Multiple access techniques allow multiple users to share the same wireless spectrum simultaneously. Common techniques include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). FDMA assigns each user a different frequency band. TDMA assigns each user time slots on the same frequency. CDMA spreads each user's signal across the entire frequency band using unique codes.

1. Multiple Access
Techniques

2.  Multiplexing:-
◦ It is the process in which no. of input signals can be
transmitted through single channel.
◦ It produces more interference or overlapping between
the signals.
◦ To avoid the interference or overlapping, the multiple
access techniques are used.

3.  It is the process in which multiple users can share
simultaneously a finite amount of radio spectrum or
wireless frequency spectrum.
 The sharing of spectrum is required to achieve a high
capacity by simultaneously allocating the available
bandwidth or available amount of channels to multiple
users

5.  If data traffic is continuous and there is small transmission delay
then reservation based multiple access is used.
 For different user different channel is assigned. If the particular user
is absent then the assigned channel remain idle and no other user
can access the channel with out the permission.
 The family of reservation-based multiple access includes frequency
division multiple access (FDMA), time division multiple access
(TDMA), and code division multiple access (CDMA)

6.  If data traffic is discontinuous then random based
multiple access is used.
 Here any user can be assigned to any channel if the
particular user is absent.
 The family of Random-based multiple access
includes ALOHA ,carrier sense multiple access
(CSMA) and Idle Signal Casting Multiple Access
(ISMA)

7.  If the data arrives in a random manner and the data
length is large, then random multiple access combined
with a reservation protocol is used as it performs better
than both random and reservation based schemes.

8.  It Provides two distinct bands of frequencies for every user.
◦ The frequency used for transmission from the base station to the
mobile station is called the forward channel (downlink
channel).
◦ The frequency used for transmission from the mobile station to
the base station is called the reverse channel (uplink channel).
 It provides simultaneous transmission channels for mobile/base
station

9.  At the base station, separate transmit and receive antennas are
used to accommodate the two separate channels
 At the mobile unit, a single antenna (with duplexer) is used to
enable transmission and reception.
 To facilitate FDD, sufficient frequency isolation of the transmit and
receive frequencies is necessary.
 Frequency separation between forward band and reverse band is
constant.
 FDD is used exclusively in analog mobile radio systems

10.  It multiplexes the Tx & Rx in one frequency at different
time slots
 A portion of the time is used to transmit and a portion is
used to receive.
 TDD is only possible for digital transmission

12.  The available radio spectrum is divided into a large
number of narrowband channels.
 Each channel is relatively narrow compared with the
coherence bandwidth.
 The channels are usually operated using FDD.
 To minimize interference between forward and reverse
links, the frequency split is made as great as possible

13.  Narrowband FDMA ---- a user is assigned a particular channel
which is not shared by other users .
 If FDD is used, the system is called FDMA/FDD.
 Narrowband TDMA ---- allows users to share the same channel
but allocates a unique time slot to each user.
 For narrowband TDMA, there generally are a large number of
channels allocated using either FDD or TDD, and each channel is
shared using TDMA.

16.  The FDMA is the simplest scheme used to provide multiple access.
 Each user is allocated a unique frequency band or channel.
 These channels are assigned on demand, and can not be shared.
 Multiple users are isolated using band pass filters.
 Frequency guard bands are provided between adjacent signal
spectra to minimize crosstalk between adjacent channels.

18.  The FDMA channel carries only one frequency band at a time.
 If an FDMA channel is not in use, then it sits idle and cannot be
used by other users . 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 frequency band per carrier.
FDMA is usually implemented in narrowband systems.
 The inter symbol interference is low as the symbol time interval is
large compared to average delay spread.

19.  The complexity of FDMA mobile systems is lower than TDMA
systems.
 FDMA systems have higher cell site system costs as compared to
TDMA systems, because of the single channel per carrier design.
 The FDMA mobile unit uses duplexers as both the transmitter and
receiver operate at the same time.
 FDMA requires tight RF filtering to minimize adjacent channel
interference.

20.  Multiple users are isolated by employing simple band
pass filters.
 Capacity is increased by reducing the information bit
rate and using an efficient digital speech coding scheme.
 Interference is low.

21.  Cross talk is produced due to adjacent channel
interference.
 Guard band produces wastage of bandwidth.
 It is only used for voice signal.

22.  In FDMA, Many channels share the same antenna at the
base station.
 The nonlinear effect is due to signal spreading in the
frequency domain which generates inter modulation (IM)
frequencies.
 Inter modulation (IM) frequencies are the undesired
frequency or extra frequency in FDMA channel.

23.  The number of channels that can be simultaneously supported in
FDMA system is given by
 Where
 Bt=Total Bandwidth or Allocated Spectrum
 Bg=Guard band
 Bc=Channel bandwidth

24. Q1 :If each cellular carrier is allocated N number of channels and the
allocated frequency spectrum is 12.5 MHz, guard band is 10 KHz
and the bandwidth of each channel is 30 KHz. Find the total number
of channels provided by the cellular carrier.
 Q2 :In the United States, the advanced mobile phone service
(AMPS) cellular operator is allocated 15 MHz for each simplex
band, and if Bt is 10 MHz, Bg is 10 KHz, and Bc is 25 KHz. Find the
number of channels available in an FDMA system.

25.  It characterizes an efficient use of frequency spectrum.

26. Q : In a first-generation AMP system where there are 395 channels of
30 kHz each in a bandwidth of 12.5 MHz, what is the multiple
access spectral efficiency for FDMA?

27.  Due to insufficient spectrum efficiency and continuous transmission of
FDMA system, TDMA systems are developed.
 In a TDMA system, each user uses the whole channel bandwidth for a
fraction of time.
 In a TDMA system, time is divided into equal time intervals called slots.
User data is transmitted in the slots and several slots make up a frame.
 Guard times are used between each user’s transmission to minimize
crosstalk between channels

29.  TDMA shares a single carrier frequency with several users,
where each user makes use of non overlapping time slots.
 Data transmission for users of a TDMA system is not
continuous.
 The handoff process is much simpler because of
discontinuous transmissions in TDMA.
 TDMA uses different time slots for transmission and
reception, so duplexers are not required.

30.  Transmission rates are generally very high as compared to FDMA
channels.
 High synchronization overhead is required in TDMA systems
because of burst (the transmission of the data during the single time
slot) transmissions.
 Low battery consumption.

32.  Frame :- It consists of a number of slots (information message),
together with a preamble, and trail bits.
 Preamble :- It contains the address and synchronization
information that both the base station and the subscribers use to
identify each other.
 Guard times :-These allow synchronization of the receivers
between different slots and frames.

33.  Extended battery life over FDMA.
 Handoff process is simpler.
 More efficient use of spectrum compared to FDMA.
 TDMA accommodates more users in the same spectrum space
than an FDMA system which improves capacity.
 Different slots are used for transmission and reception. So
duplexers are not required.

34.  TDMA requires synchronization. If the time slot synchronization is lost, the
channels may collide with each other.
 For mobiles and, particularly for handsets, TDMA on the uplink demands
high-peak power in transmit mode that shortens battery life.
 Network and spectrum planning are intensive.
 Dropped calls are possible when users switch in and out of different cells.
 Higher costs due to greater equipment sophistication.
 Equalization is required, since transmission rates are generally very high as
compared to FDMA channels.

35.  Number of channels/time slots in TDMA system is
 N … number of channels/time slots
 M … number of TDMA users per radio channel
 Btotal … total spectrum allocation
 Bguard … Guard Band
 Bc … channel bandwidth

36.  The frame efficiency η is the percentage of bits per frame which
contains transmitted data.

37.  Where bo=number of overhead bits per frame and is given by

38.  The number of bits per data channel (user) per frame is
 bc = RTf
◦ where R is the bit rate of each channel (user).
◦ The above equation also indicates the number of time slots per frame.

40.  TDMA can operate as wideband or narrowband. In the wideband
TDMA, the entire spectrum is used by each individual user. For the
wideband TDMA, multiple access spectral efficiency is given as


41.  For the narrowband TDMA schemes, the total band is divided into a number
of sub-bands, each using the TDMA technique. The multiple access
spectral efficiency of the narrowband TDMA system is given as:
 Where
Bu is the bandwidth of an individual user during his or her time slot
Nu is the number of users sharing the same time slot in the system

42. 1. In a digital cellular system such as GSM system, TDMA is used as
multiple access system. The GSM system uses a frame structure where
each frame consists of eight time slots, and each time slot contains
155.55 bits, and data is transmitted at 270.833 Kbps in the channel. Find
 (a) time duration of a bit
 (b) time duration of a slot
 (c) time duration of a frame
 (d) the duration for which a user occupying a single slot must wait
between two simultaneous transmissions

43. 2. If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits,
26 training bits, and 2 traffic bursts of 61 bits of data, find the
frame efficiency for eight time slots.
3. Consider a GSM system, which is in TDMA/FDD system that uses
the 30 MHz forward link, which is broken into number of channels
of 240 kHz. If eight speech channels are supported on a single
radio channel and if no guard band is assumed, find the number
of simultaneous users that can be accommodated in GSM.

44. 4. If normal GSM time slot consists of 6 trailing bits, 8.50 guard bits, 28
training bits, and 2 traffic burst of 58 bits of data, find the frame efficiency.
5. In the North American Narrowband TDMA cellular system, the one-way
bandwidth of the system is 12.5 MHz. The channel spacing is 30 kHz and
the total number of voice channels in the system is 395. The frame
duration is 40 ms, with six time slots per frame. The system has an
individual user data rate of 16.2 Kbps in which the speech with error
protection has a rate of 13 Kbps. Calculate the multiple access spectral
efficiency of the TDMA system.

47.  CDMA is the third multiple access technique used in cellular
systems.
 CDMA allows transmissions to occupy the entire bandwidth at the
same time without interference.
 CDMA uses spread-spectrum technique to increase spectrum
efficiency over current FDMA and TDMA systems.
 CDMA cellular technology is originally known as IS-95.
 QUALCOMM is the developer of the CDMA

49.  CDMA assigns to each user a unique code sequence that is used to
code data before transmission.
 CDMA requires fewer cell towers and provides up to five times the
calling capacity.
 CDMA also provides more than 10 times the voice traffic of earlier
analogue system (AMPS) and is the basis for 3G data transmission for
GSM carriers.
 The codes are shared by the mobile phone and the base station. The
codes are called Pseudorandom code sequences.

50.  All the users can access the entire spectrum allocation all of the
time.
 CDMA channels can handle an unspecified number of users.
 The capacity of the system depends on the quality of current calls.
 There are three types of codes generally used. They are:
◦ Walsh code
◦ Short PRN code
◦ Long PRN codes.

51.  Walsh codes: These are orthogonal codes. The spreading on
forward link is 1.2288 Mbps and on reverse link is 307.2 Kbps. 64-
bit Walsh codes are used in IS 95A and IS 95B. 128-bit Walsh
codes are used in CDMA2000.
 Short PRN code: (16 bit) are used to identify the base station
and the cell.
 Long PRN code : (42-bit code) are used to identify mobile
station on reverse link

52.  Greatest spectrum efficiency: capacity increases about 8 to 10
times that of an analogue system and 4 to 5 times that of other
digital systems, which makes it most useful in high traffic areas with
a large number of users and limited spectrum.
 CDMA improves call quality by filtering out background noise,
crosstalk, and interference.
 Soft handoff is used and it requires less power, which reduces
interference and increases capacity.

53.  Simplified frequency planning: all users on a CDMA system use the
same radio frequency spectrum.
 Random Walsh codes enhance user privacy.
 Power control increases talk time and battery life for mobile phones.

54.  Backwards compatibility ( refers to a hardware or software system
that can successfully use interfaces ) techniques are costly.
 Difficult to optimize to maximize performance.
 Low traffic areas lead to inefficient use of spectrum and equipment
resources.
 The near—far problem occurs when many mobile users share the
same channel.
 Self Jamming is observed in CDMA system due to loss of
orthogonality of PN codes or spreading sequences of different
subscribers.

59.  Cellular mobile communication uses various techniques to allow
multiple users to access the same radio spectrum at the same time.
 Spread spectrum multiple access (SSMA) is another method that
allows multiple users to access the mobile radio communications
network.
 Spreading means increasing the bandwidth of the signal.

60.  Spread spectrum technology is a wireless communication technique in
which the user’s original signal is transformed into another form that
occupies a larger bandwidth than the original signal. The process of
transformation is known as spreading.
 Depending on the way the frequency spectrum is used, four types of spread
spectrum techniques are developed.
◦ Direct sequence spread spectrum (DSSS)
◦ Frequency-hopping spread spectrum (FHSS)
◦ Time-hopping spread spectrum (THSS)
◦ Multi-carrier spread spectrum (MCSS)

61.  The DSSS is known as direct sequence code division multiple
access (DS-CDMA).
 When spectrum spreading is performed by phase modulation, then
the resultant signal is called as DSSS signal.
 In a DSSS system, the bandwidth of the baseband information
carrying signals from a different user is spread by different codes
with a bandwidth much larger than that of the baseband signals.
 The spreading codes used for different users are orthogonal or
nearly orthogonal to each other.

62.  The primary advantage of DS-CDMA is its ability to tolerate a fair amount of
interfering signals compared to FDMA and TDMA.
 In this technique, adjacent microcells share the same frequencies whereas
with FDMA/TDMA it is not feasible for adjacent microcells to share the
same frequencies because of interference.
 In both FDMA and TDMA systems, a frequency planning task is required
whenever a network changes, whereas no such frequency planning is
needed for a CDMA network since each cell uses the same frequencies.
 DS-CDMA accommodate more mobile users than FDMA/TDMA .

63.  The capacity of a DS-CDMA system depends on the following
parameters:

64.  The capacity of a DS-CDMA system is also determined by
including bandwidth efficiency factor, the capacity degradation
factor due to imperfect power control, and the number of sectors in
the cell-site antenna and is expressed as

67.  MC-DS-CDMA is the combination of OFDM and DS-CDMA .
 The DS-CDMA system suffers inter-symbol interference (ISI) and multi-user
interference (MUI) caused by multipath propagation, leading to a high loss
of performance . To avoid the problem, MC-DS-CDMA technique is
developed and it gives better performance in severe multipath conditions.
 Basically, three main designs exist in multicarrier.
◦ MC-CDMA
◦ MC-DS-CDMA
◦ MT-CDMA (Multi Tone)

68.  In MC-CDMA, the spreading code is applied across a number of orthogonal
subcarriers in the frequency domain.
 In MC-DS-CDMA, the data stream is first divided into a number of sub
streams. Each sub stream is spread in time through a spreading code and
then transmitted over a set of orthogonal subcarriers.
 In MT-CDMA the system undergoes similar operations as MC-DS-CDMA
except that the different subcarriers are not orthogonal after spreading.
 The MCDS- CDMA transmitter spreads the original data stream over
different orthogonal subcarriers using a given spreading code in the
frequency domain.