Wireless Communication - Techniques
In some cases, there is a scope of performance deterioration, which affects the output. The major cause for this might be the mobile channel impairments. To resolve this, there are three popular techniques −
Equalizer
An equalizer within a receiver compensates for the average range of expected channel amplitude and delay characteristics. In other words, an equalizer is a filter at the mobile receiver whose impulse response is inverse of the channel impulse response. Such equalizers find their use in frequency selective fading channels.
Diversity
Diversity is another technique used to compensate fast fading and is usually implemented using two or more receiving antennas. It is usually employed to reduce the depths and duration of the fades experienced by a receiver in a flat fading channel.
Channel Coding
Channel coding improves mobile communication link performance by adding redundant data bits in the transmitted message. At the baseband portion of the transmitter, a channel coder maps a digital message sequence in to another specific code sequence containing greater number of bits than original contained in the message. Channel Coding is used to correct deep fading or spectral null.
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.
Diversity
Diversity is a powerful communication receiver technique that provides wireless link improvement at a relatively low cost. Diversity techniques are used in wireless communications systems to primarily to improve performance over a fading radio channel.
In such a system, the receiver is provided with multiple copies of the same information signal which are transmitted over two or more real or virtual communication channels. Thus the basic idea of diversity is repetition or redundancy of information. In virtually all the applications, the diversity decisions are made by the receiver and are unknown to the transmitter.
Types of Diversity
Fading can be classified into small scale and large scale fading. Small-scale fades are characterized by deep and rapid amplitude fluctuations which occur as the mobile moves over distances of just a few wavelengths. For narrow-band signals, this typically results in a Rayleigh faded envelope. In order to prevent deep fades from occurring, microscopic diversity techniques can exploit the rapidly changing signal.
If the antenna elements of the receiver are separated by a fraction of the transmitted wavelength, then the various copies of the information signal or generically termed as branches, can be combined suitably or the strongest of them can be chosen as the received signal. Such a diversity technique is termed as Antenna or Space diversity.
Frequency Diversity
The same information signal is transmitted on different carriers, the frequency separation between them being at least the coherence bandwidth.
Time Diversity
The information signal is transmitted repeatedly in time at regularly intervals. The separation between the transmit times should be greater than the coherence time, Tc. The time interval depends on the fading rate, and increases with the decrease in the rate of fading.
Polarization diversity
Here, the electric and magnetic fields of the signal carrying the information are modified and many such signals are used to send the same information. Thus orthogonal type of polarization is obtained.
Angle Diversity
Here, directional antennas are used to create independent copies of the transmitted signal over multiple paths.
Space Diversity
In Space diversity, there are multiple receiving antennas placed at different spatial locations, resulting in different (possibly independent) received signals.
The difference between the diversity schemes lies in the fact that in the first two schemes, there is wastage of bandwidth due to duplication of the information signal to be sent. Thus problem is avoided in the remaining three schemes, but with the cost of increased antenna complexity.
The correlation between signals as a function of distance between the antenna elements is given by the relation −
$$\rho = J_0^2 \lgroup\frac{2\Pi d}{\lambda}\rgroup$$
Where,
