A model for local/mobile radio communications with correct packetcapture

The performance of a local/mobile radio communications system utilizing the slotted ALOHA multiple random access protocol is analyzed. The probability of correct packet capture is evaluated for a local/mobile packet radio system using ideal coherent binary phase-shift keying (BPSK) modulation. Both the near/far effect and the effect of Rayleigh fading on the probability of correct packet capture are taken into account, but the effect of thermal noise is neglected since the interference due to competing packets is dominant in practical systems. The probability of correct packet reception is evaluated for a system using spatial diversity. In addition, the effect on system performance of either convolutional coding with hard decision Viterbi decoding or binary linear block coding with hard decision decoding is evaluated. The pseudo-Bayesian algorithm that has been developed to stabilize a packet system based on the slotted ALOHA protocol at maximum channel throughput is found to be adaptable to the local mobile operating environment. For the system considered here, channel throughputs as high as 0.66 can be obtained     

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     

On the randomization of transmitter power levels to increase throughput in multiple access radio systems

To enhance the throughput of a slotted random access protocol in a radio communication system, we describe the use of a scheme in which multiple power levels are used at the transmitters. We first consider a situation in which n transmitters are simultaneously trying to send a packet to a central receiving station using a time‐slotted access protocol, like slotted ALOHA. Each of these transmitters randomly chooses one of m discrete power levels during each attempt to send a packet. One of the simultaneously sent packets can often be successfully received due to the power capture effect. We consider two types of capture models: (1) one in which the transmitter with the largest received power captures the channel, and (2) one in which the transmitter captures the channel only if its signal‐to‐interference ratio is above some threshold when received at the central station. In this paper, we determine the optimal transmit probabilities for the power levels as well as the optimal values of the power levels themselves, when their range is constrained and for cases both with and without Rayleigh fading. After determining the precise optimal power levels and probabilities for maximizing the capture probabilities (i.e., for a given n), we propose a less complex, but nearly optimal, approximate approach based on using logarithmically equi‐spaced levels. After demonstrating the closeness of our suboptimal results to the optimal results, we apply our approach to the problem of optimizing the throughput of the slotted ALOHA protocol for a case in which the input traffic is generated according to a Poisson process. Several numerical examples are presented along with a demonstration of how the optimal choice of power levels and probabilities can enhance throughput relative to previous ad hoc methodologies.     

On the randomization of transmitter power levels to increase throughput in multiple access radio systems

To enhance the throughput of a slotted random access protocol in a radio communication system, we describe the use of a scheme in which multiple power levels are used at the transmitters. We first consider a situation in which n transmitters are simultaneously trying to send a packet to a central receiving station using a time‐slotted access protocol, like slotted ALOHA. Each of these transmitters randomly chooses one of m discrete power levels during each attempt to send a packet. One of the simultaneously sent packets can often be successfully received due to the power capture effect. We consider two types of capture models: (1) one in which the transmitter with the largest received power captures the channel, and (2) one in which the transmitter captures the channel only if its signal‐to‐interference ratio is above some threshold when received at the central station. In this paper, we determine the optimal transmit probabilities for the power levels as well as the optimal values of the power levels themselves, when their range is constrained and for cases both with and without Rayleigh fading. After determining the precise optimal power levels and probabilities for maximizing the capture probabilities (i.e., for a given n), we propose a less complex, but nearly optimal, approximate approach based on using logarithmically equi‐spaced levels. After demonstrating the closeness of our suboptimal results to the optimal results, we apply our approach to the problem of optimizing the throughput of the slotted ALOHA protocol for a case in which the input traffic is generated according to a Poisson process. Several numerical examples are presented along with a demonstration of how the optimal choice of power levels and probabilities can enhance throughput relative to previous ad hoc methodologies. This revised version was published online in July 2006 with corrections to the Cover Date.     

ALOHA with capture over slow and fast fading radio channels withcoding and diversity

The effects of capture on the average system throughput and delay performance of slotted ALOHA were analyzed for slow and fast Rayleigh fading radio channels. A short-range multipoint-to-base station packet radio network is considered. It is shown that larger capture effects and thus improved network performance can be achieved with proper choice of modulation. It is also shown that the use of simple error-correcting codes improves capture. The use of selection diversity also improves the capture effect both for fast and slow fading. It is concluded that the inverse distance variability of the received signal is the main reason for the capture effect. The Rayleigh fading alone yields a very small contribution in terms of throughput; nonetheless, it helps to stabilize the system. Numerical results are presented for a slotted ALOHA system with 50 users. It is found that the maximum average throughput can be increased from about 36% to almost 60% by using channel coding and space diversity     

The effect of multipath interference on the performance of random accessing

This paper studies the effect of multipath interference on the performance of random access techniques in which the power level of a packet depends on its time of arrival. The protocols considered are pure ALOHA and slotted ALOHA. Key performance measures such as channel throughout and mean packet delay are obtained. These results are derived partially using minislot approximation and are verified via computer simulations. For practical purposes in evaluating the successful packet reception probability the signal capture effect is also taken into consideration. How seriously the multipath interference affects the two protocols are witnessed through numerical examples. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

Performance comparison of a slotted ALOHA DS/SSMA network and amultichannel narrow-band slotted ALOHA network

Throughput, delay, and stability for two slotted ALOHA packet radio systems are compared. One system is a slotted direct-sequence spread-spectrum multiple-access (DS/SSMA) network where each user employs a newly chosen random signature sequence for each bit in a transmitted packet. The other system is a multiple-channel slotted narrow-band ALOHA network where each packet is transmitted over a randomly selected channel. Accurate packet success probabilities for the code-division multiple-access (CDMA) system are computed using an improved Gaussian approximation technique which accounts for bit-to-bit error dependencies. Average throughput and delay results are obtained for the multiple-channel slotted ALOHA system and CDMA systems with block error correction. The first exit time (FET) is computed for both systems and used as a measure of the network stability. The CDMA system is shown to have better performance than the multiple-channel ALOHA system in all three areas     

The capture effect in multiaccess communications-the Rayleigh andlandmobile satellite channels

The capture phenomenon and its consequences in nonbitsynchronous mobile packet radio networks for binary phase-shift-keying (BPSK) and differential phase-shift-keying modulation are investigated. Exact values of the bit error probability for given signal-to-noise ratios of colliding BPSK signals are derived. Packet error rates, which are needed for analysis of slotted random multiple access methods, are obtained by simulation. Two kinds of mobile radio channels are considered: the Rayleigh fading channel and the land mobile satellite channel. In the latter, because of shadowing, the probability that one of several colliding data packets is correctly received can be on the same order as the probability that a single packet that is not experiencing a collision is correctly received. The influence of Reed-Solomon codes on packet error probabilities is also studied. A slotted ALOHA system using the land mobile satellite channel is analyzed. It is found that with significant shadowing, the overall system throughput may reach the point-to-point throughput. Also, the code rate cannot be optimized in a straightforward manner by assuming one single transmission at a time     

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.     

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     

Capacity of slotted ALOHA in generalised fading environments

The channel throughput of the slotted ALOHA protocol is evaluated for a Nakagami fading channel model. General expressions for different values of the fading figure m and receiver capture ratio are presented. Evaluations of these expressions show that the throughput increases as the fade depth increases     

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.     

ALOHA with Multipacket Messages and ARQ-Type Retransmission Protocols--Throughput Analysis

The throughput of slotted ALOHA systems with multipacket message transmissions is evaluated. The conventional strategy of retransmitting the entire message when collisions occur results in a reduction in throughput when compared with the single packet case. However, it is proved analytically that the use of ARQ-type retransmission protocols can provide significant performance improvement. In particular, it is shown that a simple go-back-N(GBN) protocol can offer a modest throughput increase, while a selective reject (SREJ) strategy provides exactly the same throughput as a single packet slotted ALOHA system, irrespective of message length statistics. These results motivate a new scheme (referred to as unslotted selective reject (SREJ) ALOHA) for increasing the throughput of unsynchronized random access channels. It is demonstrated that unslotted SREJ ALOHA with optimized parameters can achieve typical maximum throughputs (after accounting for all overheads) at least 25-40 percent higher than conventional pure ALOHA.     

Frequency-hop transmission for satellite packet switching and terrestrial packet radio networks

The performance of frequency-hop transmission in a packet communication network is analyzed. Satellite multiple-access broadcast channels for packet switching and terrestrial packet radio networks are the primary examples of the type of network considered. An analysis of the effects of multiple-access interference in frequency-hop radio networks is presented. New measures of "local" performance are defined and evaluated for networks of this type, and new concepts that are important in the design of these networks are introduced. In particular, error probabilities and local throughput are evaluated for a frequency-hop radio network which incorporates the standard slotted and unslotted ALOHA channel-access protocols, asynchronous frequency hopping, and Reed-Solomon error-control coding. The performance of frequency-hop multiple access with error-control coding is compared with the performance of conventional ALOHA random access using narrow-band radios.     

Dynamic Frame Length ALOHA

Adding frame structure to slotted ALOHA makes it very convenient to control the ALOHA channel and eliminate instability. The frame length is adjusted dynamically according to the number of garbled, successful, and empty timeslots in the past. Each terminal that has a packet to transmit selects at random one of thentimeslots of a frame. Dynamic frame length ALOHA achieves a throughput (expected number of successful packets per timeslot) of 0.426 which compares favorably with the1/e (approx0.368)upper bound of ordinary slotted ALOHA.     

Investigation of slotted ALOHA under Nakagami fading with synchronized and asynchronous cochannel cells

The throughput performance of slotted ALOHA (S-ALOHA) in a cellular system and Nakagami fading environment is studied. Based on the signal capture model, the effects of multiple access interference (MAI) from in-cell users and of cochannel interference (CCI) from cochannel cells are quantified analytically. Especially considered are the cases when asynchronism of cochannel cells is present, for which an approximation of the interference distribution is successfully applied to get a highly precise expression of the system throughput. Our study shows that, though the MAI level and capture effect determine the basic behavior of the S-ALOHA, CCI significantly reduces the throughput of S-ALOHA and asynchronous CCI introduces an especially severe impairment. Our analytical framework is also able to incorporate additional channel conditions and system parameters like lognormal shadowing and the cellular cluster size.     

Adaptive Retransmission Diversity with Packet Combining for Slotted DS/CDMA Packet Radio Networks

A time diversity automatic repeat-request (ARQ) scheme is investigated for slotted random access direct-sequence code-division multiaccess (DS/CDMA ALOHA) wireless packet radio networks on multipath Rayleigh fading channels. The receiver retains and processes all the retransmissions of a single data block (packet) using predetection diversity combining, instead of discarding those which are detected in error. This effectively improves the system throughput and delay characteristics especially at small values of signal-to-noise ratio (SNR) per bit. A simple and practical selection combining rule is proposed, which lends itself to a low-complexity receiver structure and specifically suitable for high data rate transmissions. Owing to the stochastic nature of the multiple access interference, the new maximum output selection diversity (MO/SD) system yields superior performance in comparison to the traditional maximum SNR selection diversity (SNR/SD) model. The bit error rate performance, throughput and the average number of transmissions required to transmit a packet successfully with and without forward error correction (FEC) are evaluated. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA without incurring a substantial penalty in terms of cost or complexity.     

The Spatial Capacity of a Slotted ALOHA Multihop Packet Radio Network with Capture

In this paper we determine throughput equations for a packet radio network where terminals are randomly distributed on the plane, are able to capture transmitted signals, and use slotted ALOHA to access the channel. We find that the throughput of the network is a strictly increasing function of the receiver's ability to capture signals, and depends on the transmission range of the terminals and their probability of transmitting packets. Under ideal circumstances, we show the expected fraction of terminals in the network that are engaged in successful traffic in any slot does not exceed 21 percent.     

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。