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Abstract
This paper is a tutorial review on important issues related to code-division multiple-access (CDMA) systems such as channel capacity, power control, and optimum codes; specifically, we consider optimum overloaded codes that achieve errorless transmission in the absence of noise for the binary and nonbinary cases. A survey of lower and upper bounds for the sum channel capacity of such systems is given in the presence and absence of channel noise. The asymptotic results for the channel capacity are also investigated. The channel capacity, errorless transmission codes, and power estimation for near-far effects are also explored. The emphasis of this tutorial review is on the overloaded CDMA systems.
Highlights
Introduction to optimum codes forcode-division multiple-access (CDMA) transmission For combating the problem of bandwidth limitation in wireless and optical CDMA systems, we are interested to use optimum codes with a large overloading factor
In real communication systems, we deal with more practical applications, and some special cases such as the binary input and binary signature CDMA systems [22] and the codes being able to detect the active users in a CDMA system [23] are considered
We will take a brief look at CDMA systems with WBE (Welsh Bound Equality) codes that are optimum for analog inputs [11]
Summary
In Multi Access Channels (MAC), additive noise and multi user interference are the main factors that cause disturbance in CDMA transmission. B Noiseless channel capacity bounds In this subsection, we will take a look at the lower and upper bounds for the sum capacity of general CDMA systems These bounds are investigated further for several special cases such as COW matrices and active user detection systems. 1 Lower bounds for the sum capacity of CDMA systems for the noiseless case In the general mode where the signature alphabets and input vectors are not binary, the authors of [20,21] first defined p and π as follows: Suppose that Iis the difference set of I and is defined as:. An interesting result that can be drawn from this figure is that the channel capacity increases almost linearly with the number of users until n reaches a certain threshold value nth In this region, the errorless transmission is achieved and this implies that overloaded signature matrices do exist for these values of m and n.
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