##Multiple access:
Multiple access schemes are used to allow many mobile users to share simultaneously a finite
amount of radio spectrum in the most efficient manner. As the spectrum is limited, so 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.
##Duplexing: In wireless or conventional telephone systems, it is possible to allow the
subscriber to send simultaneously information to the base station while receiving information
from the base station. This effect is called duplexing.
There are two types of duplexing-
1. Frequency division duplexing (FDD):
Frequency division duplexing (FDD) provides two distinct bands of frequencies for every
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, and 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 channel is constant throughout the
system, regardless of the particular channel being used.
2. Time division duplexing (TDD):
Time division duplexing (TDD) uses time instead of frequency to provide both a forward and
reverse link.
TDD allows communication on a single channel (as opposed to requiring two simplex or
dedicated channels) and simplifies the subscriber equipment since a duplexer is not required.
If the time split between the forward and reverse time slot is small, then the transmission and
reception of data appears simultaneous to the user.
Figure 8.1 illustrates FDD and TDD techniques
## Trade-offs between FDD and TDD approaches: There are several trade-offs between
FDD and TDD approaches.
- FDD is geared toward radio communications systems that provide individual radio
frequencies for each user.
- Because each transceiver simultaneously transmits and receives radio signals which vary
by more than 100 dB, the frequency allocation used for the forward and reverse channels
must be carefully coordinated with out-of-band users that occupy spectrum between these
two bands.
- Furthermore, the frequency separation must be coordinated to permit the use of
inexpensive RF technology.
- TDD enables each transceiver to operate as either a transmitter or receiver on the same
frequency.
- TDD eliminates the need for separate forward and reverse frequency bands.
- However, there is a time latency due to the fact that communications is not full duplex in
the truest sense.
- TDD generally is limited to cordless phone or short range portable access.
- TDD is effective for fixed wireless access when all users are stationary so that
propagation delays do not vary in time among users.
##
There are several different ways to allow access to the channel or to share the available
bandwidth in a wireless communication system. These includes mainly the following:
1. Frequency division multiple-access (FDMA)
2. Time division multiple-access (TDMA)
3. Code division multiple-access (CDMA)
4. Space Division Multiple access (SDMA)
5. Packet radio (PR)
Depending upon how the available bandwidth is allocated to the users, these techniques can
be grouped as narrowband and wideband systems.
1. Narrowband Systems:
The term narrowband is used to relate the bandwidth of the single channel to the
expected coherence bandwidth of the channel.
In narrowband multiple access systems, The available spectrum is divided into a large
number of narrowband channels. The channels are usually operated using FDD.
In narrow band FDMA, a user is assigned a particular channel which is not shared by
other users in the vicinity and if FDD is used then the system is called FDMA/FDD.
Narrow band TDMA allows users to use the same channel but allocated a unique
time slot to each user on the channel, thus separating a small number of users in time
on a single channel. For narrow band TDMA, there generally are a large number of
channels allocated using either FDD or TDD, each channel is shared using TDMA.
Such systems are called TDMA/FDD and TDMA/TDD access systems.
2. Wideband 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 greatly
affect the received signal within a wideband channel, and frequency selective fades
occur only in a small fraction of the signal bandwidth.
In wideband multiple access systems, a large number of transmitters are also allowed to
transmit on the same channel. TDMA allocates time slots to the many transmitters 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 time. TDMA and CDMA systems may use either FDD or TDD multiplexing
techniques.
Table-1: MA techniques used in different wireless communication systems
FDMA/FDD
Advanced Mobile Phone Systems(AMPS)
TDMA/FDD
Global System for Mobile(GSM)
TDMA/FDD
U.S. Digital Cellular(USDC)
TDMA/FDD
Japanese Digital Cellular(PDC)
FDMA/TDD
CT2 (Cordless Telephone)
Digital European Cordless FDMA/TDD
Telephone(DECT)
U.S. Narrowband Spread Spectrum (IS-95) CDMA/FDD
W-CDMA(3GPP)
CDMA/FDD
CDMA/TDD
Cdma 2000(3GPP2)
CDMA/FDD
CDMA/TDD
Frequency division multiple access (FDMA) assigns individual channels to
individual users. It can be seen from Figure 8.2 that each user is allocated a
unique frequency band or channel. These channels are assigned on demand to
users who request service. 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 the forward channel, while the
other frequency is used for the reverse channel.
The features of FDMA are as follows:
The FDMA channel carries only one phone circuit at a time.
If an FDMA channel is not in use, then 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 bandwidths of FDMA channels are relatively narrow (30 kHz) as each
channel supports only one circuit per carrier. That is, FDMA is usually
implemented in narrowband systems.
The symbol time is large as compared to the average delay spread. This
implies that the amount of intersymbol interference is low and, thus, little or
no equalization is required in FDMA narrowband 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 needed for
overhead purposes (such as synchronization and framing bits) as compared
to TDMA.
FDMA systems have higher cell site system costs as compared to TDMA systems,
because of the single channel per carrier design, and the need to use
costly bandpass filters to eliminate spurious radiation at the base station.
The FDMA mobile unit uses duplexers 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.
The number of channels that can be simultaneously supported in a FDMA system is
given by
Bt −2 B guard
N=
Bc
Where, Bt is the total spectrum allocation, Bguard is the guard band allocated at the edge of the
allocated spectrum, and B c is the channel bandwidth.
Nonlinear Effects in FDMA-
In a FDMA system, many channels share the same antenna at the base station. The power
amplifiers or the power combiners, when operated at or near saturation for maximum power
efficiency, are nonlinear. The nonlinearities cause signal spreading in the frequency domain
and generate intermodulation (IM) frequencies. IM is undesired RF radiation which
can interfere with other channels in the FDMA systems. Spreading of the spectrum results in
adjacent channel interference. Intermodulation is the generation of undesirable harmonics.
Harmonics generated outside the mobile radio band cause interference to adjacent services,
while those present inside the band cause interference to other users in the wireless system.
