Beyond 3G Techniques of Orthogonal Frequency Division Multiplexing and Performance Analysis via Simulation

Technique Principle

The essential technical principle is to divides one high speed data flow into plurality of low speed data flows. And the low-speed data flows are modulated by orthogonal frequency and transmit data simultaneously, thus wide band becomes narrow bands, and consequently, the problem of selective fading can be solved. In addition, if adding a delayed protection to generated OFDM signal, intersymbol interference can be figured out (Gong & Jia, 2002; IEEE Computer Society, 1999). The modulation principle in OFDM system is seen in Figure 2.

Figure 2.

OFDM modulation principle

After the modulating of MQAM, MPSK or DPSK, etc., taking advantage of sub-carrier orthogonality, the original signal is demodulated. The principle is seen in Figure 3.

Figure 3.

OFDM demodulation principle

One OFDM symbol includes signals which are combined by multiple modulated sub-carriers, and each sub-carrier can be modulated by PSK or QAM symbol. If N indicates the number of sub-channel, T indicates OFDM symbol density, d_i (i=0, 1, 2, …, N-1) indicates data symbol distributed to each sub-channel, and f_c indicates the carrier frequency of the No. 0, rect(t)=1, |t|≤T/2, the original OFDM symbol can be indicated as following:

OFDM output signal is described by equivalent baseband symbol: