Technical Challenges and Trends in Broadband Wireless Access at Frequencies between 2 and 11 GHz Prof. Hikmet Sari SUPELEC Plateau de Moulon, 3 rue Joliot-Curie 92192 Gif-sur- Yvette Cedex, France Abstract Broadband wireless access (BWA) is a promising technology for new operators without an existing infrastructure to reach the end users. There is a wide spectrum for BWA at millimeter-wave frequencies (above 20 GHz), but the technology is not mature enough to make these systems cost- effective for residential applications. Therefore, there is currently a strong interest in BWA at microwave frequencies between 2 and 11 GHz. BWA systems in these frequency bands are not strictly line-of-sight (LOS) and they are subjected to strong multipath propagation leading to large channel dispersions. For this reason, recent technical specifications by the IEEE 802.16 and the ETSI BRAN Groups have been based on orthogonal frequency-division multiplexing (OFDM), orthogonal frequency-division multiple access (OFDMA), and single-carrier transmission with fi-equency-domain equalization (SC/FDE), three technologies which can cope with long channel dispersions. This tutorial discusses the technical challenges of BWA at frequencies between 2 and 11 GHz and describes the basic technologies used in the recently-developed IEEE and ETSI standards. We also discuss some other potential transmission and multiple access techniques for this application. Introduction In the recently deregulated telecommunications market, broadband wireless access (BWA) appears as an attractive technology to new operators without an existing wired infrastructure, because wireless networks are quick and easy to deploy, and this reduces the time to market. The other attractive feature of BWA networks is that they involve a low initial investment that is determined by the initial customer base and the network can be easily expanded to accommodate more users as the customer base grows. In the millimeter-wave frequency spectrum between 20 and 45 GHz, there are a number of frequency bands usable for BWA. Unfortunately, rnillimetenvave radio technology is still not mature enough to make BWA systems at these frequencies viable for residential applications. Therefore, millimeter-wave B WA systems are primarily intended to offer integrated broadband services to business customers in urban or suburban areas with high user density, and their commercial deployment remains very small scale. To address the residential subscriber market and compete with digital subscriber lines (DSL) and cable modems, attention was therefore turned to microwave frequency bands between 2 and 11 GHz, where low-cost radio technologies are available. These bands include the 2.5 GHz microwave multipoint distribution service (MMDS) band in the US, the 3.5 GHz band all across Europe, and the 10 GHz band, which is available in a number of countries in Europe, Latin America, and some other regions. Although it has been a hot topic for over half a decade, fixed BWA is still in its infancy. Equipment cost is obviously one of the reasons, but this slow development can also be attributed to the lack of industry standards. In order to stimulate massive commercial deployments in the near future, the IEEE 802.16 Group in the US and the ETSI BRAN Group in Europe recently elaborated technical specifications for BWA. Each of these groups has defined separate technical specifications for the 10-66 GHz frequency band and for the 2- 1 1 GHz band, the network characteristics and the technical LVIIchallenges being quite different between these bands [ 11, [2]. The overlap between these two bands is intentional so that 11 GHz BWA systems can use either of these specifications. This tutorial addresses the propagation problems in BWA systems at microwave frequency bands between 2 and 11 GHz and discusses the technical solutions. First, in the next section, we give a brief introduction to BWA in these frequency bands and present the channel characteristics. Next, in Section 3, we describe the transmission and multiple access technologies adopted in the recent IEEE 802.16a specifications. Then, in Section 4, we discuss two other potential technologies. Finally, we give some conclusions in Section 5. Conclusions In this tutorial, we have described the basic characteristics of BWA at microwave frequencies below 11 GHz and reviewed the basic technologies that have been adopted in recent industry standards. This type of networks is based on non-LOS transmission and is subjected to multipath propagation leading in some cases to very long channel dispersions. The IEEE 802.16a specifications for this frequency band make use of OFDM, OFDMA, as well as of SC/FDE, but the ETSI specifications are based on OFDM only. We have discussed the equalization of long dispersion channels and showed that SC/TDE fails to give satisfactory performance when the number of coefficients becomes large. We have given a heuristic justification of the technologies included in the IEEE 802.16a specifications and indicated that the real issue is not OFDM vs. single-carrier transmission, but instead frequency-domain vs. time-domain equalization and signal processing. We have also analyzed the application of conventional CDMA and the more recently introduced MC-CDMA, which have interesting properties in some cases. References 111 P802.16dDI-2001, "IEEE Drafi Standard for Local & Metropolitan Area Networks - Part 16: Air Interface for 12) Draft TS 101 999: "Broadband Radio Access Networks (BRAN) HIPbRACCESS Functional Specification," [3] J.G. Proakis, "Digital Communications" (Third Edition), McGraw Hill, New York, 1995. 141 P802.1 laD6.0, "LANMAN Specific Requirements - Part 2: Wireless MAC and PHY Specifications - High Speed Physical Layer in the 5 GHz Band," IEEE 802.1 1, May 1999. [5] DTS/BRAN030003-1, "Broadband Radio Access Networks HIPERLAN Type-2 Functional Specification - Part 1 : Physical Layer," ETSI, Sophia Antipolis, Sept. 1999. [6] H. Sari, G. Karam, and I. Jeanclaude, "Transmission Techniques for Digital Terrestrial TV Broad-casting," IEEE Communications Magazine, vol. 33, pp. 100-109, February 1995. [7] H. Sari and G. Karam, "Orthogonal Frequency-