WiMAX (IEEE ) 1

Transcription

1 WiMAX (IEEE ) 1

2 Contents Overview Architecture Services and Scheduler Protocol Stack Summary 2

3 Wireless Metropolitan Area Networks A competitor to DSL Defined for carrier frequency: 2-11 GHz ( (d)), GHz (802.16a (2001)) Initial standard uses fixed, building mounted antennas - not designed for mobility IEEE e 2005 Mobile Amendment to allow movement of users Provides a high bandwidth connection High bit rates both for up- and downlink (2 to 100-Mbps) Sectored, directional antennas High gain means a greater range 3

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5 Architecture Base-station (BS) consists of Connection to network core (ATM, IP) Controller for each sector, e.g. scheduler Radio System and Antenna Some possible configurations, 4 90-degree antennas 8 45-degree antennas degree antennas Subscriber Station (SS) consists of Radio subsystem Subscriber Terminal Local Distribution system (Ethernet or WLAN ) - optional 5

6 Architecture The BS controls communication A Master-Slave configuration is used The BS controls access to the RF In the downlink frame, it transmits two maps These inform the system of when to listen, and when to transmit As transmission is scheduled, it is possible to guarantee QoS 6

7 Physical Layer Standard specifies a frequency range WiMAX initially was to work between 10 and 66-GHz (802.16a 2001) These frequencies require Line of Sight Standard extended to cover other frequencies (2-11 GHz) d ( ) lower bit rates but less expensive This allows NLOS propagation Bad Good 7

8 Physical IEEE defines 3 different PHY: Burst Single Carrier Modulation Line of sight, multi-path is not particularly important The directional antennas provide good interference discrimination WMAN OFDM a 256-carrier orthogonal frequency division multiplexing scheme. Uses TDMA mandatory for licence-exempt bands (fixed WiMAX) WMAN OFDM a 2048-carrier OFDM scheme multiple access by assigning a subnet of carriers to individual user (OFDMA) Use of advanced antenna systems supported (MIMO etc.) MIMO - Multiple Input Multiple Output 8

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10 Physical Layer Both TDD and FDD supported In TDD up- and downlink share the channel but don t transmit simultaneously FDD up- and downlink have separate channels and can transmit simultaneously, Half-duplex FDD also supported, less expensive but more complicated Both TDD and FDD allow adaptive burst profiles where modulation and coding can be dynamically assigned on burstby-burst basis 10

11 In FDD continuous and burst downlink supported Continuous allows some robustness enhancement techniques to be used (interleaving) Burst advanced robustness and capacity techniques subscriber-level adaptive burst profiling, advanced antenna systems (TDD & FDD) Adaptive modulation and coding scheme depending on SS equipment, channel and interference conditions trade-off between data rate and robustness Modulation formats QPSK, 16-QAM, or 64-QAM Offers 2, 4, or 6 bits / symbol Convolution Codes and Convolution Turbo Codes used together with Low Density Parity Check 11

12 Frames Frames The frame is the basic unit of transmission Its duration can vary on frame-by-frame basis from 2 to 20 ms A physical slot defined by 4 QAM symbols A contiguous series of slots assigned to one user is that user s data region Frame is divided into two sub-frames Downlink frame To the SS followed by Uplink fram To the BS There is a guard time between the two sub-frames Downlink-to-uplink ratio can vary from 3:1 to 1:1 In TDD uplink frame follows downlink frame In FDD frames coincident but at different frequencies 12

13 Frame Downlink frame starts with a downlink preamble (time and freequency synchronization, initial channel estimation) Frame Control Header frame configuration information (MAP message length, modulation and coding scheme, usable subcarriers) MAP messages: DL-MAP and UL-MAP (burst profile for each user: modulation and coding used) UL-MAP grants bandwidth to specific SS SS transmits in their assigned allocation using the burst profile specified by the Uplink Interval Usage Code (UIUC) 13

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15 Downlink Frame: May contain multiple bursts of varying size and type, carrying data for several users Each burst can contain concatenated fixed size or variable size packets or fragments of packets from higher leyers Uplin Frame: Traffic burst for data transmission from SS to BS Portion of subfram set aside for contention-based access ranging channel for closed loop frequency, time and power adjustments, uplink bandwidth requensts for SS, best-effort data can be sent with small amount of data for transmission Channel quality indicator channel feed back channel quality information Acknowledge channel 15

16 Adaptive Modulation and Coding in WiMAX Various modulation and coding allowed: BPSK, QPSK, 16 QAM, 64 QAM Modulation and coding can change on burst-by-burst basis Channel quality indicator used to inform BS about channel quality in downlink direction BS estimates channel quality on uplink direction BS chooses modulation and coding to maximize throughput for available signal-to-noise ratio Total 52 combinations of modulation and coding schemes defined in WiMAX as burst profiles 16

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18 MAC Layer Interface between the higher transport layer and the physical layer Takes packets form upper layer: MAC Service Data Units (MSDU) and organises them into MAC Protocol Data Units (MPDU) for radio transmission With received data does the reverse Convergence sub-layer used as an interference with variety of higher layers protocols such as ATM, Ethernet, IP and future protocols. Convergence layer supports MSDU header suppression to reduce higher layer overheads Different QoS for different types of traffic 18

19 MAC uses variable length MPDU increased flexibility (fragmentation, aggergation of MSDU allowed) 19

20 MAC frames consists of: Generic MAC header connection identifier, length of frame, presence of CRC, encryption Payload is either a transport or management message Can also contain bandwidth request or retransmission request (MAC supports ARQ) Connection Identifiers (CIS s) Connection assigned 16-bit ID used as a primary address of SS during connection Two types of connections Requiring continuously granted bandwidth Requiring bandwidth on demand 20

21 Channel Access Mechanisms MAC at BS fully responsible for allocation of bandwidth to all users in both directions In downlink it assignes BW based on needs of the incomming traffic, without involving SS For uplink allocation based on SS request SS can request BW in several ways: Polling - BS allocates dedicated or shared resources periodically to each SS, which can be used for BW request. Polling can be done individually (unicast) or in groups (multicast) when there in not enough BW to poll each SS. Contention resolution used to avoid collisions SS which already has BW not polled, it can request more BW by: sending stand-alone BW request MPDU, sending a BW request using the ranging channel, piggibacking a BW request on generic MAC packets 21

22 Quality of Service Strong QoS control achieved by using connection-oriented MAC architecture downlink and uplink connections controlled by BS BS and SS establish unidirectional logical link called connection Each connection identified by connection identifier temporary address for data transmission In addition to transport connections WiMAX MAC defines 3 management connections: Basic - short, critical messages, relating to Radio & MAC control Primary - longer, more delay tolerant messages for authentication and connection setup Secondary - Encrypted, standard-based management messages 22

23 MAC reserves additional connections for: Contention-based initial access Broadcast transmission Signalling broadcast contention-based polling of SS bandwidth needs Multicast contention-based polling SS may be instructed to join multicast polling group WiMAX also defines the concept of a service flow unidirectional flow of packets with a particular set of QoS parameters and identified by a service flow identifier QoS can include traffic priority, maximum sustained traffic rate, muximum burst rate, minimum tolerable rate, ARQ type, maximum delay, tolerated jitter, BW requests mechanisms etc. 23

24 WiMAX defines 5 scheduling services: Unsolicited Grant Service (UGS) for services generating fixed units of data periodically BS grants bandwidth of a size negotiated at the connection set-up without explicit request eliminates overhead and latency ATM constant bit rate, E1/T1 over ATM Service flow parameters: maximum sustained traffic rate, maximum latency, tolerated jitter, BW requests mechanisms Real Polling Service for services dynamic in nature (real time) Offers periodic dedicated request opportunities to meet real-time requirements SS needs to request bandwidth increase in overhead and latency Capacity granted according to real need (VoIP, streaming video) Service flow parameters: minimum reserved traffic rate, maximum sustained traffic rate, maximum latency, tolerated jitter, BW requests mechanisms 24

25 Non-real polling service delay tolerant data streams (e.g. FTP) Connections can use random access transmit opportunities for sending bandwidth requests Require variable-size data grants at a minimum guaranteed rate Service flow parameters: minimum reserved traffic rate, maximum sustained traffic rate, traffic priority, BW requests mechanisms Best effort services neither throughput nor delay are guaranteed Extended real-time variable rate real time applications such as VoIP with silecnce suppression Variable rate but require guaranteed data rate and delay 25

26 Burst Profiles WiMax can trade robustness of signal against efficiency Modulation (QPSK to 64-QAM) is one technique Also, differing levels of Forward Error Correction are used A Burst Profile is a FEC coding scheme and a modulation scheme SS that are further away are likely to use more robust profiles to protect against errors 26

27 Burst Codes QAM + FEC 16-QAM 64-QAM QAM 27

28 SS monitors the signal and knows when burst profile should change BS is the only one that can change the profile There are 2 methods of requesting profile change by SS: During station maintenance interval by sending RNG-REQ (only Grant per Connection SS) Preferred method sending downlink burst profile change request (DBPC-REQ) BS responds with DBPC-RSP accepting or rejecting request 28

29 Bandwidth requests and grants There are 2 classes of SS: Grant Per Connection (GPC) bandwidth granted explicitly to a connection and only used for that connection Grant Per SS (GPSS) granted aggregate bandwidth to SS itself bandwidth requested for connection but can use for different data, if needs to transmit data with higher QoS can request additional bandwidth to replace bandwidth stolen from lower QoS connection. Can also use BW to react more quickly to changing conditions e.g. to send DBPC-REQ Trade-off between simplicity and efficiency GPC less efficient and scalable (likelihood of multiple entry from one SS) GPSS enhances system performance only one allowed in GHz PHY 29

30 In both classes self-correcting protocol is used (not ACK): Uses less bandwidth No need for additional time no delay If bandwidth requested by SS not granted (no resources, unsuccessful request transmission etc.) after time-out the SS requests bandwidth again Most requests are incremental SS requests more BW then before Ways of requesting more BW: Continuous bandwidth demand services don t need to request BW SS with running UGS connection can use poll-me bit in the grant management SH to let BS know that it needs to be polled for BW request SS can send MAC PDU with BW request header and no payload 30

31 GPSS can use its allocated BW to send PDU BW request GPC can send it during request interval or data grant interval allocated to its basic connection Grant management SH can be used to piggyback a request for additional BW for the same connection within MAC PDU BS can issue a broadcast poll by allocating a request interval to the broadcast CID BS can multicast users finer control to contention-based polling WiMAX combines the determinism of unicast polling with responsiveness of contention-based requests and the efficiency of unsolicited BW 31

32 Channel Acquisition Upon installation SS scans its frequency list for operating channel After choosing the channel it tries to synchronise to downlink transmission by detecting frame preambles (periodic) After synchronisation SS looks for periodically broadcast information on modulation and FEC coding used on the carrier SS looks for ranging opportunities by scanning UL-MAP It picks ranging slot Sends burst with minimum power, and increases power until it receives a Ranging Response Ranging Response has timing and power information This sets up Basic and Primary Connections to SS 32

33 SS Authentication and Registration WiMAX supports variety of credentials: username/password, digital certificates, smart cards Each SS has a X.509 certificate, which are used to authenticate the SS to the BS (includes digital certificate and MAC address) If OK, BS will send Authorisation Key (AK) to SS, which allows SS to encrypt transmissions SS will register with network Establish secondary management channel 33

34 Privacy and Security Privacy is based on Privacy Key Management (PKM) Accommodates Advanced Encryption Standard 128 or 256-bit key used for deriving the cipher is generated during the authentication phase Key is refreshed periodically for additional protection 34

35 Mobility Support WiMAX envisions 4 mobility scenarios Nomadic fixed subscriber station can be reconnected from different points of attachment Portable nomadic access for a portable device (PC card) with expectation of a best-effort handover Simple mobility SS may move with speeds up to 60 km/h with brief interruptions during handoff (< 1 sec) Full mobility speeds up to 120 km/h with seamless handover (< 50 ms latency and < 1% packet loss) 35

36 Hard handoff mandatory, abrupt transfere of connection between BSs Fast BS switching SS connected with several BS form active set, but exchanges information only with one anchor BS Macro Diversity handoff similar to fast switching but SS communicates with all BS in a diversity set: In downlink multiple copies of signal transmitted and combined using diversity combining techniques, in uplink the best signal picked. Best performance, but BS need to be synchrnonised, use the same frequency and share network entry information 36

37 Frequency Reuse Can use universal frequency reuse plan (reuse factor 1) To avoid severe interference SS differentiated depending on their location SS with good SIR can use full channel BW with frequency reuse factor of 1 SS with poor SIR (close to border of the cell) are allocated nonoverlapping subchannels frequency reuse factor of 2, 3 or 4 Effective reuse factor taking fractional values greater than 1 37

38 Why WiMAX High data throughput Scalable architecture Low cost of deployment Open standard approach Intellectual Property Rights policy promoting reasonable royalty, sharing etc. Broad industry participation in technology development and production 38

39 Most important features High data rates advanced coding and modulation, MIMO antenna techniques support peak data rates up to 63 Mb/s per sector in DL and 28 Mb/s in UL in 10 MHz channel QoS mechanisms for optimal scheduling of space, frequency and time resources on frame-by-frame basis Scalability can work with different channelisation MHz to comply with variety of regulations, utilises versatility of WiMAX for specific geographic needs (Internet access in rural areas, capacity enhancement in urban) Security best types of security methods with authentication and encryption. Diverse set of user credentials SIM, Smart Cards, Digital Certificates, Username/Password schemes Mobility supports handover schemes with latency less than 50 ms enough for real-time applications 39