Wireless Communications

The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS/CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol.

https://www.cse.iitb.ac.in/~varsha/allpapers/wireless/802.11_DCF_analysis.pdf

Performance analysis of the IEEE 802.11 distributed coordination function

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

Wireless local area networks (W-LANs) have become increasingly popular due to the recent availability of affordable devices that are capable of communicating at high data rates. These high rates are possible, in part, through new modulation schemes that are optimized for the channel conditions bringing about a dramatic increase in bandwidth efficiency. Since the choice of which modulation scheme to use depends on the current state of the transmission channel, newer wireless devices often support multiple modulation schemes, and hence multiple datarates, with mechanisms to switch between them Users are given the option to either select an operational datarate manually or to let the device automatically choose the appropriate modulation scheme (data rate) to match the prevailing conditions. Automatic rate selection protocols have been studied for cellular networks but there have been relatively few proposals for W-LANs. In this paper we present a rate adaptive MAC protocol called the Receiver-Based AutoRate (RBAR) protocol. The novelty of RBAR is that its rate adaptation mechanism is in the receiver instead of in the sender. This is in contrast to existing schemes in devices like the WaveLAN II [15]. We show that RBAR is better because it results in a more efficient channel quality estimation which is then reflected in a higher overall throughput Our protocol is based on the RTS/CTS mechanism and consequently it can be incorporated into many medium access control protocols including the widely popular IEEE 802.11 protocol. Simulation results of an implementation of RBAR inside IEEE 802.11 show that RBAR performs consistently well.

/pdf/a-rate-adaptive-mac-protocol-for-multi-hop-wireless-networks-1ofa5fd9t5.pdf

A rate-adaptive MAC protocol for multi-Hop wireless networks

Reliable transport protocols such as TCP are tuned to perform well in traditional networks where packet losses occur mostly because of congestion. However, networks with wireless and other lossy links also suffer from significant losses due to bit errors and handoffs. TCP responds to all losses by invoking congestion control and avoidance algorithms, resulting in degraded end-to end performance in wireless and lossy systems. We compare several schemes designed to improve the performance of TCP in such networks. We classify these schemes into three broad categories: end-to-end protocols, where loss recovery is performed by the sender; link-layer protocols that provide local reliability; and split-connection protocols that break the end-to-end connection into two parts at the base station. We present the results of several experiments performed in both LAN and WAN environments, using throughput and goodput as the metrics for comparison. Our results show that a reliable link-layer protocol that is TCP-aware provides very good performance. Furthermore, it is possible to achieve good performance without splitting the end-to-end connection at the base station. We also demonstrate that selective acknowledgments and explicit loss notifications result in significant performance improvements.

/pdf/a-comparison-of-mechanisms-for-improving-tcp-performance-ut4fklymf7.pdf

A comparison of mechanisms for improving TCP performance over wireless links

Today's cellular systems are designed to achieve 90-95 percent coverage for voice users (i.e., the ratio of signal to interference plus noise must be above a design target over 90 to 95 percent of the cell area). This ensures that the desired data rate which achieves good voice quality can be provided "everywhere". As a result, SINRs that are much larger than the target are achieved over a large portion of the cellular coverage area. For a packet data service, the larger SINR can be used to provide higher data rates by reducing coding or spreading and/or increasing the constellation density. It is straight-forward to see that cellular spectral efficiency (in terms of b/s/Hz/sector) can be increased by a factor of two or more if users with better links are served at higher data rates. Procedures that exploit this are already in place for all the major cellular standards in the world. In this article, we describe data rate adaptation procedures for CDMA (IS-95), wideband CDMA (cdma2000 and UMTS WCDMA), TDMA (IS-136), and GSM (GPRS and EDGE).

Adaptation techniques in wireless packet data services

A code division multiple access system provides a way of allocating an increased data rate to a requesting mobile station A mobile station requesting a data rate in excess of the basic data rate sends received pilot strength data for its base station and base stations in adjacent cells The received pilot strength data is used to determine an increased data rate to be assigned to the requesting mobile station One feature assigns an increased data rate when the received pilot strength data has a predetermined relationship to an established threshold Another feature utilizes a series of threshold levels, each pair of levels associated with a different permitted data rate Using the received pilot strength data, a data rate is determined which satisfies all adjacent cell interference concerns Another feature uses average adjacent cell capacity loads rather than threshold levels, together with the received pilot strength data, to determine the appropriate increased data rate to be assigned to a user requesting an increased data rate

Code division multiple access system providing load and interference based demand assignment service to users

We propose a technique to measure channel quality in terms of signal-to-interference plus noise ratio (SINR) for the transmission of signals over fading channels The Euclidean distance (ED) metric, associated with the decoded information sequence or a suitable modification thereof, is used as a channel quality measure Simulations show that the filtered or averaged metric is a reliable channel quality measure which remains consistent across different coded modulation schemes and at different mobile speeds The average scaled ED metric can be mapped to the SINR per symbol We propose the use of this SINR estimate for data rate adaptation, in addition to mobile assisted handoff (MAHO) and power control We particularly focus on data rate adaptation and propose a set of coded modulation schemes which utilize the SINR estimate to adapt between modulations, thus improving the data throughput Simulation results show that the proposed metric works well across the entire range of Dopplers to provide near-optimal rate adaptation to average SINR This method of adaptation averages out short-term variations due to Rayleigh fading and adapts to the long-term effects such as shadowing At low Dopplers, the metric can track Rayleigh fading and match the rate to a short-term average of the SINR, thus further increasing throughput

Channel quality estimation and rate adaptation for cellular mobile radio

Existing IS-95-based CDMA systems support circuit-mode and packet-mode data services at a data rate limited to 9.6-14.4 kb/s. Fueled by the explosive growth of the Internet, applications and market opportunities are demanding that higher capacity, higher data rates, and advanced multimedia services be supported in the near future. The evolution of the IS-95 CDMA standard to higher data rates and more advanced services occurs in two steps. The first step, IS-95-B, is an enhancement to the IS-95-A standard and offers the highest possible performance without breaking current IS-95-A air interface design characteristics, thereby maintaining strict compatibility with existing base station hardware. The proposed standard will support a high data rate (64 kb/s) in both directions, to and from the mobile device. A new burst mode packet data service is defined to allow better interference management and capacity utilization. The second evolution step, cdma2000, provides next-generation capacity, data rates, and services. The cdma2000 system includes a greatly enhanced air interface supporting CDMA over wider bandwidths for improved capacity and higher data rates while also maintaining backward compatibility with existing IS-95 CDMA end-user devices. The cdma2000 system also includes a sophisticated MAC feature to effectively support very-high-data-rate services (up to 2 Mb/s) and multiple concurrent data and voice services. This article describes the design and performance of the high-data-rate service provided by second- and third-generation CDMA systems.

Evolution of wireless data services: IS-95 to cdma2000

An access terminal scans for nearby access points and maintains a candidate list of access point with which the access terminal may associate in the event the access terminal's communication with its current access point deteriorates for some reason. This search procedure may be performed in a proactive manner whereby the access terminal repeatedly performs scans and updates its list of candidate access points when it is powered on. In some aspects, the search procedure used by the access terminal may be based on a state of the wireless device. In addition, different states of the access terminal may be associated with different optimization criteria.

Sanjiv Nanda

Papers

Adaptation techniques in wireless packet data services

Code division multiple access system providing load and interference based demand assignment service to users

Channel quality estimation and rate adaptation for cellular mobile radio

Evolution of wireless data services: IS-95 to cdma2000

Wireless node search procedure