The primary benefit of an interleaver for transmission in fading environment is to provide time diversity (when used along with error-correction coding).

Figure 1 illustrates the benefits of providing an interleaver time span TILTIL size 12{T rSub { size 8{ ital "IL"} } } {}, that is large compared to the channel coherence time T0T0 size 12{T rSub { size 8{0} } } {}, for the case of DBPSK modulation with soft-decision decoding of a rate 1/2, K = 7 convolutional code, over a slow Rayleigh-fading channel.

It should be apparent that an interleaver having the largest ratio of TIL/T0TIL/T0 size 12{T rSub { size 8{ ital "IL"} } /T rSub { size 8{0} } } {} is the best-performing (large demodulated BER leading to small decoded BER). This leads to the conclusion that TIL/T0TIL/T0 size 12{T rSub { size 8{ ital "IL"} } /T rSub { size 8{0} } } {} should be some large number—say 1,000 or 10,000. However, in a real-time communication system this is not possible because the inherent time delay associated with an interleaver would be excessive.

The previous section shows that for a cellular telephone system with a carrier frequency of 900 MHz, a TIL/T0TIL/T0 size 12{T rSub { size 8{ ital "IL"} } /T rSub { size 8{0} } } {} ratio of 10 is about as large as one can implement without suffering excessive delay.

Note that the interleaver provides no benefit against multipath unless there is motion between the transmitter and receiver (or motion of objects within the signal-propagating paths). The system error-performance over a fading channel typically degrades with increased speed because of the increase in Doppler spread or fading rapidity. However, the action of an interleaver in the system provides mitigation, which becomes more effective at higher speeds

Figure 2 show that communications degrade with increased speed of the mobile unit (the fading rate increases), the benefit of an interleaver is enhanced with increased speed. This is the results of field testing performed on a CDMA system meeting the Interim Specification 95 (IS-95) over a link comprising a moving vehicle and a base station.

Typical Eb/N0Eb/N0 size 12{ {E rSub { size 8{b} } } slash {N rSub { size 8{0} } } } {} performance versus vehicle speed for 850 MHz links to achieve a frame-error rate of 1 percent over a Rayleigh channel with two independent paths

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