Wireless Personal Communications 29: 121–133, 2004. C  2004 Kluwer Academic Publishers. Printed in the Netherlands. The Role of UWB in 4G IAN OPPERMANN Centre for Wireless Communications, University of Oulu, Oulu, Finland E-mail: ian.oppermann@ee.oulu.fi 1. Introduction The world of Ultra Wideband (UWB) changed dramatically in very recent history. In the past 20 years, UWB has been used for radar, sensing, military communications and niche applications. A substantial change occurred in February 2002 when the FCC [3, 4] issued a ruling that UWB could be used for data communications as well as radar and safety applications. The band allocated to communications is a staggering 7.5 GHz; by far the largest allocation of bandwidth to any commercial terrestrial system. This allocation came shortly after the hotly contested, and very expensive, auctions for third-generation spectrum in 2000 which raised more than $ 100 billion for European governments. The FCC UWB rulings allocated 1500 times the spectrum allocation of a single Universal Mobile Telecommunication System (UMTS) licence, and, worse, the band is free to use. Therefore, it was no wonder that efforts to bring UWB into the mainstream were greeted with great hostility. First, the enormous bandwidth of the system meant UWB could potentially offer data rates of the order of Gbps. Second, the bandwidth sat on top of many existing allocations causing concern from those groups with the primary allocations. When the FCC proposed the UWB rulings, the FCC received almost 1000 submission opposing the proposed UWB rulings [25, 26]. Fortunately, the FCC UWB rulings went ahead. The concession was however that available power levels would be very low. If the entire 7.5 GHz band is optimally utilised, the maximum power available to a transmitter is approximately 0.5 mW. This is a tiny fraction of what is available to users of the 2.45 GHz Industrial, Scientific and Medical (ISM) bands such as the IEEE 802.11a/b/g standards. This effectively relegates UWB to indoor, short-range communications for high data rates (HDR), or very low data rates (LDR) for substantial link distances. Applications such as wireless UWB and personal area networks have been proposed with hundreds of Mbps to several Gbps with distances of 1–4 m. For ranges of 20 m or more, the achievable data rates are very low compared to existing Wireless Local Area Network (WLAN) systems [8, 9]. One of the enormous potentials of UWB however is the ability to move between the very HDR, short link distance and the very LDR, longer link distance applications. The trade-off is facilitated by the physical layer signal structure. The very low transmit power available invariably means multiple, low energy UWB pulses must be combined to carry 1-bit of in- formation. In principle, trading data rate for link distance can be as simple as increasing the number of pulses used to carry 1-bit. The more pulses per bit, the lower the data rate, and the greater the achievable transmission distance. The low power and HDR of UWB systems