Cellular Systems

Introduction

The scope of the research primarily encompasses the evolution of the third generation (3G) cellular systems towards what is expected to become the fourth generation (4G). At the same time, keeping in mind progressing convergence of different types of wireless networks into interconnected heterogeneous wireless networks, our interests also encompass wireless local area networks, as well as broadband wireless networks (e.g. WiMax), especially in their mobile variety.

Main emphasis of the research is on techniques enabling high-throughput, spectrally efficient, and flexibly asymmetric packet data access and their role in defining the future 4G systems. The work is primarily concerned with methods and solutions confined to the physical and MAC/RLC (medium access control and radio link control) layers of the system architecture. However, network layer related problems, such as coordinated relaying for improved coverage and capacity in very high bit rate wide-area data systems are also of interest. An integrated, cross-layer approach is applied in arriving at effective system-level solutions.

Multiple Antenna Techniques

Multiple-input multiple-output (MIMO) antenna systems can theoretically increase radio channel’s capacity by up to a factor equaling the smallest of the three numbers: # of antennas in the transmit array, # of antennas in the receive array, and # of scatterers in the channel. Coordinated use of MIMO systems can exploit the potentially higher capacity of a multiple antenna channel through the use of space-time coding. Currently two main approaches to realizing the capacity potential of these channels are popular in wireless research community: coordinated space-time codes and layered space-time transmission.
Coordinated space-time codes are quite efficient for small arrays, but extensions to larger arrays lead to a rapid growth of decoder complexity with array size and data rate. Layered space-time transmission decomposes the multiple antenna channel into parallel single-input, single-output channels, called layers (or streams). The multiple antenna receiver successively decodes these layers using linear or non-linear cross-layer interference suppression methods. Both approaches show much promise, but the latter is more scalable. It also has the advantage that available techniques such as standard error control codecs can be more easily integrated in a system design.

We are currently working on several aspects of layered space-time transmission system design. In one project receiver algorithms of reduced complexity for efficient MIMO layer separation are designed and analyzed. Different transmit and receive antenna selection algorithms are also considered to reduce inter-layer interference. These algorithms are particularly useful in an interference-limited propagation environment. In another project a non-linear transmitter pre-processing algorithm, Tomlinson-Harashima precoding, is investigated as a superior performance alternative to receiver processing. Transmitter-based non-linear pre-processing for separation of spatial layers is particularly useful on the downlink (from base station to mobile) of multi-user MIMO systems, in which the conventional receiver-based layer separation is ineffective due to the lack of coordination among mobile users. Joint transmitter-receiver processing algorithms for layer separation are also investigated, and they show much promise for downlinks of multi-user MIMO systems with multiple antenna mobiles.

The performance of multiple antenna techniques is very sensitive to the radio propagation environment, in which antenna arrays are used. While several measurement programs have been conducted to characterize the multiple antenna radio channel in indoor environments, outdoor multiple antenna measurements are much less common. The purpose of another project is to collect a series of outdoor multiple antenna measurements using base station sites of a commercial cellular network. An outdoor wireless system can use a multiple antenna array to communicate with mobile users using either MIMO spatial multiplexing or beamforming techniques. The performance of these two approaches depends very much on propagation environment. Beamforming tends to work best in regions free of scatterers, while MIMO works best with a large number of scatterers between the transmitter and receiver. The channel measurements collected as part of the project will be used in simulation to determine whether MIMO or beamforming techniques are best in a given environment.

Capacity gains due to the application of layered MIMO systems implementing spatial multiplexing can be achieved on channels characterized by rich scattering and relatively high values of signal-to-interference-plus-noise ratio (SINR). When sufficiently high values of SINR are not available, the capacity gains are not achievable, and what becomes more important is achieving acceptable reliability of transmission under relatively poor propagation conditions. If multiple antennas are available at transmitter and receiver, both transmit and receive diversity techniques can be applied to improve reliability. This approach becomes more efficient when suitable, relatively simple space-time coding techniques are applied to achieve robust transmission. As mentioned above, in the absence of sufficiently rich scattering environment, beamforming becomes an alternative to spatial multiplexing. Beamforming can be applied to reduce multiple access interference (and hence increase capacity in multiuser systems), improve robustness of transmission, or increase its range. In this context multiple antenna systems capable of adapting their mode of operation between layered MIMO, space-time coding for diversity and beamforming become relevant and desirable.

Multi-Carrier Transmission Techniques

Spread spectrum multi-carrier systems for wireless downlink packet data access combine the benefits of spread spectrum techniques with those of orthogonal frequency division multiplexing (OFDM). In multi-carrier spread spectrum (MC-SS) adapted for efficient packet data service on the downlink, multiplexing of user signals within a given cell is achieved through frequency and time division. To ensure high spectral efficiency of a multi-cell system the same carrier frequencies are used in all cells, and separation of users between cells is achieved using code division multiplexing implemented with spread spectrum modulation. Spread spectrum modulation also increases robustness to channel dispersion. The MC-SS data links may use two-dimensional (time-frequency) adaptive transmission techniques. Hence, in MC-SS systems two-dimensional allocation of radio resources becomes a possibility and multi-user diversity can be exploited in two dimensions, which leads to higher spectral efficiencies for packet transmission.

One significant disadvantage of OFDM systems is their sensitivity to phase noise and frequency offsets. We are attempting to address that problem effectively by applying non-linear transmitter pre-processing techniques (e.g. Tomlinson-Harashima precoding) to mitigate the effects of phase noise and frequency offset on system performance.

Another drawback of OFDM systems that increases their implementation costs is their high peak-to-average power ratio (PAPR). A high PAPR of modulated signals reduces power efficiency and increases cost of the transmit power amplifier. A significant effort is under way within our group to develop signal processing and coding techniques leading to the PAPR reduction.

Radio Link Adaptation and Adaptive Radio Resource Allocation Techniques

This aspect of the research program ties together the research efforts within the areas of multiple antenna and multi-carrier (MC) techniques described in the preceding paragraphs, and channels them towards the overall objective of developing efficient very high throughput cellular packet data access systems. The work encompasses the following main thrusts:

1. Multiple antenna multi-carrier adaptive systems employing link adaptation and radio resource allocation in three dimensions: time, space and frequency;
2. Adaptive multiple antenna systems capable of adaptively (depending on propagation conditions) selecting one of three modes of operation: layered MIMO, beamforming and space-time coding for diversity.

The asymmetric nature of the Internet access – usually much higher bit rate requirement for the downlink (network to user) direction of transmission than for the uplink – shifts the main emphasis and challenge of a very efficient wireless access system design to the downlink. Fortunately, the MC solution based on the OFDM approach is inherently more suitable for the downlink and at the same time is highly bandwidth efficient. Combination of OFDM with spread spectrum techniques increases robustness against multi-user interference and enables multi-cell operation. The main idea of adaptive transmission techniques is to transmit a data packet at the highest possible rate to a user experiencing favourable propagation conditions during the short transmission interval.

The adaptive transmission techniques are effective for best-effort delay-tolerant packet data services, and they turn fading of the mobile radio channel from a major impairment to an advantage, potentially enabling higher average throughput per sector on a Rayleigh or Ricean fading channel than on a Gaussian one. The MC approach makes adaptive transmission more efficient by enabling adaptation on a sub-channel basis, and making allocation of radio resources in both time and frequency possible. MC approach also enables efficient application of MIMO antenna techniques on wideband radio channels, typical for very high bit rate wireless access systems. Wideband radio channels, especially in a terrestrial mobile or nomadic environment, are normally frequency selective, which makes the application of MIMO techniques difficult. Partitioning of a wideband channel into a large number of flat fading sub-channels in an MC system enables effective application of MIMO antenna techniques, while at the same time enabling a better match of the selected modulation/coding format to channel conditions. Tight adaptation to propagation conditions can be further enhanced by scheduling packet transmissions on different groups of sub-bands to different users depending on the predicted instantaneous signal to interference ratio in individual sub-bands. Such adaptive multi-sub-band transmission of a single packet creates challenges for the design of effective hybrid ARQ schemes.

Development of three-dimensional space-time-frequency methods for allocation of radio resources for packet transmission in MIMO MC systems, and development of scheduling algorithms maximizing the average throughput per sector, and at the same time ensuring desired balance between throughput and fairness, when optimized three-dimensional allocation of resources is used, are the main long term objectives of the work.

MIMO/MISO Relays for Improved Cellular Coverage and Capacity

The use of coordinated relaying in cellular networks is considered a key technique expected to enable reasonable coverage in fourth generation (4G) wide-area wireless systems at acceptable cost. This relatively new direction of work deals with the issues of the application of multi-hop networking to wide-area cellular systems. It is focused on performance benefits available from coordinated relaying in combating heavy path loss and exploiting spatial diversity to increase very low received signal-to-interference-plus-noise ratios (SINR) typical at very high bit rates expected of 4G systems.