A review on synchronous CDMA systems: optimum overloaded codes, channel capacity, and power control

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.Keywords: Code Division Multiple Access (CDMA), Optimum codes, channel capacity bounds, near-far effects, power control I Introductioncode-division multiple access (CDMA) has been the most important multiple access technology for the 3rd generation GSM and American Cellular systems [1]. Optical CDMA systems have become an alternative multiple access for fiber optics and optical wireless systems [2][3][4].In CDMA systems, each user is assigned a unique code signature that consists of a number of chips. The signature length (also called chip rate) is defined as the number of chips in each signature code. Each user signature is multiplied by the respective data, and the transmitted vectors are added up in the common channel. The resultant vector is then observed at the received user end. In order to decode the received signal, the detector of the received user should know its own unique signature. These codes should be designed such that the cross-correlations between the code of the desired user and the codes of the other users are minimal.For the wireless case, the most well-known binary (Endnote a) code for the synchronous case is Hadamard orthogonal code that is appropriate for fully and underloaded CDMA systems. (Endnote b) But because of bandwidth limitation in the communication systems, we are interested in finding codes that can support more users than the chip rate (overloaded case). In the overloaded case, we cannot use Hadamard codes; Even random codes create interference that cannot be eliminated completely [5][6][7]. Optical orthogonal codes (OOC) [3,8] are not really orthogonal; however, using interference cancelation, we can remove interference completely. Most of the research in the overloaded case is related to code design and multi-access interference (MAI) cancelation in order to decrease the probability of error. Examples of these types of codes are pseudo random spreading [9,10], Welsh Bound Equality (WBE) codes with minimal total square correlation (TSC) [11][12][13][14], OCDMA [15][16][17], Multiple-OCDMA [18], and PN/ OCDMA [19] signature sets. None of the above codes guarantee errorless transmission in the absence of channel noise for overloaded CDMA systems. There are also some codes that are not designed upon cross-correlation and are designed such that they can provide one-to-one transfor...

Capacity Bounds of Finite Dimensional CDMA Systems with Power Allocation, Fading, and Near-Far Effects

“…In the absence of near-far effects, the channel capacity has been evaluated for real and complex inputs [3] and [4]. However, for overloaded CDMA systems with finite input alphabets, only lower and upper bounds have been evaluated [5]- [8]; a recent review of these papers is published in [9]. Asymptotic results for finite input sum capacity have been derived by [1] and [2].…”

Parallel overloaded CDMA crossbar for network on chip

“…The power consumption of this NoC switch increases with its data packet because the transition activity is more when the data is undergoing STP/PTS block, hence dynamic power consumption also increases and it is shown in Fig.7(c). The throughput ( ) is calculated as (5) where N c is the number of required clock cycles, N bpp is the number of bits in a packet, N pe is the number of received packets at the PE and t c is the clock period for complete data transmission. From Fig.…”

“…The local port is used as a processing element (PE) and is responsible for communication between the port and crossbar.This paper proposes a new method for NoC with fixed latency, reduced system cost and power consumption. Recently, CDMA is used to transfer data between the input and output in NoC switch [5]. Fig.1 depicts the structure of CDMA NoC switch.…”

Finding sub‐optimum signature matrices for overloaded code division multiple access systems

“…Alishahi et al [5] generalised their previous works [3,4] to finite non-binary discrete input and matrix entries. A review of these papers has been done by Hosseini et al in [6]. The total square correlation is used in [7] and [8] to obtain the Welch bound equality (WBE) sequences which maximises the sum channel capacity of real input synchronous CDMA systems.…”

“…For finite scale systems the actual capacity is not known, however there are lower and upper bounds for the sum capacity [4]- [6]. A review of these papers is done by Hosseini in [7].…”