IEEE 802.11n: enhancements for higher throughput in wireless LANs

This article introduces a new standardization effort, IEEE 802.11n, an amendment to IEEE 802.11 standards that is capable of much higher throughputs, with a maximum throughput of at least 100 Mb/s, as measured at the medium access control data services access point. The IEEE 802.11n will provide both physical layer and MAC enhancements. In this article we introduce some PHY proposals and study the fundamental issue of MAC inefficiency. We propose several MAC enhancements via various frame aggregation mechanisms that overcome the theoretical throughput limit and reach higher throughput. We classify frame aggregation mechanisms into many different and orthogonal aspects, such as distributed vs. centrally controlled, ad hoc vs. infrastructure, uplink vs. downlink, single-destination vs. multi-destination, PHY-level vs. MAC-level, single-rate vs. multirate, immediate ACK vs. delayed ACK, and no spacing vs. SIFS spacing.     

Digital wireless broadband corporate and private networks: RNETconcepts and applications

The radio broadband networks suffer from some specific constraints-access protocol efficiency, multipath effects, and limited available frequency bands-which have to be overcome by choosing the best possible compromises in cost/performance on physical and MAC layers. Current WLAN products (1-2 Mb/s at 2.45 GHz), which provide mainly data exchange service, are able to make seamless handover for an indoor cellular network but suffer from relatively low data rates. Even if compliant with the IEEE 802.11 future standard, they do not necessarily lead to interoperability. New generations become necessary for higher data transfer rates, real-time video, and multimedia applications compatible with the future ATM transfer mode. ETSI is proposing the HIPERLAN concept, working in the 5.3 and 17.2 GHz bands. Our contribution to this new standard, called RNET (Radio Network), uses a spread spectrum linear ramp waveform with enhanced capabilities (relative to the TDMA, CSMA/CA, and FDMA structures). The advantages of the MAC layer are highlighted and the first results of a transceiver demonstrator are given     

Design alternatives for wireless local area networks

In this paper an overview of the wireless local area network (LAN) area is provided. The two types of wireless LAN topologies used today, infrastructure and ad hoc, are presented. The requirements that a wireless LAN is expected to meet are discussed. These requirements impact on the implementation of both the Physical and MAC layer of a wireless LAN. The unique characteristics of wireless physical layers are discussed and the five technology alternatives used today are presented. MAC layer issues are discussed and the two existing standards, IEEE 802.11 and HIPERLAN 1, are examined. Polling‐based MAC protocols (RAP, GRAP) are also reviewed. Finally, an introduction is made to wireless technologies that interact with WLANs, such as personal area networking (PAN) and wireless ATM and an overview of HIPERLAN 2, a WLAN using ATM technology, is provided. Copyright © 2001 John Wiley & Sons, Ltd.     

Reactive routing evaluation using modified 802.11a with realistic vehicular mobility

Realistic mobility dynamics and underlying PHY/MAC layer implementation affect real deployment of routing protocols in vehicular ad hoc network (VANET). Currently, dedicated short range communication devices are using wireless access in vehicular environment (WAVE) mode of operation, but now IEEE is standardizing 802.11p WAVE. This work presents an in-depth simulation-based analysis of two reactive routing protocols, i.e., dynamic source routing (DSR) and ad hoc on-demand distance vector (AODV) with modified IEEE 802.11a PHY/MAC layers (comparable to 802.11p) in modified VANET mobility models (freeway, stop sign, and traffic sign) in terms of load, throughput, delay, number of hops, and retransmission attempts. Results obtained using OPNET simulator show that in urban/highway mobility scenarios, AODV??s performance with forthcoming 802.11p at high bit rate would be better than DSR in terms of high throughput, less delay, and retransmission attempts. Moreover, this comprehensive evaluation will assist to address challenges associated with future deployment of routing protocols integrated upon devices with upcoming IEEE 802.11p, concerning specific macro-/micro-mobility scenarios.     

Increasing fairness and efficiency using the MadMac protocol in ad hoc networks

The IEEE 802.11 MAC layer is known for its unfairness behavior in ad hoc networks. Introducing fairness in the 802.11 MAC protocol may lead to a global throughput decrease. It is still a real challenge to design a fair MAC protocol for ad hoc networks that is distributed, topology independent, that relies on no explicit information exchanges and that is efficient, i.e. that achieves a good aggregate throughput. The MadMac protocol deals with fairness and throughput by maximizing aggregate throughput when unfairness is solved. Fairness provided by MadMac is only based on information provided by the 802.11 MAC layer. MadMac has been tested in many configurations that are known to be unfair and compared with three protocols (IEEE 802.11 and two fair MAC protocols). In these configurations, MadMac provides a good aggregate throughput while solving the fairness issues.     

无线局域网MAC层信道利用效率分析

基于IEEE802·11g的WLAN的标称速率高达54Mb/s,而实际数据传输速率只有20Mb/s左右。从成帧效率、信道共享效率和冲突避免效率的角度对IEEE802·11g的MAC层信道利用效率进行了分析,得出这种基于CSMA/CA机制的MAC层信道利用效率较低。实测结果验证了分析结论的有效性。     

A traffic control system for IEEE 802.11 networks based on available bandwidth estimation

To support Quality of service (QoS)‐sensitive applications like real‐time video streaming in IEEE 802.11 networks, a MAC layer extension for QoS, IEEE 802.11e, has been recently ratified as a standard. This MAC layer solution, however, addresses only the issue of prioritized access to the wireless medium and leaves such issues as QoS guarantee and admission control to the traffic control systems at the higher layers. This paper presents an IP‐layer traffic control system for IEEE 802.11 networks based on available bandwidth estimation. We build an analytical model for estimating the available bandwidth by extending an existing throughput computation model, and implement a traffic control system that provides QoS guarantees and admission control by utilizing the estimated available bandwidth information. We have conducted extensive performance evaluation of the proposed scheme via both simulations and measurements in the real test‐bed. The experiment results show that our estimation model and traffic control system work accurately and effectively in various network load conditions without IEEE 802.11e. The presence of IEEE 802.11e will allow even more efficient QoS provision, as the proposed scheme and the MAC layer QoS support will complement each other. Copyright © 2006 John Wiley & Sons, Ltd.     

Hotspot wireless LANs to enhance the performance of 3G and beyond cellular networks

At present, WLANs supporting broadband multimedia communications are being developed and deployed around the world. Standards include HIPERLAN/2 defined by ETSI BRAN and the 802.11 family defined by the IEEE. These systems provide channel adaptive data rates up to 54 Mb/s (in a 20 MHz channel spacing) over short ranges up to 200 m. The HIPERLAN/2 standard also specifies a flexible radio access network that can be used with a variety of core networks, including UMTS. It is likely that WLANs will become an important complementary technology to 3G cellular systems and will typically be used to provide hotspot coverage. In this article the complementary use of WLANs in conjunction with UMTS is presented. In order to quantify the capacity enhancement and benefits of cellular/hotspot interworking we have combined novel ray tracing, software-simulated physical layer performance results, and optimal base station deployment analysis. The study focuses on an example deployment using key lamppost mounted WLAN access points to increase the performance (in terms of capacity) of a cellular network.     

基于IEEE 802.11g的光载无线网络媒体接入控制协议性能分析

This paper investigates the Medium Access Control (MAC) protocol performance in the IEEE 802.11g-over-fiber network for different payloads and fiber lengths using Direct Sequence Spread Spectrum-Orthogonal Frequency Division Multiplexing (DSSS- OFDM) and Extended Rate Physicals- Orthogonal Frequency Division Multiplexing (ERP-OFDM) physical layers using basic ac-cess mode, Request to Send/Clear to Send (RTS/CTS) and CTS-to-self mechanisms. The results show that IEEE 802.11g-over-fiber network employing the ERP-OFDM physical layer is much more efficient than that em-ploying the DSSS-OFDM physical layer, with regards to both throughput and delay. For a given maximum throughput/minimum delay, the tradeoff among the access mechanism, the fiber length, and the payload size must be considered. Our quantified results give a se-lection basis for the operators to quickly select suitable IEEE 802.11g physical layers and the different access mechanisms, and accurately predict the data throughput and delay given the specific parameters.     

Improvement of Data Rate Performance of Wireless Local Area Networks through the Use of Orthogonal Code Division Multiplexing (OCDM), Multipath Diversity,and Parallel Concatenated Channel Encoding

The widespread use of Wireless Local Area Networks (WLAN) and the desire for such products from different vendors to operate together has generated a movement towards standardization. Over the last few years, several organizations worldwide have researched and developed WLAN standards like IEEE 802.11/15, HIPERLAN, multimedia mobile access communication (MMAC), etc., to name a few. One of the important considerations in designing and marketing WLAN products is the data rate supported by them. This paper deals with the development of a modification of the Direct Sequence (DS) physical layer standard in IEEE 802.11b to allow reliable higher data rates beyond the 1–2 Mb/s supported by the standard. More precisely, this paper proposes the usage of Orthogonal Code Division Multiplexing (OCDM), a form of Code Division Multiple Access (CDMA), diversity via a rake receiver, and parallel concatenated channel encoding, in order to improve the data rate performance in a multipath Rayleigh fading environment. The proposed transceiver design is described in detail and its performance is assessed via digital computer simulation of the corresponding mathematical model.     

对HIPERLAN/2和802.11a标准的物理层分析

HIPERLAN/2和802.11a是工作在5GHz频段的两种高速无线局域网的协议标准。论述了两种标准的物理层的结构,通过软件仿真对物理层的性能进行了分析;同时也讨论了两种标准的区别以及由此引起的对吞吐量的影响。     

A novel IEEE 802.11‐based MAC protocol supporting cooperative communications

Cooperative diversity is a transmission technique, where multiple terminals form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. The concept of cooperation has already been introduced to MAC layer to design MAC protocol. But it does not take advantage of physical layer's cooperation. In this paper, we present a novel MAC protocol based on IEEE 802.11, called C‐MAC, which is able to support the basic building block of cooperative system. In other words, in C‐MAC, a source would invite a relay node into data transmission if there exits an available one. During data transmission, the source sends the signal to destination in the first time slot. The relay node will retransmit the overheard information to the destination in the second time slot. The destination combines two signals from the source and the helper to create the spatial diversity and robustness against channel fading. The C‐MAC is backward compatible to the legacy IEEE 802.11 system. The performance of C‐MAC mainly depends on physical layer's performance as it just provides the support for cooperation at the MAC layer. If the physical layer works well, C‐MAC would outperform IEEE 802.11 when considering packet error rate (PER). We also perform extensive simulation using ns‐2 with assumptive physical parameters. The results show that C‐MAC would outperform 802.11 if PER is over some threshold, e.g. when PER is 0.4, C‐MAC can achieve up to 11.5% higher throughput than IEEE 802.11. Copyright © 2011 John Wiley & Sons, Ltd.     

Throughput behavior of link adaptive 802.11 DCF with MUD capable access node

Conventional IEEE 802.11 medium access control (MAC) protocol discourages simultaneous transmission to avoid collisions. With fast advances in physical layer technologies, multi-user detection (MUD) capable receivers which can detect multiple frames from different users simultaneously become available. If we are to utilize them in today's wireless LAN, however, it is not entirely clear how we should change the MAC and how much benefit is available and can be obtained by doing so. The primary objective of this paper is to investigate such questions. We approach this objective by developing a new throughput expression for 802.11 distributed coordination function (DCF). The derived expression has been verified in simulation. We show that significant throughput gain can be garnered with slight modification in 802.11 DCF.     

5GHz无线LAN的OFDM技术和MAC协议分析

对两个工作在5GHz频段上的高速无线LAN标准——IEEE802.11a和HIPER-LAN2——进行比较,并就两者相似的物理层OFDM技术和不同的MAC子层协议进行详细分析。     

IEEE 802.11n MAC Enhancement and Performance Evaluation

The IEEE 802.11-based WiFi wireless technology is one of the most promising technologies to provide ubiquitous networking access. The IEEE 802.11 working group has always strived to improve this wireless technology through creating new amendments to the base 802.11 standard. Recently, IEEE 802.11n amendment was created to enhance 802.11 for higher throughput operation. Not only new Physical Layer enhancements are standardized, but new Medium Access Control Layer mechanism are also defined. In this paper, we examine the network performance enhancement by the proposed 802.11n MAC layer features: aggregation, block acknowledgement, and reverse direction mechanism. We implemented a new 802.11n module in the NS-2 simulation platform. The simulation results demonstrated the effectiveness of 802.11n MAC layer enhancement. VoIP performance is effectively improved with 802.11n MAC enhancement.     

New high-rate wireless LAN standards

After the IEEE 802.11 standardization group established the first wireless LAN, several efforts were started to increase data rates and also to use other bands. This article describes the new wireless LAN standards developed by IEEE 802.11, ETSI BRAN, and MMAC. The new standards are targeting data rates up to 11 Mb/s in the 2.4 GHz band and up to 54 Mb/s in the 5 GHz band     

Design and implementation of an all-CMOS 802.11a wireless LAN chipset

The tremendous growth in wireless LANs has generated interest in technologies that provide higher data rates and greater system capacities. The IEEE 802.11a standard, based on coded OFDM modulation, provides nearly five times the data rate and at least 20 times the overall system capacity compared to the incumbent 802.11b wireless LAN systems. This article describes the design challenges and circuit implementation of a two-chip set that forms a complete 802.11a solution in 0.25 /spl mu/m CMOS technology. Wherever possible, sophisticated digital signal processing techniques are used to compensate for possible analog impairments associated with integrating RF circuitry in a CMOS technology. The analog portion of the chip set implements a 5 GHz transceiver comprising all the necessary RF and analog circuits of the 802.11a standard integrated on a single chip. Some features of this IC include 22 dBm peak transmitted power, 8 dB overall receive-chain noise figure, and -112 dBc/Hz synthesizer phase noise at 1 MHz frequency offset. The digital portion of the chip set, the baseband and MAC processor, contains dual ADCs/DACs and all the digital circuits for synchronization, detection, and 802.11 MAC layer data processing. This IC delivers up to 54 Mb/s in a 20 MHz channel according to the 802.11a standard, and includes proprietary modes supporting up to 108 Mb/s in a 40 MHz channel.     

Saturation throughput analysis of IEEE 802.11e enhanced distributed coordination function

The IEEE 802.11 Task Group E will soon approve the 802.11e standard for medium access control (MAC) layer quality-of-service (QoS) enhancements to the 802.11 protocol, and it is widely believed that these enhancements will allow 802.11 technology to form the foundation of high-bandwidth vertically integrated networks. At the heart of 802.11e is a modified contention-based access mechanism, named the enhanced distributed coordination function (EDCF). In this paper, we propose and validate an analytical model for the saturation throughput of EDCF. Key to the accuracy of our model is a treatment of the postcollision period, which has been ignored by all previous 802.11 models. With results from the ns-2 simulator, we show that our model can accurately predict throughput over a wide range of scenarios, and thereby demonstrate its usefulness as a predictive tool for use in QoS provision. With context provided by our analytical model, we discuss the primary throughput differentiation mechanisms of EDCF.     

Aggregation With Fragment Retransmission for Very High-Speed WLANs

In upcoming very high-speed wireless LANs (WLANs), the physical (PHY) layer rate may reach 600 Mbps. To achieve high efficiency at the medium access control (MAC) layer, we identify fundamental properties that must be satisfied by any CSMA-/CA-based MAC layers and develop a novel scheme called aggregation with fragment retransmission (AFR) that exhibits these properties. In the AFR scheme, multiple packets are aggregated into and transmitted in a single large frame. If errors happen during the transmission, only the corrupted fragments of the large frame are retransmitted. An analytic model is developed to evaluate the throughput and delay performance of AFR over noisy channels and to compare AFR with similar schemes in the literature. Optimal frame and fragment sizes are calculated using this model. Transmission delays are minimized by using a zero-waiting mechanism where frames are transmitted immediately once the MAC wins a transmission opportunity. We prove that zero-waiting can achieve maximum throughput. As a complement to the theoretical analysis, we investigate the impact of AFR on the performance of realistic application traffic with diverse requirements by simulations. We have implemented the AFR scheme in the NS-2 simulator and present detailed results for TCP, VoIP, and HDTV traffic. The AFR scheme described was developed as part of the IEEE 802.11n working group work. The analysis presented here is general enough to be extended to proposed schemes in the upcoming 802.11n standard. Trends indicated in this paper should extend to any well-designed aggregation schemes.