This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.

A Novel Preamble Scheme for Packet-based OFDM WLAN Ying Wanga, Job Oostveenb, Alessio Filippia, and Stefan Wesemannc a

Philips Research Laboratories, High Tech Campus 37, 5656 AE, Eindhoven, the Netherlands Email: {ying.z.wang, alessio.filippi}@philips.com b TNO Information and Communication Technology, Delft, the Netherlands Email: [email protected] c Signalion Gmbhm, Dresden, Germany Email: [email protected]

Abstract— In packet-based OFDM WLAN systems (IEEE 802.11a/g/n), each packet has a fixed-length preamble for synchronization and channel estimation purposes. The preamble carries no data information and hence causes considerable overhead during transmission when the payload is short and the data rate is high. In this work, we propose a new scheme, called superimposed preamble, to improve the efficiency of 802.11a alike systems. We design a new signal structure using variable data rate and propose an efficient receiver processing scheme to perform joint training (time and frequency synchronization and channel estimation) and data detection. It is shown that the proposed superimposed preamble system achieves training performance similar to the conventional 802.11a systems while gaining in robustness because of the reduced overhead.

I. INTRODUCTION Basically every (wireless) communication system uses known signals, named pilots, for all sorts of training and calibration purposes. In orthogonal frequency division multiplexing (OFDM) packet based transmissions (e.g., IEEE 802.11a/g/n wireless LAN standards [1]), each packet starts with a so-called preamble, consisting of a few pilot OFDM symbols for synchronization and channel estimation. The preamble technique is very well suited in case the channel does not vary within the packet. However, in case of short packets, the preamble causes a significant overhead since only a small fraction of the transmitted signal contains data. For instance, in 802.11a standard, every packet starts with a preamble consisting of four pilot OFDM symbols. The four pilot OFDM symbols constitute a negligible overhead in case of long packets, but for short packets the overhead becomes significant. Unfortunately, short packets are very common in WLAN systems [2], as shown in the network traffic packet size distribution in Fig. 1. Actually, more than 50% of the traffic consists of short packets with data payload less than 128 bytes, which corresponds to five OFDM symbols at 54Mbits/s. Here we assume the highest data rate in 802.11a, since this scenario causes the largest preamble overhead. For these packets, the preamble overhead is almost half of the whole packet. For newer standards like the upcoming 802.11n, the situation would be even worse. This is because the maximal data rate will strongly increase, and the small-size data payload will be encapsulated into even fewer OFDM symbols.

IEEE network traffic packet size model

24% 32%

2% 6% 64 Bytes 65-127 Bytes 128-255 Bytes 256-511 Bytes 512-1023 Bytes 1024-1518 Bytes

14% 22%

Fig. 1. Network traffic packet size distribution [2].

It is suggested in the literature that superimposed pilots can be employed to reduce this overhead [3], [4]. In a superimposed pilot scheme, every subcarrier in every OFDM symbol carries both unknown data and known pilot. The superimposed pilots take a fraction α (0