Cross layer design for Wireless networks Kav Salamatian

Cross layer design for Wireless networks Kavé Salamatian LIP 6 -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 Interface Layer Mobile Terminal Layer Wireless Interface Layer Mobile Application Layer Internet Wireless PSTN

Radio Access Network Mobile User Equipment (e. g. Win 9 X, Palm OS) Application IP Transport (TCP, UDP, RTP) Internet Protocol (IP) Ethernet Modem Radio Access Network Server (e. g. Win. NT, Unix) Radio Access Network Radio Resource Mgmt Application IP Transport (TCP, UDP, RTP) Transport Agents Radio Access L 2 IP Internet Protocol (IP) Access Core L 2 Internet Radio Access L 1 Access Core L 1 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 l 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 l Wireless channels are a difficult and capacitylimited broadcast communications medium l Traffic patterns, user locations, and network conditions are constantly changing l Applications are heterogeneous with hard constraints that must be met by the network l 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 l l l 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 l l l 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 l Want to provide end-to-end Properties l 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 l Scheduling, adaptivity, and diversity are most powerful in the context of a crosslayer design l Energy must be allocated across all protocol layers

Multilayer Design l Hardware ¡ ¡ l Link Design ¡ l Resource allocation (power, rate, BW) Interference management Networking. ¡ l Time-varying low capacity channel Multiple Access ¡ ¡ l Power or hard energy constraints Size constraints Routing, prioritization, and congestion control Application ¡ ¡ Real time media and QOS support Hard delay/quality constraints Multilayer Design

Crosslayer Techniques l Adaptive ¡ ¡ ¡ Link, MAC, network, and application adaptation Resource management and allocation (power control) Synergies with diversity and scheduling l Diversity ¡ ¡ ¡ techniques Link diversity (antennas, channels, etc. ) Access diversity Route diversity Application diversity Content location/server diversity l 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? l How to manage its complexity? l What information should be exchanged across layers, and how should this information be used? l ¡ 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

Adaptive Modulation and Coding in Flat Fading Uncoded Data Bits Buffer g(t) l log 2 M(g) Bits One of the M(g) Points M(g)-QAM Modulator Power: S(g) g(t) To Channel Adapt transmission to channel ¡ ¡ l Point Selector 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 l They are formed by nodes with radios ¡ There ¡ is no a priori notion of “links” Nodes simply radiate energy

Nodes Cooperation l l Decode and forward Why not: Amplify and Forward Increase Signal for Receiver l Why not: Reduce Interference at Receiver l

How should node cooperates ? l Some obvious choices ¡ ¡ ¡ ¡ ¡ l 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) 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 l The point following scheduling : is achieved by the

Adaptive Rate MAC (Kumar) l Idea: Adapt transmission rate according to channel quality ¡ ¡ l 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 l 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 » l Physical channel is instable l Each transmission inject interference in the network l ¡ l Power management is needed ¡ l Interference reduce capacity Make use of multi-rate and power control on WIFI card L’architecture en couches n’est pas optimale Cross Layer approch

Maximise throughput l 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 l Routing metric ¡ Rate ¡ Interference ¡ Packet Error Rate SIR Interface characteristics Next-Hop Data-Rate Transmission power

Interference l l l 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 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)

Routing Strategy • Rate (Mbps) • Maximise • Interference (m. W) • Minimise • PER • Minimise • Power (m. W) • trade off for optimising routing parameter • NP-Complet Problème

Routingless approach Ramin & al.

Ad-Hoc Network l Ad Hoc Networks function by multi-hop transport ¡ Nodes relay packets until they reach their destinations l l ¡ Intermediate nodes relay the same information l ¡ Must of the traffic carried by the nodes is relay traffic The actual useful traffic per user pair is small Duplicated information might be received by the receiver More intelligent relaying is needed l Which packet to relay l Which information to relay • The relay nodes must only send useful information

Coding for erasure channels l MDS (Maximum Distance Separable) codes ¡ Get k packets, generates n-k redundant packets l l Each combination of k packets out of n enable to retrieve the initial packets Generating matrix • Each submatrix of ¡ is invertible Reed Solomon codes are MDS We suppose that sender generates m redundant packets l We suppose that relay generates l packets l ¡ How to choose m and l to achieve the bound

Achievability of the capacity bound for the more capable case l Choose a code length n. Knowing packet loss matix of the netwok R and can be determined. We chose then ¡ l The code 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

Comments & practical consideration Relay send only useful side information over the channel l The relay load is chosen as the minimal value which maximize the global rate l Each sender and relay can derivate the number of needed redundant packets if it know the packet loss probability matrix l The proposed scheme can be implemented very easily in Wi. Fi based wireless network ¡ Does not need any change to physical layer l

Practical implementation l 15 node distributed randomly in the environment ¡ ¡ ¡ One Sender-Receiver pair is chosen randomly each node have two cart Wi. Fi, with different frequency channels f 1 and f 2 If one node receive the packets l ¡ The relay nodes broadcast information in the environment l l It can be a relay with probability p There is not any routing protocol It is done in NS

Topology

Throughput and relay load

Toward Software radio Antenna Tx Chan Common DSP platform Interface Upcon. D/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 l l l 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 l MIMO or Ad Hoc, that’s the question? ¡ Routing l can be seen as a diversity Not shortest path !