MITP 413: Wireless Technologies Week 7

1. MITP 413: Wireless TechnologiesWeek 7 Michael L. Honig Department of ECE Northwestern University May 2004

2. The Multiple Access Problem • Use different frequencies (FDMA) • Use different time slots (TDMA) • Use different pulse shapes (CDMA) How can multiple mobiles access (communicate with) the same base station?

3. DS-CDMA Transmitter Source bits chips RF signal Modulator (e.g., QPSK) Spreader (generate chips) Ex: 100 source bits, processing gain = 10  1000 chips

4. 2G CDMA: IS-95 or cdmaOne • Introduced by Qualcomm (San Diego) • Direct-Sequence Spread Spectrum signaling • FDD • Wideband channels (1.25 MHz) • Tight, closed-loop power control • Sophisticated error control coding • Multipath combining to exploit path diversity • Noncoherent detection • Soft handoff • High capacity • Air-interface only: uses IS-41

5. CDMA vs. TDMA(early 1990s) TDMA CDMA • Proven technology • Large investment in research, development • Earlier military applications • Near-far problem • Enticing (exaggerated?) performance claims

6. TDMA vs. CDMA:Performance Critera Capacity: Users per Hz per km2 Channel conditions System assumptions Perfect power control? Modulation and coding? Complexity Power control (CDMA) Synchronization (TDMA) Equalization Frequency assignment Flexibility Integrated services (voice/data) Multimedia Variable rate/QoS

7. 3G Air Interfaces cdma2000 Wideband (W)-CDMA • Also referred to as “multicarrier” CDMA • 1X Radio Transmission Technology (RTT): 1.25 MHz bandwidth (1 carrier) • Supports 307 kbps instantaneous data rate in packet mode • Expected throughput up to 144 kbps • 3X RTT: 3.75 MHz bandwidth (3 carriers) • Data rates can exceed 2 Mbps • 1xEV (Evolutionary): High Data Rate standard introduced by Qualcomm • 1xEV-DO: data only, 1xEV-DV: data and voice • Radio channels assigned to single users (not CDMA!) • 2.4 Mbps possible, expected throughputs are a few hundred kbps • 1xEV-DV has twice as many voice channels as IS-95B • Also referred to as Universal Mobile Telecommunications System (UMTS) • European proposal to ITU (1998) • Backwards compatibility with 2G GSM and IS-136 air interfaces • Network and frame structure of GSM • ``Always on’’ packet-based data service • Supports packet data rates up to 2 Mbps • Requires minimum 5 MHz bandwidth, FDD, coherent demodulation • 6 times spectral efficiency of GSM

8. Service Providers and Technologies (2003)

9. Closed-Loop Power Control • Solves near-far problem: crucial part of CDMA cellular systems (IS-95, 3G). • Minimizes battery drain. • Complicated (increases cost) • Requires overhead: control bits in feedback channel to tell transmitter to lower/raise power. • Cannot compensate for fast fading.

10. CDMA Capacity Performance depends on Let S= Transmitted power (per user), R= information rate (bits/sec), W= Bandwidth, K= Number of users Eb= S/R N0= (Number of interferers x S)/W = ((K-1) x S)/W Therefore Eb/N0 = (W/R)/(K-1) = (Processing Gain)/(K-1) For a target Eb/N0, the number of users that can be supported is K = (Processing Gain)/(Eb/N0) + 1

11. Interference and CDMA Capacity If interference is reduced by a factor 1/g, then the target Eb/N0 can be reduced by 1/g: If W/R is large, then reducing interference by 1/g (approximately) increases the capacity by a factor of g.

12. Properties of CDMA • High capacity with power control. • Power control needed to solve near-far problem. • Robust with respect to interference. • Benefits from voice inactivity and sectorization. • No loss in trunking efficiency. • Wideband: benefits from frequency/path diversity. • No frequency assignments (eases RF planning). • Soft capacity: performance degrades gradually as more users are added. • Asynchronous • Soft handoff

13. Frequency Hopping • Properties: • Exploits frequency diversity (can hop in/out of fades) • Can avoid narrowband interference (hop around) • No near-far problem (Can operate without power control) • Low Probability of Detect/Intercept • Spread spectrum technique – can overlay • Cost of frequency synthesizer increases with hop rate • Must use error correction to compensate for erasures due to fading and collisions. • Applications • Military (army) • Part of original 802.11 standard • Enhancement to GSM • Bluetooth

14. Bluetooth: A Global Specification for Wireless Connectivity • Wireless Personal Area Network (WPAN). • Provides wireless voice and data over short-range radio links via low-cost, low-power radios (“wireless” cable). • Initiated by a consortium of companies (IBM, Ericsson, Nokia, Intel) • Standards are being developed (IEEE 802.15 ).

15. Bluetooth Specifications • Allows small portable devices to communicate together in an ad-hoc “piconet” (up to eight connected devices). • Frequency-hopped spread-spectrum in the 2.4 GHz UNII band. • Packet switching with 1600 hops/s over 1 MHz channels. • Range set at 10m. • Gross data rate of 1 Mbps (TDD), with second generation plans for 2 Mbps. • 64 kbps voice channels • Maximum asymmetric data transfer rate of 721 kbps in either direction,or 432.6 kbps symmetric link • Interferes with 802.11. • Competes with 802.11?

16. Cellular Call Setup 1. Call Request 2. Send numbers to switch 4. Request Channel/Time slot/Code 3. Page Receiver

17. Cellular Call Setup (cont.) 5. Switch assigns channels 6. Cellular conversation is set up

18. Multiple Access Control (MAC) • Fixed assignment access • Each user is assigned a dedicated channel, time slot, or code • Appropriate for circuit-switched traffic, transferring long data files • Random access: users contend for access to the channel • Users may collide, losing packets. • Sometimes can negotiate rate (bandwidth, time slots, codes) and power • Widely used in wired networks • Used in wireless networks for requesting channel/time slot/code

19. ALOHA-Based Random Access • Simple: asynchronous • Low throughput under heavy loads (maximum is 18% of incoming packets) no Packet arrives Base station Sends ack transmit Collision? yes Transmitter does not Receive ack Packet is rescheduled With random delay • Slotted ALOHA • Synchronous, maximum throughput increases to 36% • Used in GSM to reserve a time slot for voice connection • Reservation ALOHA • Contention period followed by reserved message slots

20. ALOHA Protocols

21. Carrier Sense Multiple Access (CSMA) • “Listen before talk” (LBT) protocol • Collisions occur if transmitters cannot sense the other transmission (e.g., due to large propagation delay) • Lower probability of collision/higher throughput than ALOHA • Long propagation times  more collisions • ALOHA preferred for wide area applications no Packet arrives Sense channel Transmit packet Busy? yes Delay transmission (non-persistent)

22. CSMA Example

23. Carrier Sensing • Nonpersistent: After sensing a busy channel, the terminal senses the channel after a random waiting period • Persistent: The terminal senses the channel until the channel becomes free. • 1-Persistent: After the channel becomes free, the terminal transmits immediately. • p-Persistent: The terminal transmits with probability p.

24. Binary Exponential Backoff • R is random • Time slot 2 X (maximum round trip delay) • Maximum of 16 retries no Packet arrives Base station Sends ack transmit Collision? yes Transmitter does not Receive ack Packet is rescheduled After R time slots • After 1st collision: • R=0 or 1 with equal probability • After 2nd collision: • R=0,1,2, or 3 with equal probability • After ith collision (i=1,…,10): • R is selected between 0 and 2i-1

25. Performance • Throughput (S): Average number of successful packet transmissions per unit time. • Normalized throughput: Percentage of successful packet transmissions • Average Delay (D): Average waiting time before successful transmission • Offered Traffic (G): Number of packet transmission attempts per packet time slot – includes both new arrivals and retransmissions. • Performance depends on the propagation delay across the network relative to the packet duration.

26. Throughput vs. Offered Load

27. Delay vs. Throughput

28. CSMA with Collision Detection (CSMA/CD) • Nodes detect a collision in progress, and stop transmitting before the entire packet is transmitted. • Assumes nodes can hear each other when they are transmitting. • Appropriate for wired channels. • Problems with wireless channels: • Nodes cannot transmit and receive at the same frequency at the same time. • Not all nodes may be in range of each other.

29. Station A Station B Station C Station D Hidden Terminal Problem • A is transmitting to B. • C wants to transmit to D. • C may not sense A’s transmission, causing a collision at B. Coverage area for station A.

30. Station A Station B Station C Station D Exposed Terminal Problem • B is transmitting to A. • C wants to transmit to D. • C senses B’s transmission, and does not transmit even though it would not cause interference at D. Coverage area for station B.

31. Basic Problem Carrier sensing determines whether or not there are interfering sources near the transmitter, not the receiver.

32. Solutions • Busy-tone multiple access (BTMA) • Separate control channel used to indicate that the channel is idle or busy. • An active station transmits a busy tone on the control channel. • Each receiver that senses a busy tone turns on its own busy tone. • Used in ad hoc networks. • Digital or Data Sense Multiple Access (DSMA) • Used in FDD cellular mobile data networks • Forward channel periodically broadcasts a busy/idle bit for the reverse link. • Mobile transmits if bit is in idle state; base station sets bit to busy. • Not carrier sensing: sensing is performed after demodulation. • Multiple Access with Collision Avoidance (MACA)

33. MACA Protocol (RTS/CTS) • Terminals receiving either an RTS or CTS must not transmit for the duration of the packet. (What if the terminal hears RTS but not CTS?) • Collision occurs if multiple nodes transmit an RTS, or the CTS is not heard due to other interference. • Collision  binary exponential back-off Receiver Transmitter Request to Send (RTS), packet length Clear to Send (CTS), packet length Data Ack

34. Wireless Local Area Networks (WLANs) • Low mobility, high data rates within confined region (building or campus) • Competitive with other wireless data systems (3G, fixed wireless access) • Unlicensed bands • Industrial, Scientific, Medical (ISM): 2.4 GHz • National Information Infrastructure (UNII): 5 GHz • Must accept interference, therefore uses spread spectrum signaling, or random access with collision avoidance.

35. IEEE 802.11 2.4 GHz 850 to 950 nm Diffuse IR FHSS DS-SS 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE 802.11b Extension 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK Overview of 802.11 Standard 2 Mbps 4GFSK

36. 802.11b vs. 802.11a 802.11a 802.11b • 2.4—2.485 Ghz • Interference from Microwave, Bluetooth • 3 non-overlapping channels • Max Speed 11 Mbps • Direct Sequence Spread Spectrum (DSSS) • CSMA/CA • Many products available • 5 Ghz range • 5 channels • Max Speed 54 Mbps • Orthogonal Frequency Division Multiplexing (OFDM) • CSMA/CA • Some products available (Cisco, Proxim)

37. Peer-to-Peer Configuration Single Cell Station C Station A Station B • Mobile devices are referred to as Stations. • Each Station can communicate directly with another Station. • System referred to as Independent Basic Service Set (IBSS)

38. Access Point Single Cell Station C Station A Station B Infrastructure Configuration • Access Point is analogous to a cellular Base Station • System referred to as Infrastructure Basic Service Set (BSS)

39. Extended BSS (EBSS) Configuration Distribution System (DS) BSS • Connect APs via a wired network • DS consist of Layer 2, Layer 3 devices • System referred to as Extended BSS • APs have a BSSID • System has a SSID Basic Service Area

40. Association/Disassociation STA AP Probe Request Probe Response Authenticate Associate Request Associate Response Disassociate (No ack)

41. 802.11b: Physical Layer CCK: Complementary Code Keying

42. 802.11 Protocol Architecture • DCF: CSMA/CA with additional virtual carrier sensing (ad hoc) • PCF: Point Coordination Control Function (infrastructure) • Access Point Becomes a Scheduler • Contention Free Mode • DCF is more prevalent • PCF and DCF can co-exist Logical link control Contention-free service Contention service Point Coordination Function (PCF) MAC Layer Distributed Coordination Function (DCF) Air interface (802.11/11a/11b)

43. IEEE 802.11 Medium Access Control Logic Wait for frame to transmit Medium idle? No Wait until current transmission ends Yes Wait IFS Wait IFS Still idle? No Still idle? No Yes Yes Transmit frame Exponential backoff while medium idle Transmit frame

44. Inter Frame Spacing (IFS) • Station has to detect a minimum idle time before transmit • Time depends on type of Frame, type of MAC • SIFS: Short IFS (used by ACKs, CTS) • PIFS: PCF IFS used by PCF Frames • DIFS: DCF IFS used by DCF Frames • SIFS < PIFS < DIFS • Relative values of IFS used to “prioritize” Medium Access

45. Basic Access Method DIFS Contention window PIFS DIFS Busy medium SIFS Backoff window Next frame Time Slot time Select slot using binary exponential backoff Defer access

46. 802.11 CSMA/CA

47. Distributed Control Function (DCF) Network Allocation Vector: Prevents transmissions by other terminals.

48. Point Coordination Function (PCF) • Alternative access method implemented on top of the DCF. • Stations are scheduled to transmit by AP. • Point coordinator (AP) uses PIFS when issuing polls. • Optional MAC feature – not widely available in products.

49. 802.11 Frame Structure • Total Header Size – 34 bytes • Variable packet size • Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) • Frame Control: indicates if frame is data, RTS, CTS, or other type of control. • Duration: indicates time in microseconds that the channel will be allocated for transmission (virtual carrier sensing) • Sequence Control: identifies place in packet sequence for fragmentation and reassembly • CRC: Cyclic Redundancy Check (for error detection) 2 2 6 6 6 2 6 0-2312 4 Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control

50. Power Management • Most of the time mobile devices receive data in bursts and then become idle. • To maintain session: • AP buffers download data while mobile “sleeps” • Mobiles are synchronized, and “wake up” at designated times. • AP transmits beacon announcing mobiles with buffered data • Mobile checks for buffered data; if listed, then goes active. • AP transmits buffered data. • Different from power control in cellular networks – geared towards burst data.