System identification: theory for the user
System identification: theory for the user
Using simulated annealing to design good codes
IEEE Transactions on Information Theory
Wireless Communications: Principles and Practice
Wireless Communications: Principles and Practice
Introduction to Trellis-Coded Modulation with Applications
Introduction to Trellis-Coded Modulation with Applications
On Schemes for Multriate Support in DS-CDMA Systems
Wireless Personal Communications: An International Journal
IEEE Communications Magazine
Time division multiple access methods for wireless personal communications
IEEE Communications Magazine
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A multicode direct-sequence code-division multiple-access system experienceslarge envelope variations as a result of a sum of many independently spreadsignals. However, large envelope variations is problematic because it reducesthe spectral efficiency, the efficiency of the power amplifiers and theperformance. All these effects depend on the non-linear amplifiers thatgenerally are used in handsets. It would of course be possible to use a linearamplifier but then the power efficiency is drastically reduced. In this paperwe analyze the envelope variations of a multicode signal in terms of thecrest factor and find that it increases as the square root of thenumber of used codes. As a consequence it is only possible to use themulticode scheme for a few parallel codes. To reduce the envelope variationsa precoder is introduced. This precoder is a non-linear high-rate block codeespecially designed for the set of spreading codes used. However, the precodercan be made independent of the spreading codes if a user-specific spreadingcode is concatenated with a set of Hadamard or Conference sequences. Theresulting spreadig codes are orthogonal. Also, the precoder is independent ofthe user-specific spreading code, and can thus be used for all users. Afterprecoding the crest factor is significantly reduced and the performance, dueto introduced coding gain, improved. Algorithms for the design of precoderswith both reduced envelope variation and good performance are presented.Furthermore, simulations show that a precoded multicode system outperforms anuncoded multicode system in a single-user as well as in a multiuserenvironment.