Tài liệu Mobile & wireless networking – lecture 12 4g radio access networks - geert heijenk

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4G Radio Access Networks G. Hiddink Motorola Mobile Devices Nieuwegein April 2010 Slide 1 April 2010 Contents •  •  •  •  •  •  •  •  •  WiMAX standards development 3GPP standards development OFDMA PHY basics OFDMA frame structure Protocol Stack architecture Data pipes WiMAX MAC PDU format WiMAX Bandwidth Management WiMAX Scheduling Services Slide 2 April 2010 WiMAX standards development •  •  WiMAX = “Worldwide Interoperability for Microwave Access” industry forum –  –  –  –  –  –  •  WiMAX Forum Goal –  –  •  functional entities: Mobile Stations, Base Stations, gateways, routers, .... interfaces between these entities air interface: IEEE 802.16 –  –  –  –  –  •  achieve the “world-wide interoperability” promote WiMAX as a world-wide mobile access technology Network Architecture –  –  •  Network Operators Infrastructure vendors (base stations) Mobile Station vendors (USB dongles, Customer Premises Equipment (CPE), miniPCI, cellphone) chipset vendors Test equipment vendors totaling > 350 companies “802” family member 802.3: Ethernet 802.11: WiFi 802.15.1: Bluetooth 802.15.4: Zigbee Specification status –  –  IEEE 802.16-2009 currently being deployed as "Mobile WiMAX 1.0" IEEE 802.16m being developed, supports up to 1 Gbit/s Slide 3 April 2010 WiMAX standards development (2) IEEE 802.16 Working Group tasks •  standards development for MAC and PHY layer •  includes many mechanisms, features, functionalities •  enable interoperability by specifying air interface WiMAX Forum tasks •  develop network architecture •  selects features from existing standards •  composes a ‘system profile’ (for Release 1.0, 1.5, 2.0, etcetera) •  writes certification tests to test mandatory elements of profile •  manages certification process Slide 4 April 2010 3GP standards development •  •  •  3GPP: Third Generation Partnership Project Collaboration project of organizations and companies working on 'GSM' technologies produces specifications for: –  –  –  –  –  –  –  •  GSM EDGE GPRS UMTS HSPA (HSDPA, HSUPA) LTE LTE-Advanced Responsibility can be compared to WiMAX + IEEE 802.16 jointly –  –  –  –  specify air interface (physical layer, layer 2 protocol stack) set radio performance requirements (incl radio resource measurements) specify network architecture and interfaces between network elements specify certification tests and manage certification process Slide 5 April 2010 WiMAX vs. 3GPP LTE •  WiMAX and LTE have similar goals –  –  –  –  •  develop 3G and 4G technology with throughput up to 1Gbit/s voice (circuit switched) and data (IP, packet switched) support integrated TV broadcasting support similar network elements and evolution paths (relays, home access points) therefor WiMAX and LTE share a lot of the same problems and solutions –  –  Spatial Multiplexing (MIMO) to increase throughput advanced antenna technologies to increase sensitivity and ‘link budget’ •  –  advanced information coding- and combining technologies •  –  spectrum allocations by regulatory bodies such as FCC, ETSI, ITU WiMAX and LTE target the same regulatory family, i.e. IMT 2000 and IMT Advanced this allows for reuse of domain expertise and design modules –  –  –  •  fast (inverse) FFT of size 512 to 2048 bandwidths of similar size (1.5 MHz to 20 MHz) TDD as well as FDD support required similar regulatory constraints •  •  •  Hybrid ARQ, Maximum Ratio Combining, Alamouti coding, Turbo Coders, rate matching OFDM technology •  –  –  –  beamforming techniques, radio channel estimation Digital Signal Processing design and implementation receiver and transmitter algorithms embedded protocol stack software development Motorola has not chosen for either one – but for both Slide 6 April 2010 OFDMA PHY basics •  Orthogonal Frequency Division Multiple Access –  for 10 MHz bandwidth: 1024 subcarriers –  subcarrier spacing: WiMAX 11 kHz, LTE 15 kHz •  Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD) defined –  WiMAX is mostly deployed using TDD, LTE mostly uses FDD –  in TDD, a radio frame is divided into downlink sub-frame(s) and uplink sub-frame(s) •  modulations used: QPSK, QAM-16, QAM-64 •  •  cyclic prefix to combat multipath fading when using MIMO, multiple constellations are transmitted and received simultaneously Slide 7 April 2010 Orthogonal Subcarriers •  •  •  •  OFDMA: Orthogonal Frequency Division Multiple Access at the peak of each subcarrier, all other subcarriers have amplitude zero avoids subcarrier interference subcarrier spacing: e.g. ~11 kHz (WiMAX) or 15 kHz (LTE) Slide 8 April 2010 FDM vs. OFDM Conventional Frequency Division Multiplex (FDM) Multi-carrier Modulation Technique OFDM subcarriers have a sinc (sin(x)/x) frequency response resulting in overlap in the frequency domain. This overlap does however not cause any interference due to the orthogonality of the subcarriers. Saving of the bandwidth Orthogonal Frequency Division Multiplex (OFDM) Multi-carrier Modulation Technique The OFDM receiver uses a time and frequency synchronized FFT to convert the OFDM time waveform back into the frequency domain. In this process the FFT picks up discrete frequency samples, corresponding to just the peaks of the carriers. At these frequencies, all other carriers pass through zero amplitude eliminating any interference between the subcarriers. Slide 9 April 2010 Subcarrier Usage •  Three types of subcarrier: –  data subcarrier •  constellation vector carries payload information –  pilot carrier or reference signal •  •  •  •  known vector, allows the receiver to learn the channel ("channel estimation") i.e. determine frequency response of the channel channel estimate is intrapolated and filtered over data subcarriers helps receiver to adjust data subcarrier constellation vector –  null subcarrier (not used ) •  •  •  •  e.g. used at frequency edge to build a guard band in frequency domain, signal is not 'square' i.e. frequency roll-off guard band prevents interference to neighbour spectrum users subcarriers are assigned in groups to users –  WiMAX: "subchannels" consisting of 48 subcarriers –  LTE: "Resource Blocks" consisting of 12 subcarriers •  two approaches to deal with frequency-selective fading –  exploit: schedule strongest (group of) subcarriers to a user •  •  requires frequent small-band signal quality feedback from user to base station requires base station scheduler to perform "frequency-selective scheduling" –  avoid: randomize subcarriers so that a fade does not affect the entire group •  •  all users suffer a little bit from one fade weak subcarrier's decoding errors can be repaired through redundant coding –  both approaches are used in LTE and WiMAX Slide 10 April 2010 OFDMA Symbols •  a Symbol is an interval during which constellations are constant –  allows receiver to take sufficient samples in time domain –  using Fast Fourier Transform the signal is converted to frequency domain –  this reveals the amplitude and phase of the constellation vector ("I and Q") •  •  •  WiMAX symbol duration in WiMAX: ~100 uS LTE symbol duration: ~140 uS or ~167 uS (6 or 7 symbols per 1 ms subframe) 'cyclic prefix' used to combat multi-path fading –  reflections of walls and other objects cause inter-symbol interference (ISI) –  reflections have longer path and longer delay than original (line-of-sight) signal •  •  –  –  –  –  reflection pattern sometimes called "Power Delay Profile" (PDP) pattern depends on physical environment, e.g. office, rural area, high rise buildings, suburbs channel response not stable while additional delayed reflections arrive at antenna after the last reflection has arrived, channel is stable solution: insert part of the signal to the front (time domain) at transmitter remove again at receiver Slide 11 April 2010 WiMAX TDD Frame Structure frame length: 5 ms preamble: MAP: FCH: ACKCH: CQICH: Ranging: channel estimation and cell identifier signals downlink- and uplink allocations frame control header for packet acknowledgements for Channel Quality Indications for time and power adjustments Slide 12 April 2010 LTE Frame Structure •  FDD frame structure •  TDD frame structure –  –  –  –  #1 and #6 can be special subframes or uplink or downlink others can be uplink or downlink 7 patterns are pre-defined allows base station to adapt to changing uplink / downlink loading Slide 13 April 2010 WiMAX MAC Architecture CS SAP ATM, Ethernet, IPv4, IPv6, header compression Service Specific Convergence Sublayer (CS) MAC Common Part Sublayer (MAC CPS) MAC SDU MAC MAC SAP Authentication, Key Exchange, Privacy (Encryption) Privacy Sublayer Physical Layer Slide 14 MAC PDU PHY PHY SAP Net Entry, Mobility, Packing, Fragmentation, ARQ, HARQ, Connection management, sleep mode, Idle mode and Paging, resource management OFDMA, Ranging, Power control, Tx, Rx April 2010 LTE Protocol Stack •  MAC: Medium Access Control –  Hybrid ARQ, Random Access, uplink Timing Alignment, uplink scheduling •  RLC: Radio Link Control –  ARQ, concatenation, fragmentation, reordering •  RRC: Radio Resource Control –  configuration, mobility, idle mode and paging, RRC connection management, radio measurements and reporting •  PDCP: Packet Data Convergence Protocol –  header compression, ciphering, Internet Protocol convergence •  NAS: Network Access Stratum –  EPS bearer management, authentication Slide 15 April 2010 Data pipes... •  WiMAX LTE mobile <==> base station "Connection" "Logical Channel" (MAC) "Radio Bearer" (other) mobile <==> network "Service Flow" "EPS Bearer" data pipe from mobile to base station: –  –  –  –  –  used for uplink scheduling decisions (prioritization) used for identifying destination of received data re-established after handover associated to a data pipe from mobile into the network QoS parameters •  •  •  WiMAX: average / peak bitrate, max jitter, max latency, etc. LTE: prioritized bitrate, priority data pipe from mobile into network: –  survives a handover procedure Slide 16 April 2010 WiMAX MAC PDU format 6 bytes Variable length 4 bytes Generic MAC Sub-headers PN Header (optional) (encryption) •  •  •  •  •  Variable length Payload 8 bytes 4 bytes ICV CRC (encryption) (optional) multiple MPDUs can be concatenated into one PHY burst MAC header: connection ID, PDU length sub-headers: to support fragmentation, packing, ARQ PN: packet number, for message replay detection payload –  (fragments of) higher layer packets (MSDUs) •  typically encrypted using AES –  MAC management message •  •  •  ICV: Integrity Check Value CRC: Cyclic Redundancy Check larger header overhead than LTE Slide 17 April 2010 WiMAX uplink scheduling WiMAX uses a demand-based allocation scheme •  sometimes called ‘DAMA’ – Demand Assigned Multiple Access •  MS must request "bandwidth": bytes waiting to be transmitted –  for each connection individually •  Base Station scheduler determines when to grant uplink allocations –  scheduling algorithm is an important function in Base Station –  BS signals uplink allocation in the uplink MAP –  allocation can be repetitive, e.g. for voice ("persistent scheduling") Bandwidth request methods •  unsollicited, periodic allocation (‘automatic’) •  periodic poll (BS asks “do you have data to transmit?”) –  poll consists of just enough allocation to transmit bandwidth request –  if MS has data to transmit, it sends bandwidth request PDU •  piggyback bandwidth request –  transmit bandwidth request within uplink data PDU •  CDMA code based request –  transmitted in the ranging channel –  BS responds with a poll –  long delay can interfere with TCP estimation of round trip delay Slide 18 April 2010 LTE uplink scheduling •  •  •  UE regularly reports its 'buffer status' (transmittable data in its buffers) total buffer status of up to four logical channel groups logical channel groups are prioritized –  data for one group is transmitted first, until prioritized bit rate achieved •  Buffer Status Report types: –  periodic –  regular (new data arrival) –  padding status report (when uplink room is available) •  UE can send Scheduling Request –  special uplink control signal –  transmitted in UE dedicated resource –  i.e. fast and reliable => more TCP friendly •  if UE has no SR resource, it sends in random access channel Slide 19 April 2010 WiMAX Scheduling Services •  •  determines Base Station scheduler behavior determines what methods the Mobile Station may use to request bandwidth Supported Scheduling Services •  Unsollicited Grant Service (UGS) –  –  –  •  real-time Polling Service (rtPS) –  –  –  •  typically used for voice with silence detection codecs no need to request bandwidth, but allocation size can be changed dynamically periodic allocations can be stopped to support voice silence detection QoS parameters: minimum / maximum traffic rate, latency non real-time Polling Service (nrtPS) –  –  •  variable bitrate, e.g. MPEG video MS is polled periodically QoS parameters: minimum / maximum traffic rate, latency Extended real-time Polling Service (ertPS) –  –  –  –  •  continuous bitrate, e.g. voice QoS parameters: packet inter-arrival time, latency, jitter no need to request bandwidth, fixed allocation size MS is polled periodically QoS parameters: minimum / maximum traffic rate Best Effort (BE) –  –  MS must request all bandwidth explicitly, no polling QoS parameters: maximum traffic rate Slide 20 April 2010
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