For CDMA One/IS-95 . Provide clearly the answer . In the two tables below, write

Question

For CDMA One/IS-95 . Provide clearly the answer . In the two tables below, write the techniques that cdmaOne/IS-95 uses to mitigate or eliminate the effects of slow fading, fast fading, flat fading, frequency selective fading, same cell interference, and adjacent channel interference. Possible answers are Close-loop Power Control, Open-loop Power Control, Interleaves, Walsh Codes, Long Code, Forward Error Correcting (FEC) Convolutional Codes, Short Codes. Adaptive Equalizer, TDMA, CDMA, FDD, TDD, FDMA, Wide bandwidth signal. Message Based Power Control, etc.

Explanation / Answer

Slow fading in wireless communication systems occur due to a long term fading effect which changes the mean value of the received signal. This occurs under events of shadowing or moving away from the transmitter. Slow fading is usually encountered using power control schemes either open loop power control where the base station/mobile determines the transmit power or a close loop power control where the loop between the base station and the BTS determines the transmit power. Slow fading occurs because the channel coherence time is larger than the symbol period and hence can be eliminated by time diversity duplex method as well by separating the symbol transmissions in time domain.

Fast fading is a short term fading effect which is due to the multipath propagation effect. The coherence time of the channel in this case is very small compared to the symbol period and hence the channel changes over one period of the transmit symbol. In this case the symbol is interleaved between different symbol periods by interleaving technique and retrieved at the receiver using de-interleaver and proper error correction codes. If not interleaved there are high chances that a symbol will be completely lost in a fading cycle and cannot be corrected using error correction codes.

Flat fading is a frequency non-selective fading where most of the spectrum is attenuated almost same. Flat fading can be eliminated by power control schemes only. Adaptive equalizers in conjunction with power control can do a nice job in these cases.

Frequency selective fading is characterized by different attenuation levels for different frequencies and hence can be eliminated by using wide bandwidth signals.

Same cell interference in CDMA/is-95 is encountered using 64 bit Walsh codes and long PN codes. The Walsh code is an orthogonal code having poor cross correlation and good auto correlation properties which eliminates the interference between users who use different codes. The long PN code 42 bit long is used for spreading the Walsh code signal in time. It takes almost 41.2 days for a PN code to repeat.

Adjacent cell interference is caused by multiple users reusing the same Walsh code. This can be eliminated only by PN codes which are being generated by a LSFR and 42 bits long. These codes will separate the signals in time domain and hence eliminates interference.

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