Stability of N interacting queues in random-access systems

We revisit the stability problem of systems consisting of N buffered terminals accessing a common receiver over the collision channel by means of the standard ALOHA protocol. We find that in the slotted ALOHA system queues have “instability rank” based on their individual average arrival rates and transmission probabilities. If a queue is stable, then the queue with lower instability rank is stable as well. The instability rank is used to intelligently set up the dominant systems. And the stability inner and outer bounds can be found by bounding the idle probability of some queues in the dominant system. Through analyzing those dominant systems one by one, we are able to obtain inner and outer bounds for stability. These bounds are tighter than the known ones although they still fail to identify the exact stability region for cases of N>2. The methodology used is new and holds promise for successfully addressing other similar stability problems     

Performance Analysis of Random Access Packet-Switched Code Division Multiple Access Systems

Analytical techniques for performance evaluation of synchronous random access packet switching in code division multiple access (CDMA) systems are presented. Steady-state throughput characteristics using several packet generation models are obtained. A number of example random access CDMA systems are compared in terms of their throughput versus offered traffic and utilization-delay characteristics. Numerical results indicate that appropriate use of multiaccess coding can provide utilization-delay characteristics superior to that of ALOHA. System stability is evaluated using a general finite user model, and the dynamic behavior of some example random access CDMA schemes is investigated.     

Multiple-frequency slotted ALOHA in a shadowed and Rician faded radio environment

A multiple-frequency channel is proposed to enhance the capacity of slotted ALOHA radio networks. The throughput is derived considering each packet being frequency assigned randomly in an uncorrelated shadowing and fading channel. Poisson and binomial distributions are assumed for the packet arrival models for an infinite and a finite number of users, respectively.<>     

Sensor networks with mobile access: optimal random access and coding

We consider random access and coding schemes for sensor networks with mobile access (SENMA). Using an orthogonal code-division multiple access (CDMA) as the physical layer, an opportunistic ALOHA (O-ALOHA) protocol that utilizes channel state information is proposed. Under the packet capture model and using the asymptotic throughput as the performance metric, we show that O-ALOHA approaches the throughput equal to the spreading gain with an arbitrarily small power at each sensor. This result implies that O-ALOHA is close to the optimal centralized scheduling scheme for the orthogonal CDMA networks. When side information such as location is available, the transmission control is modified to incorporate either the distribution or the actual realization of the side information. Convergence of the throughput with respect to the size of the network is analyzed. For networks allowing sensor collaborations, we combine coding with random access by proposing two coded random access schemes: spreading code dependent and independent transmissions. In the low rate regime, the spreading code independent transmission has a larger random coding exponent (therefore, faster decay of error probability) than that of the spreading code dependent transmission. On the other hand, the spreading code dependent transmission gives higher achievable rate.     

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     

Throughput analysis of CDMA systems using multiuser receivers

Throughput bounds are attained for random channel access multichannel code-division multiple-access (CDMA) systems and spread slotted Aloha systems employing multiuser receivers. It is shown that the normalized throughput of these two systems reaches 1.0 exponentially fast in the region r/K<1, where, r is the average number of simultaneous users in each channel in the random channel access multichannel CDMA system and the packet arrival rate in the spread slotted Aloha system, respectively, and K is the maximum number of users which the multiuser receiver can handle at the same time. Therefore, both of the random channel access multichannel CDMA system and the spread slotted Aloha system employing multiuser receivers can achieve perfect throughput while being stable in the region r/K=1-δ, δ>0. The maximum throughput of the random channel access multichannel CDMA systems is found as K-√(1-(1/M))KlogK-O(logK), where M is the number of channels in the system. The maximum throughput is reached when the average number of simultaneous users is r_m=K-√((1-(1/M))KlogK))+O(√(K/logK)). The maximum throughput of the spread slotted Aloha systems is K-√(KlogK)-O(log K). The maximum throughput is reached when the packet arrival of Poisson distribution has the arrival rate λ_m=K-√(KlogK)+O(√(K/logK))     

Stability and delay of finite-user slotted ALOHA with multipacket reception

The effect of multipacket reception (MPR) on stability and delay of slotted ALOHA based random-access systems is considered. A general asymmetric MPR model is introduced and the medium-access control (MAC) capacity region is specified. An explicit characterization of the ALOHA stability region for the two-user system is given. It is shown that the stability region undergoes a phase transition from a concave region to a convex polyhedral region as the MPR capability improves. It is also shown that after this phase transition, slotted ALOHA is optimal i.e., the ALOHA stability region coincides with the MAC capacity region. Further, it is observed that there is no need for transmission control when ALOHA is optimal i.e., ALOHA with transmission probability one is optimal. Next, these results are extended to a symmetric N>2 user ALOHA system. Finally, a complete characterization of average delay in capture channels for the two-user system is given. It is shown that in certain capture scenarios, ALOHA with transmission probability one is delay optimal for all stable arrival rates. Further, it is also shown that ALOHA with transmission probability one is optimal for stability and delay simultaneously in the two-user capture channel.     

Stability and throughput analysis of unslotted CDMA‐ALOHA with finite number of users and code sharing

We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than TDMA‐ALOHA. This revised version was published online in June 2006 with corrections to the Cover Date.     

Improving the performance of a slotted ALOHA packet radio networkwith an adaptive array

The use of an adaptive antenna array as a means of improving the performance of a slotted ALOHA packet radio network is presented. An adaptive array creates a strong capture effect at a packet radio terminal by automatically steering the receiver antenna pattern toward one packet and nulling other contending packets in a slot. A special code preamble and randomized arrival times within each slot allow the adaptive array to lock onto one packet in each slot. The throughput and delay performance of a network with an adaptive array are computed by applying the standard Markov chain analysis of slotted ALOHA. It is shown that throughput levels comparable to carrier sense multiple access (CSMA) are attainable with an adaptive array without the need for stations to be able to hear each other. The performance depends primarily on the number of adaptive array nulls, the array resolution, and the length of the randomization interval within each slot     

Idle-signal casting multiple access with data slot reservation(ICMA-DR) for packet radio communications

A population of terminals communicating with a central station over a packet-switched multiple access radio channel is investigated with regard to multiple access control schemes. The authors describe the ICMA-DR, which is an advanced idle-signal casting multiple access (ICMA) scheme characterized by data slot reservation. This improved central controlled multiple-access scheme for packet transmission in terrestrial radio communications is evaluated in terms of throughput traffic, throughput delay characteristics, and handling capacity. It is shown that the throughput characteristics of ICMA-DR are superior than those of ICMA or slotted ALOHA when a packet for data slot reservation is relatively short in comparison to that for upward data. Thus, it is shown that ICMA-DR is suitable for the packet radio multiple-access scheme, especially in the case where fading packet error occurs frequently and ordered traffic is heavy. The ICMA-DR scheme has been utilized for the access control channel of NTT's new 800-MHz-band high-capacity land mobile communication system since the Spring of 1988     

Throughput of DS‐CDMA/unslotted ALOHA radio networks with Markovian arrival processes

Previous works on the throughput analysis of the direct sequence‐code division multiple access/unslotted ALOHA radio network all used the Poisson arrival process (PAP). However, the interarrival times of PAP are independent, so it is not suited to model today's Internet and multimedia traffic, which have correlated interarrival times. We are motivated to use the Markovian arrival process (MAP), a more general input traffic model that captures the correlation of interarrival times. We are the first to analyze the throughput of the direct sequence‐code division multiple access/unslotted ALOHA radio network with MAP. We propose the use of MAP, which encompasses the PAP as a special case. The new MAP model basically generalizes the current traffic and queuing models of multimedia in wireless networks. Copyright © 2011 John Wiley & Sons, Ltd.     

Capacity of time-slotted ALOHA packetized multiple-access systems over the AWGN channel

We study different notions of capacity for time-slotted ALOHA systems. In these systems, multiple users synchronously send packets in a bursty manner over a common additive white Gaussian noise (AWGN) channel. The users do not coordinate their transmissions, which may collide at the receiver. For such a system, we define both single-slot capacity and multiple-slot capacity. We then construct a coding and decoding scheme for single-slot capacity that achieves any rate within this capacity region. This coding and decoding scheme for a single time slot combines aspects of multiple access rate splitting and of broadcast codes for degraded AWGN channels. This design allows some bits to be reliably received even when collisions occur and more bits to be reliably received in the absence of collisions. The exact number of bits reliably received under both of these scenarios is part of the code design process, which we optimize to maximize the expected rate in each slot. Next, we examine the behavior of the system asymptotically over multiple slots. We show that there exist coding and decoding strategies such that regardless of the burstiness of the traffic, the system is stable as long as the average rate of the users is within the multiple access capacity region of the channel. In other words, we show that bursty traffic does not decrease the Cover-Wyner capacity region of the multiple access channel. A vast family of codes, which includes the type of codes we introduce for the single-slot transmission, achieve the capacity region, in a sense we define, for multiple-slot transmissions. These codes are stabilizing, using only local information at each of the individual queues. The use of information regarding other queues or the use of scheduling does not improve the multiple-slot capacity region.     

Packet Switching in a Multiaccess Broadcast Channel: Performance Evaluation

In this paper, the rationale and some advantages for multiaccess broadcast packet communication using satellite and ground radio channels are discussed. A mathematical model is formulated for a "slotted ALOHA" random access system. Using this model, a theory is put forth which gives a coherent qualitative interpretation of the system stability behavior which leads to the definition of a stability measure. Quantitative estimates for the relative instability of unstable channels are obtained. Numerical results are shown illustrating the trading relations among channel stability, throughput, and delay. These results provide tools for the performance evaluation and design of an uncontrolled slotted ALOHA system. Adaptive channel control schemes are studied in a companion paper.     

Adaptive Spatial Filtering for an FH/SSMA Packet Radio Network with Packet Combining

In this paper, packet throughput is analyzed and simulated for a show FH/SSMA packet radio network with adaptive antenna array and packet combining in a Rayleigh fading channel with shadowing. The packet throughput is defined as the average number of captured packets per slot. To enhance the throughput performance, an adaptive spatial filtering through adaptive antenna array and a packet combining scheme are employed. As a random access protocol, slotted ALOHA is considered, and synchronous memoryless hopping patterns are assumed. A packet consists of codewords from an (n, k) RS (Reed-Solomon) code. The tap weights of an adaptive processor is updated by RLS (recursive-least-square) algorithm. From the simulation results, it is shown that a pre-processing by adaptive antenna array and a post-processing by packet combining are very effective to improve reception performance of an FH/SSMA network.     

Throughput of unslotted direct-sequence spread-spectrummultiple-access channels with block FEC coding

An analysis of unslotted random-access direct-sequence spread-spectrum multiple-access (DS/SSMA) channels with block forward error correction (FEC) coding is presented. Extending a methodology that was introduced in an earlier paper on unslotted packet code-division multiple access (CDMA) without coding, a procedure for calculating the error probability of an L-bit packet in the variable message length, FEC-coded, DS/SSMA environment is described. This procedure is then used in conjunction with appropriate flow equilibrium traffic models to compute channel throughput. Using BCH block coding as an example, the analytical model is exercised to obtain throughput versus channel traffic curves over a range of code rates, leading to an assessment of maximum achievable throughput and the associated optimum FEC code rate. The results show that the use of block FEC coding provides a significant improvement in the bandwidth-normalized channel throughput (utilization), approaching values competitive with those for comparable narrowband ALOHA channels     

MIMO-OFDM系统中跨层媒体接入控制

针对MIMO-OFDM系统,提出了一种基于时隙ALOHA协议的随机接入方案,该方案利用一个简单的中心控制器和物理层的自适应调制技术以及MIMO迫零信号检测算法,在媒体接入控制层提供多包接收能力,解决包冲突问题。在满足用户BER要求的前提下,研究了系统最大化吞吐量时,接入同一子载波的最优用户数,并为此设计最佳的接入请求概率。仿真结果表明,最优接入天线数大约为接收天线数的四分之三,此时可以获取最大的吞吐量。     

A bit-map-assisted dynamic queue protocol for multiaccess wireless networks with multiple packet reception

A novel network-assisted (signal processing based) medium access control (MAC) protocol known as the bit-map-assisted dynamic queue (BMDQ) is presented. The protocol is explicitly designed for a wireless slotted system with multiple packet reception (MPR) capability. In the proposed protocol, the traffic in the channel is viewed as a flow of transmission periods (TPs). Each TP has a bit-map (BM) slot at the beginning followed by a data transmission period (DP). The BM slot is reserved for user detection so that accurate knowledge of the active user set (AUS) can be obtained. Then, given the knowledge of the AUS and the channel MPR matrix, the number of users that can access the channel simultaneously in each packet slot in the DP is chosen to maximize the conditional throughput of every packet slot. Compared with other conventional and network-assisted MAC protocols, the proposed BMDQ protocol yields better performance. Its maximum steady-state throughput is close to the channel MPR capacity, and it can achieve the same throughput with lower traffic load and smaller delay. Performance issues are investigated analytically and via simulations.     

Power levels and packet lengths in random multiple access

Multiple-power-level ALOHA has been proposed to take advantage of the capture phenomenon in order to improve the throughput of a multiple random access system. We study the effect of the use of multiple transmission power levels and of the corresponding packet lengths on the system throughput and energy efficiency. We prove that the single-power-level system in which all transmit at the maximum allowable power level achieves both optimal throughput and energy usage efficiency under a condition on the decodability threshold value     

Random Signal Levels for Channel Access in Packet Broadcast Networks

In this paper, it is proposed to employ random multiple signal levels for channel access in packet broadcast networks. We present priority-free random access protocols that possess the advantage of capture effect. The presented schemes are applied to the slotted ALOHA, and the performance is analyzed based on a conservative capture model. Closed-form expressions for the system throughput are derived for a general two-signal level system and a generalm-signallevel system. It is shown that the maximum throughput for the twolevel system increases from 0.47 to 0.52 as the separation between the two levels increases. For them-level system, the maximum throughput increases from 0.52 to 0.66 asmincreases from three to infinity. Then a rotary-priority sure-capture random access scheme is presented, which can achieve perfect channel utilization. The time-delay characteristic and the throughput-delay tradeoff are analyzed for the simplest two-level system for which the higher level is double the lower level. The results compare favorably to those of the conventional slotted ALOHA system which employs a single signal level for packet transmission. A number of open problems are addressed.     

Performance analysis of DS-CDMA with slotted ALOHA random access for packet PCNs

In this paper, we present a discrete time Markov chain based analytical framework for the study of Direct-Sequence Code-Division-Multiple-Access (DS-CDMA) with slotted ALOHA random access protocols (DS-CDMA-S-ALOHA) for packet Personal Communications Networks (PCNs). It incorporates both the random access and the random errors associated with DS-CDMA-S-ALOHA protocols into a unified framework. The key feature is that it distinguishes between the two stages in the transmission process, namely the access stage and the reception stage, which characterize the random access and the random errors associated with DS-CDMA-S-ALOHA protocols respectively. Two DS-CDMA-S-ALOHA protocols are presented and analyzed. The performance of the protocols and the effects of the design parameters, namely the packet retransmission probability and the forward error correction code rate of the Bose-Chaudhuri-Hocquenghem (BCH) block codes are evaluated numerically and compared with a bandwidth equivalent conventional multi-channel slotted ALOHA system. The results show that, by proper design, the DS-CDMA-S-ALOHA protocols can double the throughput with respect to that of a bandwidth equivalent conventional multi-channel slotted ALOHA system.