RF Performance Testing

1. RF Performance Testing Jason Trachewsky Mark Kobayashi Tushar Moorti Broadcom Corporation J. Trachewsky, et al Broadcom

2. Topics • Knowing Received Power is Not Sufficient • Test Categories for WPP • Some Test Configurations J. Trachewsky, et al Broadcom

3. Knowing Received Power is Not Sufficient • Knowing the signal power at the receiver is helpful in determining the effect of the environment on a transmission. • However, it is the channel capacity that determines wireless performance. • PHY rate • throughput • latency • Two channels which result in identical received power can have very different capacities. • The cause is frequency-selective fading from multipath. J. Trachewsky, et al Broadcom

4. Frequency-Selective Fading • A channel is considered frequency-selective if the transmitted signal bandwidth exceeds the coherence bandwidth. • The coherence bandwidth of a channel is a statistical measure of the range of frequencies over which a channel can pass all spectral components with approximately equal gain and linear phase. 1 • The coherence bandwidth is often defined as the frequency span over which the value of the frequency correlation function is > 0.9. In this case: • where st is the r.m.s. delay spread of the channel.1 J. Trachewsky, et al Broadcom

5. Frequency-Selective Fading • For an ensemble of 25-nsec. r.m.s. delay spread channels, Bc is approximately 8 MHz. • 802.11b, 802.11g, and 802.11a channels are all generally frequency-selective. J. Trachewsky, et al Broadcom

6. Definitions For Capacity Examples 1. Equation relating power spectral density and received power. 2. Equation relating achievable channel capacity and power and noise spectral densities. is the SNR gap, which for uncoded QAM modulation is 9.8 dB at a BER of 10-7. The gap is reduced by the coding gain of the system. 3. Equation for achievable channel capacity using an N-OFDM system with sufficient guard interval and large-enough N to represent each subcarrier channel as flat. J. Trachewsky, et al Broadcom

7. Extreme Theoretical Capacity Example receiver bandwidth flat channel half-erased channel thermal noise floor identical received power for J. Trachewsky, et al Broadcom

8. Conclusions from the Half-Erased Channel • The received power through each channel is the same. • Capacity C2 is greater than C1. • This holds true even for relatively low input SNRs. J. Trachewsky, et al Broadcom

9. More Half-Erased Channel Results Equal received power case. Break-even point J. Trachewsky, et al Broadcom

10. 10 Mhz clk ethernet Channel Measurement Block Diagram UPS ARBtx0 Filt UPS Scoperx ARBtx1 Filt 10 Mhz clk • Scope provides 10Mhz reference clk for all systems • 3 long interconnect cables connect tx and rx side • Filt module includes LNA ARBtx2 Filt ARBtx3 Filt trigger I / Q gpib DC PWR ARBtxctl ethernet linux controller 10/100 hub 10/100 hub J. Trachewsky, et al Broadcom

11. Measured Channel Capacity Example J. Trachewsky, et al Broadcom

12. Measured Channel Capacity Example OFDM capacity, assuming sufficiently-long guard interval and large-enough N J. Trachewsky, et al Broadcom

13. Time- and Frequency-Selective Fading > 10-dB change in received signal power in some bins over 60 msec. J. Trachewsky, et al Broadcom

14. Topics • Knowing Received Power is Not Sufficient • Test Categories for WPP • Some Test Configurations J. Trachewsky, et al Broadcom

15. Test Categories for WPP • Deciding what parameters are to be considered is the challenge. • How do we transform user perception of performance into a set of repeatably-measurable quantities? • Throughput and Range • Visibility of APs • Delays in association • Host CPU utilization • Ability to roam without loss of connections • Etc. J. Trachewsky, et al Broadcom

16. Test Categories for WPP • Basic PHY/RF Measurements • Transmitter Parameter Measurements • TX EVM or Frame Error Rate (FER) with Golden/Calibrated Receiver • Carrier suppression • Carrier frequency settling • Receiver Parameter Measurements • RX FER vs. input power • Flat channel (controlled through cabled/shielded environment) • Controlled frequency-selective channels (known multipath power-delay profile) J. Trachewsky, et al Broadcom

17. Test Categories for WPP • Antenna measurements • cable/feed losses (S11 and S21) • gain vs. azimuth and elevation angle J. Trachewsky, et al Broadcom

18. Test Categories for WPP • MAC Layer Measurements • rate adjustment behavior • specific parameters? test conditions? • association and roaming behavior • specific parameters? test conditions? • layer-2 throughput with encryption • host CPU cycles consumed? • Layer-4 Measurements • UDP frame loss rate and latency vs. received power • flat or frequency-selective channels? • TCP throughput vs. received power • flat or frequency-selective channels? J. Trachewsky, et al Broadcom

19. Test Categories for WPP • Open-air Measurements • Open-air measurements are always subject to time-varying multipath power-delay profiles. • There is substantial variation at 2.4 and 5.5 GHz over 10s of msec. • We have established that frequency-selectivity due to multipath can result in higher-loss channels having higher capacity than lower-loss channels. • The capacity of the channel can vary rapidly. • This is a more significant problem for systems like 802.11a and 802.11g which include a large number of possible rates to better approach the practical capacity. • (The problem won’t get easier for future WLAN standards.) J. Trachewsky, et al Broadcom

20. Test Categories for WPP • Open-air Measurements • What can we do? • Try to perform as many measurements as possible with cable networks. • Perform open-air measurements in an area in which the distance between transmitter and receiver is small compared with the distance between either transmitter or receiver and any other object. I.e., strong LOS. • Give up and perform a large ensemble of measurements and gather statistics. J. Trachewsky, et al Broadcom

21. RF Transceiver RF Transceiver Power Meter Link Budget Diagram J. Trachewsky, et al Broadcom

22. Topics • Knowing Received Power is Not Sufficient • Test Categories for WPP • Some Test Configurations J. Trachewsky, et al Broadcom

23. GPIB or DIO line GPIB RF cable ethernet System Block Diagram • Main boardtest system • Transmit test system Transmit Test Filter Mux Microwave Spectrum Analyzer boardtest system controller (attenmach) J. Trachewsky, et al Broadcom

24. 5V,12V,GND GPIB GPIB or DIO line RF cable Ethernet Boardtest System controller (attenmach) DUT mux-2 coupler atten coupler mux-3 REF SA mux-6 power meter J. Trachewsky, et al Broadcom

25. DUT ATTEN ATTEN ATTEN ATTEN ATTEN Split / Combine Split / Combine Golden Node 6-dB Loss 6-dB Loss RF “Loop” Block Diagram Does WPP specify RF test fixtures? J. Trachewsky, et al Broadcom

26. s channel 2 ( = 133 nsec.) t -30 -40 -50 -60 -70 -80 path gain (dB) -90 -100 -110 -120 -130 0 50 100 150 200 250 300 350 400 450 500 delay (nsec) Multipath Channel 2 Example fixed multipath channel power-delay profile. J. Trachewsky, et al Broadcom

27. References • 1. Rappaport, T. S. Wireless Communications Principles and Practice. pp. 163-164. J. Trachewsky, et al Broadcom