1. Sept 2005
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Motivation for Multi-band UWB]
Date Submitted: [ 19 Sept, 2005]
Source: [C. Razzell] Company [Philips]
Address [1151 McKay Drive, M/S SJ48A, San Jose, CA USA]
Voice:[ ], FAX: [],
Re: [To be considered in context of TG3a down-selection]
Abstract: [Technical Contribution on pros and cons of Multi-band Approach to UWB]
Purpose: [To inform and persuade]
Notice: This document has been prepared to assist the IEEE P It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P
C. Razzell (Philips)

2. What do we mean by multi-band?
Sept 2005
What do we mean by multi-band?
Multi-band UWB signaling is simply the division in the frequency domain of a single ultra-wideband signal into multiple sub-bands.
C. Razzell (Philips)

3. Parallel vs. Sequential Multiband
Sept 2005
Parallel vs. Sequential Multiband
Parallel Pulsed Multiband
Sequential Pulsed Multiband
C. Razzell (Philips)

4. Sequential Multiband for Channel Isolation or for Modulation Capacity
Sept 2005
Sequential Multiband for Channel Isolation or for Modulation Capacity
Sequences can be fixed and used for piconet isolation or…
Variable and used to convey information (e.g. Spectral Keying™)
Sequential Multiband with Sequence Keying
C. Razzell (Philips)

5. Spectrum Definition by pulse or otherwise
Sept 2005
Spectrum Definition by pulse or otherwise
A further classification of multiband schemes needs to be made, namely between pulsed multiband and OFDM multiband approaches
Pulsed multiband transmissions use a specifically chosen, constant pulse shape to obtain the frequency domain properties for each sub-band.
OFDM can be applied in each sub-band. The occupied bandwidth and spectral shape is largely defined by the inverse Fourier transform applied in the transmitter. This technique can be considered as further frequency division of each sub-band of the multiband scheme into a further parallel multiband scheme providing a much finer degree of granularity in the frequency domain.
C. Razzell (Philips)

6. Overall Classification Scheme for Multiband UWB
Sept 2005
Overall Classification Scheme for Multiband UWB
OFDM
PULSE
OFDM
PULSE
OFDM
OFDM
Information
Information - -
bearing
bearing
(variable) Sequence
(variable) Sequence
PULSE
PULSE
Sequenced
Sequenced
Multiband
Multiband
OFDM
PULSE
OFDM
PULSE
OFDM
OFDM
Piconet
Piconet
isolation
isolation
(fixed) sequence
(fixed) sequence
Multiband
Multiband
PULSE
PULSE
OFDM
PULSE
OFDM
PULSE
OFDM
PULSE
MB-OFDM Scheme is highlighted
Parallel
Parallel
Multiband
Multiband
C. Razzell (Philips)

7. Detailed Motivation for Multi-band Approach to UWB
Sept 2005
Detailed Motivation for Multi-band Approach to UWB
C. Razzell (Philips)

8. Outline of Discussion Points
Sept 2005
Outline of Discussion Points
Tight Control of Spectrum Mask
Receiver Sampling Rate Issues
Active Circuit Bandwidth and Power Consumption
ADC and DAC Sampling Rates
Robustness to Strong Narrow-band Interferers
C. Razzell (Philips)

9. Tight Control of Spectrum Mask
Sept 2005
Tight Control of Spectrum Mask
In-band spectrum flatness directly impacts link budget
Out-of-band attenuation requirements are likely to be tough, especially outside the US
Tight control is best achieved with digital filters for control and repeatability
Direct implementation of digital filters with >GHz sampling rates needed for single-band approach is impractical
Multi-band brings down sampling rates, allowing more digital filter implementation.
C. Razzell (Philips)

10. Example requirements for digital definition of Tx Spectrum
Sept 2005
Example requirements for digital definition of Tx Spectrum
1.5GHz Single-band UWB system
Baseband LP filters need at least 1.5GHz sampling rate in I and Q channels
Additional, non-trivial analog filters needed for removal of aliases
In view of these factors, single-band UWB system designers have sometimes resorted to purely analog filtering for pulse shaping.
C. Razzell (Philips)

11. Example requirements for digital definition of Tx Spectrum
Sept 2005
Example requirements for digital definition of Tx Spectrum
500 MHz per band multiband UWB system
Baseband LP filters need only 500MHz sampling rate in I and Q channels
Excess bandwidth at the edges of the UWB spectrum is reduced by a factor N, where N is the number of sub-bands (since excess BW is given by B.a, where a is the raised cosine roll-off factor and B is the bandwidth of a sub-band).
Multi-band approach also allows the transmit power in each sub-band to be independently managed, for interference avoidance and overall in-band flatness compensation.
C. Razzell (Philips)

12. Receiver Sampling Rate Issues
Sept 2005
Receiver Sampling Rate Issues
Required length of channel matched filter is product of excess delay in channel and the sampling rate
Rate of multiply-accumulates is product of number of required taps and sampling rate
Since Fs is proportional to receiver channel bandwidth, the square law proportionality with Fs (bandwidth) becomes a very significant factor in favor of processing one sub-band at a time per the multi-band approach
C. Razzell (Philips)

13. Active Circuit Bandwidth and Power Consumption
Sept 2005
Active Circuit Bandwidth and Power Consumption
In UWB receivers for sequential multiband modulation schemes, the first down-mixing step results in a bandwidth reduction by a factor equal to the number of sub-bands employed
C. Razzell (Philips)

14. Sept 2005
Filter Feasibility
On-chip channel selectivity at baseband, is much more feasible in the multiband case, where the low-pass corner may be 250MHz (for a total IF bandwidth of 500MHz).
For the single-band case, the equivalent low-pass corner would have to be 750MHz, which means that active on-chip filtering techniques are difficult if not impossible to apply.
Similarly, the current consumption required to obtain to linear operation and high dynamic range up to 250MHz is much less than that required to reach 750MHz.
Similar arguments apply to a direct up-conversion transmitter for similar reasons. Once again, the implementation difficulty is reduced for the analog transmitter blocks until the upconversion using an agile local oscillator spreads the bandwidth to its final value.
C. Razzell (Philips)

15. ADC and DAC Sampling Rates
Sept 2005
ADC and DAC Sampling Rates
The sampling rates required in the mixed signal components will obtain a reduction by a factor equal to the number of sub-bands employed
Useful considering the need for low power converters in portable applications
Higher precision converters may be used to provide dynamic range required for strong narrow-band interference
C. Razzell (Philips)

16. Vulnerability to Strong Interferers
Sept 2005
Vulnerability to Strong Interferers 
Two Strategies:
The receiver alone adapts by inserting erasures for the symbols to be received in the interference impacted sub-band
The transmitter and receiver negotiate to adopt a smaller or different set of sub-bands, avoiding the use the heavily interfered sub-band.
C. Razzell (Philips)

17. Multipath Energy Collection in Sequential Multiband Receivers
Sept 2005
Multipath Energy Collection in Sequential Multiband Receivers
C. Razzell (Philips)

18. Sept 2005
Fundamental Conflict
BPSK and QPSK modulation schemes are preferred for UWB, but limit uncoded data rate to R=2.PRF, where PRF is the pulse repetition frequency.
For sequential multiband good energy collection, using a single receive path, requires PRF  1/tch where tch is the excess delay of the channel
Assume tch=40ns, then R  2/40 Gbps = 50Mbps.
Realizing higher data rates for TG3a means sacrificing energy collection and consequently BER performance.
Require long dwell times on each band coupled with high information density not possible with simple pulses
C. Razzell (Philips)

19. Sept 2005
Enter Multi-band OFDM
MB-OFDM benefits from all the generic multi-band advantages described above, but…
The dwell time on each sub-band is 312.5ns
Energy collection period is up to 60.6ns
Number of uncoded bits per OFDM ‘pulse’ is up to 200, due to 100 data sub-carriers
Max uncoded data rate is 200/312.5 Gbps = 640Mbps.
C. Razzell (Philips)

20. Sept 2005
Conclusions
Multiband techniques have a number of important implementation related advantages that have not been reviewed for a long time
Pulsed multiband disadvantages with respect to energy collection have been overcome by the adoption of OFDM in each sub-band
I hope that this review of the technical reasons to prefer the MB approach in general and MB-OFDM in particular has been helpful.
C. Razzell (Philips)