# frequency experienced by mobile is not f but distorted version of f: call it f

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1 Impact of mobility: Doppler frequency shift and fading First, Doppler frequency shift Set-up: mobile (e.g., car, train, pedestrian) travels in straight line at speed v mph sender transmits data on carrier frequency f Hz angle between mobile and sender is θ f sender φ mobile receiver v frequency experienced by mobile is not f but distorted version of f: call it f

2 Distorted frequency under Doppler effect: ( v ) f = f + f cos φ SOL Hence: φ = 0 deg: head-on collision frequency experienced: fastest φ = 180 deg: going on oppostive direction frequency experienced: slowest φ = 90 deg: at right angle no distortion Ex.: carrier frequency f 1.8 GHz 4 mph: 10 Hz, 40 mph: 100 Hz Depending upon PHY layer encoding: bit flips may occur

3 Second, fading fading: means that signal is changing, often weakening consider city environment with many buildings and no direct line of sight between sender and receiver Assumption: received signal is comprised of bounced off copies (i.e., echos) from buildings and other reflective obstructions called multi-path propagation

4 Clarke s fading model: if there are many echos and the echos are independent of each other then the average signal strength of the echos has a Gaussian (i.e., normal) distribution central limit theorem called Rayleigh fading

5 Thus: mobile s signal strength fluctuates erratically fading plus Doppler shift may lead to bit flips knowing the statistical properties of signal fluctuation helps with applying error correction FEC is common in wireless networks (e.g., WiFi) What about the case when the mobile (in a city environment) becomes stationary? e.g., sitting at a coffee shop and checking on pre-4g phone?

6 Outdoors: signal strength rapidly decreases with distance recall: signal diminishes as 1/d 2 plus signal fluctuation: Rayleigh fading Indoor environment: more complex distance need not be dominant factor a new factor: spatial diversity

7 Consider an empty room: large lecture room no obstructions e.g., WLAN hot spot AP sends out signal at 2.4 (802.11b/g) or 5 GHz (802.11a/n) frequency how does indoor signal reception look like?

8 Signal strength reception at table height 0.7 m: carrier frequency: 5.32 GHz channel 8 in U.S. (12 channels in 5 GHz a/n) AP->stations (5.32GHz) distance does not determine signal strength signal irregularity: called spatial diversity

9 Impact on throughput: SNR and throughput along straight line SNR [db] x-coordinate y=14m SNR: significant variation Rate[Mbps] x-coordinate y=14m good locations, bad locations can lead to unfairness and even starvation

10 Indoor office WLAN hot spot (HAAS G50):

11 Throughput share of 16 HP/Compaq pocket PCs: uplink CSMA competition Throughput (Mb/s) node 1 node 2 node 3 node 4 node 5 node 6 node 7 node 8 node 9 node 10 node 11 node 12 node 13 node 14 node 15 node Offered Load (Mb/s) significant unfairness: why?

12 Spatial diversity: Location not distance from AP is determining factor switching around the handhelds doesn t change throughput Persistent unfairness bad location remains bad location, good location remains good location What causes uneven some say chaotic signal strength distribution in indoor environments?

13 Unique feature of wireless networks: wave interference multi-path reflections (aka echos) interact constructive vs. destructive interference Ex.: constructive two sine waves both of frequency f meet constructively Ex.: destructive sin f +sinf = 2 sin f two sine waves of frequency f and f +π meet destructively sin f + sin(f + π) =0 Note: second sine wave is phase-shifted by 180 degrees multi-path reflection causes phase shifting longer distance to travel

14 Consequence indoors: good and bad reception spots cause by wave interference Significant real-world consequences: 1. Unfair throughput in WiFi hot spots depends on location: good or bad spot closeness to AP not determining factor (unless very close) unfairness in networks is a big deal a key goal of network protocols: fairness e.g., CSMA is fair if signal strengths are uniform another source of throughput unfairness in networks we have seen?

15 2. WiFi NICs are half-duplex cannot send and receive at the same time recent proposal for full-duplex: echo cancellation using 3 antennas two send antennas: how? 3. Indoor triangulation using signal strength is difficult companies: indoor employee location tracking software NIC reports RSSI (received signal strength indication) full range: RSSI Max capped by different chipset vendors (e.g., 127 Atheros) key problem: spatial diversity

16 Solutions: what can be done? First, for sliding window whose throughput decreases with RTT no good solution e.g., increasing window size too much has negative side effects Second, spatial diversity in indoor wireless networks moving around: even a little (inches) can help does increasing power help?

17 Changing carrier frequency (i.e., channels) can help: good/bad location depends on frequency Carrier frequency GHz (channel 12) qualitatively similar to channel 8 but quantitatively different

18 Consequence: good spot under channel 8 may become bad spot under channel 12, and vice versa different from channel switching to reduce overcrowding i.e., frequency reuse Ex.: old cordless phones with manual channel switch button switch to channel that no one else is using avoid multi-user interference different from spatial diversity

19 Combined with mobility mobile throughput at walking speed (HAAS corridor) 0.9 MH Throughput (Mb/s) Time (s) walking back-and-forth from AP can observe gradual distance dependence but significant short-term fluctuation not due to Doppler shift but spatial diversity i.e., moving in and out of good/bad spots

20 Long Distance Wireless Communication Principally satellite communication: Uplink/Downlink Footprint LOS (line of sight) communication for good reception Effective for broadcast/multicast services e.g., cable TV, satellite radio, atomic clock

21 Not effective for unibroadcast services: video-on-demand (VoD) and Internet service limited bandwidth what about GPS? Satellites can also be used as routers/switches satellite phones VSAT (very small aperture terminal): e.g., remote gas stations low bandwidth, specialized applications

22 Shorter Distance Wireless Communication medium: wireless MAN (IEEE ) short: wireless LAN (IEEE ) very short: wireless PAN (IEEE ) home area networks very very shot: near field communication: ISO e.g., RFID others: wireless USB, WRAN (wireless regional area network) for white spaces IEEE , 60 GHz (mainly indoor), etc. MAC protocols: OFDMA, FDMA, TDMA, CDMA, CSMA MIMO (multiple input multiple output): use multiple antennas e.g., n: unclear how useful given extra cost

23 Cellular telephony: hybrid FDMA/TDMA F3 F3 Mobile Mobile Mobile F1 F1 F2 F2 Mobile Base Station Mobile Mobile Stationary FDD & TDMA Ex.: U.S. IS-136 with 25 MHz frequency band uplink: MHz downlink: MHz 125 channels 200 khz wide each (= ) higher spectral efficiency with OFDMA

24 8 time slots within each channel (i.e., carrier frequency) TDM component total of 1000 possible user channels (124 8 realized) codec/vocoder (i.e., compression): 13.4 kbps compare with landline toll quality (64 kbps) standard

25 Cellular telephony: CDMA C3 C3 Mobile Mobile Mobile C1 C1 C2 C2 Mobile Base Station Mobile Mobile Stationary FDD & CDMA different code vector per user Ex.: IS-95 CDMA with 25 MHz frequency band uplink: MHz; downlink: MHz no separate carrier frequencies everyone shares same 25 MHz band codec: 9.6 kb/s

26 Recall: in CDMA each user gets a code vector code vectors between users: orthogonal called pseudonoise (PN) sequence or chipping code Moreover: a single data bit is encoded using r>1code bits apply FEC r is called code rate common for wireless: why?

27 Packet radio: ALOHA F1 F1 Stationary Stationary F1 F1 F1 F1 Base Station Stationary Stationary Stationary ALOHA downlink broadcast channel F 1 shared uplink channel F 1 Ex.: ALOHANET data network over radio frequency Univ. of Hawaii, 1970; 4 islands, 7 campuses

28 Norm Abramson precursor to Ethernet (Bob Metcalfe) pioneering Internet technology parallel to wired packet switching technology FM carrier frequency uplink: MHz; downlink: MHz bit rate: 9.6 kb/s contention-based multiple access: MA plain and simple needs explicit ACK frames (stop-and-wait)

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### Exercise 2 Common Fundamentals: Multiple Access

Exercise 2 Common Fundamentals: Multiple Access Problem 1: TDMA, guard time. To set up a GSM-connection, the base station (BTS) and the mobile station (MS) use the following short access burst in a TDMA-slot