We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the ...discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).
We compare the technique of using the orbital angular momentum (OAM) of radio waves for generating multiple channels in a radio communication scenario with traditional multiple-in-multiple-out (MIMO) ...communication methods. We demonstrate that, for certain array configurations in free space, traditional MIMO theory leads to eigen-modes identical to the OAM states. From this we conclude that communicating over the sub-channels given by OAM states is a subset of the solutions offered by MIMO, and therefore does not offer any additional gains in capacity.
While the use of radio technology for wireless data communications has increased rapidly, the wide variety of radio interfaces being used has made interference investigations hard to perform. With ...that in mind, we present a novel approach for analyzing packet radio communications, applicable to interfering heterogeneous networks, which leads to tractable analytical expressions. The core of the approach is an analytical framework modeling each network with individual properties for the packet types and the channel sets used, while taking path loss between all network nodes into account. Furthermore, we present a derivation of closed-form expressions for the throughput of the networks, thus allowing for the investigation of important mechanisms limiting network and system performance. The expressions enable fast and flexible analysis to be performed without extensive computer simulations or measurement campaigns. To illustrate the use of the framework and the strength of the closed-form expressions, we analyze a heterogeneous example system consisting of one IEEE 802.11b network and multiple Bluetooth networks that use multiple packet types. In the analysis, we also take the adjacent channel interference into account when calculating network throughput as functions of the number of interferers in the system
Very-large multiple-input multiple-output (MIMO), also called massive MIMO, is a new technique that potentially can offer large network capacities in multi-user scenarios, where the base stations are ...equipped with a large number of antennas simultaneously serving multiple single-antenna users on the same frequency. We investigate channel behavior for a realistic outdoor base station scenario using large arrays. Specifically we compare dirty-paper coding (DPC) capacities and zero-forcing (ZF) sum-rates when using a physically large linear array and a compact cylindrical array, both having 128 antenna elements. As a base-line reference, we use the DPC capacity and ZF sum-rate for the ideal case with independent and identically distributed (i.i.d.) channel coefficients. The investigation shows that the measured channels, for both array types, often allow us to harvest most of the capacities/sum-rates achieved in the i.i.d. case, already at about 10 base station antennas per user.
In this paper, a novel peak-to-average power reduction approach for orthogonal frequency division multiplexing (OFDM) has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is ...employed in coherent OFDM for channel estimation, and it is based on inserting known symbols spread throughout the 2D time-frequency grid. These pilot symbols are employed to simultaneously perform distortionless peak power reduction with a suboptimum technique named orthogonal pilot sequences (OPS), which reduces additional system complexity and side information compared to optimum pilot values. This proposal attains a further step over other previous works, since this set of sequences allows blind detection at the receiver without prior knowledge of any side information.
Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to ...previously considered systems. In theory, as the number of antennas increases, propagation properties that were random before start to become deterministic. Theoretical investigations with independent identically distributed (i.i.d.) complex Gaussian (Rayleigh fading) channels and unlimited number of antennas have been done, but in practice we need to know what benefits we can get from very large, but limited, number of antenna elements in realistic propagation environments. In this study we evaluate properties of measured residential-area channels, where the base station is equipped with 128 antenna ports. An important property to consider is the orthogonality between channels to different users, since this property tells us how advanced multi-user MIMO (MU-MIMO) pre-coding schemes we need in the downlink. We show that orthogonality improves with increasing number of antennas, but for two single-antenna users there is very little improvement beyond 20 antennas. We also evaluate sum-rate performance for two linear pre-coding schemes, zero-forcing (ZF) and minimum mean squared error (MMSE), as a function of the number of base station antennas. Already at 20 base station antennas these linear pre-coding schemes reach 98% of the optimal dirty-paper coding (DPC) capacity for the measured channels.
In this paper, a number of channel estimation algorithms for iterative receivers are compared for the case of an up-link orthogonal frequency division multiplexing interleave division multiple access ...(OFDM-IDMA) system. Both pilot based algorithms, used to obtain an initial estimate, as well as semi-blind decision-directed algorithms working as a component of the iterative receiver are considered. Algorithms performing either joint minimum mean square error (MMSE) channel estimation, or iterative estimation using space-alternating expectation maximization (SAGE), are evaluated. The considered algorithms differ in terms of complexity, as well as performance. The main contribution of this paper is to give an overview of different channel estimation approaches for OFDM-IDMA, where the complexity versus performance tradeoff is at the focal point. There is no single channel estimator providing the best tradeoff and our analysis shows how the system load (number of users) and the SNR influence the estimator choice.
With the increasing use of short-range wireless devices for high-data-rate communication in the shared frequency bands, the level of interference can be expected to increase. The ability to estimate ...the performance degradation of these devices due to increasing interference is, therefore, important. In this paper, the throughput of devices that perform frequency hops after each transmitted packet in order to achieve diversity is investigated. The system model allows for an analysis of systems where packets of variable durations are used, and the throughput derivation is based on the assumption that collisions result in a total loss of the data in the colliding packets. The resulting expression for the throughput is given as a function of the number of frequency channels used for frequency hopping, the number of interfering networks, the durations of the packet types available, and the probability of networks selecting a certain packet type for transmission. An approximation of the exact expression for the throughput is also derived, and the results are applied to an example system consisting of Bluetooth piconets.
The performance of a turbo code can be severely degraded if no trellis termination is employed. This paper investigates the implications of the choice of trellis termination method for turbo codes, ...and explains the origin of the performance degradation often experienced without trellis termination. An efficient method to derive the distance spectrum of turbo codes for different trellis termination methods is presented. Further, we present interleaver design rules that are tailored to each termination method. Using interleavers designed with these restrictions, we demonstrate that the performance difference between various termination methods is very small, including no trellis termination at all. For example, we demonstrate a turbo code with a 500-bit interleaver that exhibits no sign of an error floor for frame error rates as low as 10/sup -8/, even though no trellis termination is employed.
The performance of a turbo code is dependent on two code properties: its distance spectrum and its suitability to be iteratively decoded. Both these properties are influenced by the choice of ...interleaver used in the turbo encoder. This paper presents an interleaver design criterion that focuses on the performance of iterative decoding, based on the correlation properties of the extrinsic inputs. Interleavers designed with the proposed criterion achieve very competitive performances, both in terms of convergence rates and error correcting capabilities.
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