MULTIPLE ACCESS TECHNIQUES FOR WIRELESS COMMUNICATIONS
A.B.M. Siddique Hossain
Multiple Access schemes allow many mobile users to simultaneously share a finite amount of radio spectrum. The sharing of spectrum is required to achieve high capacity by simultaneously allocating the available bandwidth (or the available amount of channels) to multiple users. For high quality communications, this must be done without severe degradation in the performance of the system.
In wireless communication systems, it is often desirable to allow subscribers to simultaneously send information to the base station while receiving information from the base station. For example, in conventional telephone systems, it is possible to simultaneously talk and listen, and this effect, called duplexing, is generally required in wireless telephone systems. Duplexing may be done using frequency or time domain techniques. Frequency Division Depleting (FDD) provides two distinct bands of frequencies for each user. The forward band provides traffic from the base station to the mobile, and the reverse band provides traffic from the mobile to the base. In FDD, any duplex channel actually consists of two simplex channels. A device called a duplexer is used inside each subscriber unit and base station to allow simultaneous radio transmission and reception on the duplex channel pair. The frequency split between the forward and reverse hand provides traffic from the mobile to the base. In FDD, any duplex channel actually consists of two complex channels. A device challed a duplexer is used inside each subscriber unit and base station to allow simultaneous radio transmission and reception on the duplex channel pair. The frequency split between the forward and reverse channel is constant throughout the system, regardless of particular channel being used. Time Division Multiplexing (TDM) uses time instead of frequency to provide both a forward and reverse link. If the time split between the forward and reverse time slot is small, then the transmission and reception of data appears simultaneously to the user. Figure 1 illustrates FDD and TDD techniques. TDD allows communication on single or dedicated channel as opposed to requiring two simplex channels, and simplifies the subscriber equipment since a duplexer is not required.
Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) are the three major access techniques used to share the available bandwidth in a wireless communication system. These techniques can be grouped as narrowband and wideband systems, Depending upon how the available bandwith is allocated to the users.
The term narrowband is used to relate the bandwidth of a single channel to the operated coherence bandwidth of the channel. In a narrowband multiple access system, the available radio spectrum is channelized into a large number of narrowband channels. The channels are usually assigned using FDD. To minimize the interference between the forward and reverse links on each channel, the frequency split is made as great as possible within the frequency spectrum, while still allowing inexpensive duplexer and a common transceiver antenna to be used in each subscriber unit. In narrowband FDMA, a user is assigned a particular channel which is not shared by other user in the vicinity, and if FDD is used (that is each channel has a forward and a reverse link), then the system is called FDMAIFDD. Narrowband TDMA, on the other hand, allows user to share the same channel but allocates a unique lime slot to each user in a cyclical fashion on the channel, thus separating a small number of users in lime on a single channel. For narrowband TDMA, there generally are a large number of channels allocated using either FDD or TDD, and each channel is shared using TDMA. Such systems are called TDMAJFDD or TDMAITDD access systems.
In wideband systems, the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. Thus, multipath fading does not greatiy affect the received signal within a wideb and channel, and frequency selective fades occur in only a small fraction of the signal bandwidth. In wideband multiple access systems, the user are allowed to transmit in a large part of the spectrum. A large number of transmitters are also allowed to transmit on the same channel. TDMA allocates lime slots to many ttransmitters on the same channel and allows only one transmitter to access the channel at any instant of time, whereas spread spectrum CDMA allows all of the transmitters to access the channel at the same lime. ‘DMA and CDMA systems may use either FDD or TDD multiplexing techniques.
In addition to FDMA, TDMA and CDMA, two other multiple access schemes are used for wireless communications. These are Packed Radio (PR) and Space Division Multiple Access (SDMA).
Table 1 shows the different multiple access techniques being used in different wireless communication systems.
Table 1 Multiple Access techniques used in different
wireless communication systems
Advanced Mobile Phone System (AMPS)
Global System for Mobile (GSM)
FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)
Frequency Division Multiple Access (FDMA) assigns individual channels to individual users. Figure 2 shows that each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service.
Figure: 2 Frequency Division Multiple Access where different channels are assigned different frequency bands.
During the period of the call, no other user can share the same frequency band. In FDD systems, the users are assigned a channel as a pair of frequencies; one frequency is used for forward channel, while the other frequency is used for the reverse channel.
The features of FDMA are:
* The FDMA channel carries only one phone circuit at a time.
* If an FDMA channel is not in use, it sits idle and cannot be used by other users to increase or share capacity. It is essentially a wasted resource.
* After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously.
* The bandwidth of FDMA channels are relatively narrow (=3O Khz) as each channel supports only one circuit per carrier. That is, FDMA is usually implemented in narrow band systems.
* The symbol time is large as compared to the average delay spread. This implies that the amount of inter symbol interference is low and, thus, little or no equalization is required in FDMA narrow band systems.
* The complexity of FDMA mobile systems is lower when compared to TDMA systems, though this is changing as digital signal processing methods improve for TDMA.
* Since FDMA is a continuous transmission scheme, fewer bits are required bits overhead purposes (such as synchronization and framing bits) compared to TDMA.
* FDMA systems have higher cell site system costs compared to TDMA systems. This is due to the single channel per carrier design and the need to use costly band pass filters to eliminate spurious radiation at the base station.
* The FDMA mobile unit uses duplexes, since both the transmitter and receiver operate at the same time. This results in an increase in the cost of FDMA subscriber units and base stations.
* FDMA requires tight RF filtering to minimize adjacent channel interference.
TIME DIVISION MULTIPLE ACCESS (TDMA)
Time Division Multiple Access (TDMA) systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive. Figure 3 shows that each user occupies a cyclically repeating time slot, so a channel may be thought of as a particular time slot that reoccurs every frame, where N time slots comprise a frame.
Figure 3: Time Division Multiple Access scheme where each channel
occupies a cyclically repeating time slot.
TDMA systems transmit data in a buffer-and-burst method; thus the transmission for any user is non-continuous. This implies that, unlike FDMA system which accommodate analog FM, digital data and digital modulation must be used with TDMA. The transmission from various users are interlaced into a repeating frame structure as shown in Figure 4.
Figure: TDMA Frame Structure
A fram consists of a number of slots. Each frame is made up of a preamble, information message, and trail bills. In a TDMAITDD system, half of the slots in the fram information message are used for the forward link channels and half would be used for reverse link channels. In TDMAIFDD systems, an identical or similar frame structure is used solely for either forward or reverse transmission, but the carrier frequencies are different for the forward and reverse links. In general, TDMA/FDD systems intentionally induce sveral time slots of delay between the forward and reverse time slots of a particular user, so that duplexers are not required in the subscriber unit.
IN a TDMA fram, the preamble contains the address and synchronization information that hot the base station and the subscribers use to identify each other. Guard limes allow synchronization of the receivers between slots and frames. Different TDMA wireless standards have different TDMA frame structures. In mobile radio communications a number of TDMA carriers, each carrying users, may he assigned unique carriers to produce a TDMA/FDMA multiple access arrangement. This approach where each TDMAIFDMA operates with FDD is employed by the pan-European GSM and the American IS-54 systems. The digital European cordless telecommunication (DECT) network uses TDMAITDD/TDMA.
The features of TDDMA are:
* TDMA shares a single carrier frequency with several users, makes use of non-overlapping lime slots. The number of time slots per frame depends on several factors, such as modulation technique, available bandwidth etc.
* Data transmission for users of a TDMA system is not continuous, but occurs in bursts. This results in low battery consumption, since the subscriber transmitter can be tu rued off when in use ( which is most of te time)
- Because of discontinuous transmissions in TDMA, the handoff process is much simpler for a subscriber unit, since it is able to listen for other base stations during idle time slots. An enhanced link control, lksuch as that provided by mobile assisted handoff(MAHO) can be carried out by a subscriber by listening on an idle slot in the TDMA frame.
- TDMA uses different time slots for transmission and reception, and thus duplexers are not required. Even if FDD is used, a switch rather than a Duplixer inside the subscriber unit is all that is required to switch between transmitter and receiver using TDMA.
- Adaptive equalization is usually necessary in TDMA systems, since the transmission rates are generally very high compared to FDMA channels.
- In TDMA, the guard time should be minimized. But if transmitted signal at the edges of a time slot are suppressed sharply in order to shorten the guard time, the transmitted spectrum will expand and cause interference to adjacent channels.
- High synchronization overhead is required in TDMA systems because of burst transmissions. TDMA transmissions are slotted, and this requires the receivers to be synchronized for each data burst. In addition, guard slots are necessary to separate users and this results in the TDMA systems having larger overheads compared to FDMA.
- TDMA has an advantage in that it is possible to allocate different numbers of time slots per frame to different users. Thus bandwidth can he supplied on demand to different users by reassigning time slots based on priority.
Code Division Multiple Access (CDMA)
In Code Division Multiple Access (CDMA) systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo noise (PN) code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message.
Figure 5: Code Division Multiple Access in which each channel is assigned a unique orthogonal to PN codes used by other users.
All users in a CDMA system, as seen from Figure 5, use the same carrier frequency and may transmit simultaneously. Each user has its own pseudo-random codeword which is approximately orthogonal to all other desired codeword. The receiver performs a time correlation operation to detect only the specific desired codeword. All other codeword appear as noise due to deceleration. For detection of the message signal, the receiver needs to know the codeword used by the transmitter. Each user operates independently with no knowledge of the other’s frequency or time slot.
In CDMA, the power of multiple users at a receiver determines the noise floor after deceleration. If the power of each user within a cell is not controlled so that they do not appear equal at the base station receiver, then the near-far problem occurs. The near-far problem occurs when nearly subscriber transmitters overpower the base station receiver and drown out the received signals of far away subscribers.
The features of CDMA are:
* Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.
* Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor in a linear manner. Thus there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased, and improves as the number of users is decreased.
* If the spreading of the CDMA signal is much greater than the coherence bandwidth of the channel, multi path fading is substantially reduced hecasue the signal provides frequency diversity.
* Channel transmission rates are very high in typical CDMA systems. Consequently, the symbol (chip) duration is very short and usually much less than the channel delay spread.
* Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC (Mobile Switching Centre), which can simultaneously monitor a particular user from two or more base stations. The MSC may choose the best version of the signal at any time without switching frequencies.
* Sefl-jamming is a problem in CDMA system. Self-jamming arises from the spreading sequences of different users that are not exactly orthogonal, and hence in the dispersing of a particular PN code nonzero contributions to the receiver decision statistic for a desired user arise from the transmissions of other users in the system.
* The near-far problem occurs at a CDMA receiver if a non-desired user has a high detected power as compared to be desired user. When a base station receiver attempts to receive a particular mobile, interferes (such as out-of-cell mobiles not under power control) may capture the receiver. The ability to control the received power from each mobile (wherever it is within the cell) so that it is the same at the base station in the presence of Raleigh fading is a major problem in CDMA.
Dr A. B.M. Siddique Hossain was born in Barisal on 31st March, 1948. He obtained B. Sc. Engg. (Electrical) degree from BUET in 1969, Masters in 1971 and Ph.D. from the National Technical University of Athens, Greece in 197& He has more than 26 years of teaching and research experience and more than 15 publications both at home and abroad. Currently he is a professor and Head of the department of Electrical & Electronic Engg. in BUET