3.1 Multiple access techniques in wireless communications Frequency division multiple access technology FDMA | unit 3 multiple access techniques in wireless communications

Unit - 3

Multiple access techniques in wireless communications

3.1 Multiple access techniques in wireless communications: Frequency division multiple access technology (FDMA)

It sub-divide frequency into a several frequency band with non-overlapping.

Fig. 1 FDMA

FDMA allows user to transmit signals simultaneously to satellite transponder with the help of assigning a specific frequency to every user among a channel.

Each transaction of signal has own unique radio channel. Channel are probably 30 KHz or less used to transmit or receive channels.

Frequency allocation made by national policies. Uplink (i.e. from mobile station to base station). Downlink (i.e. from base station to mobile station) uses frequencies bands like. Uplink and downlink with frequencies mentioned below :

Uplink	890.2 MHz, 915 MHz
Downlink	935.2 MHz to 960 MHz

Uplink (UL) and downlink (DL) can be described with the help of one relation among them as follows:

fd = fu + 45 MHz

For Example :

If UL frequency is :

fu = 890 MHz + n∙ 0. 2 MHz

Then

fd = 935 MHz + n ∙ 0. 2 MHz

Fixed assigned frequency makes scheme very inflexible limits to no. of sender turn into disadvantage of FDMA.

3.2 Time division multiple access (TDMA)

It is based on time slots. TDMA more flexible than FDMA scheme.

Dynamic allocation of channel or by some fixed pattern channels get allotted for time slots to takes places synchronized communication.

Fig. 2 TDMA

Fixed channel are allocated for communication as best practice solution for wireless phone system in Medium Access Control (MAC) reserved time slot access is for crucial.

3.3 Space division multiple access (SDMA)

Fig. 3 SDMA

A cable consists of several fiber which are used as bi-directional pairs.

Fig. 4 Guard Spaces

Interference’s risk is reduced by guard space between frequency space.

It is used to assign distinct space for subscribers. It is bound to provide a base station to mobile phone user. According to availability of frequency (FDM), time slot (TDM) or code (CDM) MAC algorithm available best base station to mobile station.

SDM doesn’t need any multiplexing equipment. It gives well utilize individual physical channel by some multiplexing technique.

3.4 Code division multiple access (CDMA)

CDMA stands for Code Division Multiple Access. CDMA is a technique for spread spectrum multiple access.

Data signals are XOReD with pseudorandom code and then transmitted over a channel.

Different codes are used to modulate their signals, selection of code to modulate is important aspect as it relates with performance of system.

In CDMA, single channel uses entire bandwidth and it does not share time as all stations transmit data simultaneously. Due to this CDMA differ from FDMA and TDMA.

Let us assume we have 4 different stations A, B, C and D. All 4 stations connected to same channel. Data from station A is d1, B is d2, C is d3 and D is d4 similarly code assigned to A is C1, B is C2, C is C3 and D is C4.

To transmit data, all 4 stations have multiplication strategy i.e. either code multiply by itself. It results in 4 stations or code multiply results in 4 stations or code multiply results in 4 station of code multiply by another, it results in zero station.

Fig. 5: Communication of station with code

If station C and station D are talking to each other, D wants to listen whatever C is saying. It multiplies data on channel by C3 of station C.

As C3  C3 is 4 and rest of combination i.e. C1  C3, C2  C3 and C4  C3 are all zero.

Every station has some code, which is sequence of numbers known as chips.

Sequence should be selected in proper manner to get appropriate codes. Orthogonal sequence has some properties :

Multiplication of sequence by a scalar.

Inner product of two equal sequences.

Inner product of two different sequences.

Sequence generation uses Walsh table to generate chip sequence.

Key takeaway

Approach	SDMA	TDMA	FDMA	CDMA
Idea	Segment space into cells/sectors	Segment sending time into disjoint time-slots, demand driven or fixed patters	Segment the frequency band into disjoint sub-bands	Spread the spectrum using orthogonal codes
Terminals	Only one terminal can be active in one cell/one sector	All terminals are active for short periods of time on the same frequency	Every terminal has its own frequency. uninterrupted	All terminals can be active at the same place at the same moment uninterrupted.
Signal separation	Cell structure, directed antennas	Synchronization in the time domain	Filtering in the frequency domain	Code plus special receivers
Advantages	Very simple, increases capacity per km2	Established, fully digital, flexible	Simple, established robust	Flexible, less frequency planning needed, soft handover
Disadvantages	Inflexible, antennas typically fixed	Guard space needed (multipath propagation), synchronization difficult	Inflexible, frequencies are a scarce resource	Complex receivers, needs more complicated power control for senders
Comment	Only in combination with TDMA, FDMA or CDMA useful	Standard in fixed networks, together with FDMA/SDMA used in many mobile networks	Typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)	Still faces some problems, higher complexity, lowered expectations: will be integrated with TDMA/FDMA

3.5 Spectral efficiency of different wireless access technologies

As we know radio frequency resource is very vital in wireless communication system. Hence RF frequencies should be maximally utilized. Following are the methods used to improve the spectrum utilization.

• Cell planning

• Frequency reuse

• channel assignment

• multiple access techniques

In general, spectral efficiency depends upon following parameters.

• Channel spacing in KHz

• frequency reuse factor • cell area in Km2

• Modulation techniques

• Multiple Access techniques of types

Spectral efficiency =

3.6 Spectral efficiency in FDMA system

η = Ndata/cluster / (BWt * N * Acell)

= N * Ndata/cell / (BWt * N * Acell) channels/MHz/Km2

Where, Acell is the area of each cell

Ndata/cell = [ {(BWt - 2*Bg)/Bch} - Ncntl]/N

BWt is total bandwidth,

Bch is channel bandwidth,

Bg is guard band bandwidth,

N is number of cells per cluster

3.7 Spectral efficiency in TDMA system

Spectral efficiency of Wide band TDMA and narrow band TDMA is expressed as follows:
ηWTDMA = {(Tf - Tp - Tt)/Tf} * (Ni/Ns)

Where,

Tp = preamble period,

Tt= trailer time period,

Tf= frame duration

Ns= number of symbols in a time slot, Ni = number of information bits

ηNTDMA = {(Tf - Tp - Tt)/Tf} * (Ni/Ns) * {(Nsub * Bch)/BWt}

Nsub = Number of sub bands

Nslot = Number of slots

3.8 Spectral efficiency for DS-CDMA system.

Spectral efficiency of DS-CDMA system is expressed as follows:

η = Us * Rb/W ... bits/S/Hz

Where Rb is information bit rate

W = system bandwidth for one direction

Us =system utilization

Key takeaway

Spectral efficiency for FDMA

η = Ndata/cluster / (BWt * N * Acell)

= N * Ndata/cell / (BWt * N * Acell) channels/MHz/Km2

Where, Acell is the area of each cell

Ndata/cell = [ {(BWt - 2*Bg)/Bch} - Ncntl]/N

Spectral efficiency for TDMA

ηNTDMA = {(Tf - Tp - Tt)/Tf} * (Ni/Ns) * {(Nsub * Bch)/BWt

Spectral efficiency for DS-CDMA

η = Us * Rb/W ... bits/S/Hz

References:

[1] Wireless Communications- Principles and Practice, T S Rappaport, Pearson Education India, Second Edition.

[2] Wireless Communication and Networks, Upen Dalal, Oxford university Press, First Edition, 2015.

[3] Wireless Communication and Networks 3G and Beyond, Iti Saha Misra, Tata

McGraw Hill Education Pvt. Ltd, Second Edition, 2009.

[4] Mobile Communication Engineering – Theory and Applications W C Y Lee, TMH Publication, Second Edition, 2008.

[5] Wireless Communication, Andrea Goldsmith, Cambridge University Press, 2005

[6] Fundamentals of Wireless Communication, David Tse and Pramod Viswanath,

Cambridge University Press, 2005

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