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Cross layer design for Wireless networks Kavé Salamatian LIP6-UPMC Future Wireless Systems Ubiquitous Communication Among People and Devices Nth Generation Cellular Wireless Internet Access Wireless Video/Music Wireless Ad Hoc Networks Sensor Networks Smart Homes/Appliances Automated Vehicle Networks All this and more… Next generation network architecture Internetworking Layer Mobility Services Layer Network Service Layer Local Service Layer Access Management Radio Layer Access Layer Access Interface Layer Mobile Terminal Layer Wireless Interface Layer Mobile Application Layer Internet Wireless PSTN Radio Access Network Mobile User Equipment (e.g. Win9X, Palm OS) Application Network Server (e.g. WinNT, Unix) Radio Access Network Radio Resource Mgmt Application IP Transport (TCP, UDP, RTP) Internet Protocol (IP) Ethernet Modem Radio Access IP Transport (TCP, UDP, RTP) Transport Agents Transport Agents Radio Access L2 L2 IP Internet Protocol (IP) Access Core L2 L2 Internet Radio Access L1 L1 Access Core L1 L1 Radio-Optimized IP Networking • Transparent to TCP/IP protocols • Enables deployment of IP-based consumer applications in next generation wireless systems Ethernet ATM Separation principles Application, transport and physical layer can be separated if : No errors at physical layer No losses and delays at transport layer No fluctuations in applications rate Each layer being perfect from the point of view of other layers Application Signal Transport Packet Physical Bits Challenges Wireless channels are a difficult and capacitylimited broadcast communications medium Traffic patterns, user locations, and network conditions are constantly changing Applications are heterogeneous with hard constraints that must be met by the network Energy and delay constraints change design principles across all layers of the protocol stack These challenges apply to all wireless networks, but are amplified in ad hoc/sensor networks Why is Wireless Hard? The Wireless Channel Fundamentally Low Capacity: R< B log(1+SINR) bps Spectrum scarce and expensive Received power diminishes with distance Self-interference due to multipath Channel changes as users move around Signal blocked by objects (cars, people, etc.) Broadcast medium – everyone interferes d …And The Wireless Network Wireline Backbone Link characteristics are dynamic Network access is unpredictable and hard to coordinate Routing often multihop over multiple wireless/wired channels Network topology is dynamic Different applications have different requirements Design objective Want to provide end-to-end Properties The challenge for this system is dynamics Scheduling can help shape these dynamics Adaptivity can compensate for or exploit these dynamics Diversity provides robustness to unknown dynamics Scheduling, adaptivity, and diversity are most powerful in the context of a crosslayer design Energy must be allocated across all protocol layers Multilayer Design Hardware Power or hard energy constraints Size constraints Link Design Time-varying low capacity channel Multiple Access Resource allocation (power, rate, BW) Interference management Networking. Routing, prioritization, and congestion control Application Real time media and QOS support Hard delay/quality constraints Multilayer Design Crosslayer Techniques Adaptive techniques Link, MAC, network, and application adaptation Resource management and allocation (power control) Synergies with diversity and scheduling Diversity techniques Link diversity (antennas, channels, etc.) Access diversity Route diversity Application diversity Content location/server diversity Scheduling Application scheduling/data prioritization Resource reservation Access scheduling Key Questions What is the right framework for crosslayer design? What are the key crosslayer design synergies? How to manage its complexity? What information should be exchanged across layers, and how should this information be used? How do the different timescales affect adaptivity? What are the diversity versus throughput tradeoffs? What criterion should be used for scheduling? How to balance the needs of all users/applications? Single user example WIFI : (171,133) 0 10 -1 10 -2 10 Packet Error Rate -3 10 -4 10 -5 10 -6 10 1 2 3 4 5 6 SNR 7 8 9 10 Adaptive Modulation and Coding in Flat Fading Uncoded Data Bits Point Selector Buffer log2 M(g) Bits g(t) One of the M(g) Points M(g)-QAM Modulator Power: S(g) g(t) To Channel Adapt transmission to channel Parameters: power,rate,code,BER, etc. Capacity-achieving strategy Recent Work BSPK 4-QAM 16-QAM Adaptive modulation for voice and data (to meet QOS) Adaptive turbo coded modulation (<1 db from capacity) Multiple degrees of freedom (only need exploit 1-2) Adaptive power, rate, and compression with hard deadlines Crosslayer design in multiuser systems • Users in the system interact (interference, congestion) • Resources in the network are shared • Adaptation becomes a “chicken and egg” problem • Protocols must be distributed Wireless networks They are formed by nodes with radios There is no a priori notion of “links” Nodes simply radiate energy Nodes Cooperation Decode and forward Why not: Amplify and Forward Increase Signal for Receiver Why not: Reduce Interference at Receiver How should node cooperates ? Some obvious choices Should nodes relay packets? Should they amplify and forward? Or should they decode and forward? Should they cancel interference for other nodes? Or should they boost each other’s signals? Should nodes simultaneously broadcast to a group of nodes? Should those nodes then cooperatively broadcast to others? What power should they use for any operation? … Or should they use much more sophisticated unthought of strategies? Example: Six Node Network Capacity Regions (Goldsmith) Rij 0, ij 12,34, i j Multiple hops Spatial reuse SIC (a): Single hop, no simultaneous transmissions. (b): Multihop, no simultaneous transmissions. (c): Multihop, simultaneous transmissions. (d): Adding power control (e): Successive interference cancellation, no power control. Optimal Routing The point R12 R34 1.64 Mbps following scheduling : is achieved by the Adaptive Rate MAC (Kumar) Idea: Adapt transmission rate according to channel quality Change modulation to get higher rate if channel is good Could send multiple packets at higher rates (A suggested cscheme) Protocol details RTS/CTS and Broadcast packets sent at lowest rate Receiver measures strength of RTS Communicates rate to sender in CTS DATA and ACK at that rate Interaction with Min Hop Routing Protocol Most current routing protocols are min hop Consider DSDV for example Chooses long hops But long hops => low signal strength => low rates Switching off adaptation is better Routing based approach Luigi & al. Routing in wireless network « Shortest path approche is not optimal » Physical channel is instable Each transmission inject interference in the network Interference reduce capacity Power management is needed Make use of multi-rate and power control on WIFI card L’architecture en couches n’est pas optimale Cross Layer approch Maximise throughput Gupta & Kumar Rate Transmission range Node number Throughput To maximise throughput we have to maximise transmission rate and reduce interference generated by each packets Capacity Constraints Cross-Layer Approach Routing metric Rate Interference Packet Error Rate SIR Interface characteristics Next-Hop Data-Rate Transmission power Interference Measurement: unrealistic More neighbor => More interference More power => More interference Defining a interference replacement function I(P) Minimise I(P) => Minimise Real interference Packet Error Rate (I) IP packet IP packet MAC MAC Convolution Coder Viterbi Decoder Interleaver Deinterleaver Modulator & Scrambler Interference Noise (White or fading) Channel Single Antenna Multiple Antenna Rake Receiver Packet Error Rate (III) BER PERSIR f Pf E L Routing Strategy • Rate (Mbps) •Maximise •Interference (mW) • Minimise •PER • Minimise •Power (mW) • trade off for optimising routing parameter •NP-Complet Problème Routingless approach Ramin & al. Ad-Hoc Network Ad Hoc Networks function by multi-hop transport Nodes relay packets until they reach their destinations Must of the traffic carried by the nodes is relay traffic The actual useful traffic per user pair is small Intermediate nodes relay the same information Duplicated information might be received by the receiver More intelligent relaying is needed Which packet to relay Which information to relay • The relay nodes must only send useful information Coding for erasure channels MDS (Maximum Distance Separable) codes Get k packets, generates n-k redundant packets Each combination of k packets out of n enable to retrieve the initial packets Generating matrix C I k k Bk ( n k ) • Each submatrix of Bk( nk ) is invertible Reed Solomon codes are MDS We suppose that sender generates m redundant packets We suppose that relay generates l packets How to choose m and l to achieve the bound Achievability of the capacity bound for the more capable case Choose a code length n. Knowing packet loss matix of the netwok R and opt can be determined. We chose then k nR, l n opt The code C I k k Bk0( nk ) Bk1l is a MDS code The receiver is able to retrieve the k initial packets if it receives at least k packets from sender and relay together This happen asymptotically with large n if the rate validate the bound W X I k k Bk0( n k ) X 1 W Bk1l 1 p2 p1 W X W I k k Bk0( n k ) p W X , X1 C W Comments & practical consideration Relay send only useful side information over the channel The relay load is chosen as the minimal value which maximize the global rate Each sender and relay can derivate the number of needed redundant packets if it know the packet loss probability matrix The proposed scheme can be implemented very easily in WiFi based wireless network Does not need any change to physical layer Practical implementation 15 node distributed randomly in the environment One Sender-Receiver pair is chosen randomly each node have two cart WiFi, with different frequency channels f1 and f2 If one node receive the packets It can be a relay with probability p The relay nodes broadcast information in the environment There is not any routing protocol It is done in NS Topology 600 500 Receiver 400 300 200 100 Sender 0 0 100 200 300 400 500 600 Throughput and relay load 6 10 5 10 4 10 3 10 2 10 -3 10 -2 10 -1 10 0 10 35 30 25 20 15 10 5 0 -3 10 -2 10 -1 10 0 10 Toward Software radio Antenna Common DSP platform Tx Chan Interface UpconD/A verter Channelizer Interface Wideband transceiver MCPA Rx Chan A/D Interface Dup LNA RF/IF Network ATM I/F Cellsite controller middleware • Common technology for multiple radio platforms Conclusions Crosslayer design needed to meet requirements and constraints of future wireless networks Key synergies in crosslayer design must be identified The design must be tailored to the application Crosslayer design should include adaptivity, scheduling and diversity across protocol layers Energy can be a precious resource that must be shared by different protocol layers Coming Challenges MIMO: how to take advantage of Multiple Antenna Software Radio: How to enable adaptation of physical layer from upper layer Interesting Question MIMO or Ad Hoc, that’s the question? Routing can be seen as a diversity Not shortest path !