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Modulation Rate Adaptation in Urban and Vehicular Environments: Experimental Evaluation

This study explores rate adaptation techniques in different settings through loss- and SNR-based protocols. Findings show the impact of channel coherence time and evaluate collision/fading differentiation methods, SNR-triggered schemes, and the use of WARP for implementation.

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Modulation Rate Adaptation in Urban and Vehicular Environments: Experimental Evaluation

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  1. Modulation Rate Adaptation in Urban and Vehicular Environments: Cross Layer Implementation and Experimental Evaluation ACM MobiCom 2008 Joseph Camp and Ed Knightly All figures taken from the paper.

  2. Premise of the Paper • Rate adaptation techniques are used in to-days networks. • How good are these rate adaptation techniques in various settings. • Indoor and Outdoor. • They experimentally evaluate two types of rate adaptation strategies • The first is loss based (reaction to packet loss) -- used in 802.1 networks. • The second is SNR based (not implemented previously) -- rate changed based on perceived SNR

  3. Explicit findings • Depending on the scenario, the different protocols behave differently. • The performance is dependent on the coherence time of the channel • Coherence time is the time for which, the channel quality remains unchanged (fade duration remains unchanged). • Some interesting observations that we will discuss.

  4. Loss Triggered Rate Adaptation • With loss triggered adaptation,, the transmitter interprets the channel state based on time outs • Time-outs suggest failed delivery • Current .11 networks use some form of rate adaptation -- AMRR, SampleRate, Onoe. • The authors consider two loss triggered rate adaptation protocols.

  5. Considered Loss Trigged Adaptation Protocols • Consecutive Packet Decision Loss-Triggered Rate Adaptation (Protocol 1) • Increase the modulation rate after a number of consecutive successful transmission (10) and decrease after a number of failures (2). • Numbers chosen based on previous studies • Two way handshake (no RTS/CTS) • Historical-Decision Loss-triggered Rate Adaptation (Protocol 2) • Window of packets to select modulation rate. • Threshold for increase and decrease based on prior work (specifics in related work) • Two way handshake (no RTS/CTS)

  6. Collision/Fading Differentiation • Traditional rate adaptation schemes assume that losses are due to fading -- but they could be due to interference. • A previous effort (Wong, Lu, Yang and Bharghavan, MobiCom 06) propose a way to distinguish between losses due to fading/interference. • Use RTS/CTS upon experiencing loss. • If RTS/CTS exchange is successful, loss most likely due to channel effects. • Authors implement a similar scheme in conjunction with loss triggered rate adaptation.

  7. SNR triggered rate adaptation • With SNR triggered rate adaptation, the receiver measures the signal-to-noise ratio and informs the transmitter via the four way handshake. • In CTS message. • These protocols have not been implemented previously -- not available in the commodity 802.11 hardware. • Authors use the WARP FPGA radios (implemented by Rice University and sold by Mango Networks) to implement SNR triggered rate adaptation.

  8. SNR triggered schemes considered • SNR triggered Rate Adaptation (Protocol 3) • Signal quality feedback using CTS message. • Four way handshake used. • Equal Air-time Assurance (Protocol 4) • In traditional SNR based schemes, if nodes transmit at higher rates -- they occupy channel for less time. • Since 802.11 offers equal transmission opportunity to all nodes, high rate links will have to share channel capacity with low rate links. • Thus, even with high rate, you may get low throughput • To overcome this, they give equal air-time to all rates • If a node is transmitting at a high rate, it gets to transmit multiple consecutive packets with a single RTS/CTS Exchange • As before, four way handshake is used.

  9. WARP • Three main components • Xilinx Virtex-II FPGA • MAC protocols in C • PHY within the FPGA fabric • MIMO capable radios • up to four antennas • OFDM capable • BPSK, QPSK and 16 QAM are supported. • Ethernet port to report performance of the protocols.

  10. What else has been implemented ? • Carrier Sensing • Binary Exponential back-off • NAV -- network allocation vector to facilitate virtual carrier sensing. • Time-outs • Four way handshake - RTS/CTS DATA ACK

  11. In Lab Evaluations • Controlled setting -- use of a channel emulator (Spirent Communications) and a signal generator (Agilent ESG-D series) • Channel conditions specified in terms of : • Coherence time • Delay spread -- time between incidence of first multi-path ray to that of the last ray. • Interference • PHY layer capture -- ability to decode signal in presence of noise/interference. Ideal rate: Modulation rate with which the highest throughput is achieved (exhaustive search)

  12. Impact of Coherence time • Vary coherence time on a single Rayleigh fading channel of high average quality (avg SNR = -40 dBm) • For long coherence times, all protocols converge to same throughput -- they can track the channel when there is slow fading. • Protocol 3 suffers -- RTS/CTS overhead per high rate transmission. • Historical trigger based is best at small coherence times. • Other protocols are poor -- for different reasons!

  13. Performance with small coherence times • 100  sec coherence time. • SNR protocols overselect • Measure SNR only during RTS -- this may decrease during packet transmission. • In essence, these assume that SNR value is valid throughout packet -- not the case! • Protocol 1 (consecutive loss triggered) underselects. • Consecutive losses common. • Chooses rates that are lower than that possible.

  14. Coherence time training for SNR based adaptation • Offline measurements of performance of different modulation schemes with varying coherence time. • Depending on SNR, choose the right rate for either long or short coherence times. • Left figure with coherence time 80 milliseconds (long) and right figure 0.8 ms (short). • When coherence time is long, with increase SNR use high rates. • When coherence time is short, no benefit from using 16 QAM -- highest rate considered.

  15. The training helped! • Allows the choice of the right rates • Performance of SNR based protocols improves.

  16. Impact of multipath fading • SNR protocols are more sensitive to coherence time in the presence of multi-path fading. • Training becomes more critical.

  17. Impact of external interference • Slow fading channel and packet-sized noise (2 milliseconds). • The idle period between noise instants is varied. • With short idle periods, consecutive packet-decision protocol (Protocol 1) increases underselecting rate. • Historical packet decision less susceptible. • SNR protocols have lower overall throughput -- but choose the right rate based on the measured SNR (due to interference).

  18. Evaluating Heterogeneous links • A case with hidden terminals is considered. • The goal is to see how the different rate adaptation protocols work in the different settings. • First, create links A--> B and C--> B equal and of good quality (-45 dBm) • Then keep the quality of one of the links fixed and then vary that of the other in steps of 5 dB. • Key observation : Due to PHY layer capture, there is a mismatch in achieved throughputs. • The previous protocol that differentiates between collisions and fading (one by Wong, Lu etc.) increases the mismatch in throughputs -- why ? • Increases PHY layer capture • Increases overall throughput though!

  19. Measurement results on heterogeneous links • Notice that mismatch increases as the difference in quality of links increases. • Weaker transmitter has increased losses due to lack of RTS protection -- begins to lower rate and this leads to underselection.

  20. Outdoor experiments • Both residential and downtown Houston • Residential urban measurements: densely populated residential neighborhood with dense foliage. • Downtown measurements in streets of Houston -- buildings of tens of stories high on each side.

  21. Impact of Environment on Static nodes • First, they characterize the environment -- send UDP traffic of various packet sizes, record SNR variance to determine the coherence time. • Vehicles pass at approx 30 mph. • Coherence time -- 100 milliseconds to 80 milliseconds on average, in residential and downtown areas. • However, passing cars can drive the coherence time to as low as: • 15 milliseconds in the residential area • 300 s in the downtown area • Why ? Moving vehicles can cause perturbations in signal quality for short periods of time.

  22. Performance of different protocols: Residential Areas • 60 second tests • Coherence time is long -- multiple packets in duration • Consecutive decision underselects -- presence of mobile scatterers prevents the required 10 consecutive packet successes to raise rate. • Historical decision mechanism overselects -- parameters for window appropriate for indoor -- short for outdoor. • SNR mechanisms work well -- coherence times are long enough.

  23. Performance of different protocols: Downtown • Avg coherence time is 80 ms but as low as 300 s as cars pass. • When coherence time is short -- underselection by loss triggered protocols (due to losses) and overselection by SNR triggered protocols (assume that channel state is stable for the entire packet). • Lower number of received packets compared to residential scenario -- artifact of the channel changing much more quickly.

  24. Impact of Mobility • Goal is to evaluate the rate adaptation accuracy within the two settings • Increased fading and more dynamic channel changes with mobility. • Speeds of 20 Kph • The authors track the per packet variance in SNR to measure channel fading.

  25. Mobile experiments in residential areas • Mobile node approaches a static node and passes by. • Loss triggered protocols cannot track mobile environments • SNR protocols better adapt.

  26. Interference + Mobility • Rate decisions are affected by interference with loss triggered protocols causing them to underselect. • Rate decisions of SNR based protocols remain ok -- but lower throughput due to interference -- better protection from interference due to four way handshake.

  27. Heterogeneous links • Similar to that seen with in-lab experiments. • Collision differentiation with loss triggered protocols can increase throughput imbalance. • With SNR triggered protocols, good quality link achieves higher throughput -- but without equal air time, the SNR based protocol sustains equal throughputs for the longest period. • For results -- see paper.

  28. To summarize • Performance of rate adaptation protocols is sensitive to environment and in particular coherence time. • Depending on coherence time, different protocols behave differently -- can either use higher rates than what can be supported (overselect) or use lower rates than what can be supported (underselect). • Indoor calibrations may not be suitable for outdoor settings.

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