MSIT 413: Wireless Technologies Week 5

MSIT 413: Wireless Technologies Week 5 Michael L. Honig Department of EECS Northwestern University October 2017

Outline Diversity, MIMO Multiple Access techniques FDMA, TDMA OFDMA (LTE) CDMA (3G, 802.11b, Bluetooth) Random Access

Diversity Idea: Obtain multiple independent copies of the received signal. Improves the chances that at least one is not faded. Macroscopic (space): copies of signal are received over distances spanning many wavelengths. Microscopic (space): copies of signal are received over distances spanning a fraction of a wavelength Different types

Macroscopic Diversity Copies of signal are separated by many wavelengths.

Macroscopic Diversity MSO Copies of signal are separated by many wavelengths.

Macroscopic Diversity: Handoff Received Signal Strength (RSS) handoff threshold RSS margin time needed for handoff from right BST from left BST unacceptable (call is dropped) time

Microscopic Space Diversity Antenna 2 s2 Antenna 1 s1 Want signals s1 and s2 to experience independent fading (why?). distance between antennas should be ½ wavelength. Ex: 900 MHz, λ = c/f 1/3 meter 2 GHz, λ 0.15 meter

Multiple Antennas: Multi-Input/Multi-Output (MIMO) Channel Transmitted Data Multi-Channel Detector Estimated Data (multiple data streams) Multiple (M) antennas at receiver and transmitter Channel has multiple inputs and multiple outputs. 8

Single Transmit Antenna Transmitted Data (single stream) Multi-Channel Detector Estimated Data Multiple receiver antennas provides spatial diversity Lowers error rate Single-Input/Multiple-Output (SIMO) channel 9

Multi-Input/Single Output (MISO) Channel Transmitted Data Single-Channel Detector Estimated Data (single or multiple streams) Transmitting the same symbol from all transmitters provides transmit spatial diversity (e.g., select the best antenna, turn the others off). Practical for cellular downlink. 10

Downlink Beamforming Narrow beam focused on one user Different beams can use the same frequency! M antennas at the base station (single or multiple antennas at mobiles) Can support up to M data streams. Multi-user MIMO: multiple users on the same channel Introduced in LTE, 802.11ac 11

Orthogonal Frequency Division Multiplexing (OFDM) substream 1 Modulate Carrier f 1 source bits Split into M substreams substream 2 substream M Modulate Carrier f 2 + OFDM Signal Modulate Carrier f M

Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel Transmitted Data Multi-Channel Detector Estimated Data Multiple (M) antennas at receiver and transmitter. 13

Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel Substream 1 Substream M Multi-Channel Detector Estimated Data Multiple (M) antennas at receiver and transmitter. Transmitted data is divided into M substreams, one for each antenna. Transmit antennas are used to multiplex multiple data streams. Multiple receiver antennas (plus signal processing) are used to remove interference from the different antennas. 15

WiFi Evolution: 802.11n Technology based on OFDM with multiple antennas at the transmitter and receivers Supports data rates up to 540 Mbps 4 spatial streams, 40 MHz bandwidth Can replace USB 2.0 connections. Also important part of 802.11ac (multi-user MIMO) 17

Frequency Diversity channel gain signal power (wideband) coherence bandwidth B c Frequencies far outside the coherence bandwidth are affected differently by multipath. f 1 f 2 frequency Wideband signals exploit frequency diversity. Spreading power across many coherence bands reduces the chances of severe fading. Wideband signals are distorted by the channel fading (distortion causes intersymbol interference). 18

Time Diversity

Time Diversity Transmit multiple copies of the signal in time. Error control coding: add redundant bits Problem: slow fading Combine with power control

Path Diversity τ 1 τ 2 received signal adjust phase + Delay τ 2 - τ 1 adjust phase Called a RAKE receiver, since it rakes up (combines) the energy in the different paths. Can substantially increase the S/I. An important component of CDMA receivers. Each branch in the Rake is typically referred to as a finger.

Multiuser Diversity

Multiuser Diversity d 1 d 2 > d 1 Received power user 1 user 2 transmit to user 2 transmit to user 1 transmit to user 2 transmit to user 1 time The BST can choose to transmit to the user with the best channel. Exploits variations in signal strength across users.

Selection Diversity Antenna 2 s2 Antenna 1 s1 Received power antenna 1 antenna 2 select ant. 2 select ant. 1 select ant. 2 select ant. 1 time Choose the best signal (highest instantaneous SNR). Easy to implement (antenna switch).

Benefit of Selection Diversity (Example) Suppose that the signal on each antenna experiences independent Rayleigh fading. Determine the probability that the received signal is faded: Recall Rayleigh fading formula: Probability that the signal power is less than a x P 0 (average received power) = 1 e -a Hence the probability that the signals on both antennas are less than a x P 0 = (1 e -a ) 2 Without diversity, probability of a signal fade = 1 e -1 = 0.63 With 2-branch diversity, probability of a signal fade = 0.63 2 = 0.39

Benefit of Selection Diversity (cont.) Suppose that there are N copies of the signal (e.g., N antennas, paths, coherence bands, etc.) Probability that the signal power is less than a x P 0 (average received power) = 1 e -a Hence the probability that all N signals are less than a x P 0 = (1 e -a ) N Without diversity, probability of a signal fade = 1 e -1 = 0.63 With 4-branch diversity, probability of a signal fade = 0.63 4 = 0.16 Without diversity, Prob(signal is faded by more than 10 db) = 1 e -0.1 0.1 With diversity this probability is (1 e -0.1 ) 4 0.0001!

Coherent Combining S1 (ant. 1) S2 (ant. 2) adjust phase adjust phase + Coherent means that the phases of the two signals are estimated at the receiver and aligned. Performs better than selection combining (why?). Example: RAKE receiver Can weight the combined signals to maximize the received SNR. (How should the weights depend on the signal levels?)

Probability of Error with Fading add diversity Diversity can transform a fading channel back to a non-fading (additive noise) channel. Essential for mobile wireless communications.

The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (GSM) Code-Division (3G, Bluetooth) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

Duplexing (Two-way calls) Frequency-Division Duplex (FDD) Channel 1 Channel 2 Time-Division Duplex (TDD) Time slot (frame) 1 Time slot (frame) 2

Combinations FDMA/FDD (AMPS) TDMA/FDD (GSM) TDMA/TDD (IS-136 or 2G in the U.S.) CDMA/FDD (IS-95, CDMA2000) CDMA/TDD (3G/UMTS) Frequency-Hopped CDMA/TDD (Bluetooth) OFDMA/TDD and FDD (WiMax, 4G)

The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (IS-136, GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

uplink Cellular Spectrum (50 MHz) 824 825 835 845 846.5 849 A* A B A* B* 869 870 880 890 891.5 894 downlink AMPS (1G): 30 khz Channels 416 FDD Channels (requires 12.5 MHz): 395 FDD voice channels 21 FDD control channels

Properties of FDMA Can be analog or digital (AMPS is analog). Narrowband: channel contained within coherence bandwidth undergoes flat fading. Low capacity Best for circuit-switched (dedicated) connections. Requires guard channels for adjacent channel interference.

The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

Time Division Multiple Access Channel f 1 Frame N time slots H: Frame Header H 1 2... N H Channel f 2 H 1 2 Time slots... N H Channel f K H 1 2...... N H

Time Slot Frame H 1 2... N time slots N H H: Frame Header Preamble and synch Time Slot Data to or from user K + control information Guard time

TDMA/Time-Division Duplex H 1 2 3 4 5 6 H { { Uplink time slots Downlink time slots

Properties of TDMA Data transmission occurs in bursts. Must ensure small delays for speech. High peak to average power on reverse link. Can measure signal strength in idle time slots (e.g., for handoff). Can assign multiple time slots for higher data rates. Significant overhead/complexity for synchronization. Guard times needed between time slots for delay spread. May require an equalizer to mitigate intersymbol interference.

Global System for Mobile Communications (GSM) Originated in Europe Main objective: allow roaming across countries Incompatible with 1G systems More than an air-interface standard: specifies wireline interfaces/functions TDMA/FDMA, FDD Dynamic frequency assignment 200 khz channels 270.833 kbps

GSM Frame Structure 156.25 bits 576.92 µs Frame TS TS TS TS TS TS TS TS 0 1 2 3 4 4.615 ms 5 6 7 T n : nth TCH frame S: Slow Associated Control Channel frame I: Idle frame T 0 T 1 T 2... T 10 T 11 T 12 S T 13 T 14 T 15 T 22 T 23 T 24 I/S 120 ms Speech Multiframe = 26 TDMA frames 200 khz FDD channels divided into 8 time slots per frame Total number of available channels = (12.5 MHz 2 X Guard Band)/200 khz 100 khz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = 270.839 kbps Without overhead, data rate/user = 24.7 kbps

6.12 s GSM Time Slots Hyperframe = 2048 superframes lasts ~3 hrs 28 min 54 sec Superframe 120 ms Multiframe Frame Time slot 4.615 ms 0 1 2 3 4 5 6 7 576.92 µs 3 57 1 26 1 57 3 8.25 Normal Burst Traffic Channel (TCH) 148 bits/time slot 114 coded information bits Frame efficiency 74% (total bits overhead bits)/(total bits)

GSM Capacity Total bandwidth = 12.5 MHz, 200 khz channels è 62 channels With cell cluster size N=3 (typical), capacity is (62/3) x 8 ~ 165 users/cell

Orthogonal Frequency Division Multiplexing (OFDM) substream 1 Modulate Carrier f 1 source bits Split into M substreams substream 2 substream M Modulate Carrier f 2 + OFDM Signal Modulate Carrier f M

OFDM Spectrum Total available bandwidth Power Data spectrum for a single carrier f 1 ß 0 f 2 f 5 f 6 f 3 f 4 subchannels frequency M subcarriers, or subchannels, or tones Orthogonal subcarriers è no cross-channel interference.

OFDM vs OFDMA OFDM is a modulation technique for a particular user. OFDMA is a multiple access scheme (allows many users to access a single receiver). Can OFDM be combined other multiple access techniques?

OFDM with FDMA OFDM users are assigned adjacent frequency bands. Frequency diversity is determined by (BW of signal)/(coherence BW) OFDM vs OFDMA Overall User 1 User 2 User 3 User 4 OFDMA User subcarrier assignments are permuted across the entire available frequency band. So what?? Overall User 1 User 2 User 3 User 4

OFDM (with FDMA) OFDM users are assigned adjacent frequency bands. Frequency diversity is determined by (BW of signal)/(coherence BW) OFDMA User subcarrier assignments are permuted across the entire available frequency band. Each sub-carrier may experience independent fading. Frequency diversity is determined by the number of sub-carriers. Also provides interference diversity. OFDM vs OFDMA Overall User 1 User 2 User 3 User 4 Overall User 1 User 2 User 3 User 4

OFDM/TDMA and OFDMA OFDM/TDMA: t TDMA Each color represents a different user, which is assigned particular time slots. subchannels m TDMA\OFDMA Different sub-carriers can be assigned to different users. t N time slot Each user can be assigned a time/frequency slice. Requires a time/frequency scheduler.

WiMax OFDMA Frame Structure (TDD example) (downlink) (uplink)

Adaptive Rate Control channel gain large channel gain è higher data rate small channel gain è lower data rate f 1 f 2 frequency How can we control the rate per subchannel? Change the modulation format (e.g., choose from QPSK/16-QAM/64 QAM) Change the code rate (i.e., change the number of redundant bits) Requires feedback from receiver to transmitter

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

Code Division Multiple Access Users transmit simultaneously over the same frequency band Performance limited by interference

Two-User Example User 1: T/2 T time bits: 1 s 1 (t) -1 1 1 0 1 0 T 2T 3T 4T 5T User 2: T/2 T time s 2 (t) 1 0 1 1 0 T 2T 3T 4T 5T received signal r(t)= s 1 (t)+s 2 (t) 2 How to recover each users bits? -2 T 2T 3T 4T 5T

Chip Sequence chips User 2: T/2 T time User 2 s chip sequence (1, -1) is called a signature. chip duration T c symbol duration T=2T c s 2 (t) bits: 1 0 1 1 0 T 2T 3T 4T 5T Transmitted chips: 1-1 -1 1 1-1 1-1 -1 1

Chip Sequence chips User 1: User 1 s signature is (1, 1). T/2 T time chip duration T c symbol duration T=2T c s 1 (t) 1-1 bits: 1 1 0 1 0 T 2T 3T 4T 5T Transmitted chips: 1 1 1 1-1 -1 1 1-1 -1

Two-User Example 1 1 0 1 0 1 s 1 (t) -1 T 2T 3T 4T 5T Transmitted chips: 1 1 1 1-1 -1 1 1-1 -1 s 2 (t) 1 0 1 1 0 T 2T 3T 4T 5T Transmitted chips: 1-1 -1 1 1-1 1-1 -1 1 r(t)= s 1 (t)+s 2 (t) 2-2 T 2T 3T 4T 5T Received chips: 2 0 0 2 0-2 2 0-2 0

Correlation Given two sequences: a 1, a 2, a 3,, a N b 1, b 2, b 3,, b N The correlation between the sequences is defined as: (a 1 x b 1 ) + (a 2 x b 2 ) + (a 3 x b 3 ) + + (a N x b N ) Examples: 1 1 1 1 1 correlated with 1 1 1 1 1 = 5 1 1 1 1 1 correlated with 1-1 1-1 1 = 1 2 4 1 3 1 correlated with -2 1 8 2 0 = -4 + 4 + 8 +6 + 0 = 14 If the correlation between two sequences is zero, they are said to be orthogonal.

Correlator Receiver r(t) Sample Chips Correlate with desired user s signature Bit Decision < 0 à 0 > 0 à 1 estimated bits

Why Does This Work? amplitude A 1 s 1 Correlate with User 1 s signature signature (1,1) 2A 1 A 2 s 2 Correlate with 0 User 1 s signature The user signatures are orthogonal. Now observe that: A 1 s 1 + A 2 s 2 Correlate with User 1 s signature 2A 1

Correlator, or Matched Filter Receiver A 1 s 1 + A 2 s 2 Correlate with User 1 s signature Bit Decision < 0 à 0 > 0 à 1 User 1 s bits Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 User 2 s bits The correlator is matched to user 1 s signature s 1, and rejects s 2 (and vice versa).

Observations Users transmit simultaneously (not TDMA). Users overlap in frequency (not FDMA). Spectrum: User 1 signal bandwidth is roughly 1/T Spectrum: User 2 0 frequency signal bandwidth is roughly 1/T c = 2/T 0 frequency Bandwidth expansion (factor of 2) è spread spectrum signaling.

Users and Bandwidth Expansion To guarantee orthogonal signatures (no interference), the length of the signatures must be the number of users. Example (4 users): signature: 1 1 1 1 signature: 1 1-1 -1 User 1: T/4 3T/4 T/2 T time User 2: 3T/4 T/4 T/2 T time signature: 1-1 -1 1 signature: 1-1 1-1 User 3: 3T/4 User 4: 3T/4 T/4 T/2 T time T/4 T/2 T time The chip rate is 4 times the symbol rate, hence the bandwidth expansion is a factor of 4.

Correlator Receiver (4 users) s 1 + s 2 + s 3 + s 4 Correlate with User 1 s signature Bit Decision < 0 à 0 > 0 à 1 User 1 s bits Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 User 2 s bits Correlate with User 3 s signature Bit Decision < 0 à 0 > 0 à 1 User 3 s bits Correlate with User 4 s signature Bit Decision < 0 à 0 > 0 à 1 User 4 s bits

DS-CDMA Transmitter Source bits Spreader chips Modulator RF signal (generate chips) (e.g., QPSK) Ex: 100 source symbols, processing gain = 10 è 1000 chips Nyquist chip shape sin 2πf c t T c time Baseband signal X Passband (RF) signal

Orthogonality and Asynchronous Users s 1 (t) s 2 (t) 1-1 1 1 0 1 0 T 2T 3T 4T 5T 1 0 1 1 0 T 2T 3T 4T 5T time Asynchronous users can start transmissions at different times. Chips are misaligned è signatures are no longer orthogonal! Orthogonality among users requires: Synchronous transmissions No multipath

Correlator, or Matched Filter Receiver delay s 1 (t) + s 2 (t-τ) Correlate with User 1 s signature Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 Bit Decision < 0 à 0 > 0 à 1 Signal 1 + multiple acess interference (MAI) From user 2 Signal 2 + multiple acess interference (MAI) From user 1 The multiple access interference adds to the background noise and can cause errors. For this reason, CDMA is said to be interference-limited. Because CDMA users are typically asynchronous, and because of multipath, it is difficult to maintain orthogonal signatures at the receiver. Consequently, in practice, the signatures at the transmitter are randomly generated.

Processing Gain (PG) The performance of CDMA depends crucially on the Processing Gain: Bandwidth of spread signal / Symbol rate (minimum bandwidth needed) or equivalently, Number of chips per symbol

Processing Gain (PG) The performance of CDMA depends crucially on the Processing Gain: Bandwidth of spread signal / Symbol rate (minimum bandwidth needed) or equivalently, Number of chips per symbol Fundamental tradeoff: increasing the PG decreases the correlation between random signatures. decreases interference. increases the bandwidth of the signal.

Example IS-95 (2G CDMA) Total bandwidth = 1.25 MHz Data rate = 9.6 kbps PG 130 3G/CDMA2000 Total bandwidth = 1.25 MHz Data rate varies between 14.4. kbps (voice) up to 2 Mbps (1X-DO) PG varies from 1 to 130

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

Near-Far Problem SO THEN THE THIRD TIME I CALLED CUSTOMER SERVICE, I SAID &%$#%^

Near-Far Problem User 1 amplitude A 1 User 2 amplitude A 2 Output of correlator receiver is signal + interference. As the interferer moves closer to the base station, the interference increases. In practice, power variations can be up to 80 db! Conclusion: User 1 s signal is overwhelmed by interference from user 2!

Closed-Loop Power Control User 1 raise power lower power User 2 Base station gives explicit instructions to mobiles to raise/lower power. Needed to solve near-far problem (equalizes received powers). Introduced by Qualcomm in the late 80 s.

Closed-Loop Power Control: Properties User 1 raise power lower power User 2 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

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

Bandwidth and Multipath Resolution reflection (path 2) direct path (path 1) multipath components are resolvable signal pulse τ (delay spread) signal pulse T > τ τ T < τ Narrow bandwidth è low resolution Receiver cannot distinguish the two paths. T Wide bandwidth è high resolution Receiver can clearly distinguish two paths.

Soft Handoff (CDMA) Make before break BEFORE DURING AFTER MSC MSC MSC BSC BSC BSC BSC BSC BSC Hard Handoff (TDMA) MSC MSC MSC BSC BSC BSC BSC BSC BSC

Applications of Spread-Spectrum Cellular Military (preceded cellular applications) Wireless LANs (overlay)

Military Spread Spectrum Can hide a signal by spreading it out in the frequency domain. spread 0 frequency noise level Requires a very large PG (several 100 1000). Enemy must know spreading code (the key containing 100 s of bits) to demodulate too complicated for simple search. Spread spectrum signals have the LPI/LPD property: low probability of intercept / low probability of detect. Spread spectrum used for covertness, not multiple access. 0 frequency

Applications of Spread-Spectrum Cellular Military (preceded cellular applications) Wireless LANs (WiFi)

Spread Spectrum Underlay FCC requirements on spectrum sharing in the unlicensed (Industrial, Scientific, Medical (ISM)) bands: Listen before talk Transmit power is proportional to the square root of the bandwidth. spread spectrum signal hospital monitor telemetry frequency Spread spectrum signaling is robust with respect to a narrowband interferer. To a narrowband signal, a spread spectrum signal appears as low-level background noise.

Frequency-Hopped CDMA Idea: Hop from channel to channel during each transmission. f 5 frequency f 4 f 3 User 1: blue f 2 f 1 time slots time

Frequency-Hopped CDMA Idea: Hop from channel to channel during each transmission. frequency f 5 f 4 f 3 collision bits are lost User 1: blue User 2: red f 2 f 1 time slots time

Hop Rate Can make synchronous users orthogonal by assigning hopping patterns that avoid collisions. Fast hopping generally means that the hopping period is less than a single symbol period. Slow hopping means the hopping period spans a few symbols. The hopping rate should be faster than the fade rate (why?).

Properties of FH-CDMA 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

1. 2.

N O R T H W E S T E R N U 1. 2. N I V E R S I T Y 3.

N O R T H W E S T E R N U N I V E R S I T Y 1. 3. 2. 4.

Inventor of Frequency-Hopping Hedi Lamar, the famous actress of the 1930 s has one of the first U.S. patents on frequency hopping with co-author and composer George Antheil.

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, lowpower radios ( wireless cable). Initiated by a consortium of companies (IBM, Ericsson, Nokia, Intel) Standard has been developed (IEEE 802.15.1 ).

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. Interferes with 802.11b/g/n 1600 hops/sec over 79 channels (1 MHz channels) Range set at 10m. Gross data rate of 1 Mbps (TDD). 64 kbps voice channels Second generation (Bluetooth 2.0+) supports rates up to 3 Mbps. Competes with Wireless USB.

802.11 Random Access Access Point (AP) Station A Station C Station B Terminals send/receive messages (packets) to/from the AP at random times (i.e., when they appear).

Cellular Call Setup (Random Access) 1. Call Request 2. Send numbers to switch 3. Page Receiver 4. Request Channel/Time slot/code

Medium 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, WiFi

Carrier Sense Multiple Access (CSMA) Packet arrives Sense channel Busy? no Transmit packet yes Delay transmission (non-persistent) Listen before talk (LBT) protocol How do collisions occur?

Carrier Sense Multiple Access (CSMA) Packet arrives Sense channel Busy? no Transmit packet yes Delay transmission (non-persistent) 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 è more collisions ALOHA preferred for wide area applications

CSMA Example

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.

Hidden Terminal Problem Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D.

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

Exposed Terminal Problem Station A Station B Station D Station C B is transmitting to A. C wants to transmit to D

Exposed Terminal Problem Coverage area for station C. Station A Station B Station D Station C 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 A.

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

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)

Revealing the Hidden Terminal Station A RTS Station B Coverage area for station C. Station D Station C A sends a Request to Send (RTS) packet to B.

Revealing the Hidden Terminal CTS Coverage area for station C. Station A Station B Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C!

Revealing the Hidden Terminal data Coverage area for station C. Station A Station B Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C! C is silent for duration of A s transmission (specified in CTS)

Revealing the Hidden Terminal Coverage area for station C. Station A Station B Station D Station C What if C hears RTS, but not CTS?

Exposed Terminal Station A RTS Station B Coverage area for station C. Station D Station C C will not hear the CTS from A.

RTS Collision Station A RTS Station B Station E RTS Station C RTS messages from E and B collide à exponential backoff

Corrupted CTS Station A CTS Station B Station E Data, RTS, or CTS Station C CTS message from A is corrupted due to interference from E à exponential backoff by B

MACA Protocol (RTS/CTS) Transmitter Receiver Request to Send (RTS), packet length Clear to Send (CTS), packet length Data Ack 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 è binary exponential back-off