1. September, 2003
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Complexity and Performance Analysis of a DS-CDMA UWB System]
Date Submitted: [ 17 September, 2003]
Source: [Jaiganesh Balakrishnan, Anand Dabak, Srinivas Lingam and Anuj Batra] Company [Texas Instruments]
Address [12500 TI Blvd, Dallas, TX 75243, Texas, USA]
Voice:[ ], FAX: [ ],
Re: []
Abstract: [The following contribution provides a complexity and performance analysis of a DS-CDMA UWB System.]
Purpose: [This document presents results on the performance of a DS-CDMA system]
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
Jaiganesh Balakrishnan et al., Texas Instruments

2. Complexity and Performance Analysis of a DS-CDMA UWB System
September, 2003
Complexity and Performance Analysis of a DS-CDMA UWB System
Jaiganesh Balakrishnan, Anand Dabak, Srinivas Lingam and Anuj Batra
Texas Instruments
12500 TI Blvd, MS 8649
Dallas, TX 75243
Jaiganesh Balakrishnan et al., Texas Instruments

3. Multi-path for DS-CDMA
September, 2003
Multi-path for DS-CDMA
Multi-path reflections from different objects in a room
Light travels at ~ 1 ns a foot
DS-CDMA chip period is 731 ps
MBOK symbol period is 17.5 ns
CM3 channel reflections easily up to 40 ns
There is cross-talk between the different reflections. This is what is referred to here as multi-path
Object 1
Object 2
Reflection 1
Reflection 2
Object 3
Reflection 3
MBOK code
Cross talk between
multiple reflections
Jaiganesh Balakrishnan et al., Texas Instruments

4. Multi-path cross talk effect in DS-CDMA
September, 2003
Multi-path cross talk effect in DS-CDMA
Simulations for DS-CDMA should take into account multi-path cross talk.
Simplistic simulations for DS-CDMA can be done by just summing up the energies from the different multi-paths
However, these simulations do not take into account cross-talk between the multi-paths
Will give incorrect and OVER optimistic results
Reflection 1
Reflection 2
Reflection 3 a1 a2 a3
Jaiganesh Balakrishnan et al., Texas Instruments

5. Multi-path Performance
September, 2003
Multi-path Performance
In multi-path environments, the RMS delay spreads for a UWB channel can be large (14 ns for CM3, 25 ns for CM4). The performance of a DS-CDMA system in multi-path channel environments is determined by the following factors:
Multi-path Energy Capture:
Function of the number of RAKE fingers used.
Uncollected multi-path results in lost energy
ICI/ISI:
Degradation due to ICI/ISI depends on the severity of the channel, data rate(spreading gain) and the equalizer structure.
Jaiganesh Balakrishnan et al., Texas Instruments

6. Fading PDF Statistics of OFDM carriers versus DS-CDMA (03/344)
September, 2003
Fading PDF Statistics of OFDM carriers versus DS-CDMA (03/344)
0.1
Large proportion of deep fades that cause bit errors
4 MHz OFDM carrier BW fading
0.08
0.06
PDF - 4 MHz Fading
0.04
0.02
-30
-25
-20
-15
-10 -5 5 10
Received Energy (dB)
1.368 GHz BW DS-CDMA Fading
0.4
PDF GHz Fading
NO deep fades!
0.2
-30
-25
-20
-15
-10 -5 5 10
Received Energy (dB)
Jaiganesh Balakrishnan et al., Texas Instruments

7. OFDM versus DS-CDMA with Rate ½ k=7 Code (03/344)
September, 2003
OFDM versus DS-CDMA with Rate ½ k=7 Code (03/344)
Performance Differential for 4 MHz OFDM vs GHz DS-CDMA 10 1 2 3 4 5 6 7 8 9 10 11 12 -6 -5 -4 -3 -2
OFDM CM1
ODFM CM2 -1
OFDM CM3 10
OFDM CM4
DS CM1
DS CM2
DS CM3
DS CM4
AWGN
BER
~ dB
SNR (dB)
Jaiganesh Balakrishnan et al., Texas Instruments

8. Multi-path Energy Capture (1)
<month year>
doc.: IEEE <doc#>
September, 2003
Multi-path Energy Capture (1)
Assumptions:
Optimal timing information.
Perfect channel estimation.
Largest RAKE fingers over the entire span of the channel impulse response are selected.
No shadowing
Does not reflect degradation due to ICI/ISI.
Loss in captured energy averaged over all 100 channel realizations.
Observations:
Average loss of 2.5 dB with a 16 finger RAKE for CM4 channel environment.
Average loss of 1.3 dB with a 16 finger RAKE for a CM3 channel environment.
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

9. Multi-path Energy Capture (2)
<month year>
doc.: IEEE <doc#>
September, 2003
Multi-path Energy Capture (2)
The performance for the 90th %-ile channel realization is the metric of interest [03/031, 03/276].
03/031: Selection criteria document
03/276: CE SIG requirements
Assumptions:
No Shadowing
Does not reflect degradation due to ICI/ISI.
Observations:
90th %-ile channel realization has a loss of 3.8 dB with a 16 finger RAKE for CM4 channel environment.
90th %-ile channel realization has a loss of ~2 dB with a 16 finger RAKE for a CM3 channel environment.
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

10. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
September, 2003
Impact of cross talk (1)
Even if an infinite finger RAKE is used to collect all the multi-path energy, the DS-CDMA system will have a performance gap from AWGN due to the effect of ICI/ISI.
Assumptions for the DS-CDMA System:
Chip rate of 1368 MHz.
RRC pulse shape with 50% excess bandwidth.
R = 1/2, K = 7 convolutional code.
Information data rate of 114 Mbps  a processing gain of 12.
No back-off at the transmitter due to ripples in the PSD.
CM3 channel environment.
No shadowing.
Perfect channel estimation. (Not realistic)
Optimal finger placement for the RAKE, i.e., the locations with the largest channel taps are picked.
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

11. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
September, 2003
Impact of cross talk (2)
Assumptions for the OFDM system:
MB-OFDM system with 3-bands and a total information tone bandwidth of MHz.
R = 1/2, K = 7 convolutional code.
Maximum uncoded data rate of 640 Mbps with QPSK modulation.
Information data rate of ~110 Mbps  a frequency domain spreading factor of 3.
No overhead due to the OFDM prefix.
CM3 channel environment.
No shadowing.
Perfect channel estimation. (Not realistic)
Observations:
Infinite finger RAKE has an average degradation of 0.7 dB over AWGN due to ICI/ISI.
OFDM system loses 1 dB over AWGN due to loss in frequency diversity.
A 16 finger RAKE has a 2 dB loss over AWGN.
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

12. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
September, 2003
Impact of cross talk (3)
What happens when we look at the 90th %ile results rather than the average performance?
Observations:
The infinite finger RAKE has a ~1.7 dB degradation over AWGN performance.
The OFDM system has the same 90th %ile performance as the infinite finger RAKE.
The 16 finger RAKE has a ~4.5 dB degradation over AWGN and ~2.8 dB degradation over the OFDM system.
MB-OFDM achieves infinite rake performance with implementable complexity
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

13. Range of DS-CDMA System
<month year>
doc.: IEEE <doc#>
September, 2003
Range of DS-CDMA System
Document [03/153] does not provide results for the 90% link success probability distance. Average range results are optimistic and does not provide the complete picture.
Impact due to ICI/ISI for the 90th %ile realization is nearly 1.7 dB for the 114 Mbps data rate.
Need a margin of 3.8 dB to compensate for the impact of log-normal shadowing to obtain the range for the 90th %ile link.
* - Based on the results provided in document [03/153]
RAKE
AWGN*
Loss in captured energy
ICI/ISI + Shadowing
Total degradation
Expected Range
Infinite
21.6 m
0 dB
5.5 dB
11.4 m
16 fingers
2 dB
7.5 dB
9.1 m
5 fingers
4.5 dB
10 dB
6.8 m
MB-OFDM has a 90% link success probability of ~11.5 m
Jaiganesh Balakrishnan et al., Texas Instruments
<author>, <company>

14. Complexity comparison (based on 03/343)
September, 2003
Complexity comparison (based on 03/343)
RAKE receiver for DSSS*
Computational cost of correlators
Each Correlator requires 24 additions/subtractions at the chip-rate (e.g., 1.368GHz)
For RAKE of length 16, need 8 x 16 = 128 correlators for 16-BOK => 175,000 Madds per second !!
Computational cost of MRC (this occurs at symbol rate. E.g., 57MHz)
16 vectors of length 8 need to be multiplied by the corresponding 16 complex conjugate channel tap weights (one complex tap weight per vector).
16 x 8 x 57 = 7296 MOPS (non-trivial complex multiplies)
Overall 7296 M complex multiplies M complex adds/sec!!)
* Does not even include multi-path equalization complexity
FFT for OFDM (MBOA)
FFT part
Conservatively, this requires 8 complex multiplies and 22.4 complex adds per clock cycle at 128MHz
Looking at complex multiplies, we need 8 x 128 = 1024 MOPS
Frequency-domain EQ
100 data carriers multiplied by their respective conjugate channel taps every 312.5ns
Implies 100/ = 320 MOPS
Overall: 1344 M complex multiplies/sec
MBOA OFDM has better performance and is computationally less expensive.
Jaiganesh Balakrishnan et al., Texas Instruments