FACTS GSM (III)

 

GSM Architecture / Reference Model

 

 

FACTS GSM (II)

 

GSM Public Land Mobile Network (PLMN)

 

 

Spectral Efficiency – Revisited !

This is an extension of the earlier post What is Spectral Efficiency ?

we already discussed spectral efficiency in an earlier post, here we are revisiting spectral efficiency with respect to the mobile communication system (GSM). The overall efficiency of a mobile communication system can be estimated by knowing the modulation and multiple access spectral efficiencies separately.

A. Spectral Efficiency of Modulation:

Spectral Efficiency of Modulation

η_b modulation efficiency (channels/MHz/km² )

B_w bandwidth of the system

B_c Channel Spacing

N_c total number of the cells in the covered area

N frequency reuse factor of the system

A_c area covered by a cell (km²)

B. Multiple Access spectral efficiency:

Multiple access spectral efficiency is defined as the ration of the total time frequency domain dedicated for the voice transmission to the total time-frequency domain available to the system. It simply means that spectral efficiency is dimension less quantity with an upper limit of unity.

1. FDMA spectral Efficiency:

η_a Multiple access spectral efficiency

N_T Total number of voice channels in the covered area

2. TDMA spectral Efficiency:

τ duration of a time slot

T_f Frame duration

M_t number of time slots per frame

3. Overall spectral efficiency of TDMA and FDMA system:

4. Capacity and Frame Efficiency of TDMA system

4.i. Capacity

N_u Number of channels (mobile users) per cell

η_b Bandwidth efficiency Factor

μ Bit efficiency (“=” 2 for QPSK, =1.354 for GMSK as used in GSM)

v_f Voice activity factor (“=” 1 for TDMA system)

B_w one way Bandwidth of the System

R information bit rate plus overhead

N frequency reuse factor

4.ii. Efficiency of a TDMA frame:

The number of overhead bits per frame:

N_r Number of Referece bursts per frame

N_t Number of Traffic bursts (slots per frame)

b_r Number of overhead bits per reference bursts

b_p Number of overhead bits per preamble per slot

b_g Number of equivalent bits in each guard time interval

Total number of bits per frame:

T_f Frame duration

R_rf bit rate of the radio frequency channel

Frame efficiency

Number of channels per frame