DS-CDMA: the modulation technology of choice for UWB communications

Abstract

We show that because of drastic differences in the fading statistics between ultra wide bandwidth (UWB) multicarrier and direct sequence code division multiple access (DS-CDMA) approaches, DS easily scales to Gbps rates while multicarrier architectures have severe difficulty. Both DS-CDMA and orthogonal frequency division multiple access (OFDM) are well understood and proven modulation techniques in conventional (narrowband) commercial technologies (e.g. DS-CDMA in cell phones; OFDM in IEEE 802.11a/g). The maturity of these approaches, however, is vastly different when applied to ultrawideband (UWB) systems. Already implemented and operating in silicon, DS-CDMA architectures have proven to be the most mature and scaleable for UWB on both a theoretical as well as implementation basis. Among the proposed approaches before the IEEE 802.15.3a standards committee, the DS-CDMA transmitted waveform (which is the "thing" being standardized) is uniquely capable of serving the broadest diversity of applications. It can, for example, allow very low-cost low-power transmit-only devices (even at Gbps rates) because it requires no FFT or DAC or DSP. At the same time, receivers can incorporate varying degrees of DSP to provide scaleable power/cost versus performance. We present performance comparisons of DS-CDMA [Document IEEE.15-03/153r10, July 2003] vs. the proposed multiband MB-OFDM architecture [Document IEEE 802.15-03/267r0, July 2003] for outage range in a variety of multipath environments. Moreover, we describe how DS-CDMA UWB architectures can support robust and flexible multiuser capabilities, protect against in-band interference, and provide high-resolution ranging capabilities for safety-of-life applications.