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     

Effect of Acknowledgment Traffic on the Performance of Slotted ALOHA-Code Division Multiple Access Systems

Use of code division multiple access (CDMA) in conjunction with slotted ALOHA improves throughput, since some of the "collided" packets can be retrieved, although at the expense of enlarged bandwidth. In view of the feasibility of recovery of packets, such schemes are attractive to carry receiver-to-transmitter acknowledgment (ACK) traffic also. The presence of ACK traffic is known to significantly reduce the throughput of slotted ALOHA channels. In this correspondence, the impact of such ACK traffic on the performance of slotted ALOHACDMA schemes is examined in detail for both finite and infinite terminal population.     

Performance analysis of a slotted CDMA ALOHA network

ALOHA is a simple and efficient way of allowing many machines with bursty data streams to communicate with a central computer. For cases where machines are equally likely to transmit to one another, CDMA ALOHA, which allows for full connectivity, may be a better multiple access protocol than slotted ALOHA through a central machine. This paper first describes a model for a fully connected, full duplex, and slotted CDMA ALOHA network where the receiver-based code access protocol is used. The stations can send data to, and receive data from, different stations simultaneously. The model is analyzed using discrete-time Markov chain, and some numerical results are presented. For a system with a large number of users where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system and determine the throughput as well as the delay performance of the system when it is stable. It is shown that a CDMA slotted ALOHA network has a much better performance compared to simple slotted ALOHA networks.     

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     

Joint Delay-Power Multiple Packet Capture Scheme for Spread-Spectrum Slotted ALOHA Packet Radio Networks

In this paper, a joint delay-power multiple packet capture scheme, that can collect multiple packets simultaneously from different terminals with both delay and power captures, is presented. The corresponding joint delay-power capture probabilities for a spread-spectrum slotted ALOHA packet radio networks where all terminals use a common spreading code under Rayleigh fading with power control are derived. Throughput and delay performance of the spread-spectrum slotted ALOHA packet radio networks with the joint delay-power multiple packet capture effect are shown by our simulation results to be significantly improved compared with the existing schemes.     

Ku-Band satellite data networks using very small aperture terminals—part I: Multi-access protocols

This two-part paper presents a comprehensive overview on the technology of very small aperture terminal (VSAT)-based satellite data networks, which have received increasing attention in recent years because of a combination of technical and economic factors. Ku-band networks with customer-premise earth-stations can overcome the inherent local access bottleneck in wide-area terrestrial data networks by suitably exploiting the unique multi-access and broadcast features of satellite communication. In this part of the paper, the issue of multiple access, which is an important critical path technology in the ongoing evolution of VSAT data networks, is considered in detail. Satellite multi-access protocols are classified in terms of channel synchronization (slotted and unslotted) and the qualitative nature of message access (fixed assigned, contention and reservation). In addition to well known techniques such as ALOHA, slotted ALOHA, tree CRA, packet CDMA and demand assigned (DAMA) TDMA, a number of new approaches to efficient unslotted access, including selective reject (SREJ) ALOHA, time-of-arrival CRA and locally synchronous reservation, are discussed. The general review is followed by a detailed performance comparison of a number of candidate first-generation VSAT protocols, namely ALOHA, slotted ALOHA, SREJ-ALOHA, DAMA with TDMA reservation access and DAMA with slotted ALOHA reservation access. The performance evaluation, based on detailed simulations with VSAT equipment and transaction traffic parameters, is summarized by curves of average delay and throughput vs. number of VSATs per channel for each of the protocols considered. Higher-order differences in delay performance are illustrated with appropriate delay distribution results and the sensitivity of performance with respect to key equipment and traffic parameters is investigated. For the transaction traffic model under consideration, the results suggest the use of SREJ-ALOHA among contention techniques or DAMA with slotted ALOHA reservation among controlled access alternatives. The trade-off between contention and reservation access is shown to be more complex, depending on the combination of average delay, peak delay, transmission cost and equipment complexity suited to the particular application. In Part II

1 To published in the next issue

of this paper on Ku-band VSAT networks, the issue of overall system design is considered.     

Maximum-likelihood decoding and code combining for DS/SSMA slottedALOHA

This paper considers the combination of multiple copies of a packet to improve the performance of a slotted direct-sequence spread-spectrum multiple-access (DS/SSMA) ALOHA packet radio system with coherent binary phase-shift keying (BPSK) modulation. Both slotted DS/SSMA ALOHA with and without forward error correction (FEC) are considered. For the case with FEC, maximum-likelihood decoding with code combining is used. Code combining allows for the combination of multiple copies of the same packet (which are typically discarded), to obtain a lower code rate for that specific packet, and therefore an improved probability of successful decoding. In both cases, combining multiple copies of the same packet results in a throughput which is an increasing function over a broad range of offered load, so that the system is more reliable from the point of view of stability. In addition, combining provides a higher throughput and a smaller time delay for packet transmission. This is illustrated by means of analytical and simulation results     

Pereormance analysis of slotted ALOHA for land mobile satellite networks

The ALOHA protocol has been proposed for accessing reservation channels in demand-assigned channel access protocols for land mobile satellite communications networks. This paper provides a rigorous performance analysis of both the slotted ALOHA protocol and two-packet replication ALOHA protocol taking into account the effects of the fading multipath communications environment. Throughput, delay and stability of these two protocols are determined using a Gilbert channel error model with memory. Comparison of slotted ALOHA and two-packet replication ALOHA shows that a smaller average delay can be achieved by replication. Moreover average throughput is improved in some cases. However replication reduces stability.     

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     

Throughput performance of slotted DS/CDMA ALOHA with packetcombining over generalised fading channels

A time diversity automatic repeat-request (ARQ) scheme is investigated for a slotted narrowband DS/CDMA wireless data network over a Nakagami fading channel. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA at the expense of a slight increase in implementation complexity     

Asynchronous fiber optic local area network using CDMA and optical correlation

An experimental fiber optic local area network utilizing code division multiple access (CDMA) and fiber optic delay-line signal processing is reported. The throughput-delay characteristics are compared to ALOHA and CSMA/CD. Since CDMA allows many users to access the network simultaneously, it can achieve higher throughput with no delay.     

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.     

Design and analysis of a propagation delay tolerant ALOHA protocol for underwater networks

Acoustic underwater wireless sensor networks (UWSN) have recently gained attention as a topic of research. Such networks are characterized by increased uncertainty in medium access due not only to when data is sent, but also due to significantly different propagation latencies from spatially diverse transmitters—together, we call these space-time uncertainty. We find that the throughput of slotted ALOHA degrades to pure ALOHA in such an environment with varying delay. We therefore propose handling this spatial uncertainty by adding guard times to slotted ALOHA, forming Propagation Delay Tolerant (PDT-)ALOHA. We show that PDT-ALOHA increases throughput by 17–100% compared to simple slotted ALOHA in underwater settings. We analyze the protocol’s performance both mathematically and via extensive simulations. We find that the throughput capacity decreases as the maximum propagation delay increases, and identify protocol parameter values that realize optimal throughput. Our results suggest that shorter hops improve throughput in UWSNs.     

Performance analysis of a fully-connected, full-duplex CDMA ALOHAnetwork with channel sensing and collision detection

In cases where machines having bursty data are equally likely to transmit to one another, code-division multiple-access (CDMA) ALOHA which allows for an individual “virtual channel” for each receiving station may be a better multiple-access protocol than simple ALOHA. With the use of a “receiver-based code” multiple-access protocol, it is also possible for a station to listen to the channel of the intended receiver before transmission, and also abort transmission when it detects others transmitting on the same channel. This paper describes a model for a fully-connected, full duplex, and slotted CDMA ALOHA network where channel sensing and collision detection are used. The model is analyzed using a discrete time Markov chain and some numerical results are presented. For a system with a large number of users, where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system, and estimate the throughput as well as the delay performance of the system when it is stable. Finally, a comparison is made with a simple channel sense multiple-access with collision detection (CSMA-CD) network, showing that a substantial improvement in the performance is achieved by the proposed network     

Multiple access in wireless digital networks

General principles for the design of a multiple-access system for large numbers of terminals transmitting to a single hub station are discussed. The importance of understanding the nature of the traffic to be carried by the network is emphasized. After some discussion of multiple access options for steady traffic and for slowly varying traffic, the use of random-access protocols for rapidly varying traffic is explained. Two general random-access protocols have been used in a variety of data networks, ALOHA and CDMA. Although these two techniques have different origins and are generally thought of as separate, they are in fact but different ways of looking at the same basic signals. The author shows that the use of multiple spreading codes in a CDMA network is not necessary in order to achieve multiple access capability. A single code can greatly reduce the complexity of a CDMA system. He introduces a spread-spectrum version of an ALOHA channel (spread ALOHA) which is equivalent to a CDMA channel with a common spreading code for all users. The equivalence the author demonstrates opens the door to a variety of techniques commonly used in ALOHA channels which can significantly increase both the throughput and the efficiency of the spread-spectrum channel     

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.     

Stabilization of slotted ALOHA spread-spectrum communicationnetworks

A method is presented for stabilizing slotted ALOHA frequency-hop communication networks by controlling code rate. In particular, the stabilization of a fully connected network in which a finite number of transmitter-receiver pairs exchange packets of information encoded by Reed-Solomon codes is considered. A principle of flow balance to stability analysis is adopted, and it is shown that network stability can be ensured by controlling code rate. The requirement on code rate for ensuring network stability is examined. In addition, the mean delay and the mean channel throughput of the network are discussed     

时隙ALOHA DS/CDMA系统与多载波时隙ALOHA系统吞吐性能的比较研究

该文对同样带宽下时隙 ALOHA DS/CDMA系统和多载波时隙 ALOHA系统的吞吐量进行了理论计算、比较和仿真。结果表明,在总负载较大时,采用高纠错能力的时隙 ALOHA DS/CDMA系统可以在吞吐量上有更好的性能。但若网络负载过重,时隙ALOHA DS/CDMA系统的吞吐性能较多载波时隙ALOHA的系统下降快;码字总数受限会带来码字选择的冲突,从而降低系统的吞吐性能。     

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     

Delay distributions of slotted ALOHA and CSMA

We derive the closed-form delay distributions of slotted ALOHA and nonpersistent carrier sense multiple access (CSMA) protocols under steady state. Three retransmission policies are analyzed. We find that under a binary exponential backoff retransmission policy, finite average delay and finite delay variance can be guaranteed for G<2S and G<4S/3, respectively, where G is the channel traffic and S is the channel throughput. As an example, in slotted ALOHA, S<(ln2)/2 and S<3(ln4-ln3)/4 are the operating ranges for finite first and second delay moments. In addition, the blocking probability and delay performance as a function of r/sub max/ (maximum number of retransmissions allowed) is also derived.