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Cellular Networks and Mobile Computing COMS 6998-8, Spring 2012 Instructor: Li Erran Li ([email protected]) http://www.cs.columbia.edu/~coms6998-8/ 1/30/2012: Cellular Networks: UMTS and LTE Outline • Wireless communications basics – Signal propagation, fading, interference, cellular principle • Multi-access techniques and cellular network air-interfaces – FDMA, TDMA, CDMA, OFDM • 3G: UMTS – Architecture: entities and protocols – Physical layer – RRC state machine • 4G: LTE – Architecture: entities and protocols – Physical layer – RRC state machine 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 2 Basic Wireless Communication Information is embedded in electromagnetic radiation Lossy signal and interference Noise Recover information Transmitter 1/23/12 Receiver Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 3 Noise & Interference • Thermal Noise – Generated due to random motion of electrons in the conductor and proportional to temperature – No= KoT dBm/Hz where Ko is Boltzmann’s constant – Receiver Noise Figure – extent to which thermal noise is enhanced by receiver front end circuitry ~ 10 dB • Interference – signals transmitted by other users of the wireless network • Signal transmitted by other wireless devices from different wireless networks – Example: Microwave ovens near 802.11 network 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 4 Impact of White Gaussian Noise Shannon Capacity 10 Signal Power Capacity (bits/sec/Hz) 9 SNR = 8 Noise Power 7 6 5 C = log (1 + SNR) 4 3 2 1 0 -10 -5 0 5 10 15 20 25 30 SNR (dB) 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 5 Scattering of Signals - Multipath Fading Reflection Diffraction Absorption s2 (t ) x(t d / c)e s1 (t ) x(t )e 1/23/12 j 2 f c ( t d / c ) j j 2 f ct Cellular Networks and Mobile Computing (COMS 6998-8) Multiple paths with random phases and gains combine constructively and destructively to cause significant amplitude variations Courtesy: Harish Vishwanath 6 Impact of Mobility s2 (t ) a x(t t )e v j 2 fc cos( ) ( t t ) j Doppler Shift = v s1 (t ) x(t )e v cos( ) j 2 f ct Signal Amplitude Multipath Fading time 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 7 Flat & Frequency Selective Fading • When the multipath delay is small compared to symbol duration of the signal, fading is flat or frequency non-selective x(t tmax ) x(t ) 1 Symbol -1 1 -1 • Happens when signal bandwidth is small 1 Bx = tmax • Urban macro-cell delay spread is 10 micro seconds • When signal bandwidth is large different bands have different gains – frequency selective fading 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 8 Typical Pathloss 1.0 10.0 100.0 -50 -70 Free space : 20 dB/decade A decade : transmitter and receiver distance increase 10 times -90 -110 Urban Macro cell -40 dB/decade 1/23/12 Shadow fading Log-normal with std ~ 8 dB Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 9 Spectrum Reuse A B Ib Ia SINRa = S Sa Sa I a+ N b a and b can receive simultaneously on the same frequency band if SINRa and SINRb are above required threshold This happens if the respective transmitters are sufficiently far apart 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 10 The Cellular Principle • Base stations transmit to and receive from mobiles at the assigned spectrum – Multiple base stations use the same spectrum (spectral reuse) • The service area of each base station is called a cell • The wireless network consists of large number of cells – Example – The network in Northern NJ is about 150 base stations for a given operator • Cells can be further divided into multiple sectors using sectorized antennas • Each terminal is typically served by the “closest” base station(s) 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 11 Fixed Frequency Planning Each base is assigned a fixed frequency band g1 g4 g6 g5 g7 g2 g3 g1 g6 g4 g5 g2 g2 g3 g7 g1 g3 g2 Reuse of 3 g2 g1 g3 g5 g3 g6 g1 g3 g7 g2 g1 g3 g2 g6 g1 g4 Reuse of 1/3 Reuse of 7 – nearest co-channel interferer is in the second ring 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 12 Cellular Network Evolution CDMA 2000 IS-95 CDMA 1X-DO ANALOG FDMA IS- 136 TDMA LTE OFDM TDMA GSM 1/23/12 TDMA/CDMA EDGE TDMA Cellular Networks and Mobile Computing (COMS 6998-8) UMTS CDMA Courtesy: Harish Vishwanath 13 The Multiple Access problem • The base station has to transmit to all the mobiles in its cell (downlink or forward link) – Signal for user a is interference for user b – Interference is typically as strong as signal since a and b are relatively close – How to avoid interference? • All mobiles in the cell transmit to the base station (uplink or reverse link) – Signal from a mobile near by will swamp out the signal from a mobile farther away – How to avoid interference? 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 14 Meeting Room Analogy Simultaneous meetings in different rooms (FDMA) Simultaneous meetings in the same room at different times (TDMA) Multiple meetings in the same room at the same time (CDMA) 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 15 Frequency Division Multiple Access Guard Band Each mobile is assigned a separate frequency channel for the duration of the call Sufficient guard band is required to prevent adjacent channel interference Mobiles can transmit asynchronously on the uplink 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 16 Time Division Multiple Access Time is divided into slots and only one mobile transmits during each slot FRAME j SLOT 1 FRAME j+2 FRAME j + 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6 Guard time – Signal transmitted by mobiles at different locations do not arrive at the base at the same time 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 17 TDMA Characteristics • Discontinuous transmission with information to be transmitted buffered until transmission time – Possible only with digital technology – Transmission delay • Synchronous transmission required – Mobiles derive timing from the base station signal • Guard time can be reduced if mobiles pre-correct for transmission delay – More efficient than FDMA which requires significant guard band 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 18 Orthogonality in TDMA/FDMA Every information signal lasts a certain duration of time and occupies a certain bandwidth and thus corresponds to a certain region in the time-frequency plane frequency Granularity is determined by practical limitations time Time division and frequency division are invariant under transformation of the channel and retain the orthogonality Any orthogonal signaling scheme for which orthogonality is preserved will be a useful multiple access technique 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 19 Code Division Multiple Access • Use of orthogonal codes to separate different transmissions • Each symbol or bit is transmitted as a larger number of bits using the user specific code – Spreading • Spread spectrum technology – The bandwidth occupied by the signal is much larger than the information transmission rate – Example: 9.6 Kbps voice is transmitted over 1.25 MHz of bandwidth, a bandwidth expansion of ~100 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 20 Spread Spectrum systems frequency time time code Code orthogonality is preserved under linear transformations and hence near orthogonality is preserved under signal propagation 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 21 Orthogonal Walsh Codes Spread factor 4 Walsh Array 1 1 1 1 1 -1 1 -1 1 1 -1 -1 1 -1 -1 1 De-spreading Information Spreading bit chip Walsh Code Transmitter 1/23/12 Receiver Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 22 Power Control is critical • The dynamic range of the pathloss for a typical cell is about 80 dB • The signal received from the closest mobile is 80 dB stronger than the farthest mobile without power control – Code orthogonality is not sufficient to separate the signals - Near-far problem in CDMA – Strict orthogonality in TDMA/FDMA makes power control not critical • Power Control – Mobiles adjust their transmit power according to the distance from the base, fade level, data rate 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 23 Why CDMA? • Simplified frequency planning – Universal frequency reuse with spreading gain to mitigate interference – Interference averaging allows designing for average interference level instead of for worst case interference TDMA / FDMA 1/23/12 CDMA Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 24 Why CDMA? • Variable rate Vocoder with Power Control – Advanced data compression technology is used to compress data according to content – Typical voice activity is 55% - CDMA reduces interference by turning down transmission between talk spurts – Reduced average transmission power increases capacity through statistical multiplexing – Compensate for fading through power control - transmit more power only under deep fades avoiding big fade margins 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 25 Why CDMA? • Simple multipath combining to combat fading Each signal arriving at a different time can be recovered separately and combined coherently The resulting diversity gain reduces fading s2 (t ) x(t Tc )e j 2 fc (t Tc ) j s1 (t ) x(t )e 1/23/12 j 2 f ct Cellular Networks and Mobile Computing (COMS 6998-8) Spreading sequence in x(t ) is offset by one chip compared to spreading sequence in x(t Tc ) Courtesy: Harish Vishwanath 26 Why CDMA? Mobile can transmit and receive from multiple base stations because all base stations use the same frequency • Soft Handoff - Make-before-break handoff Signals from different bases can be received separately and then combined because each base uses a unique spreading code 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 27 What is OFDM ? Orthogonal Frequency Division Multiplexing is block transmission of N symbols in parallel on N orthogonal sub-carriers 1 T Traditional Multi-carrier Guard Band OFDM Implemented digitally through FFTs Frequency Frequency OFDM invented in Bell Labs by R.W. Chang in ~1964 and patent awarded in 1970 Widely used: Digital audio and Video broadcasting, ADSL, HDSL, Wireless LANs 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 28 High Spectral Efficiency in Wideband Signaling 1 T T large compared to channel delay spread Closely spaced sub-carriers without guard band Each sub-carrier undergoes (narrow band) flat fading - Simplified receiver processing Frequency Narrow Band (~10 Khz) coding or scheduling across sub-carriers Dynamic power allocation across sub-carriers allows for interference mitigation across cells Wide Band (~ Mhz) Sub-carriers remain orthogonal under multipath propagation 1/23/12 Frequency or multi-user diversity through Orthogonal multiple access Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 29 Reverse link Orthogonal Frequency Division Multiple Access User 1 Users are carrier synchronized to the base Differential delay between users’ signals at the base need to be small compared to T W User 2 Efficient use of spectrum by multiple users Sub-carriers transmitted by different users are orthogonal at the receiver - No intra-cell interference User 3 CDMA uplink is non-orthogonal since synchronization requirement is ~ 1/W and so difficult to achieve 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 30 Typical Multiplexing in OFDMA Each color represents a user Each user is assigned a frequency-time tile which consists of pilot sub-carriers and data sub-carriers Yellow color indicates pilot subcarriers Channel is constant in each tile Block hopping of each user’s tile for frequency diversity Time 1/23/12 Typical pilot ratio: 4.8 % (1/21) for LTE for 1 Tx antenna and 9.5% for 2 Tx antennas Cellular Networks and Mobile Computing (COMS 6998-8) Courtesy: Harish Vishwanath 31 Outline • Wireless communications basics – Signal propagation, fading, interference, cellular principle • Multi-access techniques and cellular network air-interfaces – FDMA, TDMA, CDMA, OFDM • 3G: UMTS – Architecture: entities and protocols – Physical layer – RRC state machine • 4G: LTE – Architecture: entities and protocols – Physical layer – RRC state machine 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 32 UMTS System Architecture Iu Node B RNC GMSC Node B USIM Cu ME Iub Iur HLR Node B RNC Node B UE 1/23/12 MSC/ VLR External Networks Uu SGSN UTRAN Cellular Networks and Mobile Computing (COMS 6998-8) GGSN CN 33 UMTS Control Plane Protocol Stacks GMM/SM/SMS GMM/SM/SMS Relay RRC RRC RANAP RLC RLC SCCP MAC MAC UMTS RF UMTS RF Signaling Bearer AAL5 ATM UE Uu RNS RANAP SCCP Signaling Bearer AAL5 ATM Iu SGSN UMTS User Plane Protocol Stacks Application Application Relay IP, PPP, OSP IP, I PPPP, OSP Relay PDCP PDCP Relay GTP-U GTP-U GTP-U GTP-U UDP/ TCP IP IP RLC RLC UDP/IP UDP/IP UDP/IP MAC MAC AAL5 AAL5 L2 UMTS RF ATM ATM L1 UMTS RF UE Uu UTRAN Iu SGSN UDP/IP IPL2 IP L1 Gn GGSN Gi ISP UTRAN UE UTRAN CN UMTS Terrestrial Radio Access Network Overview Two Distinct Elements : Base Stations (Node B) Radio Network Controllers (RNC) 1 RNC and 1+ Node Bs are group together to form a Radio Network Sub-system (RNS) Node B RNC Node B Handles all Radio-Related Functionality RNS Iur Iub Soft Handover Radio Resources Management Algorithms Node B Maximization of the commonalities of the PS and CS data handling RNC Node B RNS 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) UTRAN 36 UTRAN UE UTRAN CN Logical Roles of the RNC Controlling RNC (CRNC) Responsible for the load and congestion control of its own cells Node B Serving RNC (SRNC) Terminates : Iu link of user data, Radio Resource Control Signalling Performs : L2 processing of data to/from the radio interface, RRM operations (Handover, Outer Loop Power Control) Node B Drift RNC (DRNC) Performs : Macrodiversity Combining and splitting 1/23/12 CRNC RNC Node B Iu SRNC Node B UE Iur Iu Node B DRNC Node B Iu Node B SRNC Node B Iur Iu Node B UE Cellular Networks and Mobile Computing (COMS 6998-8) DRNC Node B 37 Radio Resources Management UE UTRAN CN • Network Based Functions – Admission Control (AC) • Handles all new incoming traffic. Check whether new connection can be admitted to the system and generates parameters for it. – Load Control (LC) • Manages situation when system load exceeds the threshold and some counter measures have to be taken to get system back to a feasible load. – Packet Scheduler (PS): at RNC and NodeB (only for HSDPA and HSUPA) • Handles all non real time traffic, (packet data users). It decides when a packet transmission is initiated and the bit rate to be used. • Connection Based Functions – Handover Control (HC) • Handles and makes the handover decisions. • Controls the active set of Base Stations of MS. – Power Control (PC) • Maintains radio link quality. • Minimize and control the power used in radio interface, thus maximizing the call capacity. 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 38 Connection Based Function Power Control Prevent Excessive Interference and Near-far Effect Fast Close-Loop Power Control Feedback loop with 1.5kHz cycle to adjust uplink / downlink power to its minimum Even faster than the speed of Rayleigh fading for moderate mobile speeds Outer Loop Power Control 1/23/12 UTRAN CN Outer Loop Power Control If quality < target, increases SIRTARGET UE Fast Power Control If SIR < SIRTARGET, send “power up” command to MS Adjust the target SIR setpoint in base station according to the target BER Commanded by RNC Cellular Networks and Mobile Computing (COMS 6998-8) 39 Connection Based Function UE UTRAN CN Handover Softer Handover Soft Handover A MS is in the overlapping coverage of 2 sectors of a base station Concurrent communication via 2 air interface channels 2 channels are maximally combined with rake receiver A MS is in the overlapping coverage of 2 different base stations Concurrent communication via 2 air interface channels Downlink: Maximal combining with rake receiver Uplink: Routed to RNC for selection combining, according to a frame reliability indicator by the base station Hard handover HSDPA Inter-system and inter-frequency 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 40 HSDPA UE UTRAN CN High Speed Downlink Packet Access Improves System Capacity and User Data Rates in the Downlink Direction to 10Mbps in a 5MHz Channel Adaptive Modulation and Coding (AMC) HARQ provides Fast Retransmission with Soft Combining and Incremental Redundancy Replaces Fast Power Control : User farer from Base Station utilizes a coding and modulation that requires lower Bit Energy to Interference Ratio, leading to a lower throughput Replaces Variable Spreading Factor : Use of more robust coding and fast Hybrid Automatic Repeat Request (HARQ, retransmit occurs only between UE and BS) Soft Combining : Identical Retransmissions Incremental Redundancy : Retransmits Parity Bits only Fast Scheduling Function 1/23/12 which is Controlled in the Base Station rather than by the RNC Cellular Networks and Mobile Computing (COMS 6998-8) 41 Core Network UE UTRAN CN Core Network CS Domain : Mobile Switching Centre (MSC) Switching CS transactions Visitor Location Register (VLR) Holds a copy of the visiting user’s service profile, and the precise info of the UE’s location Gateway MSC (GMSC) The switch that connects to external networks PS Domain : 1/23/12 Serving GPRS Support Node (SGSN) Similar function as MSC/VLR Gateway GPRS Support Node (GGSN) Similar function as GMSC GMSC HLR Iu-ps SGSN External Networks MSC/ VLR Iu-cs GGSN Register : Home Location Register (HLR) Stores master copies of users service profiles Stores UE location on the level of MSC/VLR/SGSN Cellular Networks and Mobile Computing (COMS 6998-8) 42 WCDMA Air Interface UE UTRAN CN Direct Sequence Spread Spectrum Spreading User 1 f Wideband f Spreading f Received User N f Wideband f Narrowband f Frequency Reuse Factor = 1 Variable Spreading Factor (VSF) Spreading : 256 Multipath Delay Profile Narrowband Code Gain Despreading t f User 1 Wideband f Spreading : 16 Wideband t 5 MHz Wideband Signal Allows Multipath Diversity with Rake Receiver 1/23/12 f f Wideband VSF Allows Bandwidth on Demand. Lower Spreading Factor requires Higher SNR, causing Higher Interference in exchange. User 2 Cellular Networks and Mobile Computing (COMS 6998-8) 43 WCDMA Air Interface (Cont’d) UE UTRAN Multiple Access Method DS-CDMA Duplexing Method FDD/TDD Base Station Synchronization Asychronous Operation Channel bandwidth 5MHz Chip Rate 3.84 Mcps Frame Length 10 ms Service Multiplexing Multiple Services with different QoS Requirements Multiplexed on one Connection Multirate Concept Variable Spreading Factor and Multicode Detection Coherent, using Pilot Symbols or Common Pilot 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) CN 44 WCDMA Air Interface (Cont’d) UE UTRAN CN • Channel concepts 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 45 WCDMA Air Interface (Cont’d) UE UTRAN CN Mapping of Transport Channels and Physical Channels Broadcast Channel (BCH) Primary Common Control Physical Channel (PCCPCH) Secondary Common Control Physical Channel (SCCPCH) Physical Random Access Channel (PRACH) Forward Access Channel (FACH) Paging Channel (PCH) Random Access Channel (RACH) Dedicated Channel (DCH) Dedicated Physical Data Channel (DPDCH) Dedicated Physical Control Channel (DPCCH) Downlink Shared Channel (DSCH) Common Packet Channel (CPCH) Physical Downlink Shared Channel (PDSCH) Physical Common Packet Channel (PCPCH) Highly Differentiated Types of Channels enable best combination of Interference Reduction, QoS and Energy Efficiency 1/23/12 Synchronization Channel (SCH) Common Pilot Channel (CPICH) Acquisition Indication Channel (AICH) Paging Indication Channel (PICH) CPCH Status Indication Channel (CSICH) Collision Detection/Channel Assignment Indicator Channel (CD/CA-ICH) Cellular Networks and Mobile Computing (COMS 6998-8) 46 WCDMA Air Interface (Cont’d) UE UTRAN CN • Code to channel allocation 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 47 Codes in WCDMA • UE UTRAN CN Channelization Codes (=short code) – Used for • channel separation from the single source in downlink • separation of data and control channels from each other in the uplink – Same channelization codes in every cell / mobiles and therefore the additional scrambling code is needed • Scrambling codes (=long code) – Very long (38400 chips = 10 ms =1 radio frame), many codes available – Does not spread the signal – Uplink: to separate different mobiles – Downlink: to separate different cells – The correlation between two codes (two mobiles/Node Bs) is low • Not fully orthogonal 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 48 RRC State Machine UE UTRAN CN • IDLE: procedures based on reception rather than transmission – Reception of System Information messages – PLMN selection Cell selection Registration (requires RRC connection establishment) – Reception of paging Type 1 messages with a DRX cycle (may trigger RRC connection establishment) Cell reselection – Location and routing area updates (requires RRC connection establishment) 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 49 RRC State Machine (Cont’d) UE UTRAN CN • CELL_FACH: need to continuously receive (search for UE identity in messages on FACH), data can be sent by RNC any time – Can transfer small PS data – UE and network resource required low – Cell re-selections when UE mobile – Inter-system and inter-frequency handoff possible – Can receive paging Type 2 messages without a DRX cycle 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 50 RRC State Machine (Cont’d) UE UTRAN CN • CELL_DCH: need to continuously receive, and sent whenever there is data – Possible to transfer large quantities of uplink and downlink data – Dedicated channels can be used for both CS and PS connections – HSDPA and HSUPA can be used for PS connections – UE and network resource requirement is relatively high – Soft handover possible for dedicated channels and HSUPA Inter-system and inter-frequency handover possible – Paging Type 2 messages without a DRX cycle are used for paging purposes 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 51 RRC State Machine (Cont’d) UE UTRAN CN • State promotions have promotion delay • State demotions incur tail times Courtesy: Feng Qian Tail Time Delay: 2s Delay: 1.5s Tail Time 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) Channel Radio Power IDLE Not allocated Almost zero CELL_FACH Shared, Low Speed Low CELL_DCH Dedicated, High Speed High Outline • Wireless communications basics – Signal propagation, fading, interference, cellular principle • Multi-access techniques and cellular network air-interfaces – FDMA, TDMA, CDMA, OFDM • 3G: UMTS – Architecture: entities and protocols – Physical layer – RRC state machine • 4G: LTE – Architecture: entities and protocols – Physical layer – RRC state machine 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 53 LTE Technical Objectives and Architecture • User throughput [/MHz]: – Downlink: 3 to 4 times Release 6 HSDPA – Uplink: 2 to 3 times Release 6 Enhanced Uplink • Downlink Capacity: Peak data rate of 100 Mbps in 20 MHz maximum bandwidth • Uplink capacity: Peak data rate of 50 Mbps in 20 MHz maximum bandwidth • Latency: Transition time less than 5 ms in ideal conditions (user plane), 100 ms control plane (fast connection setup) • Cell range: 5 km - optimal size, 30km sizes with reasonable performance, up to 100 km cell sizes supported with acceptable performance Cellular Networks and Mobile 1/23/12 Computing (COMS 6998-8) 54 LTE Technical Objectives and Architecture (Cont’d) • Mobility: Optimised for low speed but supporting 120 km/h – Most data users are less mobile! • Simplified architecture: Simpler E-UTRAN architecture: no RNC, no CS domain, no DCH • Scalable bandwidth: 1.25MHz to 20MHz: Deployment possible in GSM bands. Cellular Networks and Mobile 1/23/12 Computing (COMS 6998-8) 55 LTE Architecture • Entities and functionalities NAS security Idle state mobility handling EPS bearer control UE IP address allocation Radio bearer control Mobility Packet filtering Inter-cell RRM anchoring Connection mobility Control Cellular Networks and Mobile Computing 1/23/12 Radio admission control (COMS 6998-8) 56 LTE Control Plane Protocol Stack 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 57 LTE Data Plane Protocol Stack 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 58 Functions of eNodeB UE eNodeB CN • Terminates RRC, RLC and MAC protocols and takes care of Radio Resource Management functions – – – – – Controls radio bearers Controls radio admissions Controls mobility connections Allocates radio resources dynamically (scheduling) Receives measurement reports from UE • Selects MME at UE attachment • Schedules and transmits paging messages coming from MME • Schedules and transmits broadcast information coming from MME & O&M • Decides measurement report configuration for mobility and scheduling • Does IP header compression and encryption of user data streams Cellular Networks and Mobile 1/23/12 Computing (COMS 6998-8) 59 Functions of MME UE eNodeB CN • Mobility Management Entity (MME) functions – Manages and stores UE context – Generates temporary identities and allocates them to UEs – Checks authorization – Distributes paging messages to eNBs – Takes care of security protocol – Controls idle state mobility – Ciphers & integrity protects NAS signaling Cellular Networks and Mobile 1/23/12 Computing (COMS 6998-8) 60 Session Establishment Message Flow 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 61 Session States 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 62 LTE vs UMTS • Functional changes compared to the current UMTS Architecture PGW SGW GGSN SGSN (not user plane functions) PDN GateWay Serving GateWay MME Mobility Management Entity RNC Node B RNC functions moved to eNodeB. • No central radio controller node • OFDM radio, no soft handover • Operator demand to simplify 1/23/12 eNodeB PGW/SGW • Deployed according to traffic demand • Only 2 user plane nodes (non-roaming case) Control plane/user plane split for better scalability • MME control plane only • Typically centralized and pooled Cellular Networks and Mobile Computing (COMS 6998-8) 63 LTE PHY Basics UE eNodeB CN • Six bandwidths – 1.4, 3, 5, 10, 15, and 20 MHz • Two modes – FDD and TDD • 100 Mbps DL (SISO) and 50 Mbps UL • Transmission technology – OFDM for multipath resistance – DL OFDMA for multiple access in frequency/time – UL SC-FDMA to deal with PAPR ratio problem 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 64 Frame Structure UE eNodeB CN Frame Structure Type 1 (FDD) Frame Structure Type 2 (TDD) 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 65 Resource Grid UE eNodeB CN One downlink slot, Tslot 6 or 7 OFDM symbols Resource block : Transmission BW Resource element 12 subcarriers • 6 or 7 OFDM symbols in 1 slot • Subcarrier spacing = 15 kHz • Block of 12 SCs in 1 slot = 1 RB – 0.5 ms x 180 kHz – Smallest unit of allocation : l=0 1/23/12 l=6 Cellular Networks and Mobile Computing (COMS 6998-8) 66 2-D time and Frequency Grid UE eNodeB CN #19 #18 #17 #16 #5 #4 #3 #2 NscRB subcarriers (=12) #1 Power #0 Frequency NBWDL subcarriers 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 67 DL PHY Channels and Signals UE eNodeB CN • Signals: generated in PHY layers – P-SS: used for initial sync – S-SS: frame boundary determination – RS: pilots for channel estimation and tracking • Channels: carry data from higher layers – PBCH: broadcast cell-specific info – PDCCH: channel allocation and control info – PCFICH: info on size of PDCCH – PHICH: Ack/Nack for UL blocks – PDSCH: Dynamically allocated user data 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 68 DL Channel Mapping UE eNodeB CN P-SCH - Primary Synchronization Signal S-SCH - Secondary Synchronization Signal PBCH - Physical Broadcast Channel PDCCH -Physical Downlink Control Channel PDSCH - Physical Downlink Shared Channel 16QAM Reference Signal – (Pilot) 64QAM QPSK Time Frequency 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 69 UL PHY Signals and Channels UE eNodeB CN • Signals: generated in the PHY layer – Demodulation RS : sync and channel estimation – SRS: Channel quality estimation • Channels: carry data from higher layers – PUSCH: Uplink data – PUCCH: UL control info – PRACH: Random access for connection establishment 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 70 UL Channel Mapping 64QAM 16QAM QPSK PUSCH Demodulation Reference Signal (for PUSCH) UE eNodeB CN QPSK BPSK PUCCH Demodulation Reference Signal (for PUCCH format 0 or 1, Normal CP) Time Frequency 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 71 RRC State Machine UE eNodeB CN • Much simpler than UMTS 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 72 Summary • Cellular networks are very different from WiFi – Cannot be based on carrier sensing due to large coverage area – Path must be setup dynamically due to mobility – Need to handle charging functions and QoS • Different physical layer technologies have very different overhead during inactivity – Dedicated channels prevent others from using the channel • Frequent RRC state transitions in UE can result in high network overhead and UE battery power consumption 1/23/12 Cellular Networks and Mobile Computing (COMS 6998-8) 73 Questions?