Spectral Efficiency of Random Time-Hopping CDMA

Traditionally paired with impulsive communications, Time-Hopping CDMA (TH-CDMA) is a multiple access technique that separates users in time by coding their transmissions into pulses occupying a subset of $N_\mathsf{s}$ chips out of the total $N$ included in a symbol period, in contrast with traditional Direct-Sequence CDMA (DS-CDMA) where $N_\mathsf{s}=N$. This work analyzes TH-CDMA with random spreading, by determining whether peculiar theoretical limits are identifiable, with both optimal and sub-optimal receiver structures, in particular in the archetypal case of sparse spreading, that is, $N_\mathsf{s}=1$. Results indicate that TH-CDMA has a fundamentally different behavior than DS-CDMA, where the crucial role played by energy concentration, typical of time-hopping, directly relates with its intrinsic "uneven" use of degrees of freedom.Comment: 26 pages, 13 figure

Teletraffic Modeling of Cdma Systems

This paper presents teletraffic modeling of Code Division Multiple Access (CDMA) systems that enabled the analysis of such systems capacity. Analytical tools aided by software model that assisted in analysis of the system performance, capacity estimation, dimensioning and design of CDMA networks were achieved. This work, therefore, focused on modeling telephone traffic for analysis of CDMA cellular network capacity. We developed an analytical expression for blocking probability and consequently that for the determination and analysis of the capacity of CDMA networks. The analyses of obtained results showed how interference determined the capacity of CDMA networks and therefore proved that the capacity was not hard limited, but depended on predetermined quality of service for the network. Also, the result showed how the capacity of the network, in terms of number of subscribers, can be estimated for CDMA networks. Graphical results generated from the blocking model showed the effect of variations in interference parameters on CDMA capacity. The Erlang capacity from the model was adapted into Erlang B formula to estimate capacity in terms of channels, and the number of subscribers a typical CDMA sector could accommodate

Carrier Sense Random Packet CDMA Protocol in Dual-Channel Networks

Code resource wastage is caused by the reason that many hopping frequency (FH) sequences are unused, which occurs under the condition that the number of the actual subnets needed for the tactical network is far smaller than the networking capacity of code division net¬working. Dual-channel network (DCN), consisting of one single control channel and multiple data channels, can solve the code resource wastage effectively. To improve the anti-jamming capability of the control channel of DCN, code division multiple access (CDMA) technology was introduced, and a carrier sense random packet (CSRP) CDMA protocol based on random packet CDMA (RP-CDMA) was proposed. In CSRP-CDMA, we provide a carrier sensing random packet mechanism and a packet-segment acknowledgement policy. Furthermore, an analytical model was developed to evaluate the performance of CSRP-CDMA networks. In this model, the impacts of multi-access interference from both inter-clusters and intra-clusters were analyzed, and the mathematical expressions of packet transmission success probability, normalized network throughput and signal interference to noise ratio, were also derived. Analytical and simulation results demonstrate that the normalized network throughput of CSRP-CDMA outperforms traditional RP-CDMA by 10%, which can guarantee the resource utilization efficiency of the control channel in DCNs

On the Uplink Performance of Asynchronous LAS-CDMA

In this paper closed-form formulae are derived for characterizing the BER performance of Large Area Synchronous CDMA (LAS-CDMA) as a function of both the number of resolvable paths Lp and the maximum delay difference τmax, as well as the number of users K, when communicating over a Nakagami-m fading channel. Moreover, we comparatively studies the performance of LAS-CDMA and the traditional random code based DS-CDMA

Blind multi-user combining at the base station for asynchronous CDMA systems

This paper studies the potential benefits of antenna arrays in cellular CDMA communications and proposes a powerful scheme to undertake the array processing at the base station in CDMA mobile systems. The proposed technique exploits the temporal structure of CDMA signals. The necessary information is extracted directly from the received signals, thus no training signal orPeer ReviewedPostprint (published version

Performance of Fractionally Spread Multicarrier CDMA in AWGN as Well as Slow and Fast Nakagami-m Fading Channels

Abstract—In multicarrier code-division multiple-access (MCCDMA), the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this paper, we divide the symbol duration into a number of fractional subsymbol durations also referred to here as fractions, in a manner analogous to subbands in MC-CDMA systems. In the proposed MC-CDMA scheme, the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally spread MC-CDMA arrangement, or FS MCCDMA. Furthermore, the FS MC-CDMA signal is additionally spread in the frequency (F)-domain using a spreading code with the aid of a number of subcarriers. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels. Index Terms—Broadband communications, code-division multiple access (CDMA), fractionally spreading, frequency-domain spreading, multicarrier modulation, Nakagami fading, timedomain spreading

Broadband MC DS-CDMA Using Space-Time and Frequency-Domain Spreading

In this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS) assisted transmit diversity and frequency-domain (F-domain) spreading is investigated in the context of broadband communications over frequency-selective Rayleigh fading channels. We consider the attainable capacity extension of broadband MC DS-CDMA with the advent of using Time-Frequency-domain (TF-domain) spreading. The BER performance of STS assisted broadband MC DS-CDMA using Binary Phase Shit Keying (BPSK) modulation and TF-domain spreading is investigated by simulation for a range of parameter values. Both the correlation based single-user detector and the decorrelating multiuser detector are considered. Our study shows that the number of users supported by the broadband MC DS-CDMA system is determined by the product of the T-domain spreading factor and the F-domain spreading factor, while it is independent of the frequency diversity order. Furthermore, when multiuser detection assisted F-domain spreading is considered, the broadband MC DS-CDMA system is capable of supporting a substantially increased number of users, while maintaining a similar bit error ratio (BER) performance to that of the broadband MC DS-CDMA system using no F-domain spreading

LAS-CDMA using Various Time Domain Chip-Waveforms

LAS CDMA exhibits a significantly better performance than that of classic random code based DS-CDMA, when operating in a quasi-synchronous scenario. Classic frequency-domain raised cosine Nyquist filtering is known to show the best possible performance, but its complexity may be excessive in highchip-rate systems. Hence in these systems often low-complexity time-domain waveform shaping is considered. Motivated by this fact, the achievable performance of LAS-CDMA is investigated in conjunction with three different time-limited chipwaveforms, which exhibit an infinite bandwidth. The raised cosine time-domain waveform based DS-CDMA system is shown to achieve the best performance in the context of a strictly band-limited system, because its frequency-domain spectral side-lobes are relatively low