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Additional Techniques for Capacity and Flexibility Enhancement As shown in Chapter 1, wireless channels suffer from attenuation due to the destructive addition of multipath propagation paths and interference. Severe attenuation makes it difficult for the receiver to detect the transmitted signal unless some additional, lessattenuated replica of the transmitted signal are provided. This principle is called diversity and it is the most important factor in achieving reliable communications. Examples of diversity techniques are: — Time diversity: Time interleaving in combination with channel coding provides replicas of the transmitted signal in the form of redundancy in the temporal domain to the receiver | 6 Additional Techniques for Capacity and Flexibility Enhancement 6.1 Introduction As shown in Chapter 1 wireless channels suffer from attenuation due to the destructive addition of multipath propagation paths and interference. Severe attenuation makes it difficult for the receiver to detect the transmitted signal unless some additional less-attenuated replica of the transmitted signal are provided. This principle is called diversity and it is the most important factor in achieving reliable communications. Examples of diversity techniques are Time diversity Time interleaving in combination with channel coding provides replicas of the transmitted signal in the form of redundancy in the temporal domain to the receiver. Frequency diversity The signal transmitted on different frequencies induces different structures in the multipath environment. Replicas of the transmitted signal are provided to the receiver in the form of redundancy in the frequency domain. Best examples of how to exploit the frequency diversity are the technique of multi-carrier spread spectrum and coding in the frequency direction. Spatial diversity Spatially separated antennas provide replicas of the transmitted signal to the receiver in the form of redundancy in the spatial domain. This can be provided with no penalty in spectral efficiency. Exploiting all forms of diversity in future systems e.g. 4G will ensure the highest performance in terms of capacity and spectral efficiency. Furthermore the future generation of broadband mobile fixed wireless systems will aim to support a wide range of services and bit rates. The transmission rate may vary from voice to very high rate multimedia services requiring data rates up to 100 Mbit s. Communication channels may change in terms of their grade of mobility cellular infrastructure required symmetrical or asymmetrical transmission capacity and whether they Multi-Carrier and Spread Spectrum Systems K. Fazel and S. Kaiser 2003 John Wiley Sons Ltd ISBN .