Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Rake Span Requirements for Multi-band UWB Systems] Date Submitted: [14 May, 2003] Source: [Jaiganesh Balakrishnan et al.] Company [Texas Instruments] Address [12500 TI Blvd, MS 8649, Dallas, TX 75243] Voice:[214-480-3756], FAX: [972-761-6966], E-Mail:[jai@ti.com] Re: [] Abstract: [This document describes the rake span requirements for multi-band UWB systems.] Purpose: [For discussion by IEEE 802.15 TG3a.] Notice: This document has been prepared to assist the IEEE P802.15. 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 P802.15. Jai Balakrishnan et al., Texas Instruments

2. Rake Span Requirements for Multi-band UWB Systems Jaiganesh Balakrishnan, Anuj Batra Anand Dabak, Jerry Lin, Ranjit Gharpurey, and Simon Lee Texas Instruments12500 TI Blvd, MS 8649Dallas, TX May 14, 2003 Jai Balakrishnan et al., Texas Instruments

3. Outline • Overview of multi-path energy capture. • RAKE design parameters. • RAKE design: no group delay variations. • RAKE design: group delay variations. • Buildability issues with multiple RX chains. • Conclusions. Jai Balakrishnan et al., Texas Instruments

4. Multi-path Energy Capture • In multi-path environments, the RMS delay spreads for a UWB channel can be large (14 ns for CM3, 25 ns for CM4). • Uncaptured multi-path energy results in loss in performance of the UWB device. • One method for energy collection is to use a RAKE receiver. Jai Balakrishnan et al., Texas Instruments

5. RAKE Design Parameters • There are two main parameters that need to be considered when designing a RAKE receiver for a multi-band system: • The total number of RAKE fingers (see also 03/210r0) • Span of the RAKE receiver. • The total number of RAKE fingers determines the RX digital complexity. • Span of the RAKE receiver determines the number of analog RX chains needed. • Optimal RAKE finger placement does not need to be contiguous. • Ex: 2 RAKE fingers can be separated by a considerable number of samples. Jai Balakrishnan et al., Texas Instruments

6. Design of a RAKE • Assumption: • System #1: A 7-band multi-band system that transmits a 3.9 ns pulse once every 7.8 ns in each sub-band (132 Msps). • System #2: A 7-band multi-band system that transmits a 3.9 ns pulse once every 3.9 ns in each sub-band (264 Msps). • Impact of ISI is expected to be negligible and hence not considered. • A smaller RAKE span results in loss of collected multi-path energy. • Inherent trade-off: • Number of RX chains vs. multi-path energy collection. Jai Balakrishnan et al., Texas Instruments

7. RAKE Design: No Group Delay (1) • Assumption: • Synchronous hopping across the sub-bands. • No group delay due to front-end filtering. • Optimal timing is typically not feasible with a single receive chain: • Reason: optimal sampling time for RAKE in sub-bands #2 and #3 overlap. Impossible to do with a single receive chain. Jai Balakrishnan et al., Texas Instruments

8. RAKE Design: No Group Delay (2) • To ensure that multi-path energy is collected across all sub-bands, we need to constrain the RAKE fingers to be in the same location regardless of the sub-band. • Location is chosen to maximize the overall received energy. Jai Balakrishnan et al., Texas Instruments

9. RAKE Design: No Group Delay (3) • Assumptions: • Ideal channel estimation. • No front-end group delay variations. • Zero switching time. • Sample timing chosen to maximize collected energy for “symbol” spaced sampling (264 MHz). • Normalized the channel impulse responses to unity to remove effects of shadowing/fading. • Captured energy averaged over 100 channel realizations. Jai Balakrishnan et al., Texas Instruments

10. RAKE Design: No Group Delay (4) • Captured energy versus RAKE span for CM2 channel environment: • A 3 dB performance loss  30% loss in range. • Conclusion: to achieve less than 3 dB performance loss in CM2: • Need 2 receive chains for a 132 Msps systems • Need 3 receive chains for a 264 Msps systems. Jai Balakrishnan et al., Texas Instruments

11. RAKE Design: No Group Delay (5) • Captured energy versus RAKE span for CM3 channel environment: • A 3 dB performance loss  30% loss in range. • Conclusion: to achieve less than 3 dB performance loss in CM3: • Need 3 receive chains for a 132 Msps systems • Need 5 receive chains for a 264 Msps systems. Jai Balakrishnan et al., Texas Instruments

12. RAKE Design: Group Delay (1) • Consider a multi-band system whose operating bandwidth includes the U-NII band. • If a notch filter is used to suppress the interference from the U-NII band, then there could be significant group delay variations on sub-bands on either side of the notch. • Notch filter is one of the components that will result in group delay variations. Other components include antenna, LNA, etc. Jai Balakrishnan et al., Texas Instruments

13. RAKE Design: Group Delay (2) • Assumption: • CM1 channel environment. • Synchronous hopping across the sub-bands. • Due to group delay variations, impulse response for sub-bands 4 and 6 are delayed by 3.9 ns relative to the other sub-bands. • Conclusions: • With one RAKE finger, the loss in performance is nearly 1 dB. • However, as the number of RAKE fingers increases the additional degradation due to group delay variations is small (also true for CM2, CM3, and CM4). Jai Balakrishnan et al., Texas Instruments

14. Buildability Issues with Multiple RX Chains • Hardware penalty for multiple receive chains. • Need to duplicate entire receive chain after LNA, including mixer, VGA, channel select filter, and ADC. • Increased die size, power consumption, and cost. • Analog section does not scale with improvements in technology node. • Potential issues: • LNA loading results in a trade-off between bandwidth, power, and noise figure. • Cross-talk between receiver chains. • Increased design time. Jai Balakrishnan et al., Texas Instruments

15. Conclusions • Studied RAKE span requirements for multi-band UWB systems. • Multi-band UWB systems need multiple RX chains for CM2, CM3 and CM4 environments. • If more than 1 Rx chain is used, the impact due to group delay variations is negligible. • Multiple receive chains results in a hardware penalty and has potential implementation issues. Jai Balakrishnan et al., Texas Instruments