1,716 research outputs found
DMT Optimality of LR-Aided Linear Decoders for a General Class of Channels, Lattice Designs, and System Models
The work identifies the first general, explicit, and non-random MIMO
encoder-decoder structures that guarantee optimality with respect to the
diversity-multiplexing tradeoff (DMT), without employing a computationally
expensive maximum-likelihood (ML) receiver. Specifically, the work establishes
the DMT optimality of a class of regularized lattice decoders, and more
importantly the DMT optimality of their lattice-reduction (LR)-aided linear
counterparts. The results hold for all channel statistics, for all channel
dimensions, and most interestingly, irrespective of the particular lattice-code
applied. As a special case, it is established that the LLL-based LR-aided
linear implementation of the MMSE-GDFE lattice decoder facilitates DMT optimal
decoding of any lattice code at a worst-case complexity that grows at most
linearly in the data rate. This represents a fundamental reduction in the
decoding complexity when compared to ML decoding whose complexity is generally
exponential in rate.
The results' generality lends them applicable to a plethora of pertinent
communication scenarios such as quasi-static MIMO, MIMO-OFDM, ISI,
cooperative-relaying, and MIMO-ARQ channels, in all of which the DMT optimality
of the LR-aided linear decoder is guaranteed. The adopted approach yields
insight, and motivates further study, into joint transceiver designs with an
improved SNR gap to ML decoding.Comment: 16 pages, 1 figure (3 subfigures), submitted to the IEEE Transactions
on Information Theor
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
Resource allocation and block coding within a three-stage collaborative broadband relay network
In this work we propose a power control algorithm
for a multi-input multi-output orthogonal frequency division
multiplexing (MIMO-OFDM) multi-hop collaborative relaying
network. Using orthogonal and quasi-orthogonal block codes
with three stage processing our algorithm optimally distributes
available transmission power based on the architecture and
the channel condition at each stage so as to minimize the
end-to-end bit error rate (HER) of the entire relay network.
For high data rate applications with maximum throughput (as
expected for future OFDM systems), we employ a regenerative
relaying process, where the relays at each stage decode and
through collaborative processing, re-encode the received data
before onward transmission to the next stage and then to the
destination.
We provide simulation results 1 that confirm the performance
improvement in the end-to-end bit error rate (HER) of the network
using our explicit power allocation algorithm as compared
with equal power distribution technique
Indoor off-body wireless communication: static beamforming versus space-time coding
The performance of beamforming versus space-time coding using a body-worn textile antenna array is experimentally evaluated for an indoor environment, where a walking rescue worker transmits data in the 2.45 GHz ISM band, relying on a vertical textile four-antenna array integrated into his garment. The two transmission scenarios considered are static beamforming at low-elevation angles and space-time code based transmit diversity. Signals are received by a base station equipped with a horizontal array of four dipole antennas providing spatial receive diversity through maximum-ratio combining. Signal-to-noise ratios, bit error rate characteristics, and signal correlation properties are assessed for both off-body transmission scenarios. Without receiver diversity, the performance of space-time coding is generally better. In case of fourth-order receiver diversity, beamforming is superior in line-of-sight conditions. For non-line-of-sight propagation, the space-time codes perform better as soon as bit error rates are low enough for a reliable data link
Distributed Quasi-Orthogonal Space-Time coding in wireless cooperative relay networks
Cooperative diversity provides a new paradigm in robust wireless re- lay networks that leverages Space-Time (ST) processing techniques to combat the effects of fading. Distributing the encoding over multiple relays that potentially observe uncorrelated channels to a destination terminal has demonstrated promising results in extending range, data- rates and transmit power utilization. Specifically, Space Time Block Codes (STBCs) based on orthogonal designs have proven extremely popular at exploiting spatial diversity through simple distributed pro- cessing without channel knowledge at the relaying terminals. This thesis aims at extending further the extensive design and analysis in relay networks based on orthogonal designs in the context of Quasi- Orthogonal Space Time Block Codes (QOSTBCs).
The characterization of Quasi-Orthogonal MIMO channels for cooper- ative networks is performed under Ergodic and Non-Ergodic channel conditions. Specific to cooperative diversity, the sub-channels are as- sumed to observe different shadowing conditions as opposed to the traditional co-located communication system. Under Ergodic chan- nel assumptions novel closed-form solutions for cooperative channel capacity under the constraint of distributed-QOSTBC processing are presented. This analysis is extended to yield closed-form approx- imate expressions and their utility is verified through simulations. The effective use of partial feedback to orthogonalize the QOSTBC is examined and significant gains under specific channel conditions are demonstrated.
Distributed systems cooperating over the network introduce chal- lenges in synchronization. Without extensive network management
it is difficult to synchronize all the nodes participating in the relaying between source and destination terminals. Based on QOSTBC tech- niques simple encoding strategies are introduced that provide compa- rable throughput to schemes under synchronous conditions with neg- ligible overhead in processing throughout the protocol. Both mutli- carrier and single-carrier schemes are developed to enable the flexi- bility to limit Peak-to-Average-Power-Ratio (PAPR) and reduce the Radio Frequency (RF) requirements of the relaying terminals.
The insights gained in asynchronous design in flat-fading cooperative channels are then extended to broadband networks over frequency- selective channels where the novel application of QOSTBCs are used in distributed-Space-Time-Frequency (STF) coding. Specifically, cod- ing schemes are presented that extract both spatial and mutli-path diversity offered by the cooperative Multiple-Input Multiple-Output (MIMO) channel. To provide maximum flexibility the proposed schemes are adapted to facilitate both Decode-and-Forward (DF) and Amplify- and-Forward (AF) relaying. In-depth Pairwise-Error-Probability (PEP) analysis provides distinct design specifications which tailor the distributed- STF code to maximize the diversity and coding gain offered under the
DF and AF protocols.
Numerical simulation are used extensively to confirm the validity of the proposed cooperative schemes. The analytical and numerical re- sults demonstrate the effective use of QOSTBC over orthogonal tech- niques in a wide range of channel conditions
Distributed space time block coding in asynchronous cooperative relay networks
The design and analysis of various distributed space time block coding
schemes for asynchronous cooperative relay networks is considered
in this thesis. Rayleigh frequency flat fading channels are assumed to
model the links in the networks, and interference suppression techniques
together with an orthogonal frequency division multiplexing type transmission
approach are employed to mitigate the synchronization errors
at the destination node induced by the different delays through the
relay nodes.
Closed-loop space time block coding is first considered in the context
of decode-and-forward (regenerative) networks. In particular, quasi orthogonal
and extended orthogonal coding techniques are employed for
transmission from four relay nodes and parallel interference cancellation
detection is exploited to mitigate synchronization errors. Availability
of a direct link between the source and destination nodes is studied,
and a new Alamouti space time block coding technique with parallel
interference cancellation detection which does not require such a direct
link connection and employs two relay nodes is proposed. Outer
coding is then added to gain further improvement in end-to-end performance
and amplify-and-forward (non regenerative) type networks
together with distributed space time coding are considered to reduce
relay node complexity.
Novel detection schemes are then proposed for decode-and-forward
networks with closed-loop extended orthogonal coding which reduce
the computational complexity of the parallel interference cancellation.
Both sub-optimum and near-optimum detectors are presented for relay
nodes with single or dual antennas. End-to-end bit error rate simulations
confirm the potential of the approaches and their ability to
mitigate synchronization errors. A relay selection approach is also formulated
which maximizes spatial diversity gain and attains robustness
to timing errors.
Finally, a new closed-loop distributed extended orthogonal space
time block coding solution for amplify-and-forward type networks which
minimizes the number of feedback bits by using a cyclic rotation phase
is presented. This approach utilizes an orthogonal frequency division
multiplexing type transmission structure with a cyclic prefix to mitigate
synchronization errors. End-to-end bit error performance evaluations
verify the efficacy of the scheme and its success in overcoming synchronization
errors
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