To summarize some common ISI mitigation techniques:

• Nyquist Requirement

Even in the absence of multipath induced fast fading in the channel, the filter components in the transmitter and the receiver can cause ISI at the detector.  Nyquist determined that the minimum bandwidth required to support a given symbol rate (not bit rate) Rs without incurring filter induced ISI is Rs/2.  To achieve this the system transfer function frequency response must be rectangular, yielding a sinc(x) impulse response in time (via the inverse Fourier Transform).  It is of course impossible to implement a perfectly rectangular filter (infinite bandwidth), so it is common to employ various forms of baseband pulse shaping filters.

• Equalization

One way to mitigate the effects of multipath induced fast fading in the operating channel is to compensate for the distortions in the receiver using an equalization filter.  The transversal filter is a common type of linear equalizer.  There are various techniques and algorithms used for optimizing the tap weights of the digital filter.  The choice of algorithm is a function of the severity of the ISI as well as the metric chosen for optimization (Peak Distortion Criterion, MSE Criterion etc...)  The MSE Criterion is the Mean Square Error between the desired signal and the output of the equalizer.  If the ISI causes very deep fades at the receiver, then linear equalizers might not be able to compensate.  In this case, nonlinear equalizers can be used.

• Frequency Diversity

Another way to mitigate the effects of multipath induced fast fading in the operating channel is to use a form of frequency diversity.   One specific type of frequency diversity can be implemented as a Correlation (Matched Filter) receiver.  This implementation is called a Rake Receiver.  If the transmitted signal has a bandwidth (W) greater than the Coherence Bandwidth (  1/Delay Spread ), then the individual multipath components can be resolved (down to 1/W) and coherently combined.  In this case, the information in the multipath components is used to advantage rather than allowed to cause degradation in the symbol detection logic.  This is called multipath processing gain.

• OFDM

OFDM is one realization of a multi-carrier transmission technique.  As seen in the technology matrix in our Wireless Workshop, the OFDM transmission scheme has been chosen by both the 802.11a committees for the Wireless ATM standard as well as several European standards organizations.  This is due to the fact that OFDM has some properties that make it inherently resistant to ISI.  This makes it an attractive choice for the high data rate (small symbol period) Wireless LANs, which are by nature, very susceptible to ISI.   We will focus on OFDM in the next installment of this series.

Towards completeness, the following techniques can be employed to combat other types of channel impairments (including multipath induced flat fading or signal degradation due to AWGN for example).

• Channel Coding

Channel coding can be used to overcome the effects of signal degradation brought about by many RF impairments, including AWGN, without transmitting at higher power or with larger antennas (higher gain).  Channel coding can be used for error detection only (CRC, Automatic Retransmission Query) or for error detection and correction (FEC).  Some channel coding schemes insert redundancy into the data stream (bandwidth expanding) like FEC, while others do not (CRC).  In either case, the goal is to improve the BER performance as a function of SNR.  Some modern techniques couple the coding and modulation schemes together (Trellis Coded Modulation) at the expense of decoder complexity.

Frequency Diversity

The same information can be transmitted on different carriers to overcome the effects of multipath induced flat fading (as long as the separation between carriers is greater than the Coherence Bandwidth ( 1/Delay Spread).  This is another type of multi-carrier transmission technique.   This is in contrast to separating the original information into pieces and modulating it onto orthogonal carriers, then combining, which leads to OFDM.

Path (space) diversity

It is common practice to use diversity antenna arrays (comprised of one or more antennas) at the base station to mitigate fading.  If the receive antenna elements (or antennas) are spaced an appropriate number of wavelengths apart, then it is likely that not all antennas will be in a null.  This is useful for mitigating the basic constructive / destructive effects of multipath induced flat fading.

Time diversity

The same information can be transmitted in different time slots to overcome the effects of multipath induced flat fading.  It is likely that across time slots, the signal will fade differently (again, dependent upon the frame and timeslot structure).

Polarization diversity

The same information can be transmitted with different polarizations to overcome the effects of multipath induced flat fading.  It is likely that the signal will fade differently as a function of polarization as well.

There are many combining techniques available to complement these various diversity schemes, each with it's associated tradeoffs (see comprehensive references above).

• Smart Antennas

Smart antennas can be used to suppress the effects of interference by placing nulls in the direction of the interferer.  Both the depth of the null as well as the operating bandwidth can be controlled at the expense of complexity and cost.

The next installment in this series will explore OFDM.  OFDM is a transmission technique that is inherently resistant to ISI.   OFDM will no doubt play a large role in the emerging wireless LAN/MAN standards and 4G network deployments.