124 research outputs found
SGD Frequency-Domain Space-Frequency Semiblind Multiuser Receiver with an Adaptive Optimal Mixing Parameter
A novel stochastic gradient descent frequency-domain (FD) space-frequency (SF) semiblind multiuser receiver with an adaptive optimal mixing parameter is proposed to improve performance of FD semiblind multiuser receivers with a fixed mixing parameters and reduces computational complexity of suboptimal FD semiblind multiuser receivers in SFBC downlink MIMO MC-CDMA systems where various numbers of users exist. The receiver exploits an adaptive mixing parameter to mix information ratio between the training-based mode and the blind-based mode. Analytical results prove that the optimal mixing parameter value relies on power and number of active loaded users existing in the system. Computer simulation results show that when the mixing parameter is adapted closely to the optimal mixing parameter value, the performance of the receiver outperforms existing FD SF adaptive step-size (AS) LMS semiblind based with a fixed mixing parameter and conventional FD SF AS-LMS training-based multiuser receivers in the MSE, SER and signal to interference plus noise ratio in both static and dynamic environments
Chip level decision feedback equalizer for CDMA downlink channel
In most commercial wideband code division multiple access (W-CDMA) systems, the transmitted signal in the downlink channel is spread by orthogonal codes to accommodate different users. However, frequency selective fading destroys the orthogonality and causes multiple access interference (MAI). The rake receiver has been proposed for a receiver in the downlink channel. Although providing reasonable performance due to path diversity, the rake receiver does not restore the orthogonality. As a result, the MAI is still present at the output of the rake receiver and CDMA system becomes an interference limited system. Thus, a better approach to provide MAI suppression shall be considered. The chip level linear equalizer followed by a despreader is an attractive alternative receiver to restore the orthogonality and to suppress the MAI. However, the performance of the chip level linear equalizer depends on the spectral characteristic of the channel and may not be satisfactory for some channels. To overcome this difficulty, the chip level decision feedback equalizer can be used. To improve the performance further, multiple-input multiple-output (MIMO) channels by using multiple antennas can also be employed. Recent research in information theory shows that multiple antennas can mitigate multipath fading in a wireless channel, increase the information capacity of wireless communication systems dramatically, and improve the reliability of the communications over wireless channel.In this thesis, the chip level decision feedback equalizer for CDMA downlink channel with multþle antennas is investigated. The work includes the design of the chip level decision feedback equalizer when the space time spreading scheme and the Alamouti scheme are employed at the transmitter. Theoretical and simulation results show significant performance gains compared to the rake receiver and the chip level linear equalizer.Thesis (M.Eng.Sc.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2004
Interference Suppression in WCDMA with Adaptive Thresholding based Decision Feedback Equaliser
WCDMA is considered as one of the 3G wireless standards by 3GPP. Capacity calculation shows that WCDMA systems have more capacity compared to any other multiple access technique such as time division multiple access (TDMA) or frequency division multiple access (FDMA). So it is widely used. Rake receivers are used for the detection of transmitted data in case of WCDMA communication systems due to its resistance to multipath fading. But rake receiver treat multiuser interference (MUI) as AWGN and have limitation in overcoming the effect of multiple access interference (MAI) when the SNR is high. A de-correlating matched filter has been used in this thesis, which eliminates and improves system performance. But the given receiver works well only in the noise free environment. A DFE, compared to linear equaliser, gives better performance at severe ISI condition. The only problem in this equalisation technique is to select the number of symbols that are to be fed back. This thesis gives an idea on multiple symbol selection, based on sparity where an adaptive thresholding algorithm is used that computes the number of symbols to feedback. Simulated results show a significant performance improvement for Regularised Rake receiver along with thresholding in terms of BER compared to a rake receiver, de-correlating rake receiver and regularised rake receiver. The performance of the receiver in different channels is also analysed
Performance study of air interface for broadband wireless packet access
Ph.DDOCTOR OF PHILOSOPH
Efficient Radio Resource Allocation Schemes and Code Optimizations for High Speed Downlink Packet Access Transmission
An important enhancement on the Wideband Code Division Multiple Access
(WCDMA) air interface of the 3G mobile communications, High Speed Downlink
Packet Access (HSDPA) standard has been launched to realize higher spectral
utilization efficiency. It introduces the features of multicode CDMA transmission
and Adaptive Modulation and Coding (AMC) technique, which makes radio resource
allocation feasible and essential. This thesis studies channel-aware resource
allocation schemes, coupled with fast power adjustment and spreading code optimization
techniques, for the HSDPA standard operating over frequency selective
channel.
A two-group resource allocation scheme is developed in order to achieve a
promising balance between performance enhancement and time efficiency. It only
requires calculating two parameters to specify the allocations of discrete bit rates
and transmitted symbol energies in all channels. The thesis develops the calculation
methods of the two parameters for interference-free and interference-present
channels, respectively. For the interference-present channels, the performance of
two-group allocation can be further enhanced by applying a clustering-based channel
removal scheme.
In order to make the two-group approach more time-efficient, reduction in
matrix inversions in optimum energy calculation is then discussed. When the
Minimum Mean Square Error (MMSE) equalizer is applied, optimum energy allocation
can be calculated by iterating a set of eigenvalues and eigenvectors. By
using the MMSE Successive Interference Cancellation (SIC) receiver, the optimum
energies are calculated recursively combined with an optimum channel ordering
scheme for enhancement in both system performance and time efficiency.
This thesis then studies the signature optimization methods with multipath
channel and examines their system performances when combined with different
resource allocation methods. Two multipath-aware signature optimization methods
are developed by applying iterative optimization techniques, for the system
using MMSE equalizer and MMSE precoder respectively. A PAM system using
complex signature sequences is also examined for improving resource utilization
efficiency, where two receiving schemes are proposed to fully take advantage of
PAM features. In addition by applying a short chip sampling window, a Singular
Value Decomposition (SVD) based interference-free signature design method is
presented
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