Due to the simplicity and superior error performance over other MUDs in fading environment, in this thesis, we analyze the characteristics of SIC. Then we propose practical techniques and architectures of pilot-channel-aided SIC over uplink WCDMA systems in the hope of making this technique widely used.
It has been shown that the ordering method has a great effect on the performance of SIC [15], [52], [58], [69]. Our proposed method is presented in Chapter 3, and three ordering methods for SIC in the uplink of WCDMA systems over multipath fading channels are discussed and compared in the aspect of the implementation issues and error performance related parameters. In addition to consider the single-rate system, a generalized pilot-channel aided SIC scheme is presented to apply to multirate communications.
To overcome drawbacks of the well-known SIC, several techniques have been proposed.
First, sophisticated detection schemes are sensitive to channel estimation accuracy, so are ICs due to error propagation from stage to stage. When the pilot-channel signals in Q-channel and the traffic-channel signals in I-channel are scrambled by complex scrambling codes and transmitted simultaneously such as that in the uplink of WCDMA systems [75], pilot channel can be employed to reduce channel estimation errors. However,
the traffic-channel signals are always interfered by other users’ pilot and traffic signal even without any fading. This interference between I-channel and Q-channel can also be alleviated with interference cancellation techniques. Pilot-channel signal removal (PCSR) technique combining with RAKE receiver [27] and PIC [36] is used to alleviate the interference from other users’ pilot signals. In Chapter 3, we propose that the pilot-channel signal of all users are removed from the received signal followed by the SIC for data detection at the cost of slight increase in processing delay. Second, a pipeline scheme is proposed in Chapter 4 to shorten processing delay [68]. Third, it is shown in Chapter 4 that even with the equal power control profile, the SIC with properly chosen ordering method still outperforms multistage PPIC [67].
In addition to considering pure SIC in uplink WCDMA systems, we further propose an adaptable scheme with the ability to adjust its structure according to the environment and channel condition. The scheme combines serial (SIC) and parallel (PIC) interference cancellation, and the processing delay and computational complexity can be adjusted based on system loading and required performance. Compared with SIC and PIC, the proposed pilot-channel aided adaptable scheme shows better performance over both ICs with reasonable hardware while it needs shorter processing delay than SIC. The interference between data channel signals and pilot channel signals under multipath fading channel are also taken into consideration. This results in better quality both on channel parameter estimation and user data detection.
To extend the SIC for application in turbo-coded systems, an iterative IC with ordered SIC at front-end is proposed in Chapter 5. Except for the timing information, all parameters used in the proposed scheme are estimated from the received signal. To avoid unnecessary computations, we utilize the ordering information obtained from SIC front-end to propose a stopping criterion with high efficiency and low complexity. After finding the correct CBs,
bits in correct blocks are hard-decisioned, re-encoded and removed from the correlated input signal. Only the bits in incorrect blocks should proceed to the next outer iteration.
Also, information of the CB correctness is utilized to decide if the next outer iteration should be done. As a result, huge amount of computational complexity can be saved with BER improvement.
The thesis is organized as follows. The WCDMA system especially the physical layer and the system model is described in Chapter 2. In Chapter 3, the scheme of pilot-channel aided SIC employing three cancellation-ordering methods are presented and compared in the aspect of implementation complexity and error performance. This scheme can be applied to multirate systems. To make the SIC a practical scheme, in Chapter 4 we develops two advanced techniques including pipeline SIC and adaptable IC. Iterative IC receiver with SIC at front-end in turbo-coded systems as well as low-complexity stopping criterion are presented in Chapter 5. Finally, conclusions are given in Chapter 6.
In Fig. 1-2, a block diagram of the generalized communication systems is shown. The block where our thesis is concerned is filled with gray color.
Fig. 1-1 The global access of IMT-2000
Source encoder
Source decoder
Data modulator
Data demodulator
Spread spectrum
spreader
Spread spectrum despreader
Power amplification
Receiver front end Waveform
channel noise
Spreading code generator
Timing and
synchro-niztion
Channel estimation Information
source
Information sink
Channel encoder
Channel decoder Encryptor
Decryptor
Fig. 1-2 Block diagram of the generalized communication systems
Chapter 2
Overview of WCDMA Systems
WCDMA technology was created in the 3GPP group. In Europe, the 3G mobile communication system is called the Universal Mobile Telecommunications System (UMTS), whereas the terrestrial radio access system is referred to as the UMTS Terrestrial Radio Access (UTRA), which is why WCDMA is called UTRA FDD (Frequency Duplex Division) and TD-CDMA (Time Duplex Division) is called UTRA TDD in Europe. After the completion of the Release ’99 specifications, minor adjustments are made, and so called Release 4 was completed in March 2001. The high speed downlink packet access (HSDPA) and IP-based transport layer to achieve high throughput, reduce delay and achieve high peak rates were included in Release 5 which was completed in Mar 2002 for the WCDMA radio aspect. For Release 6 completed in Mar 2005, the high speed uplink packet access (HSUPA) and multimedia broadcast multicast service (MBMS) were introduced. The HSUPA aims at providing significant enhancements in terms of user experience (throughput and delay) and/or capacity, and coverage, while MBMS enables the ability to deliver audio and video data to multiple users simultaneously. The HSDPA and the HSUPA are briefly described in Appendix A and Appendix B, respectively. Readers can refer to [34] for more details. The next step in the evolution is Release 7 which is currently under development. The techniques including MIMO, and 3.84 Mcps and 7.68 Mcps TDD enhanced uplink where the first 1.28 Mbps TDD mode originally from CWTS (China) was included in 3GPP Release 4.
In this chapter, we describe system model used in this thesis. First, the physical layer of WCDMA technology is briefly introduced. Then the transmitter in the uplink and multipath
fading channel followed by the RAKE receiver used in the overall thesis are presented.