3 research outputs found
Cooperative diversity in CDMA networks
Spatial diversity is one of the well known diversity methods used in combating fading channels. Recently, cooperative diversity has been widely studied in literature as a spatial diversity technique. Different from multiple-input multiple-output (MIMO) systems, each user in the cooperative network is employed with a single transmit/receive antenna. In this thesis, we propose a cooperative diversity technique for asynchronous direct sequence code division multiple access (D8-CDMA) over frequency selective slow fading environment. First we assume the single cooperation relay case, where the bit-error-rate performance of the system is studied for both cases of perfect and imperfect inter-user channel (user-relay link). In order to mitigate the multi-access interference (MAI), decorrelator multiuser detectors are introduced at both relay and base station sides. Its effect on performance is studied and compared to the performance of the conventional matched filter receiver. Additionally, the performance of the system is studied and compared for different multi-path diversity scenarios in the inter-user and uplink channel. Furthermore, a coded multi-relay cooperation technique is proposed, where channel coding is introduced to minimize errors over the inter-user channel. All users are embedded with convolutional encoder and a Viterbi decoder. We study the performance of the coded system for different number of cooperating relays and over different multi-path diversity scenarios. Both simulation and analytical results are compared. Finally, we conclude that for a communication network to benefit from the cooperation diversity technique, a reliable communication link between active users and the cooperating relays should be secured (inter-user channel). We show that for an active user cooperating with V relays over a P -path frequency-selective fading channel, the expected diversity degree is P ( V +1
High Capacity CDMA and Collaborative Techniques
The thesis investigates new approaches to increase the user capacity and improve the error
performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation
and collaborative spreading and space diversity techniques. Collaborative Coding Multiple
Access (CCMA) is also investigated as a separate technique and combined with CDMA. The
advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the
uplink and downlink are proposed and evaluated.
Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The
practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive
approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM)
property of digital signals to blindly suppress interference during the despreading process and obtain
amplitude estimation with minimum mean squared error for use in cancellation stages. Two
new blind adaptive receiver designs employing successive and parallel interference cancellation
architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respectively,
are presented. These techniques have shown to offer near single user performance for large
number of users. It is shown to increase the user capacity by approximately two fold compared
with conventional IC receivers. The spectral efficiency analysis of the techniques based on output
signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore,
an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a
simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI
without any knowledge of channel amplitudes and allows large number of users compared with
equal gain and maximum ratio combining techniques normally used in practice.
New user collaborative schemes are proposed and analysed theoretically and by simulations
in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First,
a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA
are designed and analysed. Next, a new user collaborative scheme with successive interference
cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce
MAI and achieve improved diversity. To further improve the performance of C-SIC under high
system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed.
It is shown to minimize the residual MAI, leading to improved user capacity and a more robust
system. It is known that collaborative diversity schemes incur loss in throughput due to the need of
orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel
near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time
encoding methods. At the receiver collaborative joint detection is performed to separate each
paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth
as inter-user channel SNR becomes high.
A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA
for both the downlink and uplink. First, collaborative group spreading technique for the downlink
of overloaded CDMA system is introduced. It allows the sharing of the same single spreading
sequence for more than one user belonging to the same group. This technique is referred to as
Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative
coding is used for each group to form a composite codeword signal of the users and then a
single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite
codeword is carried out to extract the user’s own information while maintaining a high SINR performance.
To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions,
Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative
coding multiple access and space-time coding principles. A new scheme for uplink of CDMA
using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When
users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved
by grouping multiple users to share the same spreading sequence and performing MUD on per
group basis followed by a low complexity ML decoding at the receiver. This approach has shown
to support much higher number of users than the available sequences while also maintaining the
low receiver complexity. For improved performance under highly correlated channel conditions,
T-user collaborative coding is also investigated within the CS-CDMA-UL system
Performance of Cooperative CDMA with Successive Interference Cancellation
We investigate the BER and achievable rate of user cooperation schemes in practical uplink CDMA channels with multiple access interference (MAI). It is shown that when the system loading increases, cooperation alone becomes less effective if simple matched filters (MF) followed by combining from each partners' signals are employed for obtaining decision variables. By performing successive interference cancellation (SIC) for each received signals from the partners and then using the maximum ratio combining technique, the diversity gain and hence the uplink capacity is enhanced. We further analyse the output decision variable signals and also provide a simplified bound on achievable rate based on Gaussian Approximation of MAI signals. Illustrative simulation results are given, which confirm that the proposed scheme using SIC achieves much improved diversity and error performance under high system loading and near far conditions