Module 4: Wireless Metropolitan and Wide Area Networks SMD161 Wireless Mobile Networks Kaustubh S. Phanse Department of Computer Science and Electrical Engineering Luleå University of Technology Lecture objectives Define wireless metropolitan and wide area networks Cellular networks Some background and history System architecture System design issues Mobility management IEEE 802.16 WiMax Motivation Physical and MAC layers SMD161 Wireless Mobile Networks 2 1
References T. Rappaport, Wireless Communications: Principles and Practice, Prentice Hall, 1996. W. C. Y. Lee, Mobile Cellular Telecommunications: Analog and Digital Systems, McGraw-Hill Publications, 2nd ed., 1995. C. Eklund, R. B. Marks, K. Stanwood and S. Wang, IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access, IEEE Communications Magazine, June 2002. S. J. Vaughan-Nichols, Achieving Wireless Broadband with WiMax, IEEE Computer, June 2004. SMD161 Wireless Mobile Networks 3 Metropolitan area wireless networks Broadband wireless connectivity (for the last-mile) Mostly fixed and low mobility IEEE 802.16 Infrastructure Residential broadband (DSL/cable alternative) Backhaul for local hotspots SMD161 Wireless Mobile Networks 4 High speed enterprise wide network 2
Wide area wireless networks Enable connectivity over national, continental or global level Seamless connectivity at high speed mobility Relatively low bandwidth (for now, higher bandwidth is expensive) GSM/UMTS, satellite systems Infrastructure Satellite systems Mobile cellular systems SMD161 Wireless Mobile Networks 5 Cellular systems have come a long way... Mobile Telephone Service (MTS) in New York (1976) Total of 33 channels covering an area of 50 miles in diameter Divided into three systems: MTS, MJ and MK systems MJ system served 225 customers with another 2400 on waiting list MK system served 225 customers with another 1300 on waiting list Overall, poor performance, but high demand and high blocking probability during busy hours SMD161 Wireless Mobile Networks 6 3
CDMA TDMA FDMA Evolution of mobile telecommunications systems CT0/1 AMPS NMT CT2 IS-136 TDMA D-AMPS GSM PDC IS-95 cdmaone GPRS cdma2000 1X 1G 2G 2.5G 3G SMD161 Wireless Mobile Networks 7 EDGE IMT-FT DECT IMT-SC IS-136HS UWC-136 IMT-DS UTRA FDD / W-CDMA IMT-TC HSDPA UTRA TDD / TD-CDMA IMT-TC TD-SCDMA IMT-MC cdma2000 1X EV-DO 1X EV-DV (3X) Cellular subscribers SMD161 Wireless Mobile Networks 8 4
System architecture of a cellular network Radio sub-system MS: Mobile station Another network Internet BS: Base-station : Base-station controller VLR GMSC MSC HLR MSC VLR Network switching sub-system MSC: Mobile switching centre HLR: Home location resgiter VLR: Vistor location register BS MS BS MS GMSC: Gateway MSC SMD161 Wireless Mobile Networks 9 Radio sub-system (radio access network) Connectivity between mobile stations and base-stations Radio resource management VLR Another network GMSC Internet Setup, maintenance and release of channels Call admission control Micro-mobility management MSC MSC HLR VLR Call/session handover between base-stations SMD161 Wireless Mobile Networks 10 5
Network and switching sub-system (core network) Another network Internet Gateway MSC VLR MSC MSC HLR VLR Connectivity between radio access networks and other infrastructure networks Mobile switching centre (MSC) Storage of user data and macro-mobility management Home location register (HLR) Visiting location register (VLR) Service provisioning SMD161 Wireless Mobile Networks 11 Subscriber identity Subscriber identity module (SIM) Personalized chip card to be inserted in the mobile station Stores specific user data Telephone number, called Mobile subscriber ISDN number (MSISDN) User identity, called International mobile subscriber Identity (IMSI) Secret keys for encryption Service support Address and phone book Inbox (for storing SMS) Recently called and received phone numbers, etc. SMD161 Wireless Mobile Networks 12 6
Routing call to mobile user MSISDN 1 Public switched telephone network (PSTN) MSRN 4 GMSC 2 MSISDN TMSI 7 MSC 5 MSRN 3 MSRN HLR TMSI 8 TMSI 8 TMSI 6 VLR MSISDN: Mobile Subscriber ISDN Number 9 TMSI MSRN: Mobile Station Roaming Number TMSI: Temporary Mobile Subscriber Identity SMD161 Wireless Mobile Networks 13 Handover (or handoff) Transfer of an ongoing call or session from one base-station to another When user moves from coverage of the old base-station into the coverage of a new one Should be transparent to the user New resources (channel) should be allocated by the new base-station Proper design of handover algorithm crucial for seamless mobility Generally not standardized; up to the network operator SMD161 Wireless Mobile Networks 14 7
Handover strategies Controlled by the MSC Based on the received signal strength indicator (RSSI) at the base-station = Prhandoff Prminimum usable If is too small, may not allow enough time for handover resulting in a dropped call If is too large, it may cause unnecessary handovers Mobile assisted handover (MAHO) Mobile station makes handover decision based on received signal strength of its current base-station and neighboring basestations SMD161 Wireless Mobile Networks 15 Handover strategies Mobile assisted handover (MAHO) Received power BS old Received power BS new HO_MARGIN MS MS BS old BS new SMD161 Wireless Mobile Networks 16 8
Types of handover MSC MSC MSC Intra handover Intra MSC handover Inter MSC handover Inter technology handover, e.g., GSM to UMTS SMD161 Wireless Mobile Networks 17 System design issues Cell shape Why hexagonal? Approximation to simplify modeling and analysis Ideal omni-directional isotropic propagation Real non-isotropic propagation SMD161 Wireless Mobile Networks 18 9
Frequency reuse Space division multiple access (SDMA) Efficient use of limited spectrum bandwidth f 3 f 2 f 5 f 4 f 1 f 3 f 2 f 6 f 7 f 5 f 4 f 1 SMD161 Wireless Mobile Networks 19 Frequency reuse Cellular system with: Total number of duplex channels = S Divided into a group of N cells k of these channels are allocated to each cell So, total number of duplex channels can be expressed as S = k x N The N cells which collectively use the complete set of available frequencies is called a cluster The factor N is called the cluster size SMD161 Wireless Mobile Networks 20 10
Frequency reuse If a cluster is replicated M times, then the total number of duplex channels C represents the system capacity and is given by C = M x k x N = M x S Based on hexagonal geometry, N can only have values which satisfy the following equation N = i2 + ij + j2 where i and j are non-negative integers SMD161 Wireless Mobile Networks 21 Frequency reuse distance calculation Given the total area to be covered, the frequency reuse distance D is a function of the cluster size (and the cell size) f 3 f 2 f 5 f 4 f 1 f 3 f 2 f 6 D f 7 f 5 f 4 f 1 Co-channel reuse factor is expressed as D/R = 3N = Q SMD161 Wireless Mobile Networks 22 11
Frequency reuse patterns f7 f4 f2 f7 f2 f6 f1 f3 f2 f6 f1 f2 f4 f1 f3 f5 f3 f1 f3 f5 f4 f4 f2 f4 f7 f2 f6 N = 4 (i = 2, j = 0) f1 f3 f5 f4 N = 7 (i = 2, j = 1) SMD161 Wireless Mobile Networks 23 Co-channel interference If io is the number of co-channel (i.e., using the same frequency) interfering cells, then signal-to-interference ratio (SIR) is expressed as S/I = S / (sum of received power from io interfering cells) If distance D to all interfering cells is equal, then S/I = ( 3N n ) / io where n is the path loss exponent SMD161 Wireless Mobile Networks 24 12
System capacity Trunking (also known as oversubscription) Accomodate large number of subscribers in a limited radio spectrum Exploit statistical behavior of users (i.e., not all users are expected to use the network simultaneously) Grade of service (GOS) Metric to measure performance of a trunked system Ability of a user to access a trunked system during busiest hours Expressed in Erlangs (one Erlang is the traffic intensity carried by channel that is completely busy, e.g., one call-hour per hour) SMD161 Wireless Mobile Networks 25 System capacity Average duration of a call = H Average number of calls per unit time = µ Traffic intensity of a user Au is expressed as Au = µ x H For a system containing U users, the total traffic intensity is A = U x Au Assuming the traffic is equally distributed over C channels, the traffic intensity per channel is Ac = (U x Au) / C SMD161 Wireless Mobile Networks 26 13
Blocking probability Erlang B Blocked Calls Clear system Erlang C Blocked Calls Delayed system SMD161 Wireless Mobile Networks 27 Improving system capacity Cell splitting Improve utilization of spectrum efficiency Subdividing a congested cell into smaller cells (called microcell) Each microcell has its own base-station (smaller tranmission range) Permanent cell splitting Dynamic cell splitting Microcells Picocells SMD161 Wireless Mobile Networks 28 14
Improving system capacity Sectorization Base-stations use directional antennas to transmit in a specified sector 1 2 3 1 2 3 2 3 1 4 6 5 2 3 1 4 6 5 120 deg. sectoring 60 deg. sectoring SMD161 Wireless Mobile Networks 29 802.16: Background IEEE 802.16 standard (aka 802.16-2001) Approved in 2001 (published in April 2002) WirelessMAN air interface for wireless metropolitan area networks (MANs) Market potential and usage scenarios Provide broadband wireless access to businesses and homes Alternative to wired access technologies like fibre optics, cable and DSL Cover broad geographical areas at low cost SMD161 Wireless Mobile Networks 30 15
802.16: Background Communication between a central base station and a receiver installed on a building with exterior antenna The receiver will connect to individual users through in-building LANs, e.g., Ethernet, WiFi, Future standards may allow direct communication between base-station and user device (e.g., laptop, PDA) IEEE Source: S. J. Vaughan-Nichols, Achieving Wireless SMD161 Wireless Mobile Networks 31 Broadband with WiMax, IEEE Computer, June 2004. 802.16: Background Some initial products and deployments starting 2003 Forecasts predict exponential growth WiMax forum Certification of 802.16 compliant products WiMax: Worldwide interoperability for microwave access SMD161 Wireless Mobile Networks 32 16
802.16: Protocol layer structure Service Specific Convergence Sublayer MAC Layer MAC Common Part Sublayer Security Sublayer Physical Layer Transmission Convergence Sublayer Physical Layer SMD161 Wireless Mobile Networks 33 802.16: Physical layer Support for multiple frequency bands and hence multiple transmission ranges and bandwidth 10 to 66 GHz 802.16-2001 Direct line of sight between transmitter and receiver Single carrier modulation Up to 75 Mbps per channel (on both uplink and downlink) 2-11 GHz 802.16a (2001) No line-of-sight required (better penetration of barriers) Single and multiple carrier modulation (OFDM) More flexibility with point-to-multipoint transmissions Support for mesh deployment SMD161 Wireless Mobile Networks 34 17
802.16: Enhancements 802.16b Use of spectrum in the 5 and 6 GHz frequeny range Enhancements for supporting quality of service (QoS) 802.16c Details added to 802.16-2001 (10 to 66 GHz) Encourage more consistent implementation and interoperability 802.16d Minor enhancements to 802.16a Creates system profiles for compliance testing 802.16e Support (e.g., fast handover) for communication between basestation and mobile users moving at vehicular speeds SMD161 Wireless Mobile Networks 35 802.16: Physical layer Frequency Division Duplexing (FDD) Uplink and downlink use different frequencies Time Division Duplexing (TDD) Both uplink and downlink share the same frequency Standard supports both full duplex and half duplex transceivers Time Division Multiplexing (TDM) Allow base-station (BS) to communicate simultaneously with multiple subscriber stations (SS) SMD161 Wireless Mobile Networks 36 18
802.16: Physical layer Burst single carrier modulation QPSK 16-QAM 64-QAM WirelessMAN-OFDM 256-carrier OFDM TDMA for multiple access WirelessMAN-OFDMA 2048-carrier OFDM Multiple access provided by assigning a set of carriers to each receiver SMD161 Wireless Mobile Networks 37 802.16: Physical layer Adaptive burst profiles Transmission parameters such as modulation and FEC settings can be modified for each SS on a frame-to-frame basis Downlink Interval Usage Code (DIUC) Uplink Interval Usage Code (UIUC) Radio link control (RLC) Controls power control, ranging and transition from one burst profile to another Ranging request (RNG-REQ) Initial power leveling and ranging request made by the SS Ranging response (RNG-RSP) Power, ranging and timing adjustments recommended by BS SMD161 Wireless Mobile Networks 38 19
TDD frame structure Downlink Subframe Uplink Subframe Frame j-1 Frame j Frame j+1 SMD161 Wireless Mobile Networks 39 FDD frame structure Downlink (frequency m) Frame j-1 Frame j Frame j+1 Uplink (frequency n) Frame j-1 Frame j Frame j+1 SMD161 Wireless Mobile Networks 40 20
802.16: MAC layer Connection-oriented All traffic including inherently connectionless traffic is mapped into a connection Each connection is identified by a connection identifier (CID) Reserved CIDs for management, broadcasts, Provides ability to map QoS and transmission parameters for every connection Each connection is associated with a service flow SMD161 Wireless Mobile Networks 41 802.16: MAC layer Each SS has a unique 48-bit MAC address Mainly serves as equipment identifier Primary addresses used during operation are the CIDs Upon initialization, SS is assigned three management connections in each direction Transfer of short time-critical MAC and radio link control messages Transfer longer, more delay-tolerant messages, e.g., used for authentication and connection set-up Transfer management related messages, e.g., SNMP, DHCP, TFTP SMD161 Wireless Mobile Networks 42 21
802.16: Channel access At the beginning of every frame, the BS transmits the downlink map (DL-MAP) and uplink map (UL-MAP) messages UL-MAP defines uplink channel access and UIUC for the uplink subframe DL-MAP defines the DIUC for the downlink subframe SMD161 Wireless Mobile Networks 43 802.16: Downlink subframe structure Used only in FDD systems SMD161 Wireless Mobile Networks 44 IEEE Source: C. Eklund, R. B. Marks, K. Stanwood and S. Wang, IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access, IEEE Communications Magazine, June 2002. 22
802.16: Uplink subframe structure IEEE Source: C. Eklund, R. B. Marks, K. Stanwood and S. Wang, IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access, IEEE Communications Magazine, June 2002. SMD161 Wireless Mobile Networks 45 802.16: QoS support QoS support defined in the form of four service flows Unsolicited grant service (UGS) for CBR real-time traffic such as voice over IP Real-time polling service (rtps) for VBR real-time traffic such as audio/video streaming Non-real-time polling service (nrtps) for VBR non-real-time traffic that expects better than best effort service, e.g., high bandwidth FTP Best effort (BE) for traffic that does not require QoS support Bandwidth allocation Grant per connection (GPC) Grant per SS (GPSS) SMD161 Wireless Mobile Networks 46 23