Problem Statement and Literature Survey

From the Table 1-1, it is clearly that the IEEE 802.11 receiver needs to be multi-mode design to support both non-CP SCBT (802.11b) and OFDM (802.11 a/n) modes. In practice, most of the implementations of multi-mode receivers use dedicated hardware to support both non-CP SCBT and OFDM systems as shown in Fig 1-2. However, those dedicated modules are difficult to be shared and merged for different modes, which leads high cost and large die size in implementations. For example, the different packet format and coding scheme between non-CP SCBT and OFDM may lead difficult hardware sharing in synchronization, equalization and data decoding modules. Moreover, the less hardware sharing introduces complex control path and cost inefficiently in receiver design. Although the software-defined radio (SDR) solution [15]-[16] implemented with digital signal processors (DSPs), central processing unit (CPU) and field programmable gate array (FPGA) provides the flexible solutions for multi-mode applications, the designers still need to consider the complex data path introduced by different signal formats, i.e. time-domain signals in the pre-fast Fourier transform (pre-FFT) phase and frequency-domain signals in the post-FFT phase. The complex data path might cause bad influences in the pipeline design, e.g., data hazards, hard to identify pipeline stages and complex instruction sets,

which results in higher clock rate and more complex control circuit in hardware implementation [17]. To avoid the switch of data path between time and frequency domains, handling all signals in frequency domain after conventional ADC followed by FFT is investigated in the research [18].

Recently, the concept of FD receiver with FD-ADC technology [19]-[23] is proposed as a potential solution for a multi-standard receiver. Unlike the typical OFDM receiver architecture based on conventional analog-to-digital conversion (ADC) and time-domain (TD) synchronization followed by FFTs and data decoding in frequency domain, a FD receiver directly handles all signal processes in frequency domain. To reduce signal formats transformation between domains and increase the hardware sharing between two modes, this dissertation adopts the FD receiver architecture to support for WLAN multi-mode applications.

Based on the FD-ADC technology, Fig. 1-3 shows the proposed multi-mode FD receiver architecture. To realize this FD receiver, two problems need to be overcome:

symbol synchronization and non-CP SCBT equalization over frequency domain.

Although a number of methods for symbol synchronizations [24]-[32] have been proposed, they are all developed based on TD-ADC based receiver which decides the symbol boundary by a sliding window before FFT. However, FD ADC transforms a segment of continuous time-domain signal to a set of frequency coefficients. Since

those synchronization algorithms performs sample-by-sample search for correlation peak within the sliding window, they can not function with FD-ADC technology properly. Therefore, the synchronization problem of the FD receiver architecture is to detect symbol boundary with frequency coefficients of FD-ADC outputs. For OFDM-based packets, the channel frequency response (CFR) can be obtained from the frequency response of received OFDM symbol divided by the frequency response of the frequency domain training symbol (long training symbol) due to the existence of cyclic-prefix (CP) [33]. For equalization of non-CP SCBT over frequency domain, a single-carrier frequency-domain equalizer (SC-FDE) is developed to eliminate FFT aliasing without a circular property in some approaches [34]-[35], e.g., overlap-and-save and overlap-and-add methods. Yet, additional DFT units were included—hardware complexity of the multi-mode FD receiver may increase significantly. Thus, one of the major challenge for multi-mode integrations is to make equalizers as compact as possible, i.e., consolidation of non-CP SCBT, SISO OFDM and MIMO OFDM. Finally, it is also important to reduce the complexity of MIMO detection module for a low cost receiver design. Numbers of methods have proposed in the literature [36]-[43]. However, those methods include some of the following drawbacks, i.e., poor performance, unacceptable complexity, not favored for VLSI implementation. Therefore, the design of a low complexity MIMO detection method

is investigated in this dissertation.

Figure 1-2: Block diagram of the conventional 802.11 b/g/n/ac multi-mode receiver.

Modern output

Equalizer with MIMO detection

Figure 1-3: The proposed FD receiver architecture for SCBT and OFDM systems, where dash lines are the control signals, thin solid lines are the synchronization and channel estimation paths, and bold solid lines represent the data paths.

Besides the multi-mode integration issue, another important issue for modern WLAN applications is to provide cost effective and high rate networks over a wider coverage area. From radio technique perspective, MIMO scheme can provide the high date rate transmission yet it also requires good link quality which means the coverage of high rate transmission is very small. As shown in Fig. 1-4, the coverage can be extended by deploying an IEEE 802.11s WMN. Therefore, the knowledge related to the development and deployment of an IEEE 802.11s system becomes emergency topic for WLAN research filed recently. Although Prior studies, such as the mesh on XO-laptop for One Laptop per Child (OLPC) [44] and the open80211s project for Linux, has evaluated the network performance of the IEEE 802.11s mesh, few studies examine system architectures and mesh stability. Moreover, conventional mesh deployment [8], [45]-[50] focuses on the outdoor environment, which regards the WMNs as backbone networks. For WLAN application, the backhaul network is usually considered as an indoor deployment. However, indoor and outdoor WMNs possess distinguishable attributes and limitations. To the best of our knowledge, only a little previous work focuses on indoor WMNs [45]-[46].

(a)

(b)

Figure 1-4: (a) the conventional WLAN infrastructure. (b) relay-based WLAN infrastructure.