwidth and time, necessitating both guard bands and guard time.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

_________________
" SILENCE IS CONSENT "

Jim N7UAP - Bellingham, WA / InterceptRadio.com