Question: a.) In the two tables below, write the techniques that GSM uses to mit

Question

Question: a.) In the two tables below, write the techniques that GSM 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, Interleaver, 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. Slow Fading Fast fading Flat Fading Frequency Selective Fading Same cell interference Adjacent cell interference

Explanation / Answer

Frequency selective fading - to combat distortion we use

Equalization can mitigate the effects of channel-induced ISI brought on by frequency-selective fading.The process of equalizing for mitigating ISI effects involves using methods to gather the dispersed symbol energy back into its original time interval. In effect, an equalizer is an inverse filter of the channel. If the channel is frequency selective, the equalizer enhances the frequency components with small amplitudes and attenuates those with large amplitudes. The goal is for the combination of channel and equalizer filter to provide a flat composite-received frequency response and linear phase.

Direct-sequence spread-spectrum (DS/SS) techniques can be used to mitigate frequency-selective ISI distortion because the hallmark of spread-spectrum systems is their capability of rejecting interference, and ISI is a type of interference.Frequency-hopping spread-spectrum (FH/SS) can be used as a technique to mitigate the distortion caused by frequency-selective fading, provided that the hopping rate is at least equal to the symbol rate.

Orthogonal frequency-division multiplexing (OFDM) can be used for signal transmission in frequency-selective fading channels to avoid the use of an equalizer by lengthening the symbol duration.

Pilot signal is the name given to a signal intended to facilitate the coherent detection of waveforms. Pilot signals can be implemented in the frequency domain as in-band tones , or in the time domain as digital sequences that can also provide information about the channel state and thus improve performance in fading conditions .

Fast fading-

Fast-fading distortion calls for the use of a robust modulation (noncoherent or differentially coherent) scheme that does not require phase tracking, and reduces the detector integration time . Another technique is to increase the symbol rate, W 1/Ts, to be greater than the fading rate, fd 1/T0, by adding signal redundancy. Error-correction coding can also provide mitigation; instead of providing more signal energy, a code reduces the required Eb/N0 for a desired error performance. For a given Eb/N0 with coding present, the error floor out of the demodulator will not be lowered, but a lower error rate out of the decoder can be achieved . Thus, with coding, one can get acceptable error performance and in effect withstand a large error floor from the demodulator that might have otherwise been unacceptable.

Flat fading - to combat loss in SNR

Slow fading - to combat loss in SNR

After implementing a mitigation technique to combat signal distortion due to frequency-selective fading or fast fading, the next step is to use diversity methods to move the system operating point . The term diversity is used to denote the various methods available for providing the receiver with uncorrelated renditions of the signal of interest. “Uncorrelated” is the important feature here, since it would not help the receiver to have additional copies of a signal if the copies are all equally poor.

Time diversity—Transmit the signal on L different time slots with time separation of at least T0. When used along with error-correction coding, interleaving is a form of time diversity

Frequency diversity—Transmit the signal on L different carriers with frequency separation of at least f0. Bandwidth expansion is a form of frequency diversity

Spread spectrum—Systems in which the signal bandwidth is much narrower than the coherence bandwidth of the channel have no means to resolve the different multipath contributions. Such contributions interfere to create the fading conditions.

Frequency-hopping spread-spectrum (FH/SS) is sometimes used as a diversity mechanism. The GSM system uses slow FH (217 hops/s) to compensate for cases in which the mobile unit is moving very slowly (or not at all) and experiences deep fading due to a spectral null.

Spatial diversity is usually accomplished through the use of multiple receive antennas, separated by a distance of at least 10 wavelengths when located at a base station (and less when located at a mobile unit). Signal-processing techniques must be employed to choose the best antenna output or to coherently combine all the outputs. Systems have also been implemented with multiple transmitters, each at a different location, as in the Global Positioning System (GPS). •

Polarization diversity is yet another way to achieve additional uncorrelated samples of the signal .

Any diversity scheme can be viewed as a trivial form of repetition coding in space or time. However, some techniques for improving the loss in SNR in a fading channel are more efficient and more powerful than repetition coding. Error-correction coding represents a unique mitigation technique, because instead of providing more signal energy it reduces the required Eb/N0 needed to achieve a desired performance level. Error-correction coding coupled with interleaving [16, 21–26] is probably the most prevalent of the mitigation schemes used to provide improved system performance in a fading environment. Note that the time-diversity mechanism obtained through interleaving relies on the vehicle motion to spread the errors during the fading. The faster the speed of the mobile unit, the more effective the interleaver. The interleaver is less effective at slow speeds .

Co- channel interference or same channel interference

Adjacent Channel interference -

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