Wireless Communication Systems @CS.NCTU Lecture 10: Rate Adaptation - PowerPoint PPT Presentation

Wireless Communication Systems @CS.NCTU Lecture 10: Rate Adaptation Frequency-Aware Rate Adaptation (MobiCom’09) Lecturer: Kate Ching-Ju Lin ( 林靖茹 )

Motivation • The bandwidth supported in 802.11 is getting wider ⎻ 20MHz in 802.11a/b/g ⎻ 40MHz in 802.11n ⎻ 80-160MHz in 802.11ac • 802.11 adopts OFDM, which partitions the wideband channel to subcarrier • Frequency-selective fading ⎻ Different subcarriers experience independent fading due to the multipath effect ⎻ Different frequencies exhibit very different SNRs ⎻ But the transmitter can assign one rate to the entire band 2

Frequency Diversity 30 25 20 SNR (dB) 15 10 5 0 -40 -20 0 20 40 Freq (Mhz) • The SNRs of different frequencies can differ by as much as 20dB • Different receivers prefer different frequencies 3

Key Features of FARA 30 54Mb/s 25 20 SNR (dB) 15 10 5 6Mb/s 0 -40 -20 0 20 40 Freq (Mhz) • Allow a receiver to measure the SNR of each sub-channel • Instead of assigning the same rate to the entire band, allows each sub-channel to pick the optimal rate matching its SNR 4

SNR-based Adaptation Minimum Required SNR Modulation Coding < 3.5 dB Suppress subband 3.5 dB BPSK 1/2 5.0 dB BPSK 3/4 5.5 dB 4-QAM 1/2 8.5 dB 4-QAM 3/4 12.0 dB 16-QAM 1/2 15.5 dB 16-QAM 3/4 20.0 dB 64-QAM 2/3 21.0 dB 64-QAM 3/4 • Maintain a SNR-to-rate lookup table • The sender transmits few symbols at the lowest bit-rate for all sub-channels • The receiver selects the highest rate for each sub- channel corresponding to the SNR of that sub-channel ⎻ Discard the sub-channels if SNR is too low to support the lowest rate 5

Rx-based Adaptation • The receiver is in charge of ⎻ Measuring the channel ⎻ Selecting the rate ⎻ Responding to the AP • To decrease the feedback overhead, embed the rate information in ACK • Perform some optimization to reduce the size of the embedded information 6

FARA in Frequency-Aware MAC • Further combine FARA with the frequency-aware MAC protocol to leverage frequency diversity • Instead of communicating with one receiver at a time, serve N (2-5) receivers concurrently ⎻ Randomly select N receivers with queued packets ⎻ Assign each sub-channel to a proper receiver ⎻ All the N receivers occupy the entire band 30 25 20 SNR (dB) 15 assign to the blue receiver 10 assign to the green receiver 5 0 -40 -20 0 20 40 Freq (Mhz) 7

Performance • Compare with SampleRate in 20MHz and 100MHz channel 35 35 200 SampleRate SampleRate SampleRate 180 FARA FARA FARA 30 30 160 25 25 140 ate (Mbps) Rate (Mbps) Rate (Mbps) 120 20 20 100 Rate 15 15 8 0 80 60 10 10 40 5 20 0 0 A1 A2 A3 A4 A5 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 A1 A2 A3 A4 A5 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 Location Location location location 100MHz 20MHz Throughput gain is especially large as the band is wider 8

Wireless Communication Systems @CS.NCTU Lecture 10: Rate Adaptation Predictable 802.11 Packet Delivery from Wireless Channel Measurements (SIGCOMM’10) Kate Ching-Ju Lin ( 林靖茹 )

Motivation 30 25 20 SNR (dB) 15 10 5 0 -40 -20 0 20 40 Freq (Mhz) • Again, different frequencies experience different channel condition à frequency-selective • Why not FARA? ⎻ Need hardware modification 10

Traditional SNR-based Adaptation • SNR-based rate adaptation is usually inaccurate because we ⎻ Assume frequency-flat fading ⎻ Select the bit-rate based on “average SNR” across subcarriers • However, this will over-estimate the channel quality because ⎻ A packet will fail to pass the CRC check even if only a few bits are in error due to frequency-selective fading

Traditional model: Packet SNR • Traditional theory well maps the channel condition (SNR) to the corresponding bit-error rate (BER) ✓ d min ◆ ⎻ e.g., in BPSK √ BER = Q = Q ( 2 SNR ) √ 2 N 0 ⎻ But, this only work for a narrow band channel • The average SNR over all sub-carriers is not a good representation of a wideband channel ⎻ Why? The channel condition is not a linear function ⎻ The losses in a few subcarriers would lead to packet errors 12

Traditional model: Packet SNR PRR 83%, SNR 30.2dB 45 PRR 78%, SNR 27.1dB PRR 74%, SNR 18.2dB SNR (dB) 35 PRR 80%, SNR 16.5dB Packet SNR 25 Errors 15 5 -28 -14 0 14 28 Subcarrier index • Packet SNR: Average power of a link / Noise power • Due to frequency-selective fading, a link could have a higher packet SNR, but also have a high bit-error rate 13

Effective SNR (ESNR) • Can we find a metric that can be used to ⎻ Represent a wideband channel ⎻ Estimate the BER of the whole packet à Effective SNR (ESNR) • Average SNR vs. Effective SNR ⎻ Total power of a link vs. Useful power of a link PRR 83%, SNR 30.2dB 45 PRR 78%, SNR 27.1dB PRR 74%, SNR 18.2dB SNR (dB) 35 PRR 80%, SNR 16.5dB Packet SNR 25 Effective SNR 15 5 14 -28 -14 0 14 28 Subcarrier index

Effective SNR (ESNR) • Benefits ⎻ Can accurately estimate the packet delivery rate of packets ⎻ Pick a single bit-rate that maximizes the packet delivery rate or the effective throughput in a wideband channel • How to calculate? ⎻ Reuse the theoretical channel model derived in the textbook ⎻ Find the expected BER of a link ⎻ Then, convert it back to the effective SNR narrow-band SNR narrow-band BER effective SNR packet BER 15

Effective BER and Effective SNR • First calculate the average BER of a selected modulation k across all subcarriers i BER e ff ,k = 1 X BER k (SNR i ) N • Convert it back to the effective SNR ESNR k = BER − 1 k (BER e ff ,k ) Bits/Symbol ( k ) Modulation BER k ( ρ ) � √ 2 ρ � Q BPSK 1 � √ ρ � Q QPSK 2 ⇣p ⌘ 3 4 Q ρ / 5 QAM-16 4 ⇣p ⌘ 7 12 Q ρ / 21 QAM-64 6 BER k -1 (): the inverse function BER k () 16

ESNR-based Rate Adaptation • ESNR can be thought of the equivalent SNR of a wideband flat-fading channel • Hence, now we are able to use ESNR to find the optimal rate by looking up the SNR-to-rate mapping table 17