Broadband Wireless Techniques

T HE rapid proliferation of the Internet and the strong growth of mobile telephony promise a key role for “broadband” and “wireless” in future telecommunications. For over a decade, the efforts to combine the multimedia capabilities of “broadband” and the mobility permitted by “wireless” have stimulated worldwide research and standards activities. These efforts have led to three emerging system approaches: the so-called “third generation” cellular systems, currently undergoing a global standardization known as International Mobile Telecommunications, in the year 2000 (IMT-2000); wireless local area networks (WLAN’s), of which IEEE 802.11 and ETSI HIPERLAN are examples; and broadband fixed point-to-multipoint wireless access systems, of which IEEE 802.16 and ETSI HIPERACCESS are examples. The three system approaches are complementary: the wide-area cellular systems enable greater mobility through handoff and roaming, whereas WLAN’s offer higher data rates in local areas as a result of coverage area restriction and reduced multipath delay spread, and broadband wireless access systems offer high data rates to customers in wider urban and suburbanareas. Broadband is essentially about data rate. Yet the physical limitations and impairments on wireless channels—bandwidth constraints, multipath fading, noise, and interference—present a fundamental technical challenge to reliable high data rate communication. Evolving wireless systems must rely on advances in signal processing, coding, antenna, device, and access technologies to increase the link capacity while addressing the power, size, and complexity constraints of the portable units. The current bit rate targets for IMT-2000 are 384 Kbit/s for wide area coverage and 2 Mbit/s for local area (microcellular) coverage. For WLAN’s, the IEEE 802.11 has specified high-speed extension physical layers with bit rates ranging up to 11 Mbit/s at 2.4 GHz (IEEE 802.11B) and 24 Mbit/s or higher, similar to HIPERLAN, at 5 GHz (IEEE 802.11A). The IEEE 802.16 committee was very recently formed to set standards for broadband fixed point-to-multipoint systems typically operating in bands between 10 GHz and 66 GHz and offering bit rates of several Mbit/s to customers (in competition with other broadband access media such as copper and coax lines and satellites). In terms of the air interface techniques, the two predominant techniques for IMT-2000 are wideband code division multiple access (CDMA) and time division multiple access (TDMA) based on enhanced data services for GSM evolution (EDGE). Techniques proposed for WLAN’s include orthogonal frequency division multiplexing (OFDM) for IEEE 802.11A and HIPERLAN and complementary code keying (CCK) for IEEE 802.11B. See [1]–[6] for details on the proposed standards.