Question

How can you compensate for the impact of the Doppler Effect in cellular system? Explain.

Answer 1

Doppler shift affects wireless communications by creating fading in the signal. Fading is unpredictable attenuation that occurs at undesired space and time coordinates. As fading caused by Dopper shift is undesirable, Following techniques are used to minmise the effect of fading in cellular system :-

a) Diversity

Diversity technique is used to compensate for fading channel impairements , and usually implementated by using 2 or more receving antennas. Diversity is usually employed to reduce the depth and duration of the fades experienced by a receiver in a flat fading channel. These techniques can be employed at both base station and mobile receivers. Spatial Diversity is the most widely used diversity technique. In this technique multiple antennas are strategically spaced and connected to common receiving system. While one antenna sees a signal null, one of the other antennas may see a signal peak, and the receiver is able to select the antenna with the best signal at any time.

Space Diversity or Antenna diversity: It is one of the most popular diversity technique used in wireless systems. The direct path between the transmitter (BS) & receiver (MS) is not guaranteed & the possibility of a number of scatters in the mobile environment suggests a Rayleigh fading signal (small-scale fading). Multiple signals from spatially separated antenna (BS) is received on the mobile, the best signal is picked by the mobile using the diversity technique as shown in Figure 1.. It is also used in the base station (BS) design. At each cell site, multiple BS receiving antennas are used to provide diversity reception.

Selection Diversity: This is simplest diversity technique where demodulators are used to give diversity branches whose gains are adjusted to provide the same average SNR for each branch. The receiver branch having the highest instantaneous SNR is connected to the demodulator. In practice the branch with the largest (S+N)/N is used, as it is difficult to measure SNR alone.

b) CHANNEL CODING :-

Channel coding improves Mobile communication Link performance by adding redundant data bits (called as error coorection/detection bits) in the transmitted message.At the baseband portion of transmitter, a channel coder maps a digital message sequence into a another specific code sequence containing greater number of bits than orginal contained in the message.The coded message is then modulated for transmission in wireless channel .Channel coding is used by receiver to detect or correct some (or all) of the errors introduced by the channel in a particular sequence of message bits.The added coding bits lower the raw data transmission rate through the channel .there are two types of channel codes : Block codes and convolutional codes .

c) Equalization

ISI (Inter Symbol Interference) has been identified as one of the major obstacles to high speed data transmission over mobile radio channels. If the modulation bandwidth exceeds the coherence bandwidth of the radio channel (i.e., frequency selective fading), modulation pulses are spread in time, causing ISI.

An equalizer at the front end of a receiver compensates for the average range of expected channel amplitude and delay characteristics. As the mobile fading channels are random and time varying, equalizers must track the time-varying characteristics of the mobile channel and therefore should be time varying or adaptive. An adaptive equalizer has two phases of operation: training and tracking.

Training Mode

Initially a known, fixed length training sequence is sent by the transmitter so that the receiver equalizer may average to a proper setting. Training sequence is typically a pseudo-random binary signal or a fixed, of prescribed bit pattern.

The training sequence is designed to permit an equalizer at the receiver to acquire the proper filter coefficient in the worst possible channel condition. An adaptive filter at the receiver thus uses a recursive algorithm to evaluate the channel and estimate filter coefficients to compensate for the channel.

Tracking Mode

When the training sequence is finished the filter coefficients are near optimal. Immediately following the training sequence, user data is sent.

When the data of the users are received, the adaptive algorithms of the equalizer tracks the changing channel. As a result, the adaptive equalizer continuously changes the filter characteristics over time.