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What is spectral efficiency of FDMA, TDMA, CDMA access schemes

This page covers basics of spectral efficiency with equation. It describes spectral efficiency of different wireless access techniques FDMA,TDMA and CDMA.

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 Following are the two ways to represent the spectral efficiency.

Data channel per unit bandwidth per unit coverage area( measured in channels/MHz/Km2 )

Total data traffic per unit bandwidth per unit area. ( measured in Erlangs/MHz/Km2 )

Spectral efficiency of 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

Spectral efficiency of 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, Tp= 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

Spectral efficiency of 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