A novel spread slotted Aloha system with channel load sensingprotocol

This paper presents a novel spread slotted Aloha system with channel load sensing protocol (CLSP). CLSP is an effective scheme to improve the throughput performance in spread unslotted Aloha systems. In spread slotted Aloha systems, however, it does not make sense to utilize CLSP because the slot size is usually the same as the packet size. The slot size of the proposed system is set less than the packet size, thereby enabling the authors to apply CLSP and improving the throughput performance. Another feature of the proposed system is that the system is not likely affected by the time difference between channel load sensing and timing of packet access, which they call the access timing delay. Throughput performance of the proposed system is evaluated in the presence of the access timing delay and a significant increase of the throughput is shown compared with that of spread unslotted Aloha with CLSP     

The throughput of some wireless multiaccess systems

We compute the throughput of some multiaccess wireless systems for delay-tolerant data communications, characterized by an infinite population of uncoordinated users accessing a common channel. The channel is affected by block fading, and the channel state is perfectly known to the receiver but unknown to the transmitters. To cope with multiaccess interference (MAI) and fading, the users employ retransmission of erroneously received packets. We consider unspread and randomly spread (code-division multiple-access (CDMA)) systems with decentralized (single-user) decoding and a system where the receiver employs joint multiuser decoding. The following conclusions can be drawn from our analysis: (a) unspread systems with packet retransmission outperforms CDMA systems with conventional detection, but are outperformed by CDMA with linear minimum mean-square error (MMSE) detection. (b) For all systems based on single-user decoding (SUD), there exists a threshold value of (E/sub b//N/sub o/) below which the throughput is maximized by an infinite number of users per dimension transmitting at vanishing rate, and above which the throughput is maximized by a finite average number of users per dimension transmitting at nonvanishing rate. Moreover, as (E/sub b//N/sub o/) increases, the optimal average number of users per dimension tends to one. In this sense, we say that the optimized systems "self-orthogonalize." (c) For the system based on joint multiuser decoding, a simple slotted ALOHA strategy is able to recover the throughput penalty due to fading in the limit for high (E/sub b//N/sub o/), while an incremental redundancy (INR) strategy recovers the fading penalty for any (E/sub b//N/sub o/).     

采用两种信道接入机制的认知无线电网络

时隙Aloha和CSMA是有效的随机接入协议,它们能组成更先进的媒体接入协议。研究了采用时隙Aloha和CSMA的认知无线电网络。其中,主用户比次用户有更高的优先权,次用户需检测信道,避免干扰主用户。因此,主用户采用时隙Aloha接入信道,次用户采用CSMA感知时隙Aloha的时隙,在空闲时隙传输数据包。     

Optimal power and retransmission control policies for random access systems

We consider in this study dynamic control policies for slotted Aloha random access systems. New performance bounds are derived when random access is combined with power control for system optimization, and we establish the existence of optimal control approaches for such systems. We analyze throughput and delay when the number of backlogged users is known, where we can explicitly obtain optimal policies and analyze their corresponding performance using Markov Decision Process (MDP) theory with average cost criterion. For the realistic unknown-backlog case, we establish the existence of optimal backlog-minimizing policies for the same range of arrival rates as the ideal known-backlog case by using the theory of MDPs with Borel state space and unbounded costs. We also propose suboptimal control policies with performance close to the optimal without sacrificing stability. These policies perform substantially better than existing "Certainty Equivalence" controllers.     

A micro-cellular CDMA system over slow and fast Rician fading radiochannels with forward error correcting coding and diversity

The microcellular radio environment is characterized by a Rician fading channel. The use of a slotted code division multiple access (CDMA) scheme is considered in single- and multi-microcell systems. The throughput and delay performance of a slotted CDMA network are analyzed for slow and fast Rician fading radio channels using differential phase shift keying (DPSK) modulation. The application of selection diversity (SD) and maximal ratio combining (MRC) improve the performance for both slow and fast fading. It is also shown that the use of forward error correcting (FEC) codes enhances the system performance. Computational results are presented for maximum rms delay spread in the order of 2 μs and data rates of 32 and 64 kbit/s. A comparative analysis of macro-, micro- and pico-cellular CDMA systems is also presented     

On the instability of slotted aloha with capture

We analyze the stability properties of slotted Aloha with capture for random access over fading channels with infinitely-many users. We assume that each user node knows only its own uplink channel gain, and uses this decentralized channel state information (CSI) to perform power control and/or probability control. The maximum stable throughput (MST) for a general capture model is obtained by means of drift analysis on the backlog Markov chain. We then specialize our general result to a signal-to-interference-plus-noise ratio (SINR) capture model. Our analysis shows that if the channels of all users are identical and independently distributed (i.i.d.) with finite means, the system is unstable under any kind of power and probability control mechanism that is based only on decentralized CSI.     

Performance of CDMA random access systems with packet combining in fading channels

We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance.     

Price-based rate control in random access networks

We study a price-based rate control mechanism for random access networks. The mechanism uses channel feedback information to control the aggregate packet arrival rate. For our analysis, we use the standard slotted Aloha model with an infinite set of nodes. We show that the resulting Markov chain is positive recurrent. In addition, we characterize the throughput and delay at the operating point of the system and show how the operating point can be set a priori by appropriately choosing the control parameters. We illustrate our results using numerical experiments.     

Analysis of Channel-aware Multichannel ALOHA in OFDMA Wireless Networks

Orthogonal frequency division multiple access (OFDMA) systems provide multiple channels that can be accessed via random access schemes. In this paper a channel-aware multichannel random access, based on local channel state information (CSI), was investigated and a multichannel slotted ALOHA scheme was proposed accordingly. Also an analytical investigation of total system throughput and the queue state evolution of generic node in the network were present by assuming the channel has been modeled by means of a two state Markov chain. Through the theoretical model and simulation results, we confi rm that the proposed algorithm has the advantage of high throughput and low access delay.     

A Sequential Algorithm for Joint Parameter Estimation and Multiuser Detection in DS/CDMA Systems with Multipath Propagation

A direct sequence (DS) spread spectrum code division multiple access (CDMA) communication system is considered where several users transmit data symbols over different multipath channels. The main objective of this work is the proposal of a sequential algorithm for joint parameter estimation and multiuser data detection. The computationally prohibitive maximization of the log-likelihood function is replaced by a sequential scheme using the multistage detector for recovering the data symbols and the expectation-maximization algorithm for estimating the channel parameters. The performance of the resulting multiuser receiver is evaluated via Monte Carlo simulations using both synthetic as well as measured channel impulse responses. A comparison of the performance with analytical expressions for the single-user single-path case shows that the proposed system is capable of eliminating the near-far effect existing in conventional DS/CDMA systems.     

Genetic algorithm assisted joint multiuser symbol detection andfading channel estimation for synchronous CDMA systems

A novel multiuser code division multiple access (CDMA) receiver based on genetic algorithms is considered, which jointly estimates the transmitted symbols and fading channel coefficients of all the users. Using exhaustive search, the maximum likelihood (ML) receiver in synchronous CDMA systems has a computational complexity that is exponentially increasing with the number of users and, hence, is not a viable detection solution. Genetic algorithms (GAs) are well known for their robustness in solving complex optimization problems. Based on the ML rule, GAs are developed in order to jointly estimate the users' channel impulse response coefficients as well as the differentially encoded transmitted bit sequences on the basis of the statistics provided by a bank of matched filters at the receiver. Using computer simulations, we showed that the proposed receiver can achieve a near-optimum bit-error-rate (BER) performance upon assuming perfect channel estimation at a significantly lower computational complexity than that required by the ML optimum multiuser detector. Furthermore, channel estimation can be performed jointly with symbol detection without incurring any additional computational complexity and without requiring training symbols. Hence, our proposed joint channel estimator and symbol detector is capable of offering a higher throughput and a shorter detection delay than that of explicitly trained CDMA multiuser detectors     

The throughput of hybrid-ARQ protocols for the Gaussian collisionchannel

In next-generation wireless communication systems, packet-oriented data transmission will be implemented in addition to standard mobile telephony. We take an information-theoretic view of some simple protocols for reliable packet communication based on “hybrid-ARQ,” over a slotted multiple-access Gaussian channel with fading and study their throughput (total bit per second per hertz) and average delay under idealized but fairly general assumptions. As an application of the renewal-reward theorem, we obtain closed-form throughput formulas. Then, we consider asymptotic behaviors with respect to various system parameters. The throughput of automatic retransmission request (ARQ) protocols is compared to that of code division multiple access (CDMA) with conventional decoding. Interestingly, the ARQ systems are not interference-limited even if no multiuser detection or joint decoding is used, as opposed to conventional CDMA     

On exploiting idle channels in opportunistic multichannel ALOHA

Opportunistic multichannel ALOHA is a random access protocol that can exploit both multiuser and multichannel diversities by taking the advantage of fading channel. However, this protocol is inefficient as inherited from contention based random access in the sense that there is a noticeable number of idle channels in each time slot. To reduce this wastage, we propose a simple extension to the transmission policy so that these idle channels can be exploited, which in consequence leads to a significant improvement in term of maximum throughput.     

Complexity of Verdu optimum multiuser detection algorithm inmultichannel CDMA systems

A statistical characterization of the complexity function of the Verdu optimum multiuser detection (VOMD) algorithm is presented for a communication system employing a finite number of randomly accessed orthogonal channels and a finite number of simultaneous users. Multichannel code-division multiple-access (CDMA) systems are proposed. It is proved that the probability, in which the individual channel complexity is greater than A^r(1+α), approaches zero exponentially fast as the average number of simultaneous users in each channel increases, where A is the modulation alphabet size and α>0. When the number of simultaneous users is large, the complexity of applying the VOMD algorithm to each individual channel is negligible when compared with the complexity of applying the same algorithm directly to the traditional single-channel CDMA system supporting the same number of simultaneous users. The probability distribution of the joint complexity function of the aggregate system is found. It is shown that when the number of simultaneous users is large, the joint complexity function is negligible compared with applying the VOMD algorithm directly to the traditional single-channel CDMA system supporting the same number of simultaneous users. Therefore, a multichannel CDMA communication system can support a comparable population of simultaneous users to the traditional single-channel CDMA system of comparable bandwidth, while reducing the complexity of optimum multiuser detection to a practical level     

A novel random wireless packet multiple access method using CDMA

Random packet CDMA, a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or spread ALOHA, and provides better spectral efficiencies.     

Distributed approaches for exploiting multiuser diversity in wireless networks

In wireless fading channels, multiuser diversity can be exploited by scheduling users to transmit when their channel conditions are favorable. This leads to a sum throughput that increases with the number of users and, in certain cases, achieves capacity. However, such scheduling requires global knowledge of every user's channel gain, which may be difficult to obtain in some situations. This paper addresses contention-based protocols for exploiting multiuser diversity with only local channel knowledge. A variation of the ALOHA protocol is given in which users attempt to exploit multiuser diversity gains, but suffer contention losses due to the distributed channel knowledge. The growth rate of the sum throughput for this protocol is characterized in a backlogged system under both short-term and long-term average power constraints. A simple "fixed-rate" system is shown to be asymptotically optimal and to achieve the same growth rate as in a system with an optimal centralized scheduler. Moreover, asymptotically, the fraction of throughput lost due to contention is shown to be 1/e. Also, in a system with random arrivals and an infinite user population, a variation of this ALOHA protocol is shown to be stable for any total arrival rate, given that users can estimate the backlog.     

Opportunistic multichannel Aloha: distributed multiaccess control scheme for OFDMA wireless networks

The authors consider multiaccess control for the uplink in orthogonal-frequency-division-multiple-access wireless networks, where subcarriers are grouped into clusters. A multichannel random access, based on local channel state information (CSI), was investigated, and an opportunistic multichannel Aloha (OMC-Aloha) was proposed accordingly. A key step is to build a mapping from a user's CSI to its transmission probability and subcarrier allocation. For the sake of comparison, the throughput of the optimal centralized scheduling was also characterized by using the Extreme-Value Theory of order statistics. The authors show that the OMC-Aloha is asymptotically order optimal, in the sense that the only performance loss compared to the optimal centralized scheduling is due to the contention inherent in random access. In addition, they examine the proportional fairness in heterogeneous systems.     

Slotted ALOHA multiple access and error control coding for land mobile satellite networks

This paper considers a satellite network with data messages being transmitted by land mobile users according to slotted Aloha multiple access. The mobile communication links suffering from multipath fading and signal shadowing are modelled as Gilbert-Elliott channels. FEC block coding is used to correct transmission errors. The maximum achievable information throughput and the mean packet delay are derived from a combined analysis of the multiple access and FEC/ARQ protocol. The results show that the additional overhead necessary for FEC is outweighed by the benefit in throughput and delay. Finally, the capture effect and its consequences are discussed.     

Multichannel random access in OFDMA wireless networks

Orthogonal frequency-division multiple access (OFDMA) systems are considered promising candidates for implementing next-generation wireless communication systems. They provide multiple channels that can be accessed via random access schemes. However, traditional random access schemes could result in an excessive amount of access delay. To address this issue, we develop a fast retrial scheme that is based on slotted Aloha and exploits the structure of OFDMA. A salient feature of this scheme is that when collisions occur instead of retrials occuring randomly in time, they occur randomly in frequency, i.e., the scheme randomly selects the subchannels for retrial. To further achieve fast access, retrials are designed to follow the 1-persistent type, i.e., no exponential backoff. To achieve the maximum throughput, we limit the maximum number of allowed retrials according to the load condition. We also consider the issue of designing for an appropriate reuse factor for random access channels in order to overcome the intercell interference problem in OFDMA multicell environments. Our finding is that full sharing, i.e., a reuse factor of one, performs best for given random access channels. Through analysis and simulation, we confirm that our fast retrial algorithm has the advantage of high throughput and low access delay, and the full sharing policy for random access channels shows high throughput as well as low collision.     

Random access with large propagation delay

Random access to a packet broadcast channel with large propagation delay is investigated. A protocol is presented that combines slotted ALOHA random access with the use of forward-error-correction (FEC) across transmitted packets. Expressions for the throughput, delay, and drift of this protocol are derived. Numerical studies and asymptotic analyses of the drift indicate that the protocol has a maximum throughput of e^-1 and exhibits bistability and saturation behavior similar to that of slotted ALOHA with immediate feedback. However, unlike ALOHA, bistability and saturation in the code protocol can be eliminated with the proper choice of protocol parameters without increasing the packet delay. It is further shown that, when compared to slotted ALOHA, the code protocol typically achieves a higher throughput and lower delay at system equilibrium with no loss in maximum throughput