Dan Avidor

Effect of microdiversity and correlated macrodiversity on outages in a cellular system

We focus on maximum ratio combining at each base station and switching between base stations (BSs) as a simple macrodiversity technique. We obtain analytical results for pointwise outage probabilities for systems using one or a combination of both techniques to cover a desired area, assuming a certain correlation model for the set of path losses of the links connecting a terminal to the receiving BSs. Pointwise outage probabilities are averaged over the entire region of interest to get an estimate of the outage in the desired region. A comparison of micro- and macrodiversity schemes in terms of the outage gives insights as to the tradeoff between the two forms of diversity in the design of a cellular system.

Connectivity, Power, and Energy in a Multihop Cellular-Packet System

In this paper, we study a large network of subscriber stations (SSs) with certain common wireless capabilities and base stations (BSs) having direct connections to the wired infrastructure in addition to common wireless capabilities. SSs can communicate with the outside world only through the BSs. Connections to SSs without a direct (i.e., a single hop) wireless connection to any BS are established, if possible, through other SSs serving as wireless repeaters. The locations of the SSs and the BSs follow independent homogeneous planar Poisson processes. The propagation channels exhibit signal attenuation with distance and log-normal shadowing. We evaluate exactly the probability of an SS to have a direct wireless connection to any of the BSs and a lower bound on the t-hop (t arbitrary) outage probability of an SS. We then define the minimal hop-count routing algorithm and calculate the mean number of hops for routes connecting SSs to BSs, when a maximum hop-count constraint is imposed. We compute next the probability distribution of the transmit power under the assumption of perfect power control. We conclude by calculating a bound for the total mean transmit energy required to transfer a data packet from an SS to a BS over a minimal hop-count route and show that this energy is significantly lower than the corresponding value in a single-hop network operating at the same outage probability

Transmit Power Distribution of Wireless Ad Hoc Networks with Topology Control

We study the impact of several topology control schemes on the transmit power of nodes in a wireless packet data network, where the nodes are randomly distributed over a large area according to a Poisson point process, and the propagation channels are subject to fading. Topology control has been proposed as a technique to improve the performance of multi-hop networks, e.g. ad hoc networks and sensor networks. It amounts to adjusting the transmit power of each node independently so as to optimize certain performance measures, such as throughput, connectivity, lifespan of networks of battery-powered nodes, simplifying the routing algorithms, etc. Many such algorithms use the pattern of immediate neighbors observed by each node as the basis for power adjustment. Most published research on topology control is based on a simplistic radio propagation model, where the area covered by a transmitter is a perfect disk centered at the transmitter. Similarly, the self interference of the network, if considered, is caused only by transmitters located inside such a disk centered at the receiver. With this propagation model, the statistical properties of the communication range are easily derived from the desired number of one-hop neighbors (assuming that the latter is known, and is the only criterion to be satisfied). It is not always trivial to derive the resulting statistical properties of the node transmit power when a certain pattern of neighbors is desired in a fading environment. However, this is the information required when the lifespan of a network of battery-powered devices is of interest. In this paper we calculate the statistical properties of the nodes transmit power in networks produced by several topology control algorithms, when the propagation channels are subject to fading.

On the impact of the soft handoff threshold and the maximum size of the active group on resource allocation and outage probability in the UMTS system

We study the effect of two control parameters of the universal mobile telecommunication system handoff algorithm on resource allocation and the outage probability of the forward link. We thereby provide information on the effect of the parameters on the probability distributions of the number of base stations (BSs) supporting a mobile station (MS), the number of MSs supported by a single BS (per carrier), and the maximal cell size, subject to a given probability of downlink outage on the cell boundary. Denoting the set of BSs supporting an MS in soft handoff as the active group, the first parameter n/sub A/ is the maximum allowed size of the active group. The second, denoted /spl tau/, is the upper limit on the difference between the local mean received power from the dominant BS and that from any other member of the active group (in decibels), where the term dominant refers to the BS with the lowest path loss to the MS. We assume that the MS is equipped with a Rake receiver capable of performing maximal ratio combining of the signals it receives from the transmitting BSs. We present general analytical derivations along with results derived for specific situations through simulations and numerical integration.

Downlink dimensioning for the HSDPA standard

We present an analytic derivation of downlink dimensioning for a wireless network employing the “High Speed Downlink Packet Access” (HSDPA) standard [Moulsley, Conference Publication No. 477, IEE 2001], currently being developed by the 3rd Generation Partnership Project (3GPP) [http://www.3gpp.org] as part of the next stage in the evolution of the WCDMA standard. We determine the maximum possible cell radius such that the probability of outage, Pout, at the cell’s “worst” location does not exceed a preset value under full load condition. The results of the paper are presented in the form of curves obtained by numerical integration of integral expressions. The results can be used, for instance, to find the increase in cell radius achievable by a certain reduction in the threshold value of the Signal to Interference plus Noise Ratio (SINR) defining outage.

On asymptotically fair transmission scheduling over fading channels with measurement delay

We examine the effect of measurement delays on the throughput of certain downlink data packet systems operating over Rayleigh fading channels. In these systems the base station (BS) schedules a single user at a time and transmits at a rate which is based on measurements of the BSs received power performed by the user. The same subject was also discussed in a recent paper, titled Asymptotically fair transmission scheduling over fading channels by Berggren and Jantti (2004). They studied the effects of measurement delays on the operation of the scheduler. As a consequence of the delay, it is possible that by the time the BS actually transmits data, the propagation channels might have changed, such that the schedulers choice no longer conforms to the desired scheduling policy, resulting in loss of system throughput and multi-user gain. Berggren and Jantti assume that perfect link adaptation is nevertheless always assured, i.e., that the BS transmits at a rate that matches precisely the current state of the selected users channel. However, measurement delays lead also to mismatch between the data rate the BS transmits to the chosen user, and the current state of the propagation channel to that user. We believe that this mismatch must also be taken into account. In this letter we propose to use a backoff factor and analyze its mitigating effect on losses introduced by measurements delay in a Rayleigh fading environment. We then show the resulting gain in system throughput for optimal choices of the backoff factor

Connectivity and minimum total transmit energy per packet between any pair of nodes in a bounded wireless ad-hoc network subject to fading

We investigate an ad hoc wireless network where node locations are distributed according to a homogeneous Poisson process. All nodes are equipped with identical wireless transceivers that require a certain minimal received power for proper reception. Our link model depends on the length of the link and on random log-normal fading. Each node may function as a source and destination of data packets, and may also serve as a repeater. We consider paths containing at most two hops (i.e., use at most one repeater) between the source node and the destination node. We provide exact analytic results for (a) the probability that no such path exists given the peak transmit power of a node, and (b) the distribution of the total transmit energy required per data packet. We provide figures showing the benefit of allowing two hops over just one

On the uplink SIR of a mobile in soft handoff with applications to BLER estimation

We focus on the uplink and derive the joint probability density function of the signal to interference (+noise) ratios (SIRs) at the base stations (BSs) for the general case of n BSs supporting a mobile in soft handoff, each using maximal ratio combining with m-fold diversity. We then calculate the correlations between the SIRs for some example cases. The insights gained here lead to a fresh approach to the estimation of the block error rate (BLER) at the output of the frame selector (FS), which is the usual measure of quality of service in UMTS. For low specified BLER (e.g., 0.1%), the method of counting cyclic redundancy code errors requires an impractically large number of data blocks for an accurate estimate of BLER. We propose to estimate the BLER at the FS output based on measurements performed by the individual BSs. The initial setting of the target SIR in the outer loop power control can then be selected in the same way in order to expedite BLER convergence to the desired setting.

On the impact of soft handoff threshold and maximum size of the active Group on BS transmit power in the UMTS system

This paper investigates the effect of two control parameters of the Universal Mobile Telecommunications System (UMTS) handoff algorithm on the total peak base station (BS) transmit power required per mobile station (MS) to satisfy a prescribed outage probability. Denoting the set of BSs supporting an MS in soft handoff (SH) as the active group, the first parameter n is the maximum allowed size of the active group. The second, denoted /spl tau/, is the upper limit on the difference between the path loss of the dominant BS and that of any other member of the active group, where the term dominant refers to the BS with the lowest path loss to the MS. It was assumed that the MS is equipped with a Rake receiver capable of performing maximal ratio combining of the signals it receives from the transmitting BSs. General analytical derivations along with results derived for specific situations through numerical integration are presented.

Two-hop Relaying in Random Networks with Limited Channel State Information

In this paper we study two-hop cooperative diversity relaying in random wireless networks. In contrast to most work on cooperative diversity relaying where the relay node positions or average channel characteristics are given as parameters, we formulate the problem recognizing that node positions, as well as channel fading states of the channels among the nodes, are random. We propose a simple protocol which requires minimal a priori knowledge of node positions and channel fading states. Our protocol assumes that each node in the vicinity of the source knows its average link gain to the destination. The source transmits a packet, and then chooses relays from the nodes that received the packet correctly. We also assume that the maximum number of relays that can be used for each packet is limited by M, which represents bandwidth and delay constraints. Assuming that the relay nodes are distributed according to 2-dimensional Poisson point process, we analytically study the performance of the protocol as a function of node density, fading parameters and node transmit powers. We consider both maximal ratio combining and selection combining at the destination.