87 research outputs found
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
Multicarrier CDMA: A very promissing multiple access scheme for future wideband wireless networks
International audienceIn this paper, multicarrier techniques are studied in the context of the future wideband wireless networks. After a brief presentation of the third generation mobile networks, MC-CDMA systems are considered for the downlink of the future high rate cellular networks. The performance of different mono-user and multi-user detection techniques are compared with the help of Monte Carlo simulations over a frequency selective Rayleigh channel. Thus, the efficiency of MC-CDMA as a very promising multiple access and robust modulation scheme is successfully demonstrated for the downlink of the future wideband mobile networks. Finally, the first results concerning the performance of Multicarrier CDMA technique combined with space-time block coding in order to build a Multiple Input Multiple Output/MC-CDMA system are presented over a Rayleigh channel
Space-Time Signal Design for Multilevel Polar Coding in Slow Fading Broadcast Channels
Slow fading broadcast channels can model a wide range of applications in
wireless networks. Due to delay requirements and the unavailability of the
channel state information at the transmitter (CSIT), these channels for many
applications are non-ergodic. The appropriate measure for designing signals in
non-ergodic channels is the outage probability. In this paper, we provide a
method to optimize STBCs based on the outage probability at moderate SNRs.
Multilevel polar coded-modulation is a new class of coded-modulation techniques
that benefits from low complexity decoders and simple rate matching. In this
paper, we derive the outage optimality condition for multistage decoding and
propose a rule for determining component code rates. We also derive an upper
bound on the outage probability of STBCs for designing the
set-partitioning-based labelling. Finally, due to the optimality of the
outage-minimized STBCs for long codes, we introduce a novel method for the
joint optimization of short-to-moderate length polar codes and STBCs
Iterative receivers and multichannel equalisation for time division multiple access systems
The thesis introduces receiver algorithms improving the performance of TDMA mobile radio systems. Particularly, we consider receivers utilising side information, which can be obtained from the error control coding or by having a priori knowledge of interference sources. Iterative methods can be applied in the former case and interference suppression techniques in the latter.
Convolutional coding adds redundant information into the signal and thereby protects messages transmitted over a radio channel. In the coded systems the receiver is usually comprised of separate channel estimation, detection and channel decoding tasks due to complexity restrictions. This suboptimal solution suffers from performance degradation compared to the optimal solution achieved by optimising the joint probability of information bits, transmitted symbols and channel impulse response. Conventional receiver utilises estimated channel state information in the detection and detected symbols in the channel decoding to finally obtain information bits. However, the channel decoder provides also extrinsic information on the bit probabilities, which is independent of the received information at the equaliser input. Therefore it is beneficial to re-perform channel estimation and detection using this new extrinsic information together with the original input signal.
We apply iterative receiver techniques mainly to Enhanced General Packet Radio System (EGPRS) using GMSK modulation for iterative channel estimation and 8-PSK modulation for iterative detection scheme. Typical gain for iterative detection is around 2Â dB and for iterative channel estimation around 1Â dB. Furthermore, we suggest two iteration rounds as a reasonable complexity/performance trade-off. To obtain further complexity reduction we introduce the soft trellis decoding technique that reduces the decoder complexity significantly in the iterative schemes.
Cochannel interference (CCI) originates from the nearby cells that are reusing the same transmission frequency. In this thesis we consider CCI suppression by joint detection (JD) technique, which detects simultaneously desired and interfering signals. Because of the complexity limitations we only consider JD for two binary modulated signals. Therefore it is important to find the dominant interfering signal (DI) to achieve the best performance. In the presence of one strong DI, the JD provides major improvement in the receiver performance.
The JD requires joint channel estimation (JCE) for the two signals. However, the JCE makes the implementation of the JD more difficult, since it requires synchronised network and unique training sequences with low cross-correlation for the two signals.reviewe
Iterative multiuser receivers for coded DS-CDMA systems
The introduction of cellular wireless systems in the 1980s has resulted in a continuous and
growing demand for personal communication services. This demand has made larger capacity
systems necessary. With the interest from both the research community and industry in wireless
code-division multiple-access (CDMA) systems, the application of multiuser detection (MUD)
techniques to wireless systems is becoming increasingly important. MUD is an important area
of interest to help obtain the significant increase in capacity needed for future wireless services.
The standardisation of direct-sequence CDMA (DS-CDMA) systems in the third generation of
mobile communication systems has raised even more interest in exploiting the capabilities and
capacity of this type of technology. However, the conventional DS-CDMA system has the major
problem of multiple-access interference (MAI). The MAI is unavoidable because receivers
deal with information which is transmitted not by a single source but by several uncoordinated
and geographically separated sources. As a result, the capacity of these systems is inherently interference
limited by other users. To overcome these limitations, MUD emerges as a promising
approach to increase the system capacity.
This thesis addresses the problem of improving the downlink capacity of a coded DS-CDMA
system with the use of MUD techniques at the mobile terminal receiver. The optimum multiuser
receiver scheme is far too computational intensive for practical use. Therefore, the aim of this
thesis is to investigate sub-optimal multiuser receiver schemes that can exploit the advantages
of MUD but also simplify its implementation. The attention is primarily focused on iterative
MUD receiver schemes which apply the turbo multiuser detection principle. Essentially this
principle consists of an iterative exchange of extrinsic information among the receiver modules
to achieve improved performance.
In this thesis, the implementation of an iterative receiver based on a linear MUD technique and
a cancellation scheme over an additive white Gaussian noise (AWGN) channel is first proposed
and analysed. The interference analysis shows that good performance is achieved using a lowcomplexity
receiver structure. In more realistic mobile channels, however, this type of receiver
suffers from the presence of higher levels of interference resulting in poor receiver performance.
The reason for this is that in such scenarios the desired signals are no longer linearly separable.
Therefore, non-linear detection techniques are required to provide better performance. With
this purpose, a hybrid iterative multiuser receiver is investigated for the case of a stationary
multipath channel. The incorporation of antenna arrays is an effective and practical technique to
provide a significant capacity gain over conventional single-antenna systems. In this context, a
novel space-time iterative multiuser receiver is proposed which achieves a large improvement in
spectral efficiency and performance over multipath fading channels. In addition, it is shown that
this architecture can be implemented without a prohibitive complexity cost. The exploitation
of the iterative principle can be used to approach the capacity bounds of a coded DS-CDMA
system. Using the Shannonâs sphere packing bound, a comparison is presented to illustrate how
closely a practical system can approach the theoretical limits of system performance
Iterative Receiver for MIMO-OFDM System with ICI Cancellation and Channel Estimation
As a multi-carrier modulation scheme, Orthogonal Frequency Division Multiplexing (OFDM) technique can achieve high data rate in frequency-selective fading channels by splitting a broadband signal into a number of narrowband signals over a number of subcarriers, where each subcarrier is more robust to multipath. The wireless communication system with multiple antennas at both the transmitter and receiver, known as multiple-input multiple-output (MIMO) system, achieves high capacity by transmitting independent information over different antennas simultaneously. The combination of OFDM with multiple antennas has been considered as one of most promising techniques for future wireless communication systems. The challenge in the detection of a space-time signal is to design a low-complexity detector, which can efficiently remove interference resulted from channel variations and approach the interference-free bound. The application of iterative parallel interference canceller (PIC) with joint detection and decoding has been a promising approach. However, the decision statistics of a linear PIC is biased toward the decision boundary after the first cancellation stage. In this thesis, we employ an iterative receiver with a decoder metric, which considerably reduces the bias effect in the second iteration, which is critical for the performance of the iterative algorithm. Channel state information is required in a MIMO-OFDM system signal detection at the receiver. Its accuracy directly affects the overall performance of MIMO-OFDM systems. In order to estimate the channel in high-delay-spread environments, pilot symbols should be inserted among subcarriers before transmission. To estimate the channel over all the subcarriers, various types of interpolators can be used. In this thesis, a linear interpolator and a trigonometric interpolator are compared. Then we propose a new interpolator called the multi-tap method, which has a much better system performance. In MIMO-OFDM systems, the time-varying fading channels can destroy the orthogonality of subcarriers. This causes serious intercarrier interference (ICI), thus leading to significant system performance degradation, which becomes more severe as the normalized Doppler frequency increases. In this thesis, we propose a low-complexity iterative receiver with joint frequency- domain ICI cancellation and pilot-assisted channel estimation to minimize the effect of time-varying fading channels. At the first stage of receiver, the interference between adjacent subcarriers is subtracted from received OFDM symbols. The parallel interference cancellation detection with decision statistics combining (DSC) is then performed to suppress the interference from other antennas. By restricting the interference to a limited number of neighboring subcarriers, the computational complexity of the proposed receiver can be significantly reduced. In order to construct the time variant channel matrix in the frequency domain, channel estimation is required. However, an accurate estimation requiring complete knowledge of channel time variations for each block, cannot be obtained. For time- varying frequency-selective fading channels, the placement of pilot tones also has a significant impact on the quality of the channel estimates. Under the assumption that channel variations can be approximated by a linear model, we can derive channel state information (CSI) in the frequency domain and estimate time-domain channel parameters. In this thesis, an iterative low-complexity channel estimation method is proposed to improve the system performance. Pilot symbols are inserted in the transmitted OFDM symbols to mitigate the effect of ICI and the channel estimates are used to update the results of both the frequency domain equalizer and the PICDSC detector in each iteration. The complexity of this algorithm can be reduced because the matrices are precalculated and stored in the receiver when the placement of pilots symbols is fixed in OFDM symbols before transmission. Finally, simulation results show that the proposed MIMO-OFDM iterative receiver can effectively mitigate the effect of ICI and approach the ICI-free performance over time-varying frequency-selective fading channels
Transmitter based techniques for ISI and MAI mitigation in CDMA-TDD downlink
The third-generation (3G) of mobile communications systems aim to provide enhanced voice,
text and data services to the user. These demands give rise to the complexity and power consumption
of the user equipment (UE) while the objective is smaller, lighter and power efficient
mobiles. This thesis aims to examine ways of reducing the UE receiverâs computational cost
while maintaining a good performance.
One prominent multiple access scheme selected for 3G is code division multiple access. Receiver
based multiuser detection techniques that utilise the knowledge of the downlink channel
by the mobile have been extensively studied in the literature, in order to deal with multiple
access and intersymbol interference. However, these techniques result in high mobile receiver
complexity.
Recently, work has been done on algorithms that transfer the complexity from the UE to the
base station by exploiting the fact that in time division duplex mode the downlink channel can
be known to the transmitter. By linear precoding of the transmitted signal the user equipment
can be simplified to a filter matched to the userâs spreading code. In this thesis the problem
of generic linear precoding is analysed theoretically and a method for analytical calculation
of BER is developed. The most representative of the developed precoding techniques are described
under a common framework, compared and classified as bitwise or blockwise. Bitwise
demonstrate particular advantages in terms of complexity and implementation but lack in performance.
Two novel bitwise algorithms are presented and analysed. They outperform significantly
the existing ones, while maintain a reduced computational cost and realisation simplicity.
The first, named inverse filters, is the Wiener solution of the problem after applying a minimum
mean squared error criterion with power constraints. The second recruits multichannel adaptive
algorithms to achieve the same goal. The base station emulates the actual system in a cell
to converge iteratively to the pre-filters that precode the transmitted signals before transmission.
The advantages and the performance of the proposed techniques, along with a variety of
characteristics are demonstrated by means of Monte Carlo simulations
Physical and Link Layer Implications in Vehicle Ad Hoc Networks
Vehicle Ad hoc Networks (V ANET) have been proposed to provide safety on the
road and deliver road traffic information and route guidance to drivers along with
commercial applications. However the challenges facing V ANET are numerous. Nodes
move at high speeds, road side units and basestations are scarce, the topology is
constrained by the road geometry and changes rapidly, and the number of nodes peaks
suddenly in traffic jams. In this thesis we investigate the physical and link layers of
V ANET and propose methods to achieve high data rates and high throughput.
For the physical layer, we examine the use of Vertical BLAST (VB LAST) systems
as they provide higher capacities than single antenna systems in rich fading
environments. To study the applicability of VB LAST to VANET, a channel model was
developed and verified using measurement data available in the literature. For no to
medium line of sight, VBLAST systems provide high data rates. However the
performance drops as the line of sight strength increases due to the correlation between
the antennas. Moreover, the performance of VBLAST with training based channel
estimation drops as the speed increases since the channel response changes rapidly. To
update the channel state information matrix at the receiver, a channel tracking algorithm
for flat fading channels was developed. The algorithm updates the channel matrix thus
reducing the mean square error of the estimation and improving the bit error rate (BER).
The analysis of VBLAST-OFDM systems showed they experience an error floor due to
inter-carrier interference (lCI) which increases with speed, number of antennas
transmitting and number of subcarriers used. The update algorithm was extended to
VBLAST -OFDM systems and it showed improvements in BER performance but still
experienced an error floor. An algorithm to equalise the ICI contribution of adjacent
subcarriers was then developed and evaluated. The ICI equalisation algorithm reduces
the error floor in BER as more subcarriers are equalised at the expense of more
hardware complexity.
The connectivity of V ANET was investigated and it was found that for single lane
roads, car densities of 7 cars per communication range are sufficient to achieve high
connectivity within the city whereas 12 cars per communication range are required for
highways. Multilane roads require higher densities since cars tend to cluster in groups.
Junctions and turns have lower connectivity than straight roads due to disconnections at
the turns. Although higher densities improve the connectivity and, hence, the
performance of the network layer, it leads to poor performance at the link layer. The
IEEE 802.11 p MAC layer standard under development for V ANET uses a variant of
Carrier Sense Multiple Access (CSMA). 802.11 protocols were analysed
mathematically and via simulations and the results prove the saturation throughput of
the basic access method drops as the number of nodes increases thus yielding very low
throughput in congested areas. RTS/CTS access provides higher throughput but it
applies only to unicast transmissions. To overcome the limitations of 802.11 protocols,
we designed a protocol known as SOFT MAC which combines Space, Orthogonal
Frequency and Time multiple access techniques. In SOFT MAC the road is divided into
cells and each cell is allocated a unique group of subcarriers. Within a cell, nodes share
the available subcarriers using a combination of TDMA and CSMA. The throughput
analysis of SOFT MAC showed it has superior throughput compared to the basic access
and similar to the RTS/CTS access of 802.11
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