The IEEE 802.11 WLAN uses a 64 sub-channel implementation of multicarrier modulation (OFDM). 48 subscribers are used for information transmission, 4 subscribers for pilot tones are used for synchronization and 12 are reserved. Each sub-channel has a symbol rate of 250 kbps. The occupied bandwidth is 20 MHz. Find the bandwidth of a sub-channel. What is modulation efficiency? What is user symbol rate? If 16-QAM modulation is used, what is the user data rate if the information bits are encoded with the rate of ¾? If the guard time between two transmitted symbols is 800 ms, what is the time utilization efficiency of the system?

Mumbai University > Electronics and Telecommunication > Sem8 > Wireless Networks

Marks: 10M

Year: May 2014

i. Total number of subcarriers =48+12+4 = 64

ii. Bandwidth of sub channels = $\frac{\text{Occupied B.W}}{\text{(Total number of subcarriers)}}=\frac{(20×10^6)}{64}=312.5KHz $

iii. Modulation efficiency =$\frac{\text{(Subchannel Symbol rate}}{\text{(Bandwidth of subchannel)}}=250/312.5=0.8 \text{symbols/sec/}Hz$

iv. User symbol transmission rate =Information subcarrier * Sub-channel Symbol rate

= 48 * 250 = 1.2Msps

v. User bit per symbol = 4 for 16-QPSK modulation

vi. User data rate = (3/4) * 4 * 1.2 = 3.6Mbps

vii. Symbol duration = $\frac{1}{(250×10^3 )}$ = 4000 ns

viii. Time utilization efficiency = 4000/4800 = 0.83