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 data rates, with mechanisms to switch between them. Users are given the option to either select an operational data rate 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 reected 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.

A major challenge in operation of wireless communications systems is the efficient use of radio resources. One important component of radio resource management is power control, which has been studied extensively in the context of voice communications. With increasing demand for wireless data services, it is necessary to establish power control algorithms for information sources other than voice. We present a power control solution for wireless data in the analytical setting of a game theoretic framework. In this context, the quality of service (QoS) a wireless terminal receives is referred to as the utility and distributed power control is a non-cooperative power control game where users maximize their utility. The outcome of the game results in a Nash equilibrium that is ine#cient. We introduce pricing of transmit powers in order to obtain Pareto improvement of the non-cooperative power control game, i.e. to obtain improvements in user utilities relative to the case with no pricing. Specifically, we consider a pricing function that is a linear function of the transmit power. The simplicity of the pricing function allows a distributed implementation where the price can be broadcast by the base station to all the terminals. We see that pricing is especially helpful in a heavily loaded system.

For wireless cellular and ad hoc networks with QoS constraints, we propose a suite of problem formulations that allocate network resources to optimize SIR, maximize throughput and minimize delay. The distinguishing characteristics of these resource allocation formulations is that, by using convex optimization, they accommodate a variety of realistic QoS and fairness constraints. Their globally optimal solutions can be computed efficiently through polynomial time interior point methods, even though they use nonlinear objectives and constraints.

Recently it was proposed to adapt several transmission methods, including modulation, power control, channel coding and antenna diversity to rapidly time variant fading channel conditions. Prediction of the channel coefficients several tens-to-hundreds of symbols ahead is essential to realize these methods in practice. We describe a novel adaptive long range fading channel prediction algorithm (LRP) and its utilization with adaptive transmission methods. This channel prediction algorithm computes the linear Minimum Mean Squared Error (MMSE) estimates of future fading coefficients based on past observations. This algorithm can forecast fading signals far into the future due to its significant memory span, achieved by using a sufficiently low sampling rate for a given fixed filter size. The LRP is validated for standard stationary fading models, and tested with measured data and with data produced by our novel realistic physical channel model. This model accounts for the variation of the amplitude, frequency and phase of each reflected component of the fading signal. Both numerical and simulation results show that long range prediction makes adaptive transmission techniques feasible for mobile radio channels.

We investigate the high spectral efficiency capabilities of a cellular data system that combines the following: 1) multiple transmit signals, each using a separately adaptive modulation; 2) adaptive array processing at the receiver; and 3) aggressive frequency reuse (reuse in every cell). We focus on the link capacity between one user and its serving base station, for both uncoded and ideally coded transmissions. System performance is measured in terms of average data throughput, where the average is over user location, shadow fading, and fast fading. We normalize this average by the total bandwidth, call it the mean spectral efficiency, and show why this metric is a useful representation of system capability. We then quantify it, using simulations, to characterize multiple-input multiple-output systems performance for a wide variety of channel conditions and system design options.

Wireless local-area networks are becoming increasingly popular. This is due, in part, to the recent availability of devices capable of communicating at data rates approaching that of conventional wired networks. These high rates are made possible through new modulation and coding techniques that dramatically increase bandwidth efficiency. However, maintaining reliable communication at higher data rates requires more signal power. Consequently, wireless devices often support multiple data rates, providing the user the ability to choose the rate that best suits their application. Alternatively, an automatic rate adaption mechanism may be used. Rate adaption is the process of automatically selecting the rate that gives the optimum throughput for the channel conditions. Although rate adaption mechanisms for cellular wireless networks have been studied at length, few have been proposed for wireless local-area networks. This paper presents one such mechanism: a rate adaptive MAC protocol based...

. We propose and analyze the performance of an algorithm for integrated power control and adaptive modulation/coding to achieve a specified range of packet error rate for real-time applications in broadband wireless packet-switched networks. The algorithm applies the Kalman-filter method [1] for power control, and adapts packet transmission to an appropriate modulation level, according to the packet error requirement, and the radio and interference conditions. A new criterion for maintaining stable transmission power is derived. Based on the criterion, the proposed technique performs the link adaptation and adjusts transmission power to achieve the specified packet error rate. The effectiveness of the proposed method is demonstrated by several numerical examples. 1 INTRODUCTION Customers' demand for broadband network services has been growing significantly as telecommuting and Internet access become increasingly popular. In the very near future, broadband services are also exp...

Wireless broadband access is becoming increasingly popular in the telecommunications market due to the projected demand for flexible and easily deployable highspeed connections. In order to adhere to the volatility of the wireless medium, the adoption of sophisticated adaptation techniques is required. In this paper, we investigate the problem of enhancing channel throughput by performing resource assignment and reuse with adaptation of physical layer parameters. We propose an algorithm to allocate channels to users with different rate requirements, while appropriately adjusting the modulation level and transmission power, based on instantaneous channel quality. Our algorithm constructs the cochannel set of users in a sequential manner, by utilizing a criterion which is based on the induced and received amounts of interference for a user and the contribution in throughput increase. Although illustrated in the context of TDMA/TDD, the proposed technique can be applied in systems which support different multiple access and signaling schemes with orthogonal channels (e.g. OFDMA, CDMA). Our results indicate a considerable increase in throughput per utilized channel under such adaptive techniques.

Abstract—We present a general framework to quantify the data throughput capabilities of a wireless communication system when it combines: 1) multiple transmit signals; 2) adaptive modulation for each signal; and 3) adaptive array processing at the receiver. We assume a noise-limited environment, corresponding to either an isolated cell or a multicell system whose out-of-cell interference is small compared with the thermal noise. We focus on the user data throughput, in bits per second/Hertz (bps/Hz), and its average over multipath fading, which we call the user spectral efficiency. First, an analysis method is developed to find the probability distribution and mean value of the spectral efficiency over the user positions and shadow fadings, both as a function of user distance from its serving base station and averaged over the cell coverage area. We assume fading conditions and receiver processing that lend themselves to closed-form analysis. The resulting formulas are simple and straightforward to compute, and they provide a number of valuable insights. Next, we run Monte Carlo simulations, both to confirm the analysis and to treat cases less amenable to simple analysis. A key contribution of this paper is a simple formula for the mean spectral efficiency in terms of the propagation exponent, mean signal-to-noise ratio at the cell boundary, number of antennas, and type of coding. Under typical propagation conditions, the mean spectral efficiency using three transmit and three receive antennas ranges from 19.2 bps/Hz (uncoded) to 26.8 bps/Hz (ideally coded), highlighting the potential benefits of multiple transmissions combined with adaptive techniques. This is much higher than the spectral efficiencies for a link using a single transmitter and a threefold receive diversity under the same conditions, where the range is from 8.77 bps/Hz to 11.4 bps/Hz. Moreover, the latter results are not nearly as practical to achieve, as they call for large signal constellations that would be highly vulnerable to impairments. Index Terms—Adaptive modulation, antenna arrays, fading channels, land mobile radio cellular systems, multiple-input multiple-output.

The scarce radio spectrum imposes hard limitations on design of cellular radio systems. To provide communication services with high capacity and good quality of service requires powerful methods for sharing the radio spectrum in most efficient way. In practice, all sharing methods introduce interference, which is proportional to the transmitter powers. The transmitter power control is a key technique to balance the received signal strength and the interference power, which in turn enables more efficient sharing. Emerging