Directional Antennas for Wireless Networks Romit Roy Choudhury

Several Challenges, Protocols Internet Applications Several Challenges, Protocols

Omnidirectional Antennas Internet Omnidirectional Antennas

IEEE 802.11 with Omni Antenna RTS = Request To Send CTS = Clear Y S RTS D CTS X K

IEEE 802.11 with Omni Antenna silenced M Y silenced S Data D ACK X K silenced

IEEE 802.11 with Omni Antenna `` Interference management `` silenced silenced M A silenced `` Interference management `` A crucial challenge for dense multihop networks F silenced Y silenced C silenced S Data D ACK G silenced silenced X K B silenced D silenced silenced

Managing Interference Several approaches Dividing network into different channels Power control Rate Control … New Approach … Exploiting antenna capabilities to improve the performance of wireless multihop networks

From Omni Antennas … E silenced silenced M A silenced F silenced Y G silenced silenced X K B silenced D silenced silenced

To Beamforming Antennas Y C S D G X K B D

To Beamforming Antennas Y C S D G X K B D

Today Antenna Systems  A quick look New challenges with beamforming antennas

Antenna Systems Signal Processing and Antenna Design research Several existing antenna systems Switched Beam Antennas Steerable Antennas Reconfigurable Antennas, etc. Many becoming commercially available For example …

Electronically Steerable Antenna [ATR Japan] Higher frequency, Smaller size, Lower cost Capable of Omnidirectional mode and Directional mode

Switched and Array Antennas On poletop or vehicles Antennas bigger No power constraint

Antenna Abstraction 3 Possible antenna modes Omnidirectional mode Single Beam mode Multi-Beam mode Higher Layer protocols select Antenna Mode Direction of Beam

Antenna Beam Energy radiated toward desired direction Pictorial Model Main Lobe (High gain) A A Sidelobes (low gain) Pictorial Model

Directional Reception Directional reception = Spatial filtering Interference along straight line joining interferer and receiver C C Signal Signal A B A B Interference D Interference D No Collision at A Collision at A

Will attaching such antennas at the radio layer yield most of the benefits ? Or Is there need for higher layer protocol support ?

We design a simple baseline MAC protocol (a directional version of 802.11) We call this protocol DMAC and investigate its behavior through simulation

DMAC Example Remain omni while idle Nodes cannot predict who will trasmit to it Y S D X

DMAC Example Assume S knows direction of D RTS Y S D X

X silenced … but only toward direction of D DMAC Example RTS CTS RTS Y S DATA/ACK D X silenced … but only toward direction of D X

Performance benefits appear obvious Intuitively Performance benefits appear obvious

However … Throughput (Kbps) Sending Rate (Kbps)

Clearly, attaching sophisticated antenna hardware is not sufficient Simulation traces revealed various new challenges Motivates higher layer protocol design

Antenna Systems  A quick look New challenges with beamforming antennas

New Challenges [Mobicom 02] Self Interference with Directional MAC

Unutilized Range Longer range causes interference downstream Offsets benefits Network layer needs to utilize the long range Or, MAC protocol needs to reduce transmit power Data A B C D route

New Hidden Terminal Problems New Challenges II … New Hidden Terminal Problems with Directional MAC

New Hidden Terminal Problem Due to gain asymmetry Node A may not receive CTS from C i.e., A might be out of DO-range from C CTS RTS Data A B C

New Hidden Terminal Problem Due to gain asymmetry Node A later intends to transmit to node B A cannot carrier-sense B’s transmission to C CTS RTS Data Carrier Sense A B C

New Hidden Terminal Problem Due to gain asymmetry Node A may initiate RTS meant for B A can interfere at C causing collision Collision Data RTS A B C

New Hidden Terminal Problems New Challenges II … New Hidden Terminal Problems with Directional MAC

New Hidden Terminal Problem II While node pairs communicate X misses D’s CTS to S  No DNAV toward D Y S Data Data D X

New Hidden Terminal Problem II While node pairs communicate X misses D’s CTS to S  No DNAV toward D X may later initiate RTS toward D, causing collision Collision Y S Data D RTS X

New Challenges III … Deafness with Directional MAC

Deafness Node N initiates communication to S S does not respond as S is beamformed toward D N cannot classify cause of failure Can be collision or deafness M Data S D RTS N

Channel Underutilized Collision: N must attempt less often Deafness: N should attempt more often Misclassification incurs penalty (similar to TCP) M Data S D RTS N Deafness not a problem with omnidirectional antennas

Deafness and “Deadlock” Directional sensing and backoff ... Causes S to always stay beamformed to D X keeps retransmitting to S without success Similarly Z to X  a “deadlock” Z DATA RTS S D RTS X

The bottleneck to spatial reuse New Challenges IV … MAC-Layer Capture The bottleneck to spatial reuse

Capture Typically, idle nodes remain in omni mode When signal arrives, nodes get engaged in receiving the packet Received packet passed to MAC If packet not meant for that node, it is dropped Wastage because the receiver could accomplish useful communication instead of receiving the unproductive packet

Capture Example A B C D A B C D B and D beamform to receive arriving signal Both B and D are omni when signal arrives from A

Take Away Message Technological innovations in many areas Several can help the problem you are trying to solve Although gains may not come by plug-and-play New advances need to be embraced with care. Directional/Beamforming antennas, a case study Gains seemed obvious from a high level Much revisions to protocols/algorithms needed Some of the systems starting to come out today Above true for projects you will do in class

Rate Control in Wireless Networks Romit Roy Choudhury Rate Control in Wireless Networks

Recall 802.11 RTS/CTS + Large CS Zone Alleviates hidden terminals, but trades off spatial reuse E RTS F CTS A B C D

Recall Role of TDMA

Recall Beamforming Ok No e Silenced Node f f a b c a b c d d Omni Communication Directional Communication e Silenced Node f f a b c a b c d d Simultaneous Communication No Simultaneous Communication Ok

Also Multi-Channel Current networks utilize non-overlapping channels Channels 1, 6, and 11 Partially overlapping channels can also be used

Also Data Rates Benefits from exploiting channel conditions Rate adaptation Pack more transmissions in same time

What is Data Rate ? Number of bits that you transmit per unit time under a fixed energy budget Too many bits/s: Each bit has little energy -> Hi BER Too few bits/s: Less BER but lower throughput

802.11b – Transmission rates 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time Highest energy per bit Lowest energy per bit 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time Optimal rate depends on SINR: i.e., interference and current channel conditions

Some Basics C = B * log2(1 + SINR) Eb / N0 = SINR * (B/R) Friss’ Equation Shannon’s Equation Bit-energy-to-noise ratio C = B * log2(1 + SINR) Eb / N0 = SINR * (B/R) Varying with time and space How do we choose the rate of modulation Leads to BER

Static Rates SINR Time # Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly (i.e., E b / N 0 > thresh) most of the times # Failure in some cases of fading  live with it

Adaptive Rate-Control SINR Time # Observe the current value of SINR # Believe that current value is indicator of near-future value # Choose corresponding rate of modulation # Observe next value # Control rate if channel conditions have changed

Rate and Range Rate = 10 B C E A D

Rate and Range There is no free lunch  talking slow to go far B C E A

Will carrier sense range vary with rate Any other tradeoff ? Will carrier sense range vary with rate

Total interference Carrier sensing estimates energy in the channel. Rate = 10 B C E A D Rate = 20 Carrier sensing estimates energy in the channel. Does not vary with transmission rate

Bigger Picture Rate control has variety of implications Any single MAC protocol solves part of the puzzle Important to understand e2e implications Does routing protocols get affected? Does TCP get affected? …

A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks Gavin Holland HRL Labs Nitin Vaidya Paramvir Bahl UIUC Microsoft Research MOBICOM’01 Rome, Italy © 2001. Gavin Holland

Background Current WLAN hardware supports multiple data rates 802.11b – 1 to 11 Mbps 802.11a – 6 to 54 Mbps Data rate determined by the modulation scheme

Problem Modulation schemes have different error characteristics 8 Mbps 1 Mbps BER But, SINR itself varies With Space and Time SNR (dB)

Mean Throughput (Kbps) Impact Large-scale variation with distance (Path loss) 8 Mbps Path Loss SNR (dB) Mean Throughput (Kbps) 1 Mbps Distance (m) Distance (m)

Small-scale variation with time (Fading) Impact Small-scale variation with time (Fading) Rayleigh Fading SNR (dB) 2.4 GHz 2 m/s LOS Time (ms)

Question Which modulation scheme to choose? SNR (dB) SNR (dB) 2.4 GHz 2 m/s LOS Distance (m) Time (ms)

Answer  Rate Adaptation Dynamically choose the best modulation scheme for the channel conditions Desired Result Mean Throughput (Kbps) Distance (m)

Design Issues How frequently must rate adaptation occur? Signal can vary rapidly depending on: carrier frequency node speed interference etc. For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis Coherence time of channel higher than transmission time

Adaptation  At Which Layer ? Cellular networks Adaptation at the physical layer Impractical for 802.11 in WLANs Why?

Adaptation  At Which Layer ? Cellular networks Adaptation at the physical layer Impractical for 802.11 in WLANs For WLANs, rate adaptation best handled at MAC Why? RTS/CTS requires that the rate be known in advance D C B A CTS: 8 RTS: 10 10 8 Sender Receiver

Who should select the data rate? B

Who should select the data rate? Collision is at the receiver Channel conditions are only known at the receiver SS, interference, noise, BER, etc. The receiver is best positioned to select data rate A B

Previous Work PRNet Pursley and Wilkins Periodic broadcasts of link quality tables Pursley and Wilkins RTS/CTS feedback for power adaptation ACK/NACK feedback for rate adaptation Lucent WaveLAN “Autorate Fallback” (ARF) Uses lost ACKs as link quality indicator

Lucent WaveLAN “Autorate Fallback” (ARF) DATA 2 Mbps Effective Range 1 Mbps Sender decreases rate after N consecutive ACKS are lost Sender increases rate after Y consecutive ACKS are received or T secs have elapsed since last attempt

Performance of ARF SNR (dB) Time (s) Rate (Mbps) Time (s) Dropped Packets Rate (Mbps) Time (s) Failed to Increase Rate After Fade Attempted to Increase Rate During Fade Slow to adapt to channel conditions Choice of N, Y, T may not be best for all situations

RBAR Approach Move the rate adaptation mechanism to the receiver Better channel quality information = better rate selection Utilize the RTS/CTS exchange to: Provide the receiver with a signal to sample (RTS) Carry feedback (data rate) to the sender (CTS)

Receiver-Based Autorate (RBAR) Protocol CTS (1) RTS (2) 2 Mbps 1 Mbps D DATA (1) RTS carries sender’s estimate of best rate CTS carries receiver’s selection of the best rate Nodes that hear RTS/CTS calculate reservation If rates differ, special subheader in DATA packet updates nodes that overheard RTS

Performance of RBAR SNR (dB) Time (s) Rate (Mbps) Time (s) Rate (Mbps) ARF Time (s)

Question to the class There are two types of fading Short term fading Long term fading Under which fading is RBAR better than ARF ? Under which fading is RBAR comparable to ARF ? Think of some case when RBAR may be worse than ARF

Rate Selection and Fairness

Motivation Consider the situation below A B C

Throughput Fairness vs Temporal Fairness Motivation What if A and B are both at 56Mbps, and C is often at 2Mbps? Slowest node gets the most absolute time on channel? A B C Timeshare A B C Throughput Fairness vs Temporal Fairness

Rate Selection and Scheduling Goal Exploit short-time-scale channel quality variations to increase throughput. Issue Maintaining temporal fairness (time share) of each node. Challenge Channel info available only upon transmission

Approaches RBAR picks best rate for the transmission Not necessarily best for network throughput Idea: Wireless networks have diversity Exploiting this diversity can offer benefits Transmit more when channel quality great Else, free the channel quickly

Opportunistic Rate Control Idea (OAR) Basic Idea If bad channel, transmit minimum number of packets If good channel, transmit as much as possible D C A B Data Data Data Data A Data Data Data Data C

Why is OAR any better ? 802.11 alternates between transmitters A and C Why is that bad D C A B Data Data Data Data A Data Data Data Data C Is this diagram correct ?

Why is OAR any better ? Bad channel reduces SINR  higher Tx time Fewer packets can be delivered D C A B Data Data Data A Data Data C

These are only first cut ideas … Much advanced research done after these, covered in wireless courses However, important to understand these basics, even for mobile application design