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Wireless ad hoc networks: cross layer opportunities NSF workshop Washington DC Aug 27-28 Mario Gerla Computer Science Dept, UCLA www.cs.ucla.edu Ad hoc networking Current Status Leading Applications • Tactical battlefield: – no infrastructure • Civilian emergency: – infrastructure, if present, was destroyed • Critical Requirements: scalability, survivability, 100% reliable, QoS, jam protection, etc • Non critical: Cost, Standards, Privacy SATELLITE COMMS SURVEILLANCE MISSION SURVEILLANCE MISSION UAV-UAV NETWORK AIR-TO-AIR MISSION STRIKE MISSION COMM/TASKING COMM/TASKING Unmanned Control Platform COMM/TASKING RESUPPLY UAV-UGV NETWORK MISSION FRIENDLY GROUND CONTROL (MOBILE) Manned Control Platform Tactical Ad Hoc Network Emerging Landscape : “Opportunistic” Ad Hoc networks Recreational, commercial, education applications • Vehicle networks • Workgroups (eg, sharing 3G via Bluetooth) • Massive Network games • Patient monitoring Access to Internet? • available, but - “bypass it” with “ad hoc” if too costly or inadequate Tolerant to delays: DTNs Critical: Cost, Privacy, security, standards Car to Car communications for Safe Driving Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 65 mph Vehicle type: Cadillac XLR Acceleration: - 5m/sec^2 Curb weight: 3,547 lbs Coefficient of friction: .65 Speed: 75 mph Driver Attention: Yes Acceleration: + 20m/sec^2 Etc. Coefficient of friction: .65 Alert Status: None Driver Attention: Yes Etc. Alert Status: None Alert Status: Inattentive Driver on Right Alert Status: Slowing vehicle ahead Alert Status: Passing vehicle on left Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 45 mph Vehicle type: Cadillac XLR Acceleration: - 20m/sec^2 Curb weight: 3,547 lbs Coefficient of friction: .65 Speed: 75 mph Driver Attention: No Acceleration: + 10m/sec^2 Alert Status: Passing Vehicle on left Etc. Coefficient of friction: .65 Driver Attention: Yes Etc. Co-operative Download: Car Torrent Internet Vehicle-Vehicle Communication Exchanging Pieces of File Later Vehicular Sensor Network (VSN) Roadside base station Vehicle-to-roadside Inter-vehicle communications communications VSN-enabled vehicle Sensors Systems Video Chem. Storage Proc. Personal Networking: BlueTorrent B B B C C C A A A D D D Patient Monitoring Nurses upload patient data; share data files in P2P mode 1. Future expectations on wireless network research • Network layer more tightly coupled with applications – Content sharing, environement sensing • Besides data forwarding, additional services: – Location aware service discovery, – content based routing; – P2P networking – Data collection, processing, filtering, storage, dissemination • Network layer design must interact with: – applications – PHY Layer 2. Major recent advances/breakthroughs in the physical layer • Cognitive radios (spectrum scavenging) • MIMOs (for flexible topology designs; interference mitigation etc) • Cooperative radios • Multi radio devices (BT, 802.11, 3G, etc) 3. Algorithms must adjust to PHY layer “PHY layer aware” MAC, Network and Transport designs Examples (based on MIMO): • Topology control • A MIMO aware MAC protocol: SPACE-MAC • Multi-path Routing & MIMO • TCP & MIMO MIMO Topology Control/Routing • Topology control: – Exploit mode flexibility to dynamically shape topology – Meet different customer requirements Topology with high capacity Topology with low capacity links: disconnected network links: fully connected network 300Mbps 10Mbps SPACE2 MAC When A wishes to transmit to B B 1) A sends RTS to B; F and F D learn about A 2) B responds with CTS; F and D learn about B D A SPACE MAC (cont) B 3) F and D beamform such F that signals from/to B and A are nulled; then, A and B start talking 4) After A and B pair is established, F and D pair D A also can talk Two-Path Routing using MIMO relay nodes S R sender receiver • S sends two independent streams simultaneously to R • Assume 2 antennas at each node (but extendible to systems with more antennas). MIMO yields 6-fold throughput gain S R sender receiver • In the traditional relay mode, the capacity is C/3. • Simulcast achieves 6-fold throughput increase. TCP and MIMO in Ad Hoc Networks • Consider three flows in the same wireless domain • As the flows get closer to each (100, 100) other: FTP 1 (600, 100) – Interference builds up – Throughput decreases (350, 350) – Fairness suffers FTP 2 • Can MIMO Help? (0, 700) (350, 700) (700, 700) (350, 1050) FTP 3 (100, 1300) (600, 1300) TCP over SPACE MAC (MIMO) Distance = 400m (interference range) 3 FTP/TCP Flows Flow 1 Flow 2 Flow 3 T hroughput (Kbits/s) 350 300 250 200 150 100 50 0 802.11 SPACE-MAC MAC Protocol Fig 0. The throughput of 3 FTP/TCP flows with the distance between flows being 400m TCP over SPACE MAC (MIMO) Distance = 350m (tx range) 3 FTP Flows Flow 1 Flow 2 Flow 3 Throughput (Kbits/s) 350 300 250 200 150 100 50 0 802.11 Space-MAC MAC Protocol Fig 0. The throughput of 3 FTP/TCP flows with the distance between flows being 350m Identify gaps • Question: How to exploit the wealth of PHY emerging technology? • Do not limit your scope to LINK capacity gains • Look for cross layer optimization opportunities at all layers: – MAC – Network (routing, topology control, multicast, bandwidth scavenging, etc) – transport, – applications and PHY layer The End Thank You Simul-Cast Brian Choi Mario Gerla MIMO System Model weight vector w1 = [w11 w21 … wm1] W s(t)WHVH = r(t) V Assumptions • Fading is flat (i.e. freq. independent). • Channel is symmetric and quasi-static. • Two subchannels - control channel and data channel • Rich scattering - H is full-rank • Antenna’s capable of transmitting and receiving signals simultaneously. • We ignore additive channel noise. • Perfect sychronization between nodes Two Path Routing Problem relay nodes S R sender receiver • S sends two independent streams under two paths simultaneously to R. • Assume 2 antennas at each node (but extensible to systems with more antennas). The 6-fold Benefit of MIMO S R sender receiver • If C = (capacity of a point-to-point link) in the traditional relay mode, the capacity is C/3. • Simulcast achieves 6X throughput increase. Sender s(t) = [s1(t) s2(t)] B s1(t) A s2(t) C • A wants to send a stream (s1(t)) to B and another stream (s2(t)) to C simultaneously. Sender: Linear Coding WB s(t) = [s1(t) s2(t)] HAB rB1(t) B s1(t) rB2(t) A s2(t) WA rC1(t) HAC C rC2(t) • B receives rB(t) = s(t)WAHABWBH. • For B to recover s1(t), B must consume 2 degrees of freedom. Sender: Pre-coding rB1(t) B rB2(t) s1(t) A s2(t) rC1(t) C rC2(t) • If A knows the channel and the steering matrices of B and C, then A can precode its data such that s1(t) is received at rB1(t), s2(t) is received at rC1(t), without interfering each other. • B and C needs to comsume only one DOF each. Dirty Paper Coding wB1 rB1(t) B rB2(t) s1(t) A s2(t) wC1 rC1(t) C rC2(t) Let H = HABwB1H = QR HACwC1H QR factorization, Q = unitary, R = upper triangular Dirty Paper Coding • Let r(t) = [rB1(t) rC1(t)]. Then r(t) = s(t)H. • Multiply s(t) by QH, such that s’(t) = s(t)QH. • Then r(t) = s’(t)H = s(t)QHH = s(t)QHQR = s(t)R • rB1(t) = s1(t)R1,1 (no interference) • rC1(t) = s1(t)R1,2 + s2(t)R2,2 • Sender can estimate this interference and subtract it from s(t) before transmitting. (interference!) Relay Node used to send a stream to the next node used to receive data from the previous node • There is one DOF left for us to use. We use it to simultaneously relay the received data to the next node. • We set weight vectors such that they are orthogonal to each other. Receiver A HAR R HBR B • This reduces to the problem of spatial multiplexing. • If R knows the channels and the weight vectors used for both streams, then R can decode the received data. Network-wise Benefit S R sender receiver • If C = (capacity of a point-to-point link) in the traditional relay mode, the capacity is C/3. • Simulcast achieves 6X throughput increase. Multiple Paths • We can run OLSR-type of routing protocol for the nodes to pre-determine the paths. • This suggests a cross-layer approach (between network layer and MAC layer). Summary • With MIMO and Pre-coding techniques, one can effectively reduce the DOF consumption at the receiving nodes. • We can utilize the idle DOF to relay the data simultaneously. • With two independent simultaneous paths, we can achieve up to 6X throughput increase.
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