Alister G. Burr

Survey of channel and radio propagation models for wireless MIMO systems

This paper provides an overview of the state-of-the-art radio propagation and channel models for wireless multiple-input multiple-output (MIMO) systems. We distinguish between physical models and analytical models and discuss popular examples from both model types. Physical models focus on the double-directional propagation mechanisms between the location of transmitter and receiver without taking the antenna configuration into account. Analytical models capture physical wave propagation and antenna configuration simultaneously by describing the impulse response (equivalently, the transfer function) between the antenna arrays at both link ends. We also review some MIMO models that are included in current standardization activities for the purpose of reproducible and comparable MIMO system evaluations. Finally, we describe a couple of key features of channels and radio propagation which are not sufficiently included in current MIMO models.

Capacity bounds and estimates for the finite scatterers MIMO wireless channel

We consider the limits to the capacity of the multiple-input-multiple-output wireless channel as modeled by the finite scatterers channel model, a generic model of the multipath channel which accounts for each individual multipath component. We assume a normalization that allows for the array gain due to multiple receive antenna elements and, hence, can obtain meaningful limits as the number of elements tends to infinity. We show that the capacity is upper bounded by the capacity of an identity channel of dimension equal to the number of scatterers. Because this bound is not very tight, we also determine an estimate of the capacity as the number of transmit/receive elements tends to infinity which is asymptotically accurate.

Providing multimedia communications services from high altitude platforms

System level design considerations for high altitude platforms operating in the mm-wave bands are examined. Propagation effects in these bands are outlined, followed by a brief introduction to different platform scenarios. Ground-based and platform-based fixed wireless access scenarios are considered, and it is shown that using a platform, a single base station can supply a much larger coverage area than a terrestrial base station. The effects on performance of platform displacement from its desired location with both fixed and steerable antennas are also examined. It is shown that steerable antennas are of most use when fixed stations are immediately below the platform, with no benefit for fixed stations on the edge of coverage. The bandwidths required to serve several traffic distributions (suburbs and city centre based) are evaluated using the Shannon equation. It is shown that capacity can be constrained when users are located in the city centres, despite longer line of sight paths to users out in the suburbs. The effects of temporal changes in the spatial traffic distribution are investigated. It is shown that bandwith requirements can be reduced if the platform moves to track these changes. Copyright

Layered Design of Hierarchical Exclusive Codebook and Its Capacity Regions for HDF Strategy in Parametric Wireless 2-WRC

This paper addresses a hierarchical decode-and-forward (HDF) strategy in the wireless two-way relay channel (2-WRC). This strategy uses a Hierarchical eXclusive Code (HXC) that allows full decoding of the hierarchical symbols at the relay. The HXC represents two data sources only through the exclusive law and requires side information on the complementary data at the destination (which naturally holds for the 2-WRC). The HDF strategy has the advantage over classical medium-access control (MAC) stage relaying with joint decoding in that its rate region extends beyond the classical MAC region. We present a layered design of the HXC codebook, which uses an arbitrary outer state-of-the-art capacity-approaching code [e.g., low density parity check (LDPC)] and an inner layer with an exclusive symbol alphabet. We provide basic theorems, showing that this scheme forms an HXC, and we also evaluate its alphabet-constrained rate regions. The rate regions depend on the relative channel phase parameters. Some channel rotation leads to catastrophic violation of the exclusive law. However, these values appear only in a limited range of phases and have only a mild impact on the mean capacity for some component symbol constellations.

Signal detection for orthogonal space-time block coding over time-selective fading channels: a PIC approach for the G/sub i/ systems

One major assumption in all orthogonal space-time block coding (O-STBC) schemes is that the channel remains static over the entire length of the codeword. However, time selective fading channels do exist, and in such case the conventional O-STBC detectors can suffer from a large error floor in the high signal-to-noise ratio (SNR) cases. This paper addresses such an issue by introducing a parallel interference cancellation (PIC) based detector for the G/sub i/ coded systems (i=3 and 4).

Receiver design for orthogonal space-time block coding for four transmit antennas over time-selective fading channels

A key assumption in all orthogonal space-time block coding (O-STBC) schemes is that the channel remains static over the length of the codeword. However, time selective fading channels do exist, and in such case the conventional O-STBC receiver will not function properly: even a relatively small variation in channel state can result in a large error floor. This paper presents an effective solution to this issue for the case of four transmit antennas. At the receiver, a simple zero forcing decoder is derived. To improve the performance of the decoder under certain channel conditions, the original O-STBC encoder is also modified accordingly. Computer simulations confirmed the effectiveness of the new procedure.

LMDS from high altitude aeronautical platforms

System level design considerations for high altitude aeronautical platforms operating in the LMDS band are examined. Propagation effects in the LMDS band are outlined, followed by a brief introduction to different platform scenarios. Ground-based and platform-based fixed wireless access scenarios are considered, and it is shown that using a platform, considerably longer link lengths can be used. The effects on performance of platform displacement from its desired location with both fixed and steerable antennas are also examined. It is shown that steerable antennas are of most use when fixed stations are immediately below the platform, with no benefit for fixed stations on the edge of coverage.

Reducing call dropping in distributed dynamic channel assignment algorithms by incorporating power control in wireless ad hoc networks

Methods of substantially reducing call dropping in networks which use distributed dynamic channel assignment (DDCA) schemes are discussed. Interference and received power thresholds coupled with power control are used to maintain performance, without the need for intra-cell handoffs. It is shown that the schemes reduce call dropping and increase capacity compared to those using fixed transmitter power. The schemes are developed with the aid of mathematical analysis and a pictorial model. Results are presented which show that call dropping may be virtually eliminated in shadowing environments with the median transmitter power being reduced by 15 dB. The various call dropping mechanisms are discussed, and it is suggested that the residual level of call dropping is principally a result of multiple additional call arrivals close to an active link. Methods to make further reductions in the call dropping probability are also proposed.

Performance of MIMO systems with combined polarization multiplexing and transmit diversity

Spatial multiplexing and space-time block codes are promising techniques that effectively exploit multiple-input multiple-output (MIMO) transmission to achieve a higher data rate and more reliable communication, respectively. On the other hand, the use of dual-polarized antennas in MIMO systems has emerged as a cost- and space-effective alternative, where two spatially separated uni-polarized antennas can be replaced by a single antenna element employing orthogonal polarizations. In this paper, we investigate the use of dual-polarized antennas in a combined spatial multiplexing and space-time block coded system. We present a hybrid transmission scheme that employs transmit diversity over the same polarization of the two transmit dual-polarized antennas and transmits the co-channel space-time coded signals on the orthogonal polarizations. We evaluate the system performance for both the Rayleigh fading channel and the suburban environment of a personal communication system (PCS) at 1800 MHz. It is shown that the performance of PM-STBC approaches that of STBC in the correlated Ricean fading environment and it saves one receive antenna.

A General Upper Bound to Evaluate Packet Error Rate over Quasi-Static Fading Channels

We propose a new analytical approach to evaluate the average packet error rate (PER) of a conventional packet transmission system over a quasi static fading channel, by presenting an integral inequality lemma. The basic idea of the approach is that, given the PER for the AWGN channel as a function of signal-to-noise ratio (SNR), the average PER over Rayleigh fading channel can be generally upper bounded by a quite simple inequality, i.e.,1 - exp(-wo/γ̅), for both coded and uncoded schemes, where wo, defined by an integral expression, corresponds exactly to the inversion of coding gain; and this bound is tight in the high SNR region or for long packet systems. We further apply the integral inequality to extend our research to more general Nakagami-m fading channel.