1,183 research outputs found
Space-time coding techniques with bit-interleaved coded modulations for MIMO block-fading channels
The space-time bit-interleaved coded modulation (ST-BICM) is an efficient
technique to obtain high diversity and coding gain on a block-fading MIMO
channel. Its maximum-likelihood (ML) performance is computed under ideal
interleaving conditions, which enables a global optimization taking into
account channel coding. Thanks to a diversity upperbound derived from the
Singleton bound, an appropriate choice of the time dimension of the space-time
coding is possible, which maximizes diversity while minimizing complexity.
Based on the analysis, an optimized interleaver and a set of linear precoders,
called dispersive nucleo algebraic (DNA) precoders are proposed. The proposed
precoders have good performance with respect to the state of the art and exist
for any number of transmit antennas and any time dimension. With turbo codes,
they exhibit a frame error rate which does not increase with frame length.Comment: Submitted to IEEE Trans. on Information Theory, Submission: January
2006 - First review: June 200
Super-orthogonal space-time turbo codes in Rayleigh fading channels.
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.The vision of anytime, anywhere communications coupled by the rapid growth of
wireless subscribers and increased volumes of internet users, suggests that the
widespread demand for always-on access data, is sure to be a major driver for the
wireless industry in the years to come. Among many cutting edge wireless
technologies, a new class of transmission techniques, known as Multiple-Input
Multiple-Output (MIMO) techniques, has emerged as an important technology
leading to promising link capacity gains of several fold increase in data rates and
spectral efficiency. While the use of MIMO techniques in the third generation (3G)
standards is minimal, it is anticipated that these technologies will play an important
role in the physical layer of fixed and fourth generation (4G) wireless systems.
Concatenated codes, a class of forward error correction codes, of which Turbo codes
are a classical example, have been shown to achieve reliable performance which
approach the Shannon limit. An effective and practical way to approach the capacity
of MIMO wireless channels is to employ space-time coding (STC). Space-Time
coding is based on introducing joint correlation in transmitted signals in both the
space and time domains. Space-Time Trellis Codes (STTCs) have been shown to
provide the best trade-off in terms of coding gain advantage, improved data rates and
computational complexity.
Super-Orthogonal Space-Time Trellis Coding (SOSTTC) is the recently proposed
form of space-time trellis coding which outperforms its predecessor. The code has a
systematic design method to maximize the coding gain for a given rate, constellation
size, and number of states. Simulation and analytical results are provided to justify the
improved performance. The main focus of this dissertation is on STTCs, SOSTTCs
and their concatenated versions in quasi-static and rapid Rayleigh fading channels.
Turbo codes and space-time codes have made significant impact in terms of the
theory and practice by closing the gap on the Shannon limit and the large capacity gains provided by the MIMO channel, respectively. However, a convincing solution
to exploit the capabilities provided by a MIMO channel would be to build the turbo
processing principle into the design of MIMO architectures. The field of concatenated
STTCs has already received much attention and has shown improved performance
over conventional STTCs. Recently simple and double concatenated STTCs
structures have shown to provide a further improvement performance. Motivated by
this fact, two concatenated SOSTTC structures are proposed called Super-orthogonal
space-time turbo codes. The performance of these new concatenated SOSTTC is
compared with that of concatenated STTCs and conventional SOSTTCs with
simulations in Rayleigh fading channels. It is seen that the SOST-CC system
outperforms the ST-CC system in rapid fading channels, whereas it maintains
performance similar to that in quasi-static. The SOST-SC system has improved
performance for larger frame lengths and overall maintains similar performance with
ST-SC systems. A further investigation of these codes with channel estimation errors
is also provided
Frequency Domain Hybrid-ARQ Chase Combining for Broadband MIMO CDMA Systems
In this paper, we consider high-speed wireless packet access using code
division multiple access (CDMA) and multiple-input multiple-output (MIMO).
Current wireless standards, such as high speed packet access (HSPA), have
adopted multi-code transmission and hybrid-automatic repeat request (ARQ) as
major technologies for delivering high data rates. The key technique in
hybrid-ARQ, is that erroneous data packets are kept in the receiver to
detect/decode retransmitted ones. This strategy is refereed to as packet
combining. In CDMA MIMO-based wireless packet access, multi-code transmission
suffers from severe performance degradation due to the loss of code
orthogonality caused by both interchip interference (ICI) and co-antenna
interference (CAI). This limitation results in large transmission delays when
an ARQ mechanism is used in the link layer. In this paper, we investigate
efficient minimum mean square error (MMSE) frequency domain equalization
(FDE)-based iterative (turbo) packet combining for cyclic prefix (CP)-CDMA MIMO
with Chase-type ARQ. We introduce two turbo packet combining schemes: i) In the
first scheme, namely "chip-level turbo packet combining", MMSE FDE and packet
combining are jointly performed at the chip-level. ii) In the second scheme,
namely "symbol-level turbo packet combining", chip-level MMSE FDE and
despreading are separately carried out for each transmission, then packet
combining is performed at the level of the soft demapper. The computational
complexity and memory requirements of both techniques are quite insensitive to
the ARQ delay, i.e., maximum number of ARQ rounds. The throughput is evaluated
for some representative antenna configurations and load factors to show the
gains offered by the proposed techniques.Comment: Submitted to IEEE Transactions on Vehicular Technology (Apr 2009
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Reconfigurable rateless codes
We propose novel reconfigurable rateless codes, that are capable of not only varying the block length but also adaptively modify their encoding strategy by incrementally adjusting their degree distribution according to the prevalent channel conditions without the availability of the channel state information at the transmitter. In particular, we characterize a reconfigurable ratelesscode designed for the transmission of 9,500 information bits that achieves a performance, which is approximately 1 dB away from the discrete-input continuous-output memoryless channelâs (DCMC) capacity over a diverse range of channel signal-to-noise (SNR) ratios
Block-fading channels at finite blocklength
This tutorial paper deals with the problem of characterizing the maximal achievable rate R(n; Ï” )at a given blocklength n and error probability Ï” over block-fading channels. We review recent results that establish tight bounds on R(n; Ï” )and characterize its asymptotic behavior. Comparison between the theoretical results and the data rates achievable with the coding scheme used in LTE-Advanced are reported
Collaborative HARQ Schemes for Cooperative Diversity Communications in Wireless Networks
Wireless technology is experiencing spectacular developments, due to the emergence of interactive and digital multimedia applications as well as rapid advances in the highly integrated systems. For the next-generation mobile communication systems, one can expect wireless connectivity between any devices at any time and anywhere with a range of multimedia contents. A key requirement in such systems is the availability of high-speed and robust communication links. Unfortunately, communications over wireless channels inherently suffer from a number of fundamental physical limitations, such as multipath fading, scarce radio spectrum, and limited battery power supply for mobile devices. Cooperative diversity (CD) technology is a promising solution for future wireless communication systems to achieve broader coverage and to mitigate wireless channelsâ impairments without the need to use high power at the transmitter. In general, cooperative relaying systems have a source node multicasting a message to a number of cooperative relays, which in turn resend a processed version message to an intended destination node. The destination node combines the signal received from the relays, and takes into account the sourceâs original signal to decode the message. The CD communication systems exploit two fundamental features of the wireless medium: its broadcast nature and its ability to achieve diversity through independent channels. A variety of relaying protocols have been considered and utilized in cooperative wireless networks. Amplify and forward (AAF) and decode and forward (DAF) are two popular protocols, frequently used in the cooperative systems. In the AAF mode, the relay amplifies the received signal prior to retransmission. In the DAF mode, the relay fully decodes the received signal, re-encodes and forwards it to the destination. Due to the retransmission without decoding, AAF has the shortcoming that noise accumulated in the received signal is amplified at the transmission. DAF suffers from decoding errors that can lead to severe error propagation. To further enhance the quality of service (QoS) of CD communication systems, hybrid Automatic Repeat-reQuest (HARQ) protocols have been proposed. Thus, if the destination requires an ARQ retransmission, it could come from one of relays rather than the source node. This thesis proposes an improved HARQ scheme with an adaptive relaying protocol (ARP). Focusing on the HARQ as a central theme, we start by introducing the concept of ARP. Then we use it as the basis for designing three types of HARQ schemes, denoted by HARQ I-ARP, HARQ II-ARP and HARQ III-ARP. We describe the relaying protocols, (both AAF and DAF), and their operations, including channel access between the source and relay, the feedback scheme, and the combining methods at the receivers. To investigate the benefits of the proposed HARQ scheme, we analyze its frame error rate (FER) and throughput performance over a quasi-static fading channel. We can compare these with the reference methods, HARQ with AAF (HARQ-AAF) and HARQ with perfect distributed turbo codes (DTC), for which correct decoding is always assumed at the relay (HARQ-perfect DTC). It is shown that the proposed HARQ-ARP scheme can always performs better than the HARQ-AAF scheme. As the signal-to-noise ratio (SNR) of the channel between the source and relay increases, the performance of the proposed HARQ-ARP scheme approaches that of the HARQ-perfect DTC scheme
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