Choice of OFDM parameters


Choice of OFDM parameters

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


Choice of OFDM parameters

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


( 1

 

) ,

 


T g

T g

 

SNR loss

0 .

97 dB

Wireless Communication Technologies 2.5.2


Choice of OFDM parameters

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 .


















0 2 4 6 8

Eb/No (dB)

10 12 14 16

3 Wireless Communication Technologies 2.5.2

Choice of OFDM parameters

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

Example 2.5-2 Choice of OFDM parameters

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 .


 dB.

The subcarrier spacing is now the inverse of 4.8-0.8=4μs,

Wireless Communication Technologies 2.5.2


Example 2.5-2 Choice of OFDM parameters (2/2)

To achieve 20 Mbps, each OFDM symbol has to carry


× 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