Beamforming and Synchronization Algorithms Integration for OFDM HAP-Based Communications Daniele Borio, 1 Laura Camoriano, 2 Letizia Lo Presti, 1,3 and Marina Mondin 1,3 High Altitude Platforms (HAPs) are currently experiencing a great interest as potential solutions for the growing demand of wireless broadband applications, but their rapid motion and the characteristics of the stratospheric channel, which may cause strong attenuation, multipath fading and Doppler shift, can strongly impact their performances. The described scenario requires the use of beamforming techniques, able to provide medium to high gains in a mobile environment. Given these premises, this paper discusses the conjugation of OFDM techniques with smart antennas for HAP-based communications, stressing the need for integration of the different functional blocks of an OFDM receiver. In fact, in order to face a time varying scenario, adaptive techniques are needed, and only a careful joint design of time and frequency synchronization with beamforming algorithms can guarantee reliable communication. The detailed and integrated receiver design described in the paper is based on the OFDM version of the IEEE 802.16a transmission standard. KEY WORDS: HAP; beamforming; synchronization; OFDM; multipath. 1. INTRODUCTION The wireless communication landscape is experiencing a continuous growth thanks to market boosts and to a rapid technological development. Wireless MAN (Metropolitan Area Networks) are becoming a competitive alternative to wired applications, most of all in a context where users require higher and higher bit-rates, available everywhere and at any time. New standards, like IEEE802.16 and its amendments [1 3], and new technologies, like WiMax [4,5], are currently under development, trying to answer to these new market demands. In order to increase the system flexibility and coverage, the possibility of transmitting Wireless 1 Dipartimento di Elettronica, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy. 2 NavSAS group, Istituto Superiore Mario Boella, Via Pier Carlo Boggio 61, 10138 Torino, Italy. 3 Tel: +39-0110904033/0110904023; Fax: +39-0110904099; E-mail: {letizia.lopresti,marina.mondin}@polito.it. MAN signals from high altitude platforms (HAPs) has being suggested [6,7], possibly exploiting the commercially more promising standards like WiMax. The considered wireless applications can however be extremely limited by the presence of fading caused by multipath propagation, that consists of multiple reflected signals arriving at the receiver with different delays. These echoes cause intersymbol interference (ISI), and, combined, they can produce fading. This effect is more and more severe as the distance range or the data rate of the system increase. OFDM [8,9] is a transmission technique especially tailored for situations where a high data rate is to be transmitted over a channel with a relatively large maximum delay. Anyway, when the delay of the received signals is larger than the guard interval, ISI may cause severe degradations in the system performance. To solve this problem, a multiple antenna array can be used at the receiver, not only for spectral efficiency or gain enhancement, but also for interference
suppression. Different beamforming algorithms for OFDM modulation have been developed [10 13]. In case of pilot-based transmission standards, the information embedded in the OFDM frame and carried by pilot tones is used to estimate optimum weights for the spatial filtering operated by the smart antenna. In order to work properly, the smart antenna needs reliable information from pilot tones, i.e. the OFDM frame should be correctly synchronized, the Doppler shift should be accurately estimated and compensated and finally the OFDM receiver should be able to correctly recover at least the pilot tones. These operations are generally disturbed by the same multi-path that the smart antenna should mitigate. From these simple considerations emerges the need to integrate the different functional blocks of the receiver: synchronization, Doppler compensation and beam-forming algorithms should cooperate in order to reach optimal performances. In this paper we describe a possible integration of the beamforming algorithms with the other functional blocks of the OFDM receiver, within a HAP-based wireless scenario characterized by the presence of different reflected rays, Additive White Gaussian Noise (AWGN) and Doppler shift. The receiver performs the OFDM synchronization and Doppler shift compensation, and impairments due to bad clock recovery are also taken into account. The synchronization method is based on the algorithm described in [14]. Beamforming operations are performed using a pre-fft method [13] based on a VSS- LMS (Variable Step Size LMS) [15,16]. The above referenced papers present only simplified working scenarios, where everything is ideal except the issues each paper is focused on: no multipath is considered and no integration between different blocks is performed. Our study is aimed at the integration of all the receiver blocks in order to provide a complete, operational, all-inclusive product. Since our intent is to prove the feasibility of an integrated receiver, we have employed a linear smart antenna for simplicity, but the introduction of a planar one is straightforward. This study has been carried out to evaluate the feasibility of the implementation of the IEEE 802.16 standard for mobile communications from HAPs to users in fast movement (up to 300 km/h), like public transportation vehicles. The rapidly changing scenario renders the use of smart antennas at the receiver extremely interesting, because of the important impact of the Doppler shift and of other phenomena linked to a fast movement. Furthermore, the rapid movement may generate a time-varying scenario in which multipath is constantly changing, so that the use of the OFDM technique may not be sufficient to combat the signal degradation. In these conditions smart antennas have proven to be more efficient than classical steerable antennas in providing a rapid tracking of the Line Of Sight (LOS) signal. For this type of applications the high costs generally implied by the use of smart antennas are not a limit, given the high cost of the HAP itself. In particular, this study has been carried out within the CAPANINA project [6], whose scenario involves broadband communications between HAPs and fast trains, both structures suitable to host sophisticated machines. The paper is organized as follows: Sections 2 and 3 recall the basic principles of OFDM and beam-forming, describing the adopted smart antenna architecture and the pre-fft algorithm. Section 4 describes the synchronization and the Doppler compensation algorithm, their performances, and the degradations due to multipath. Finally in Section 5 the whole system is described, highlighting the interconnections among the different functional blocks and the way in which the beamformer output can be used to improve synchronization and Doppler compensation performances and to rapidly adapt to time-varying scenarios. In Section 6 simulation results are reported. BER curves are obtained and compared with the performances of the smart antenna in ideal conditions. Conclusions are discussed in Section 7. The described receiver scheme has been submitted for a patent. 2. SIGNAL AND CHANNEL MODEL In this section we briefly introduce the expressions of the considered signals. The OFDM parameters are: N the number of data samples in the OFDM frame, excluding the cyclic prefix; Ng the number of samples of the cyclic prefix; NT =N+Ng. In the transmitter the input binary data are serial-to-parallel converted and mapped on one of the IEEE 802.16a QAM type constellations. The obtained N samples are IFFT transformed and a block of N samples, called OFDM symbol, is generated (Figure 1). The generic kth OFDM symbol can be written as [17]: