HY539: Mobile Computing and Wireless Networks

1. HY539: Mobile Computing and Wireless Networks Lecture 4: Radio Transmission

2. Roadmap • Physical layer overview • Problems in wireless transmissions • Access methods • 802.11 physical layer (Chapter 10)

3. Shannon’s limit • For a channel without shadowing, fading, or ISI, the maximum possible data rate on a given channel of bandwidth B is R=Blog2(1+SNR) bps, where SNR is the received signal to noise ratio

4. Adds redundancy to protect the digital information from noise and interference Bits mapped to signal (analog signal waveform) e.g., GFSK e.g., TDMA, CDMA

5. Multipath Propagation Wall Scattering Receiver Transmitter Cabinet Diffraction (Shadow Fading) Reflection Wall

6. Mobile radio channel • A single direct path between the base station and the mobile is seldom the only physical means for propagation Hence, the free space propagation model is inaccurate when used alone • Two-ray ground reflection model considers both the direct path and a ground reflected propagation path between transmitter and receiver • Reasonably accurate for predicting the large-scale signal strength over distances of several km for mobile radio systems that use tall tower (heights which exceed 40m) or for line-of-sight micro-cell channels in urban environment

7. Node 3 Node 1 Node 2 Hidden node problem From the perspective of node 1, node 3 is hidden If node 1 and node 3 communicate simultaneously, node 2 will be unable to make sense of anything Node 1 and node 3 would not have any indication of the error because the collision was local to node2

8. Node 3 Node 1 Node 2 Fading problem Node 1 and 3 are placed such that their signal is not strong enough for them to detect each other’s transmissions, and yet their transmissions are strong enough to have interfered with each other at node 2

9. Carrier-Sensing Functions • Physical carrier-sensing • Expensive to build hardware for RF-based media Transceivers can transmit and receive simultaneously only if they incorporate expensive electronics • Hidden nodes problem • Fading problem • Virtual carrier-sensing • Collision avoidance: stations delay transmission until the medium becomes idle • Reduce the probability of collisions Undetectable collisions

10. Carrier-Sensing Functions Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) • Virtual sensing MAC layer • control messages (RTS, CTS, ACK) • network allocation vector (NAV) to ensure that atomic operations are not interrupted • Different types of delay depending on the priority of the frame (e.g., SIFS, DIFS, backoff)

11. Question: [answers @ log] • Transceivers that transmit and receive simultaneously

12. Modulation techniques Code Division Multiple Access (CDMA) , • Direct Sequence Spread Spectrum (DSSS) • Frequency Hopping (FH), • Orthogonal Frequency Division Multiplexing (OFDM)

13. IEEE 802.11 family • 802.11b: Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping (FH), operates at 2.4GHz, 11Mbps bitrate • 802.11a: between 5GHz and 6GHz uses orthogonal frequency-division multiplexing, up to 54Mbps bitrate • 802.11g: operates at 2.4GHz up to 54Mbps bitrate • All have the same architecture & use the same MAC protocol

14. CDMA assigns a different code to each node Codes orthogonal to each other (i.e inner-product = 0) Each node uses its unique code to encode the data bits it sends Nodes can transmit simultaneously Multiple nodes per channel Their respective receivers correctly receive a sender’s encoded data bits assuming the receiver knows the sender’s code in spite of interfering transmissions by other nodes. Code Division Multiple Access (CDMA)

15. CDMA Example Sender Zi,m=di*cm Data bits d0=1 d1=-1 Spread code 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 Time slot 1 Time slot 0 1 1 1 1 1 1 1 1 Channel output -1 -1 -1 -1 -1 -1 -1 -1

16. CDMA Example • When no interfering senders, the receiver would receive the encoded bits and recover the original data bit, di, by computing di= —SZi,m*cm • Interfering transmitted bit signals are additive M 1 M m=1

17. Frequency Hopping Timing the hops accurately is the challenge Frequency slot 5 User A 4 User B 3 2 1 0 Time slot

18. Modulation techniques • DSSS • As CDMA except all mobile hosts and base stations use the same chipping code • Spreads the energy in a signal over a wider frequency range • Spreading ratio (i.e., number of chips) should be as low as possible to meet design requirements and avoid wasting bandwidth • FH divides the ISM band into a series of 1-MHz channels • Divides hopping sequences into non-overlapping sets • Any two members of a set are orthogonal hopping sequences • OFDM • Distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.

19. 802.11 direct-sequence • Uses the Barker sequence (11-bit sequence) • It is applied to each bit in the stream by a modulo-2 adder: • when 1 is encoded, all the bits in the spreading code change; • when 0 is encoded, they stay the same

20. 802.11 Media Access Protocol • Coordinates the access & use of the shared radio frequency • Carrier Sense Multiple Access protocol with collision avoidance (CSMA/CA) • Physical layer monitors the energy level on the radio frequency to determine whether another station is transmitting and provides this carrier-sensing information to the MAC protocol • If channel is sensed idle forDIFS, a station can transmit • When receiving station has correctly & completely received a frame for which it was the addressed recipient, it waits a short period of time SIFS and then sends an ACK

21. Carrier-Sensing Functions • IEEE 802.11 to avoid collisions Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) • MAC layer • RTS, CTS, ACK • network allocation vector (NAV) to ensure that atomic operations are not interrupted • Different types of delay Short Inter-frame space (SIFS): highest priority transmissions (RTS, CTS, ACK) DCF inter-frame space (DIFS): minimum idle time for contention-based services EIFS: minimum idle time in case of “erroneous” past transmission

22. RTS/CTS clearing (1) RTS Node 1 Node 2 Node3 Node 1 (3) Frame RTS (2) CTS Time (4) ACK CTS frame Node 2 ACK RTS: reserving the radio link for transmission RTS, CTS: Silence any station that hear them

23. Positive acknowledgement of data transmission Node 1 Node 2 Time frame ACK 802.11 allows stations to lock out contention during atomic operation so that atomic sequences are not interrupted by other Hosts attempting to use the transmission medium

24. 802.11 Media Access Protocol • If channel is sensed busy will defer its access until the channel is later sensed to be idle • Once the channel is sensed to be idle for timeDIFS, the station computes an additional random backoff time and counts down this time as the channel is sensed idle. When the random backoff timer reaches zero, the station transmits its frame • Backoff process to avoid having multiple stations immediately begin transmission and thus collide

25. RTS Frame Sender CTS ACK Receiver SIFS SIFS DIFS SIFS NAV (RTS) NAV NAV(CTS) Using the NAV for virtual carrier sensing (eg 4-8KB) (e.g.10ms) Contention Window Access to medium deferred NAV is carried in the headers of CTS & RTS

26. Backoff with DCF • Contention window (or backoff window) follows the DIFS • Window is divided in time slots • Slot length is medium-dependent • Window length limited and medium-dependent • Hosts pick a random slot and wait for that slot before attempting to access the medium; • All slots are equally likely selections • Host that picks the first slot (earlier number) wins • Each time the retry counter increases, the contention window moves to the next greatest power of two

27. 31 slots DIFS Initial Attempt Previous Frame 1st retransmission 63 slots DIFS Previous Frame 2nd retransmission 127 slots DIFS Previous Frame 3rd retransmission 255 slots DIFS Previous Frame Contention window size Slot time:20s The contention window is reset to its minimum size when frames are transmitted successfully, or the associated retry counter is reached and the frame is discarded