What is TDMA?

Time Division Multiple Access (TDMA) is a multiple access technique used by several cellular communication systems.

Multiple access techniques specify how signals from different sources can be combined efficiently for transmission over a given radio frequency band and then separated at the destination without mutual interference. Multiple access techniques enable many users to share the available spectrum in an efficient way.

TDMA is a common multiple access technique employed in digital cellular systems. It allows a number of users to access a single frequency channel by allocating unique time slots to each user within each channel.

The geographic service area is divided into cells with diameters of 2 to 50 km. Each cell is allocated with a number of radio frequency (RF) channels, such that transmitters in adjacent cells operate on different frequencies to avoid interference. However, since transmission power and antenna height in each cell is relatively low, cells that are sufficiently far apart can reuse the same frequencies without interfering with one another.

As a mobile communication system, a cellular system should provide the capability to handoff calls in progress, as the mobile user moves between the cells. Handoffs should be made as transparent as possible to the user in terms of call interruptions and call failures.

A cellular communication system consists of three basic parts: mobile telephones, Base Stations (BS) and a Mobile Switching Center (MSC). Mobile telephones communicate by radio signals to nearby base stations. The base station converts these radio signals for transfer to an MSC via a wired or wireless communication link. The MSC coordinates and routes calls to other mobile telephones or to wired telephones that are connected to the Public Switched Telephone Network (PSTN).

Generally, a fixed amount of spectrum is allocated for a cellular system by a national regulator (For example, the Federal Communications Commission in the United States). Thus, Multiple Access techniques are necessary in order to allow many users to share the available spectrum in an efficient way.

Multiple Access techniques specify the way signals from different sources are to be combined efficiently for transmission over a given radio frequency band and then separated at the destination without mutual interference.

There are three basic multiple access techniques in use in cellular systems:

1. Frequency Division Multiple Access (FDMA)
2. Time Division Multiple Access (TDMA)
3. Code Division Multiple Access (CDMA)

The most conventional method of multiple access is frequency division multiple access. In FDMA, users share the available spectrum in the frequency domain. The signals from different users are transmitted on carriers using different RF center frequencies. The simplest scheme within FDMA architecture is the one where a separate carrier for each channel is provided, which is known as Single Channel Per Carrier (SPSC). This scheme is very efficient because it allows channels to be used in a demand-assigned mode. Within a cell all the channels are available to all the users, and the channel assignment is carried out on a first-come-first-serve basis.

Interference from adjacent channels (within a cell) is limited by the use of guard bands between the bands allocated for different channels.

FDMA is the multiple access technique that was used by the first analog cellular systems that were implemented.

Time division multiple access techniques, that are utilized in many digital cellular systems, rely on the fact that the data signals have been digitized in order to accommodate information from several users within one frequency channel (unlike FDMA that uses one frequency per channel).

In time division multiple access techniques the available spectrum is partitioned into narrow frequency bands (as in FDMA), which in turn are divided into time slots. A user is assigned a time slot that permits access to the frequency channel for the duration of the time slot. Thus, the user can access the frequency channel for a small duration of time periodically.

In case of TDMA systems, guard bands are needed both between frequency channels and between time slots.

Code division multiple access systems are spread spectrum systems in which all the users are permitted to transmit simultaneously, operate at the same nominal frequency and use the entire system's spectrum.

Because all the users can transmit simultaneously throughout the all system frequency spectrum, a private code must be assigned to each user, so that his transmission can be identified. This privacy is achieved by the use of spreading codes (also called pseudo-random noise codes or PN codes) that are orthogonal codes - codes with very low cross correlation among themselves. The information from an individual user is modulated by means of the unique PN code assigned to each user. All the PN-modulated signals from different users are then transmitted over the entire CDMA frequency channel. At the receiving end, the desired signal is recovered by despreading the signal with a copy of the PN code for the individual user. All the other signals (belonging to other users), whose PN-codes do not match that of the desired signal, are not despread and as a result are perceived as noise by the receiver.

In CDMA, because users are permitted to transmit simultaneously and operate at the same nominal frequency, very little dynamic synchronization is needed, as opposed to FDMA and TDMA, where frequency and time management is critical. Furthermore, since signals in CDMA use the entire system's spectrum, no guard bands of any kind are needed.

Time division multiple access takes advantage of the digitization of the signals in order to accommodate information from several users within one frequency channel.

Nyquist's sampling theorem assures that if a band limited signal with bandwidth W is sampled at rate of at least Ws=2*W then the signal can be fully reconstructed from its samples and no information is lost. Thus, the signal's samples can be transmitted instead of the signal itself. But now, the time between transmitted samples can be utilized to transmit samples of other signals in order to increase the capacity of the frequency channel. This is a simplified conceptual description of how TDMA works.

More elaborately, when a user wishes to transmit an analog signal (voice), the signal is sampled, quantized and digitized in a process that is called PCM-Pulse Code Modulation (If the signal is already digitized (data signal) this is unnecessary). As a result the signal is converted into a stream of digital information. The stream is compressed by a digital speech code into bursts 1/n of their original length. The burst takes only 1/n of the airtime required to transmit the original audio signal, leaving n-1/n of the time for the other u. The digital burst is then modulated into the channel's frequency and in the time slot that was allocated for the user the burst is transmitted. The channel's frequency and the allocated time slot for the user are informed to the mobile user by the base station when the call is set up via a control channel.

TDMA is a store and burst system. Incoming user traffic is stored in memory and when a user's turn comes up, this accumulated traffic is transmitted in a digital burst.
In TDMA, the transmission is divided to frames, which contain several time slots. In each time slot, a different user transmits his digital burst. This method of multiplexing that combines data streams by assigning each stream a different time slot in a set is called Time Division Multiplexing (TDM) (this technique is also used in T1/E1 channels).

The number of time slots in a frame (n) is standard dependent. Effectively, TDMA implementations that use n:1 multiplexing (i.e. divide the channel's given bandwidth into n time slots) increase capacity by n. North American cellular standards IS-54 and IS-136, for example, triple the capacity of cellular frequencies by dividing a 30-kHz channel into three time slots, enabling three different users to occupy it at the same time.

Currently, systems are in place that allows six times capacity. In the future, with the utilization of hierarchical cells, intelligent antennas, and adaptive channel allocation, the capacity should approach 40 times analog capacity.

TDMA substantially improved upon the efficiency of analog cellular. However, like FDMA, it had the weakness that it wasted bandwidth: the time slot was allocated to a specific conversation whether or not anyone was speaking at that moment.

An enhanced version of TDMA (ETDMA) improve the bandwidth utilization by assigning time slots to users dynamically instead of waiting to determine whether a user is transmitting. ETDMA sends data through those pauses which normal speech contains. When a user has something to transmit, he puts one bit in a buffer queue. The system scans the buffer, notices that the user has something to transmit, and allocates bandwidth accordingly. If a user has nothing to transmit, he is skipped. So, instead of being arbitrarily assigned, time is allocated according to need. If partners in a phone conversation do not speak over one another, this technique can almost double the bandwidth utilization efficiency of TDMA, making it almost 10 times as efficient as analog transmission.

The wireless industry began to explore converting the existing analog network to digital as a means of improving capacity back in the late 1980s.

In 1988, the Cellular Telecommunications Industry Association (CTIA) developed a guideline for the next generation of cellular technology called User performance Requirements, and Telecommunication Industry Association (TIA) used this guideline to create a TDMA digital standard, called IS-54. The first version of IS-54 specification identified the basic parameters (for example, time slot structure, type of radio channel modulation and message formats) needed to begin designing TDMA cellular equipment. But IS-54 lacks some basic features that were introduced in the first commercial TDMA phones. Soon, IS-54 REV A was born to correct errors and to add some basic features (such as call id) to the TDMA standard.

In 1989, CTIA chose TDMA over Motorola's FDMA (today known as Narrowband Analog Mobile-Phone Service [NAMPS]) narrowband standard as the technology of choice for existing 800 MHz cellular markets and for emerging 1.9-GHz markets. With the growing technology competition applied by Qualcomm in favor of CDMA and the realities of the European global system for mobile communications standard, the CTIA decided to let carriers make their own technology selection.

Because of its adoption by the European standard, the Japanese Digital Cellular (JDC) and North American Digital Cellular (NADC), TDMA and its variants are currently the technology of choice throughout the world. However, over the last few years, a debate has convulsed the wireless community over the respective merits of TDMA and CDMA.

The TDMA system is designed for use in a range of environments and situations, from hand portable use in a downtown office to a mobile user traveling at high speed on the freeway. The system also supports a variety of services for the end user, such as voice, data and fax short message services and broadcast messages. TDMA offers a flexible air interface, providing high performance with respect to capacity, coverage, and unlimited support of mobility and capability to handle different types of user needs.

In 1991, IS-54 REV B added features such as authentication, voice privacy, and a more capable caller ID with great benefit to the user. Digital TDMA still evolve beyond IS-54 REV B, so a new standard is needed to cover specification of all these features. That is IS-136 (see applications).

There are three different applications of the TDMA RF technology:

The IS-136 evolved from the TDMA standard IS-54. The IS-54 standard identified the critical parameters (e.g. time slot structure, type of radio channel and modulation) and included like calling number identity, voice privacy, message waiting indicator etc. In order to include further features such as enhanced battery life, the digital control channel athe IS-136 series were developed. Because of the evolution process IS-136 phones can operate in an analog or digital environment. Digital traffic channels are divided into frames with six time slots. Every communications channel consists of two 30-Khz wide channels (from the cell site to the mobile phone [forward] and vice versa). The time slots between the forward and reverse channels are related so that the mobile phone cannot simultaneously transmit and receive.

The Global System for Mobile communications (GSM) is a European digital cellular standard that uses a different form of TDMA than IS- 136.

A primary feature of the GSM system is its use of a single type of digital radio channel. Each 200-Khz wide GSM digital radio channel is divided into frames with eight time slots. Every GSM channel consists of radio channels, a forward channel and a reverse channel. Again the mobile cannot transmit and receive simultaneously. The GSM system also uses portable Subscriber Identity Module (SIM) cards that contain the identity of the customer.

PDC is a cordless communications standard, developed in Japan to provide high quality, cheap and flexible communications solutions. PDC uses a micro-cell configuration with a base station range of 100-300 meters in diameter. Because of the low transmit power, smaller and lighter handsets with longer talk and standby times are supported. PDC also supports a 32kbits/sec data link, which enables fixed-line quality voice and high rate data services.

BS - The base station is a multicircuit transceiver located at the center of a cell whose primary purpose is to handle all incoming and outgoing calls within the cell. The base station relays the mobile's signal to the MSC via wireline.

CDMA - Code division multiple access. CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CTIA - Cellular telecommunications industry association

Error Correction and Detection Coding - The process of coding digital information before it is transmitted on a noisy channel, in order to allow error correction capability or error detection capability. Error correcting codes add redundancy to the information that allows the correction (or detection) of several errors.

FDMA - Frequency division multiple access. FDMA is a basic technology in the analog Advanced Mobile Phone Service, the most widely installed cellular phone system installed in North America. With FDMA, each channel can be assigned to only one user at a time.

GSA - Geographic Service Area - A service area of cellular system.

GSM - Global system for mobile communications. GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot.

Handoff -The process by which subscribers traveling throughout the system coverage area are switched from cell-to-cell (and different channels) with better coverage for that particular area when poor-quality conversation is detected.

JDC - Japanese digital cellular

Modulation - Modulation is the addition of information to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals.

MSC - Mobile switching center - The switching office that all base station cell sites connect to. The MSC in turn interfaces to the PSTN. Control of all cell sites, all subscriber records, statistics, and billing is maintained at the MSC.

NADC - North American digital cellular

NAMPS - Narrowband analog mobile-phone service

PCM - Pulse Code Modulation. Process in which the modulating signal is sampled, and the magnitude of each sample (with respect to a fixed reference) is quantized and converted by coding to digital signal.

PDC - Personal Digital Cellular - a cordless communications standard, developed in Japan to provide high quality, cheap and flexible communications solutions.

PSTN - Public switched telephone network

SIM - Subscriber Identity Module - a card that contains the identity of the customer.

Spectrum - The electromagnetic radiation spectrum is the complete range of the wavelengths of electromagnetic radiation, beginning with the longest radio waves (including those in the audio range) and extending through visible light.

Speech Coding - process that enables to transmit, store or synthesize speech at given quality using fewer bits. Bit rate reduction is accomplished by taking advantage of the redundancies in the speech signal, the high correlation between samples of the speech signal and the perceptual limitations of the human ear.

Spread Spectrum - Communication technique in which the intended signal is spread over a bandwidth in excess of the minimum bandwidth required for the transmission of the signal.

SPSC - Single Channel Per Carrier - The simplest scheme within FDMA architecture is the one where a separate carrier for each channel is provided.

TDM - Time Division Multiplexing is a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel.

TDMA - Time division multiple access

TIA - Telecommunication industry association

1. IS-136 TDMA - Technology, Economics and Services\ Lawrence J.Harte, Charles A.Jacobs, Adrian D.Smith
2. Foundations of Mobile Radio Engineering\ Michel Daoud Yacoub
3. Mobile and Personal Communication Systems and Services\ Raj Pandya