Chapter 3
Introduction to Frame Synchronization
In several communication systems including WiMAX and WiFi, packet aggregation techniques at intermediate protocol layers of the protocol stack have been proposed, where small packets are aggregated into larger packets or bursts in order to reduce the overhead due to headers and increase the throughput. These aggregated packets are then forwarded by the transmitter to lower protocol layers. As an example, packet aggregation techniques at intermediate protocol layers have been studied recently [85] in the context of WiFi stan- dard. It is also called frame aggregation if it is deployed at the lower layers of the protocol stack, e.g., at PHY layer. In this thesis, without distinguishing where the aggregation process is performed, be it at the PHY, MAC, IP, or at higher layer, it is called packet aggregation and the single aggregated packet is named a burst.
Aggregation process has similar functionality as the multiplexing scheme proposed in [15; 16], where DL packets are multiplexed into a single packet. Gateway combines multiple packets into a single multiplexed packet, and multicasts the multiplexed packet to the wireless end stations. The demultiplexer in each end station extracts its respective data and forwards them to the application. This has been shown to reduce the overhead of multiple packets and to improve capacity.
The price to be paid is a higher sensitivity of the large packets to transmission errors.
Thus, aggregation has two effects: on one side, it increases the useful throughput in quite a large amount, but, on another side, any error upon reception of the burst header results in the loss of the complete burst, composed of many individual packets. When the header of burst has been correctly received, efficient packet alignment becomes very important, since if some packets are not correctly delineated, a large amount of bits has to be retransmitted.
In such situation, if the delay is constrained (situations where one cannot afford retrans- mission of large aggregated packets) or when retransmitting bursts in error is not possible (broadcasting), synchronization algorithms are the relevant techniques, which can be used to properly recognize and maintain packet boundaries. In case the network fails to properly recognize the packet boundaries, it is said that the packet network is out of synchronism.
In this thesis, we assume that symbol (bit) synchronization has already been performed at PHY layer [86] and explore packet-level synchronization or Frame Synchronization (FS).
This chapter provides an introduction to FS. No new results are reported in this chap- ter, only the results for the state-of-the-art FS techniques are presented. The robust FS techniques proposed in this thesis are presented in Chapters 4-6.
Payload
SW/Header Payload
Payload SW/Header SW/Header
Figure 3.1: Aggregated packets
3.1 Introduction
FS is an important problem arising at various layers of the protocol stack of several communication systems. Important examples include the sequence synchronization at a spread-spectrum receiver, which is required before initiating any communication between the end points [87]. FS can be very critical in transmission of data through wireless links where, due to the use of e.g., powerful error correcting codes, the receiver is designed to work with very low SNRs. FS is often required at the PHY layer, to recover the payload and the side information (headers, preambles, etc...) of the PHY packets or frames. Therefore, FS has received continuous attention since many years [88; 89; 90; 91; 17; 21]. Initially, FS has mainly been considered at the PHY layer, albeit this problem may also occur at upper layers of the protocol stack. An example is video transmission where information is usually organized in packets, and decoding requires the knowledge of their exact boundaries. This is the case,e.g., in the MPEG-4 video stream [92; 93], where synchronization markers are aperiodically embedded in the bitstream.
The FS may become difficult when the received burst is corrupted by noise. To facilitate the process of FS at the receiver, often a Synchronization Word (SW) pattern is injected periodically into the data stream (continuous transmission) or appended at the beginning of each packet (packet transmission) [88; 89; 90]. In the absence of SW, the Header Error Control (HEC) field of the packet header can be employed [17]. Figure 3.1 shows a typical packet aggregation format with SW or header prefixed to the payload, now, the fundamental problem is to achieve FS, i.e., finding the exact starting location of the SW/header.
In several situations, e.g., when the SW/header is erroneous itself, the performance of FS degrades. In general, the performance of FS depends on the probability of missed detection, i.e., missing a correct SW/header, and on the probability of False Alarm (FA) or emulation, i.e., false detection of the start of SW/header. It is thus important for FS algorithms to minimize these probabilities. The miss detections can occur due to the channel noise in the SW/header, while the FAs are due to the fact that sometimes the random data plus noise can be interpreted as a SW/header. False alarms can occur either in the case when data symbols are coincident with the SW/header pattern or due to the channel noise, even if data symbols are different from the SW/header pattern. A single FA or miss detection can cause loss of two consecutive packets.
In the scenario under investigation in this thesis, as discussed in Chapter 2, our main concern is to perform FS at the MAC layer to localize the MAC packets, which are of variable length. Nevertheless, we will also provide a brief overview of the fixed-length FS techniques from the literature, because of their similitude with the variable-length FS techniques.
The rest of this chapter is organized as follows: The structure of the fixed-length or variable-length packet is provided in Section 3.2. The state-of-the-art FS techniques for the fixed-length packet network and the variable-length packet network are presented in Section 3.3 and Section 3.4, respectively. Finally, the simulation results for the variable-
length FS techniques are presented in Section 3.5, followed by the conclusions in Section 3.6.
3.2 Packet Structure
The two main conceptual packet networks that are common today are the fixed-length networks, such as the ATM network, and the variable-length packet networks, such as the WiMAX/WiFi network. In the fixed-length packet networks, the sizes of the packets are the same as well as their internal structure, where as in the variable-length packet networks, the sizes of the packets are variable. These packets are usually separated from each other by well defined indicator flags, packet headers, synchronization markers or SWs, which are located at the beginning of the payloads.
Let us define some notations on the structure of the variable-length or fixed-length packets. Consider then-th packet of variable-length packet network (or fixed-length packet network) at a given protocol layer. This packet is assumed to containλn=`h+`p,n bits, where the leading `h bits represent the fixed-length packet header or SW represented by hn and the remaining `p,n bits are the variable-length (or fixed-length) payload, denoted bypn. The payload is assumed to be generated by a binary symmetric source. The HEC, which can be a CRC or checksum, is assumed to be present in the last`cbits of the header and covers only the header (i.e.,`h−`cbits) without covering the payload.
3.3 Fixed-length State-of-the-art FS techniques
In order to understand the variable-length FS, let us first briefly explain the FS for the fixed-length packets. In the pioneering works [88; 89; 90] on FS to delimit fixed- length packets in the bit stream, regular spaced fixed patterns or SW are assumed to be present, as is the case, e.g., at the PHY layer of Digital Video Broadcasting-Handheld (DVB-H) [94] for MPEG2 transport stream packets. At the receiver, after recovering timing information, sampled received values are typically correlated with a SW and FS is accomplished by examining the correlation values [88; 90]. This type of FS method, which is generally referred to as the correlation rule, has been popular because of its simplicity in implementation and acceptable performance. For example, in [88], FS is performed by maximizing the correlation between the SW and the received data.
FS can also be achieved using optimal rules such as the Maximum-Likelihood (ML) rules and their various modifications [89; 91]. These rules outperform the correlation rules at the expense of additional computation. In [89] the optimal statistic for FS has been proposed for the Additive White Gaussian Noise (AWGN) case, taking into account the presence of data around the SW. Extensions to more sophisticated transmission schemes and code-aided FS techniques can be found in [91; 95; 96; 97]. A performance loss is experi- enced when the frequency or phase error exists in the channel. Under such circumstances, improved correlation metrics [98; 99] provide robustness to frequency and phase errors.
Optimum and low-complexity sub-optimum techniques in the presence of phase offset due to imperfect carrier phase estimation are proposed in [100], where possible extensions to higher order modulations and fading channels are also provided. The above-mentioned FS techniques work on-the-fly,i.e., only a small portion of the burst is processed at each time to perform FS.
However, the most common fixed-length packet network, i.e., ATM network, has an HEC present in the header, and FS is achieved through HEC search instead of SW local-