Consider an OFDM implementation that uses 15 kHz subcarriers and use an OFDM sym

ID: 3588274 • Letter: C

Question

Consider an OFDM implementation that uses 15 kHz subcarriers and use an OFDM symbol of 2048 subcarriers. The nominal cyclic prefix accounts for a 7% guard time. The extended cyclic prefix can use up to 25%. 1500 subcarriers can be used for data transmission. The rest are needed for pilot and null subcarriers. The transmission bandwidth is 20 MHz using 8 QAM modulation.

How many guard symbols does the nominal CP add?

How many guard symbols does the extended CP add?

What is the data rate for the nominal CP?

What is the data rate for the extended CP?

Explanation / Answer

MATLAB

Given parameters in the spec:

N=64; %FFT size or total number of subcarriers (used + unused) 64

Nsd = 48; %Number of data subcarriers 48

Nsp = 4 ; %Number of pilot subcarriers 4

ofdmBW = 20 * 10^6 ; % OFDM bandwidth

Derived Parameters:

deltaF = ofdmBW/N; % Bandwidth for each subcarrier - include all used and unused subcarries

Tfft = 1/deltaF; % IFFT or FFT period = 3.2us

Tgi = Tfft/4; % Guard interval duration - duration of cyclic prefix - 1/4th portion of OFDM symbols

Tsignal = Tgi+Tfft; % Total duration of BPSK-OFDM symbol = Guard time + FFT period

Ncp = N*Tgi/Tfft; %Number of symbols allocated to cyclic prefix

Nst = Nsd + Nsp; % Number of total used subcarriers

nBitsPerSym=Nst; %For BPSK the number of Bits per Symbol is same as number of subcarriers