
There are three main requirements to start with bandwidth , bit rate , and delay spread .

The delay spread directly dictates the guard time . As a rule, the guard time should be about two to four times the rootmean-squared delay spread.

This value depends on the type of coding and modulation .

Higher order QAM is more sensitive to ICI and ISI than
QPSK; while heavier coding obviously reduces the sensitivity to such interference.
Wireless Communication Technologies 2.5.2
1


Now that the guard time has been set, the symbol duration can be fixed.

The signal in the guard time will be removed at the receiver, this means that the energy is wasting away which results in the
SNR loss .

To minimize the SNR loss caused, it is desirable to have the symbol duration much larger than the guard time.

SNR loss can be calculated as follows:
SNR loss
 
10 log
10
( 1
 
) ,
 
T
T g

T g
 
SNR loss

0 .
97 dB
Wireless Communication Technologies 2.5.2
2


Following figure shows the simulation of the SNR loss, it can
P e
 
4 be seen that the SNR loss is about 1 dB for .
16-QAM
10
0
16-QAM (AWGN)
Tg=0.25*T
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
0 2 4 6 8
Eb/No (dB)
10 12 14 16
3 Wireless Communication Technologies 2.5.2


However, the symbol duration can not be arbitrarily large , because a large symbol duration means more subcarriers with a smaller subcarrier spacing, then, more sensitivity to phase noise and frequency offset , and increasing peak-to-average power ratio .

Hence, a practical design choice is to make the symbol duration at least five times the guard time , which implies a 1dB SNR loss because of the guard time.
 After the symbol duration and guard time are fixed, the number of subcarriers follows directly as the required 3-dB bandwidth divided by the subcarrier spacing, which is the inverse of the symbol duration less the guard time.
4 Wireless Communication Technologies 2.5.2


Please design a system with the following requirements:
(1) Bit rate: 20 Mbps
(2) Tolerable delay spread: 200 ns
(3) Bandwidth: <= 16 MHz.
【 Sol.
】
The delay-spread requirement of 200 ns suggests that 800 ns is a safe value for the guard time.
By choosing the OFDM symbol duration 6 times the guard s
4 .
8
 dB.
The subcarrier spacing is now the inverse of 4.8-0.8=4μs,
Wireless Communication Technologies 2.5.2
5

To achieve 20 Mbps, each OFDM symbol has to carry
20Mbps
× 4.8μs = 96 bits of information.
To do this, one is to use 16-QAM together with rate 1/2 coding to get 2 bits per symbol per subcarrier.
In this case, 48 subcarriers are needed to get the required 96 bits per symbol.
It has the additional advantage that an efficient 64-point radix-4 IFFT/FFT can be used, leaving 16 zero subcarriers (or virtual carriers ) to provide oversampling necessary to avoid aliasing or other applications, such as carrier frequency offset
(CFO) estimation .
The total bandwidth is 64
×
250k = 16MHz which fills the bandwidth requirement.
Wireless Communication Technologies 2.5.2
6