It is well known that channel fading phenomenon brings many challenges for reliable tetherless communications due the significant probability of channels being in deep fade [2]. The informa- tion theoretic aspects of fading channels and various approaches for increasing the capacity have been thoroughly investigated in [67]. Despite the challenges, it has been reported in [2] that the information theoretic capacity of fading multiple antenna channels exceeds manyfold the capacity achievable in single antenna Gaussian channels. The asymptotical limit on achievable rates when multiple antennas are used both at transmitter and receivers also called MIMO channels are shown in [35].
Though the employment of antenna arrays is very effective in achieving diversity, mobile nodes however can not afford multiple antennas due to their size and power limitations. This condition also holds true for base-station receivers in many cases, as adequate antenna spacing to ensure uncorrelated fading may not be possible due to limited space available at the basestation. For wideband systems, such as CDMA, frequency diversity of multipath channels can be exploited, however its existence is purely based on the scattering effect of source signal over many paths and hence may not be practical in sparse environments with limited scattering [2]. Furthermore, slow fading environments, such as low mobility of users, may render the use of temporal diversity ineffective. Therefore, independent of any other diversity schemes, the use of some form of space diversity is always desirable for increasing the reliability of communications of fading channels.
Recently, the idea of cooperative diversity is becoming increasingly popular in both cellular and ad-hoc wireless networks due to its ability to provide significant diversity gain without having antenna arrays at the both end of the links [7, 75, 8, 76]. Although the technique is relatively new, the motivation behind it, traces back to early works on the use of inactive user nodes as relays to facilitate the reception of desired node’s signals as investigated by Muelen in [77]. The
capacity analysis of such networks is investigated by Cover [78]. Sendonaris, Erkip and Aazhang [7] developed the idea of relay channels further for multiuser CDMA cellular networks termed as, ‘User Cooperation Diversity’ and provided its capacity and outage probability analysis with practical implementations in cellular environments. Laneman investigated different variants of cooperative diversity protocols with space-time coding and time diversity in his dissertation [8]. Furthermore, the use of coding within cooperation is investigated by Hunter and Nosratinia in [75]. A more comprehensive review on different cooperative diversity schemes can be found in a paper by Aggelos et.al. in [79], where they also propose an interesting technique called ‘Opportunistic Relaying’ based on selecting best path from the group of relays. The diversity multiplexing aspects of the different cooperative schemes is also presented. The basic idea of cooperative diversity that has been receiving so much attention recently, is described next.
The conventional transmit/receive diversity schemes require L antennas to provide L-order diversity. The same L-order diversity can also be achieved if there are L user nodes cooperating among each other so that the information can be transmitted from L antennas of the user nodes. The diversity gain is however achieved at the expense of extra bandwidth as information of each user node has to be exchanged over all user nodes.
An example of a simple two antenna diversity reception and an equivalent two user cooperation diversity scheme that achieves the same diversity gain are shown in Figure 2.13. As it can be seen from the figure that, the cooperative scheme requires at least two symbol periods to complete one cycle of cooperation. The essence of the cooperative scheme using the system model of (2.14) with L = 1 can be listed as follows:
• Period 1: User 1(U1) and User 2 (U2), also called partners transmit their data {s1, s2} using
different time-slot, frequency or orthogonal sequence to the basestation receiver (D) via their independent channels {g1d, g2d}; the received signal at D this period is r = s1g1d+
s2g2d. At the same time, the users process the received signals from each other via inter-user
channels {g12, g21}to decode the transmitted information {s1, s2} and obtain their estimates
{s01, s02}
• Period 2: The partners transmit the estimated data {s0
2, s01} via the partners’ channels {g1d, g2d}
to the basestation receiver (D) to form r0 = s02g1d+ s01g2d. After receiving two signals
r, r0, the base-station performs maximum ratio combining of signals i.e. rg1d∗ + r0g∗2dand rg∗2d+r0g1d∗ , respectively. If inter-user channel quality is good, symbols of users are decoded perfectly at the partners i.e. {s01, s02} = {s1, s2}. The soft estimates of symbols are now
˜
s1 =|g1d|2+ |g2d|2 s1 and ˜s2 =|g2d|2+ |g1d|2 s2and hard decision on the resulting
It is noted from the above protocol that, the cooperation achieves full second order diversity as the quality of inter-user channels improves. Note that the protocol assumed no interference between the transmitted signals from User 1 and User 2. A multiuser system such as CDMA uplink however suffers from many practical problems, and hence, simple protocol described above is clearly inadequate to fully exploit the space diversity from the user cooperation.
2.8.1 User Cooperation Diversity: Practical Issues
Bandwidth Efficiency Loss
The significant gain in BER of cooperative diversity scheme however comes at the cost of reduced bandwidth efficiency due to the need to decode and forward process required at the co- operating nodes that usually operate in half duplex mode. The seminal work by Sendonaris et. al. in [7] has mentioned this problem and assumed that it may be possible that the cooperating nodes could simultaneously transmit and receive (full duplex), given that transmit antenna gains are known and canceled from the received signal [78]. Despite the assumption of full duplex op- eration, the scheme in [7] incurs loss in bandwidth efficiency to achieves the diversity gain from node cooperation. This issue is addressed by Reberio et. al in [80] and Verdhe and Reynolds in [76] by using higher number of additional relays and use of superposition of user’s signals.
A new bandwidth cooperative diversity is proposed in this thesis to address the issue of band- width efficiency in uplink fading multiple access channels. Like other bandwidth efficient schemes [80, 76], the proposed scheme [81] uses the same total system resources such as time slots, system bandwidth, transmit power, however the scheme makes use of the idea of collaborative transmis- sion and reception [82]. Under this scheme, two or more users (collaborators) are grouped and they share the same spreading sequence for the transmission of their own data and that of the partners. The discussion on the idea of collaborative transmission and reception is carried in the subsection 5.2. The ‘Collaborative Diversity’ scheme itself is described in details in 4.5.
Multiuser Interference
The cooperative diversity for uplink CDMA proposed by Sendonaris et.al. in [7] assumed the use of orthogonal sequences and, hence, MAI is not an issue in the analysis of their schemes. The cooperation concept has also found widespread application in node to node ad-hoc communica- tions, using relays where orthogonal channels are used, so that simple receivers can be employed [79, 83, 75, 8]. However, the existence of MAI due to non-orthogonal sequences of users in uplink CDMA is an important practical issue that can not be ignored. An investigation of cooperative diversity for a single user in uplink CDMA with multiple relay nodes (idle users) with the use of non-orthogonal spreading sequences at the nodes is carried out by [84]. Where it is concluded that cooperation alone does not ensure that full-order diversity can be achieved in such environments,
and hence, more effective MAI suppressing receiver is incorporated in their scheme. The effects of MAI on the error performance and capacity in multiuser CDMA when users cooperate with their partners is an area that is investigated in this thesis. To put our work in context, in the sequel, some recent works carried out in this new area by different researchers will be reviewed and a brief overview and main findings of our schemes employing user cooperation with multiuser SIC receivers will be provided.
2.8.2 Combined User Cooperation and Multiuser Detection
The work of Fang and Hanzo [84] provides some insight into the problem of achieving coop- erative diversity in CDMA under MAI effects. To provide an effective solution to the problem, joint study of different CDMA multiuser detection schemes and cooperative diversity is essential. However, there exists very little work addressing the problems of cooperation diversity in realistic multiuser environment with non-trivial MAI [20, 85, 18]. This is because reduction of MAI re- quires more advanced MUD receivers which complicates the idea of cooperative diversity. Since the complexity of MUD schemes play important roles in feasibility of their implementation in realistic wireless systems, the use of low complexity MUD techniques such as parallel/successive interference cancellation seems more attractive than linear MUD schemes [50].
Considering the issues of cooperative diversity discussed earlier, multiuser cooperative diver- sity schemes using two different successive interference cancellation techniques are proposed in Chapter 4. The SIC receivers are known as low complexity receivers with proven robustness in fading and nearfar user signal environments [65, 63, 53, 16]. First, the use of conventional SIC [53] is investigated in our cooperative diversity scheme [20]. The performance of this scheme is compared with a SIC without cooperation and that of cooperative diversity without interference cancellation. It has been found that under low user loading condition, the use of SIC with coop- erative diversity achieves near full diversity order. However, as the number of users increases, the performance of the scheme shows notable degradation in BER. The main reason for such degraded performance can be explained from the problem of inaccurate MAI estimation and cancellation of convectional SIC as reported in [65]. This motivates us to incorporate the improved CMA-SIC receiver of [16] within the cooperative scheme to obtain better MAI estimates and interference can- cellation. It will be shown that, the use of CMA-SIC can significantly improve the performance under high user loading conditions.