Philip J. Fleming

Mobile WiMAX systems: performance and evolution

Mobile WiMAX was successfully adopted by ITU as one of the IMT-2000 technologies in November 2007. Since then mobile WiMAX (a.k.a. IP-OFDMA) has officially become a major global cellular wireless standard along with 3GPP UMTS/HSPA and 3GPP2 CDMA/ EVDO. Mobile WiMAX is an OFDM-based technology available for deployment today, and new WIMAX devices come to market at much reduced cost than that of current 3G solutions. Currently over 260 service providers are deploying fixed, portable and mobile WiMAX networks in 110 countries. This article provides an overview of the mobile WiMAX system and its performance under various configurations, channel conditions, and types of data traffic. Furthermore, the article provides an overview of mobile WiMAX evolution.

WiMAX Overview and System Performance

In this paper, the performance of WiMax network with DL SIMO, DL MIMO and UL SIMO is analyzed via system simulations. Two data traffic models are considered, namely Web browsing (HTTP) and full buffer sessions, operating in multi path fading channels such as the ITU pedestrian-B and vehicular-A channels. The control channel coverage and reliability of WiMAX using realistic overhead is also analyzed in this paper.

HEAVY TRAFFIC APPROXIMATIONS FOR A SYSTEM OF INFINITE SERVERS WITH LOAD BALANCING

We consider an exponential queueing system with multiple stations, each of which has an infinite number of servers and a dedicated arrival stream of jobs. In addition, there is an arrival stream of jobs that choose a station based on the state of the system. In this paper we describe two heavy traffic approximations for the stationary joint probability mass function of the number of busy servers at each station. One of the approximations involves state-space collapse and is accurate for large traffic loads. The state-space in the second approximation does not collapse. It provides an accurate estimate of the stationary behavior of the system over a wide range of traffic loads.

Estimates of multicarrier CDMA system capacity

As CDMA systems reach capacity, infrastructure providers are extending them by offering multicarrier capability. The capacity of an n-carrier CDMA system should be at least n times the capacity of a single-carrier system. We estimate the additional capacity that can be achieved by using carrier assignment disciplines. We produce the estimates from the CDMA System Static Simulator by preprocessing the simulator inputs and post-processing its outputs. The estimates are of capacities of multicarrier systems using simple carrier assignment disciplines, and of wideband CDMA systems. The latter estimates constitute an upper bound on the capacity of a multicarrier system with any carrier assignment discipline.

Robust Channel Estimation and Detection for Uplink Control Channel in 3GPP-LTE

In this paper, we present improved channel estimation and detection methods applicable for control channel in 3GPP-LTE. The physical uplink control channel (PUCCH) carries the vital information to the evolved-Node-B (eNB) such as Hybrid-ARQ (HARQ) ACK/NACK corresponding to the downlink assignment and channel quality information (CQI) for resource allocation. A robust detection of control channel helps in supporting higher data rates with low latency to the user-equipments (UEs). The PUCCH is a narrow-band code multiplexed channel and hence the inter-user interference becomes the performance limiting factor. This interference becomes severe in the presence of timing errors (leading to loss in code orthogonality) and near-far problem. The improved receiver presented in this paper employ the complex exponential functions for channel estimation and a joint minimum mean squared error (MMSE) data detection and hence are robust to timing errors and near-far problem. Also, we exploit the pilot structure for reducing the complexity of the joint channel estimation. Simulation results provided shows that the proposed receiver breaks the error floor encountered in the single user receiver.

DS/SS CDMA slotted access channel performance analysis

The performance analysis of the slotted access channel for DS/SS CDMA cellular systems with and without multipath combining is presented. Three major factors that affect access channel performance are collision, blocking, and frame erasure rate (FER). All three factors can cause retransmission which results in lower throughput and longer delay. An iterative method is presented to estimate the effective arrival rate in a system without combining. The access rate sequence generated by the iteration is monotone increasing and converges if the system is stable. Using this result, a stability condition is determined in terms of the arrival rate of new messages and the FER. We modify the standard slotted ALOHA analysis so that it applies to a system with combining. The study reveals the trade-off between the messages received per slot and the FER.<<ETX>>

Efficient Monte-Carlo simulation of a product-form model for a cellular system with dynamic resource sharing

There are many ways for users to share the radio spectrum allocated to a cell in a cellular phone system. We analyze a commonly proposed scheme wh ere the cell is divided into s sectors. Each sector has exclusive access to a certain number of channels. The remaining channels reside in a “common pool” and are shared among the sectors. The smallest unit of bandwidth that can be borrowed from the common pool is a “carrier,” which consists of c channels. When viewed as a multidimensional birth-death process, the steady-state distribution of the number of active channels in each sector has a “product form,” but because the state space is large and has a nonlinear boundary, direct calculation of quantities of interest is usually impractical. Ross and Wang have developed a Monte-Carlo technique that applies to our problem. We significantly improve the efficiency of their technique when applied to our problem by including certain (nonlinear) control variates. The kinds of control variates we use can be applied to other loss systems as well. We also explore the effect of importance sampling for our system. In many cases the variance reduction achieved from the combination of importance sampling and control variates is far greater than from either method alone. For systems with blocking probabilities in the range 0.001 to 0.1, the variance of the system-blocking probability estimator can be reduced by several orders of magnitude.

Asymptotic results for voice delay in packet networks

This paper investigates the performance of packet networks carrying many low bit-rate real-time sources of traffic. We are primarily motivated by applications to the design of wide-area transmission links carrying coded voice traffic between the base sites and the core network of a cellular system. The large number of such links and distances they span lead to significant recurring costs for cellular operators forcing them to operate these links at very high utilization. The system model is composed of a highly utilized transmission link fed by a large number of voice sources. The voice sources are characterized by their correlation structure and relative time-offset with respect to each other. We present simulation and analytical results for this system emphasizing the impact of system utilization, correlation structure, etc. The analysis is based on two different approaches (1) Gaussian queue analysis and (2) heavy traffic analysis. The analysis reflects the statistical properties of the voice sources, in particular, their correlation structure. A key analytical result is that the delays in the system can be characterized by two scaled exponential distributions: one for small delays and the other for large delays.

Load balancing in mixed services CDMA systems with delayed information

Intelligent assignment of voice and data calls to RF carriers in CDMA systems can increase spectral efficiency. This paper proposes policies for realizing this benefit in systems where there are multiple call classes, arbitrary call hold time distributions, and state information is only periodically available. A significant amount of additional benefit results from these policies when an estimate of the call hold times distribution is used in the call assignment decision. Both analysis and simulation are presented.

Two-law approximation for voice delay in packet networks

Traditionally, voice has been transported using circuit switched networks, the Public Switched Telephone Network (PSTN) for example. However, driven by the ubiquity of the Internet and the development of low bit-rate digital voice codecs there has been increasing focus on using packet-switched networks for voice traffic. We focus on one such application. Our model comprises of a relatively slow packet link (between 1.5 and 5.0 Mbps) being utilized for voice traffic. Large number (of the order of 100) voice sources are multiplexed on this link. For such a link we obtain delay distributions seen by a voice source. Specifically, we exploit key characteristics of the model, such as the large number of sources to obtain a heavy traffic approximation for the system. The key result is that the delays can be well approximated by concatenation of two exponential distributions. We also provide valuable insight into how the delay distribution is connected with the statistical properties of the voice sources, in particular their correlation behavior. Our analytical results are validated with simulation results.