Xiaoxin Qiu

On the performance of adaptive modulation in cellular systems

Adaptive modulation techniques have the potential to substantially increase the spectrum efficiency and to provide different levels of service to users, both of which are considered important for third-generation cellular systems. In this work, we propose a general framework to quantify the potential gains of such techniques. Specifically, we study the throughput performance gain that may be achieved by combining adaptive modulation and power control. Our results show that: (1) using adaptive modulation even without any power control provides a significant throughput advantage over using signal-to-interference-plus-noise ratio (SINR) balancing power control and (2) combining adaptive modulation and a suitable power control scheme leads to a significantly higher throughput as compared to no power control or using SINR-balancing power control. The first observation is especially important from an implementation point of view. Adjusting the modulation level without changing the transmission power requires far fewer measurements and feedback as compared to the SINR-balancing power control or the optimal power control. Hence, it is significantly easier to implement. Although presented in the context of adaptive modulation, the results also apply to other variable rate transmission techniques, e.g., rate adaptive coding schemes, coded modulation schemes, etc. This work provides valuable insight into the performance of variable rate transmission techniques in multi-user environments.

Radio resource allocation in fixed broadband wireless networks

We consider use of fixed broadband wireless networks to provide packet services for telecommuting and Internet access. Each cell is divided into multiple sectors, each of them served by a sector antenna colocated with the base station (BS), and user terminals also use directional antennas mounted on the rooftops of homes or small offices and pointed to their respective BS antennas. To support a target data rate of 10 Mb/s, a bandwidth of several MHz is required. Since radio spectrum is expensive, the bandwidth needs to be reused very aggressively. Thus, efficient strategies for frequency reuse and managing cochannel interference are critically important. We propose several algorithms for dynamic radio-resource allocation in the fixed wireless networks. In particular, a method to be referred to as the staggered resource allocation (SRA) method uses a distributed scheduling algorithm to avoid major sources of interference while allowing concurrent packet transmission and meeting signal-to-interference objectives. The performance of the method is studied by analytic approximations and detailed simulation. Our results show that the combination of directional antennas plus the SRA method is highly effective in controlling cochannel interference. For reasonable system parameters, the SRA method delivers a throughput in excess of 30% per sector while permitting a given frequency band to be reused in every sector of every cell. It also provides satisfactory probability of successful packet transmission. In addition, a simple control mechanism can be applied in the method to improve performance for harsh radio environments.

Link adaptation and power control for streaming services in EGPRS wireless networks

Using the MPEG-4 advanced audio coder (AAC) music as an example of streaming applications, we investigate the improvement of error performance for the streaming service by link adaptation and power control techniques in an enhanced general packet radio services (EGPRS) cellular network. A low packet error rate and variability are essential in providing a short error-burst length so that error concealment techniques can be effectively applied to music packets. We study the effects of a combined link adaptation and power control scheme (referred to as the error-based scheme) for achieving a target error rate and reducing error variability. By simulation, we compare the error performance of the error-based scheme at both the EGPRS block and AAC frame level with another adaptation algorithm (referred to as the throughput-based scheme) with a goal of maximizing overall network throughput. It is found that when offered with a similar traffic load, the former scheme can provide noticeable improvement of music quality over the throughput-based scheme. To achieve a similar AAC frame error rate, our results also show that the error-based scheme can increase the link throughput over the throughput-based scheme by 66.7% in one of our examples. These results reveal that by aiming at required error targets and thus reducing error variability, the error-based scheme for link adaptation and power control are helpful in improving quality and capacity for streaming applications.

Quasi-static resource allocation with interference avoidance for fixed wireless systems

We propose a novel intercell interference management technique, called quasi-static resource allocation with interference avoidance (QRA-IA), for fixed broadband wireless systems with narrow sectors or equipped with fixed or adaptive array antennas. The basic idea of QRA-IA is for every base to periodically turn off each of its beams (sectors) for a certain amount of time. This periodic turn off introduces a predictable nonuniformity in a terminals performance and therefore permits each terminal to identify a preferred time period for transmission. QRA-IA requires that each terminal sense the interference over time, select the preferred transmission periods, and report these to the base. The resource allocation algorithm at the base can then use this information for scheduling transmissions to the terminals. The base can also use this information for obtaining an appropriate beam-off sequence. A graph theory model is used to show that an acceptable beam-off sequence exists for each base. A simple, distributed, and measurement-based beam-switching algorithm is designed for the system to find these sequences autonomously. We demonstrate via simulations that a system with QRA-IA provides a marked improvement in the packet error rate performance of terminals over the same system without QRA-IA. This improvement translates directly to improved coverage and throughput and also reduces the burden on higher layer protocols to ensure fairness and quality of service.

RLC/MAC design alternatives for supporting integrated services over EGPRS

The Enhanced General Packet Radio Services is one of the proposals submitted to the IMT-2000 initiative of the ITU for third-generation wireless services. EGPRS is also the evolutionary path chosen by the Universal Wireless Communications Consortium, leading toward the convergence of GSM and IS-136 standards for their next-generation wireless systems. We discuss the feasibility of supporting integrated services, such as packet voice, Web browsing, and best-effort data, using EGPRS. We first describe the relevant capabilities offered by the currently proposed standard, and then outline additional capabilities at the radio link control/medium access control layer that are necessary to support integrated services in a spectrally efficient manner. In summary, these needed capabilities are: fast uplink access during an ongoing session; fast resource assignment for both uplink and downlink; and the ability to differentiate services at the base station subsystem. To realize these capabilities, we propose enhancements to the RLC/MAC layer. These enhancements include a new set of packet control channels (namely, a fast packet access channel in the uplink, and a fast packet access grant channel and a fast packet polling channel in the downlink); uplink access protocols that utilize these control channels; and a modified fast uplink access channel structure. These changes would enable EGPRS to offer a wide range of services through a single packet-based network. Similar ideas are also applicable to other third-generation systems.

Resource assignment in a fixed broadband wireless system

We propose a time slot reuse partitioning (TRSP) scheme for resource assignment in packet-switched fixed TDMA wireless networks. The TSRP scheme is based upon two key ideas. The first one is for the system to have more than one coexisting reuse pattern in the time domain. The second is to match a terminal or application to an appropriate reuse pattern, so as to guarantee the required quality of service (QoS). We define and then simulate a specific implementation of the TRSP scheme for an integrated voice and broadband data system. The results show that this scheme provides good performance for both voice and data.

Transmission of streaming data over an EGPRS wireless network

We investigate the feasibility of transmitting MPEG-2 AAC (Advanced Audio Coder) music over packet-switched bearers of an Enhanced General Packet Radio Services cellular network. Streaming data applications like AAC music differ from conversational voice in that they permit a larger transmission delay and delay jitter than the former. In addition, AAC music can tolerate a higher frame error rate than voice by using error concealment techniques at the receiver. This creates the opportunity to transmit AAC music over data-optimized, high-throughput and nominally high-error rate, wireless networks, by using appropriate performance enhancing techniques. In particular, we investigate the performance with the following techniques: (i) link-layer retransmissions; (ii) dynamic packet assignment; and (iii) packet interleaving or shuffling. We find that all the above techniques are desirable in order to simultaneously achieve a high spectral efficiency and the required quality. While this paper is focused on one application, the conclusions and insights are applicable to most streaming audio and video applications.

Providing differentiated services in EGPRS through packet scheduling

This paper discusses the feasibility of offering multiple service classes, e.g., premium and basic, in the 3G EGPRS (Enhanced General Packet Radio Services) system using different scheduling mechanisms. We investigate weighted round-robin type scheduling for differentiating the service classes together with two types of packet schedulers: radio-aware packet schedulers at the radio access network, and conventional packet schedulers at the edge of the core network. The results show that for the EGPRS-like system simulated: (i) incorporating radio link conditions and radio resource management into the scheduling can improve the overall delay performance by more than 20%; (ii) a weighted round-robin scheduling to differentiate service classes may not be suitable for an interference limited environment. Our ongoing work will consider using multiple time-slot assignments for providing differentiated services.

Network-assisted resource management for wireless data networks

We propose a framework for network-assisted radio resource management in wireless data networks. This type of radio resource management techniques offer implementation and capacity benefits compared to conventional, interference-measurement based, dynamic channel assignment (DCA) algorithms. The basic idea is to use interbase signaling to shift most of the burden of the resource allocation from the air interface to the backbone infrastructure. By exchanging channel assignment as well as other relevant information in real time through the backbone network, each base can calculate the impact of a resource assignment on the system. As a result, rapid interference measurements, which are typically needed to implement DCA schemes, are replaced by a limited amount of path loss measurements and the computation of interference conditions by the base stations. This significantly reduces the measurement and over-the-air signaling requirements, and can also provide an opportunity for a better optimization of the system performance. We focus on two specific algorithms: network-assisted least-interference-based dynamic packet assignment (NA-LI-DPA) and network-assisted dynamic packet assignment with throughput optimization (NA-DPA). NA-LI-DPA closely resembles a least-interference-based dynamic channel assignment algorithm, and NA-DPA attempts to further improve the overall system throughput. The algorithms, as defined, are appropriate for a best-effort data service, where the primary goal is to provide a higher throughput. However, it will be clear from the discussion that it is also feasible to alter the algorithms to optimize performance metrics other than throughput, e.g., to ensure a certain quality of service. We show through simulation that, for a system like enhanced general packet radio service (EGPRS) system, NA-DPA can provide a throughput that is 50% higher than random packet assignment, and 25% higher than that obtained by conventional DCA algorithms.

Some performance results for the downlink shared channel in WCDMA

We study the data performance of WCDMA systems using the downlink shared channel (DSCH) by investigating the impact of loading, rate adaptation and power control for the typical urban (TU) channel. We conclude that, using DSCH, high speed data transmissions can be achieved if the offered traffic load is well controlled. We discuss a heuristic rate adaptation algorithm that adapts the transmission rate based upon the perceived user performance in previous frames. We show that the performance of the rate adaptation is significantly better than that for the fixed-rate allocation policy.