Wireless Mesh Networks: First part: an Overview

1. Wireless Mesh Networks:First part:an Overview 2-nd Workshop on WOMEN Project University of Rome “La Sapienza”, INFOCOM Dept. (Faculty of Engineering) Rome September 8-th, 2006

2. Outline Wireless Mesh Network: Definition and Characteristics Wireless Mesh Networks: application scenarios MAC layer solutions currently adopted Slotted Seeded Channel Hopping (SSCH) Network layer solutions currently adopted Dynamic Source Routing (DSR) Conclusions and future researches

3. Wireless Mesh Networks: Definition A Wireless Mesh Network is a multi-hop distributed mesh topology system, with self-configuration and self-organization capabilities, where each node is potentially able to forward Informative Unitstoward other nearby nodes I.F. Akyildiz, X. Wnag, W. Wang, “Wirless Maesh Netowrks: a survey” , Computer Netwroks No.47, pp. 445-487, 2005.

4. Wireless Mesh Networks: Characteristics (1/2) 1. Auto-configuration: all network nodes are designed to self-discover their neighbors and paths without needing of any centralized network entity • Auto-organization: nodes can autonomously resolve Out-of-Service events, due to temporary off or congested radio links, by exploiting the Mesh Topology • Scalability: the covered area can be extended by simply adding new nodes to the current Mesh Network • Mobility: the nodes can move on a limited area and keep the connectivity with (at least) a network node

5. Wireless Mesh Networks: Characteristics (2/2) • Mesh Clients: mobile and peripheral nodes able to communicate with other nodes only through radio interfaces. Minimal routing functions are solved by them. Moreover, they are power constrained, typically low cost and developed on already existing Wireless Cards (e.g., 802.11a/b/g Network Interface Cards (NIC) ) • Mesh Routers: nodes with minimum (or null) mobility, constituting the network backbone, with radio interfaces towards the mesh clients and mesh routers and wired interfaces towards the outside network. They are not power constrained, can process the most of network traffic and results more expensive than the mesh clients. • Additional features of the Wireless Mesh Networks: Currently there is no standard, and open questions are related to the security aspects and to proper MAC protocol developments

6. Wireless Mesh Networks: architectures (1/3)(Infrastructure/backbone) IEEE 802.16 • This architecture is composed by mesh routers which are employed for the wireless backbone and mesh clients are excluded by the mesh topology • Connections among the mesh routers are realized with IEEE802.16 technology • Mesh routers function also as gateway for Internet access

7. Wireless Mesh Networks: architectures (2/3)(Client-Mesh) IEEE 802.11 • This architecture is composed by self-configured Mesh Clients with routing functions • It represents the mesh network operating in ad-hoc mode • Currently wireless links are IEEE 802.11 based

8. Wireless Mesh Networks: architectures (2/3)(Hybrid-Mesh) IEEE 802.16 • This architecture given by combing the two previous ones • Mesh Clients can access at the network through mesh routers as well as directly with other mesh clients IEEE 802.11

9. Application Scenarios (1/2) Community Networking Broadband home networking • Low cost alternative to link difficult areas to be cabled • Alternative to IEEE 802.11 and Bluetooth standards

10. Application Scenarios (2/2) They can be view as a low cost solution of wide band access networks Metropolitan Wireless Mesh Networks

11. MAC layer solutions • Mac protocol for Wireless Mesh Networks has to consider several differences with those employed by the WLANs: 1) The multi-hop environment 2) All the architectures are distributed and each node is involved to the cooperation of the network traffic management • Currently, the proposed MAC protocols are mainly based on two methods: • “Virtual” MAC protocols working on the top of existing MAC protocols P.Bahl, R. Chandra, “SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in IEEE 802.11 Wireless ad-hoc networks” • Innovative MAC protocols with features similar to those proposed for ad-hoc wireless networks J.W. Kim, N. Bambos, “Power Efficient MAC scheme using Channel Probing in Multirate Wireless Ad-hoc Networks“

12. SSCH is a protocol for ad-hoc wireless networks using IEEE 802.11 standard and exploits IEEE 802.11 MAC layer service It can be simply implemented via software into a device equipped with a Wireless Network Interface Card (NIC) and IEEE 802.11 standard compliant Its task is to extend the channelization of the IEEE 802.11a (13 channels), IEEE 802.11 b/g (11 channels) standard to the ad-hoc networks so to increase the throughput of each node Each node is equipped with a Channel Scheduler for the channel/frequency hopping Each node is equipped with a Packet Scheduler where the flow management is given by per-neighbor FIFO queues which are maintained in a priority queue ordered by perceived neighbor reachability Slotted Seeded Channel Hopping(SSCH)

13. SSCH Performances Disjoint flows • As the number of flows increases, SSCH considerably exceeds the IEEE 802.11a performances Non-disjoint flows

14. Dynamic Source Routing (1/2) • Routing protocol for ad-hoc wireless networks with low mobility nodes • Differently to “Distance vector” or “Link State” based protocols, Dynamic Source Routing does not use periodic routing advertisement messages • It is based on Source Routing technique: before transmitting, each node evaluates the nodes’ sequence (hop) through which the packets are forwarded toward the destination node Route Discovery • A control procedure is adopted for the correct packet reception and is based on data link acknowledgement between two adjacent nodes Route Maintenance

15. Routing length between a factor 1.01 and 1.09 from the optimal case DSR Performances • Overhead: ratio from 1.01 to 2.6 from the optimal case

16. Conclusions Wireless Mesh Networks are considered as a flexible, performing and low cost alternative to current WLANs Currenlty, the proposed solutions are of proprietary type (MIT, Roofnet, Nokia, Mesh Connectivity Layer) and are essentially based on the IEEE 802.11 a/b/g standards The MAC (SSCH) and routing (DSR) protocols currently adopted result to be extremely simple to be implemented By the end of 2006 IEEE 802.11s Mesh standard is expected to be ratified

17. Wireless Mesh Networks:Second Part:Publications

18. WOmEn Project First Publication Title of Paper :“Optimized Power Allocation for Multi-Antenna Systems impaired by Multiple-Access Interference and Imperfect Channel-Estimation” Authors: E.Baccarelli, M.Biagi, C.Pelizzoni, N.Cordeschi Accepted on IEEE Tr. On Vehicular Technology

19. Outline System Model (Wireless MIMO channel) Mean Mutual Information Power-Constrained Maximization of the Mean Mutual Information System Nodes Interaction: the Game Theory Approach Spatial-Power Allocation Multi-Antenna (SPAM) Game for Ad -hoc networks SPAM game-vs.-collision-free Access strategies Conclusions

20. Tx2 Rx1 Rx2 Tx1 Rx0 Tx0 System Model-(MIMO Wireless Ad-Hoc Network) (1/2)

21. System Model (2/2) Multiple Access Interference (MAI) Tx0-Rx0 Reference link

22. Payload Phase (Tx0-Rx0 reference link) • The overall observed signal vector during the payload phase • The informationstream is power constrained as:

23. The choice of is finalized to reach the system capacity User Information Throughput and Capacity We adopt Gaussian distributed input signals for computing the following information throughput: Under some conditions we have derived the Gaussian Throughput is equal to the Capacity

24. Mean Mutual Information Such expression is valid under some conditions we have derived and reported into the Paper

25. Problem: evaluate and Maximization of the User Information Throughput • It has been derived the Power Allocation Algorithm in order to find the optimal expressions of P*(1)….P*(s). It reduces to the Water Filling Approach when perfect Channel estimation is considered.

26. Modelling of the Nodes Interaction (1/2) Game Theory Approach The Game Theory is adopted in order to consider the node interaction and the dynamic ad –hoc network topology F.R. Farrokhi, etc… “Link-Optimal Space-Time Processing with Multiple Transmit and Receive Antennas” IEEE Commmunications Letters, Vol.5 March 2001.

27. Modelling of the Nodes Interaction (2/2)Game Theory Approach • MIMO ad-hoc network may be modelled as Noncooperative • Strategic Game • - set of pair; (players set) • - Action Set of node ; • - Utility Function of node . There have been found Existence and Uniqueness Conditions for the Nash Equilibrum

28. Spatial Power Allocation for Multi-Antenna (SPAM) Systems Setup Phase (Eigenvalues and Nash Equilibrium ) While Evaluate 3. Shape 4. 5. If go to 6 else go to 2 6. Evaluate the Throughput

29. SPAM Game-vs.-collision free access strategies(Examples of Throughput Regions for anhexagonal network) SNR=5dB, t=r=4. F.R. Farrokhi, etc… “Link-Optimal Space-Time Processing with Multiple Transmit and Receive Antennas” IEEE Commmunications Letters, Vol.5, March 2001.

30. Conclusions • The information throughput has been expressed in closed form for the general case of imperfect channel estimations and spatially colored MAI. • The power allocation and spatial shaping have been accomplished via the SPAM Game. • 3. The SPAM game is fully distributed, asyncronous, scalable access schemes. • 4. The SPAM game allows point-to-point throughput higher than those attainable via conventional orthogonal (e.g., collision-free) access schemes.

31. WOmEn Project Second Publication Title of Paper :“Interference Suppression in MIMO Systems for ThroughputEnhancement and Error Reduction” Authors: E.Baccarelli, M.Biagi, C.Pelizzoni, N.Cordeschi Proc. of IEEE International Wireless Communications and Mobile Computing Conference, 3-6 July 2006, Vancouver, pp.611-616.

32. Outline Reference MIMO Model Transmission Rate and Error Rate in multi-user environment Throughput enhancement and error reduction via interference cancellation Performances Impact on MAC and Routing of the pursued aproach Conclusions and work in progress

33. Reference MIMO Model We consider a scenario where a mesh router receives information bits in the presence of multi-user interference from different mesh clients

34. Reference MIMO Model The packet structure

35. Reference MIMO Model The received sequence, once acquired information about channel state (perfect CSI assumed) and interference is (under no CSI at the Tx)

36. spatially white noise Possible spatial coloration Induced by the channel Orthogonal STBC of n-th user Reference MIMO Model The interference is “generally” spatially colored since it depends on topology, so this last heavily influences the behavior of the system

37. Transmission Rate and Error Rate in multi user environment Double goal: High throughput with Low BER Requirement:High interference suppression capability since the transmission is simultaneous (collision) The final goal should be to transmit at high rate with very low bit error rates By defining the “net throughput” also known as “gooput” We have to maximize throughput and minimize error probability

38. Transmission Rate and Error Rate in multi user environment The throughput enhancement in the sense of “information rate” can be achieved by estimating interference at receiver side and by subtract it since interference reduces the capacity region At the same time, the error reduction can be obtained by reducing the effect of interference that, generally, increases BER

39. interference noise Throughput enh. and error red. via interference cancellation The transmitter avoids to transmit for TL slots so the receiver can estimate the statistical feature of V (multi-user interference) and the linear estimator is given by Reference signal And channel

40. Throughput enh. and error red. via interference cancellation The performances in terms of estimation error variance can be evaluated in the following way

41. Throughput enh. and error red. via interference cancellation The error variance reduces itself to for high noise and/or high level of reference signal for low noise and low level of reference signal

42. Throughput enhancement and error via interference cancellation The parameter that influences the performance is the SIR after cancellation

43. Throughput enhancement and error via interference cancellation To increase network rate means

44. Throughput enhancement and error via interference cancellation to reduce the error probability means to minimize q takes into account for the cardinality of modulation format

45. Performances performances in terms of BEP for different q (modulation formats)

46. Performances Net throughput for t=2, and different values of r

47. i.s. i.s. How interference Suppression can aid MAC? Approaches as CSMA usually tries to avoid collisions The packet is NOT dropped (MIMO) The packet is dropped (SISO)

48. Impact on MAC For sure the effect of interference suppression SIMPLIFIES the MAC procedures and the architecture The MAC may operate in severe interference suppression conditions that means when interference is comparable with the main signal

49. Impact on Routing Preliminary results show that MIMO system allows power saving strategies (SISO same performance with low power emission) and interference suppression allows us to consider quasi-orthogonal transmission This suggests that multi-hop approach is unnecessary in this operating conditions

50. Conclusions Interference suppression allow the system to e more simple at upper layers Without decreasing transmission rate (due to multi-user interference) we are able to assure good performances in terms of BER This does not require severe hardware complexity.