White Space Networking with Wi-Fi like Connectivity White Space Networking with Wi-Fi like Connectivity Paramvir Bahl, Ranveer Chandra, Thomas Moscibroda, Rohan Murty, Matt Welsh SIGCOMM 2009 2009-07-29 Wonjun Yoon

Contents Introduction Charactering white spaces WhiteFi design Evaluation Conclusion

Introduction White space In its ruling the FCC frequencies allocated to a broadcasting service but not used locally Unused portions of the UHF spectrum 512MHz~698MHz (180MHz) Offers the potential for substantial BW Long transmission range In its ruling the FCC The use of the unlicensed devices is permitted White space wireless devices must not interfere with incumbents(TV broadcast , wireless mics transmission) Includes TV broadcasts and wireless microphone transmission Federal Communications Commission (FCC) is an independent agency of the United States government

Introduction How can we form the network like Wi-Fi on top of the white space? WhiteFi Wi-Fi like system constructed on top of UHF white spaces Adaptively configures itself to operate in the most efficient part of the available white spaces

Characterizing white spaces 1. Spatial variation 2. Spectrum fragmentation 3. Temporal variation

Spatial variation Spectrum availability is different according to the location In terms of a wide scale Rural – small set of TV channel is used Urban- large set of TV channel is used In terms of a small scale Wireless mics are used in any lecture rooms

Spectrum fragmentation ISM bands are a contiguous chunk of spectrum UHF white spaces is fragmented due to the presence of incumbents The amount of fragmentation depends on a density of TV station The needs of variable channel widths The use of variable channel widths has better performance The use of variable channel widths introduces two new challenges It makes channel assignment more challenging It increases the time taken for nodes to discover APs WhiteFi 에서는 fragmentaion 특성때문에 variable channel width를 사용한다. 물론 fixed channel width 사용할수도 있지만 spectrum availability를 최대화하기 위해서 그렇다. Challenge의 이유에 대해서는 뒤에서 더 자세히 설명하도록 하겠다.

Temporal variation Spectrum availability is different according to the time White space devices must avoid interfering with primary users (in particular, wireless mics transmissions) We can’t predict when the wireless mics is used

White spaces design 1. Spectrum Assignment 2. AP discovery 3. Handling disconnection

WhiteFi design Terms Channel UHF channel A range of the UHF spectrum on which a WhiteFi AP or client communicates (F,W) F=center frequency, W=width of the channel (5,10 or 20MHz) UHF channel Each 6MHz wide , primary users’ channel range

Spectrum Assignment(1/4) Considering points AP must pick a channel that is free for all its clients AP has to decide on the best possible channel width The AP and client maintains A spectrum map Bit-vector {u0,…,uk} where, each ui=weather UHF channel is in use by an incumbent user Airtime utilization vector {A0,…..,Ak} Estimate of the airtime utilization on each UHF channels Spectrum map and airtime utilization vector is measured by scanner at each node using SIFT technique Ai= 채널 i가 사용되고 있는 정도

Spectrum Assignment(2/4) New channel probing Clients Periodically transmit this information(spectrum map, airtime utilization vector) to the AP AP Step1 Determine the set of UHF channels available at all of the nodes OR operation of the clients’ and AP’s spectrum maps Step2 Consider each possible channel (F,W) in the available white spaces by using the matric MCHam(F,W) Step2 다른 AP 의 방해를 가장 적게 받으면서 throughput 높은 채널

Spectrum Assignment(3/4) Probability that a node will be able to transmit on the channel c Multichannel airtime metric the metric predicts a throughput on the channel I will omit the explanation of the detail expression I just explain the main concepts. Pn(c)= 노드 n에서 채널 c를 이용할수 있는 정도 (로) MCHAMn(F,W)=노드 n에서 채널 c를 이용했을때의 예상 throughput의 정도 MCHAM(F,W)가 높을 수록 ( F,W)의 채널 throughput 높을 것이다. 가능한 채널 조합에 대한 각각의 채널에서의 pn(c) 구해서 곱한 것

Spectrum Assignment(4/4) Channel selection Each node send the MCHam(F,W) to AP AP evaluates MCHam(F,W) for each possible channel (F,W) in the available white spaces AP select the channels that maximizes : AP broadcasts the new channel to its clients

AP discovery(1/2) Wi-Fi WhiteFi By scanning each channel and listening for beacons WhiteFi There are too many channel combinations due to the variable channel widths Nodes have to reduce the number of scanning channel (F,W) by using SIFT

AP discovery(2/2) SIFT(Signal Interpretation before Fourier Transform) 1.Discover the channel widths and presence of the beacon by sampling just raw signal without decoding the packet 2. After that, the node decode beacon  It makes reduce AP discovery time!! I will omit the detail algorithm of the SIFT 패킷의 길이를 통해 ACK 인지 DATA packet 인지 판단가능 SIFT 의 핵심은 decoding 하지 않고 raw signal 가지고 beacon 있는지 찾아냄

Handling disconnections When AP or the client detect the primary user, channel must be vacated AP maintains a separate 5MHz backup channel One of nodes detects a primary user on the main channel The node switches to the backup channel Transmits a series of chirps Chirps are the packets that contain information on the white spaces available at that node Primary user가 나타나면 채널 옮겨 가야하는데 어디로 옮겨갈까 하는 문제입니다.

Evalution

Accuracy of SIFT fraction of the number of packets detected by SIFT  SIFT detects nearly all the packets for every channel width

Time to Discover APs Time to discover AP is reduced when it uses SIFT technique

Spectrum assignment WhiteFi achieves close to optimal performance for varying degree of background traffic Background (x axis) is the number of AP/client pairs 하나의 클라이언트에 대해 측정한 결과이다. Background 노드수가 늘어나도 방해받지 않는 채널 선택한다면 5Mhz 크기 작으므로 계속적으로 throuphput 같을 수 있다. 30개 노드 넘어가면 throughput 떨어질것이다.

Conclusions White space has special characteristics Spatial variation Spectrum fragmentation Temporal variation WhiteFi solve the special problems such as Frequency allocation , AP discovery, handling disconnection in white space

APPENDIX

Appendix -SIFT SIFT(Signal Interpretation before Fourier Transform) 1. Detects the presence of an AP 2. Determine its channel width mechanism 1. sample raw signal of 8 MHz band unit 2. beacon duration and the SIFS interval are measured => inversely proportional to the channel width 3. Determine channel width (F E, W) where E= W/2 (AP send beacon=>SIFS interval=> CTS-to-self) (Client detect that pattern) 4. decode the signal with changing center frequency until finding the exact center frequency 5. the node can discover AP by decoding beacon through the exact channel 패킷의 길이를 통해 ACK 인지 DATA packet 인지 판단가능

Hardware platform-KNOWS To support WhiteFi, the KNOWS hardware prototype was developed UHF translator Converts 2.4GHz signal to the 512-698MHz, vice versa Scanner Samples the UHF spectrum to detect the presence of primary users PC Transmits and receives packets over the UHF bands Has 2.4GHz Wi-Fi card Controls the scanner 한마디로 기본적으로는 wifi 프로토콜 이용하는데 witespace에서 특성맞게 네트워킹 할수 있도록 기능 첨가한 하드웨어 이다.