Inside UWB design: A tutorial

First, each information bit is not necessarily associated with a single transmitted wavelet. A group of bits can be associated with one of a family of orthogonal, direct sequence codes. A train of successive wavelets is then transmitted, each having as its individual bi-polar modulation, the polarity of its associated "chip" in the sequence.

At the receiver, signal processing is used to determine which of the orthogonal, direct sequence codes was most likely to have been transmitted. With this approach, both the detrimental effects of short duration fades and long runs of identical bits are addressed.

Multi-band UWB
For channel bonding ultra-wide band communications, two or more OFDM signals are used to transmit the high speed data stream from one source. Individual bits can be assigned to one of the 802.11a channels in blocks or individually, and concatenated coding techniques can be applied before and after the bit assignments.

All the DSP functions required in an OFDM channel such as interleaving, mapping, and IFFT are required for the transmitter, and the inverse functions for the receiver. Although this approach provides a much higher density of bits-per-hertz and a much reduced spectral bandwidth than direct sequence ultra-wide band, it is much more complicated in terms of RF and analog circuitry, with an associated higher cost.

MBUWB requires designers to go back to traditional RF architectures, advanced synthesizer techniques, and all the difficulties associated with handling OFDM signals over a narrowband radio.

On the other hand MB-UWB minimizes some of the UWB challenges mentioned above such as ADC power consumption, wideband matching (and associated power dissipation), CW interference issues, difficulties in the generation and control of a wideband spectrum, achieving the very precise timing required, and handling a high speed complex digital baseband (although this last point is debatable).

Unlike DS-UWB, MB-UWB benefits from the possibility of using a 1x over sampling ADC and DAC (1GS/s) to reduce power consumption while handling large signal interferers (which could be as high as 30dB). MB-UWB performance depends on the ability of designing a LO capable of being able to hop quickly (less than 10ns).

In summary, from a performance point of view, MB-UWB provides higher data throughput and faster acquisition time, interference avoidance (obtained by attenuating or not using particular sub-bands), and some flexibility through multiple access.

Silicon technology
A single chip CMOS MB-UWB radio is about 40% smaller than a WLAN .11a/g IC, with a significant reduction achieved in the transmit path. The benefit of a full CMOS MB-UWB IC is lower cost than a multi-chip solution combining CMOS and SiGe-BiCMOS technology. It also provides a smaller footprint. On the other hand, a single chip solution has an issue of possible analog yield and performance limitation (due to digital noise appearing in-band and RF filter issues). A MB-UWB based on a multi-chip SiGe/CMOS solution provides an optimum trade-off between technology match and performance with minimum power consumption, but does not answer the need for low cost.

Initially designed using SiGe-BiCMOS technology, UWB radios are transitioning to CMOS to benefit from lower cost in terms of cents/sq mm. UWB radios are highly dependent on the digital portion of the baseband section of the IC and therefore CMOS makes a significant contribution to the overall cost of a solution.

In addition, the carrier free nature of the UWB transmission is based on the use of high-speed pulses, making CMOS high speed switching techniques requirements.

Conclusions
UWB radio specifications give an impression of simplicity through the lack of requirements for frequency translation and filtering. In actuality, radio requirements for UWB are very demanding. This paper has discussed several issues that are unique to ultra-wide band communications links.

The distortions imposed by the RF analog circuits and antennas on the UWB signal have been introduced. Furthermore, the self-interference of the UWB signal as a result of short-range multi-path has been discussed. An overview of strategies and difficulties for the acquisition and synchronization of the UWB signal has been presented. Finally, unique coding techniques specific to Direct Sequence and TD / FDMA UWB signals have been discussed.

For each of these issues, DSB techniques are available to ensure a high throughput at low error rate for the UWB signal. The challenge is to maintain a reasonable computational load at a reasonable speed. UWB radio development involves co-design and detailed trade-offs. Only a multidisciplinary approach will deliver a UWB radio with the optimum cost, performance and power consumption.

Cedric Paillard is the vice president for semiconductor research at TECHInsights and . Jim Wight is a consultant for the firm..

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