300952
Wireless and Mobile Networks
Lecture 7. Cellular Wireless Networks
Lecture objectives
• Understand basic principles of cellular (mobile) networks
• Understand techniques to increase cell capacity
• Understand the handoff process
• Understand key characteristics of each legacy mobile generation (1G-3G)
• Understand the purpose, motivation, and elements of 4G and LTE-Advanced
• Understand goals of future 5G mobile systems
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PRINCIPLES OF CELLULAR NETWORKS
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Cellular Networks
• Revolutionary development in data communications and telecommunications
• Foundationofmobilewireless
– Telephones, smartphones, tablets, wireless Internet, wireless applications
• Supports locations not easily served by wireless networks or WLANs
• Four generations of standards – 1G: Analog
– 2G: Still used to carry voice
– 3G: First with sufficient speeds for data networking, packets only – 4G: Truly broadband mobile data up to 1 Gbps
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Cellular Network Organization
• Use multiple low-power transmitters (100 W or less)
• Areas divided into cells
– Each served by its own antenna
– Served by base station consisting of transmitter, receiver, and control unit
– Band of frequencies allocated
– Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern)
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CELLULAR GEOMETRIES
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Frequency Reuse
• Adjacent cells assigned different frequencies to avoid interference or crosstalk
• Objective is to reuse frequency in nearby cells
– 10 to 50 frequencies assigned to each cell
– Transmission power controlled to limit power at that frequency escaping to adjacent cells
– The issue is to determine how many cells must intervene between two cells using the same frequency
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FREQUENCY REUSE PATTERNS
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Approaches to Cope with Increasing Capacity
• Adding new channels
• Frequency borrowing – frequencies are taken from adjacent cells by
congested cells
• Cell splitting – cells in areas of high usage can be split into smaller cells
• Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels
• Network densification – more cells and frequency reuse – Microcells – antennas move to buildings, hills, and lamp posts – Femtocells – antennas to create small cells in buildings
• Interference coordination – tighter control of interference so frequencies can be reused closer to other base stations
– Inter-cell interference coordination (ICIC)
– Coordinated multipoint transmission (CoMP)
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CELL SPLITTING AND SECTORING
Image source: http://www.geofffox.com/MT/archives/tag/omnidirectional-antenna
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OVERVIEW OF CELLULAR SYSTEM
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Steps in an MTSO Controlled Call between Mobile Users
• Mobile unit initialization
• Mobile-originatedcall
• Paging
• Callaccepted
• Ongoingcall
• Handoff
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EXAMPLE OF MOBILE CELLULAR CALL
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Additional Functions in an MTSO Controlled Call
• Callblocking
• Calltermination
• Calldrop
• Calls to/from fixed and remote mobile subscriber
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Mobile Radio Propagation Effects
• Signalstrength
– Must be strong enough between base station and mobile unit to maintain signal
quality at the receiver
– Must not be so strong as to create too much co-channel interference with channels in another cell using the same frequency band
• Fading
– Signal propagation effects may disrupt the signal and cause errors
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Handoff Performance Metrics (1)
• Cell blocking probability – probability of a new call being blocked
• Call dropping probability – probability that a call is terminated due to a
handoff
• Call completion probability – probability that an admitted call is not dropped before it terminates
• Probability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequate
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Handoff Performance Metrics (2)
• Handoff blocking probability – probability that a handoff cannot be successfully completed
• Handoff probability – probability that a handoff occurs before call termination
• Rate of handoff – number of handoffs per unit time
• Interruption duration – duration of time during a handoff in which a
mobile is not connected to either base station
• Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur
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Handoff Strategies Used to Determine Instant of Handoff
• Relativesignalstrength
• Relative signal strength with threshold
• Relative signal strength with hysteresis
• Relative signal strength with hysteresis and threshold
• Prediction techniques
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Handoff Between Two Cells
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Power Control
• Reasons to include dynamic power control in a cellular system
– Received power must be sufficiently above the background noise for effective
communication
– Desirable to minimize power in the transmitted signal from the mobile
• Reduce co-channel interference, alleviate health concerns, save battery power
– In SS systems using CDMA, it’s necessary to equalize the received power level from all mobile units at the BS
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Types of Power Control
• Open-loop power control
– Depends solely on mobile unit
– No feedback from BS
– Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength
• Closed-loop power control
– Adjusts signal strength in reverse channel based on metric of performance
– BS makes power adjustment decision and communicates to mobile on control channel
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Traffic Engineering
• Ideally, available channels would equal number of subscribers active at one time
• In practice, not feasible to have capacity handle all possible load
• For N simultaneous user capacity and L subscribers – L < N – nonblocking system
– L > N – blocking system
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Blocking System Performance Questions
• Probability that call request is blocked?
• What capacity is needed to achieve a certain upper bound on
probability of blocking?
• What is the average delay?
• What capacity is needed to achieve a certain average delay?
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FIRST GENERATION SYSTEMS (ANALOG)
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First-Generation Analog
• Advanced Mobile Phone Service (AMPS)
– In North America, two 25-MHz bands allocated to AMPS • One for transmission from base to mobile unit
• One for transmission from mobile unit to base
– Each band split in two to encourage competition
– 30kHz channels
• 21 control channels (data channels at 10kbps – FSK) • 395 voice channels (FM, no encryption)
– Frequency reuse exploited
– FDMA
• 1G in Australia: 1987-1999
Motorola DynaTAC 8000X (c. 1984)
Image credit: Redrum0486 – http://en.wikipedia.org/wiki/File:DynaTAC8000X.jpg (license: CC BY-SA 3.0)
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AMPS Operation
• Subscriber initiates call by keying in phone number and presses send key
• MTSO verifies number and authorizes user
• MTSO issues message to user’s cell phone indicating send and receive traffic channels
• MTSO sends ringing signal to called party
• Party answers; MTSO establishes circuit and initiates billing information
• Either party hangs up; MTSO releases circuit, frees channels, completes billing
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SECOND GENERATION SYSTEMS (TDMA AND CDMA)
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Differences Between First and Second Generation Systems
• Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital
– Using FDMA/TDMA or CDMA
• Encryption – all second generation systems provide
encryption to prevent eavesdropping
• Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception
• Channel access – second-generation systems allow channels to be dynamically shared by a number of users
• GSM in Australia: 1993-2017 (Vodafone: 31 March 2018)
• CDMA in Australia: 1999-2008
Nokia 3310 (c. 2000)
Image credit: Discostu – https://en.wikipedia.org/wiki/File:Nokia_3310_blue.jpg (public domain)
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Global System for Mobile Communications (GSM)
• FDMA/TDMAapproach
• Developed to provide a common second-generation technology for
Europe
– Over 6.9 billion subscriber units by the end of 2013
• Mobile station communicates across the Um interface (air interface) with base station transceiver in the same cell as mobile unit
• Mobile equipment (ME) – physical terminal, such as a telephone or PCS – ME includes radio transceiver, digital signal processors and subscriber identity
module (SIM)
• GSM subscriber units are generic until SIM is inserted
– SIMs roam, not necessarily the subscriber devices
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Generalized Packet Radio Service (GPRS)
• Phase 2 of GSM
• Provides a datagram switching capability to GSM
– Instead of sending data traffic over a voice connection which requires setup, sending data, and teardown
– GPRS allows users to open a persistent data connection
– Also has a new system architecture for data traffic
– 21.4 kbps from a 22.8 kbps gross data rate
– Can combine up to 8 GSM connections • Overall throughputs up to 171.2 kbps
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Enhanced Data Rates for GSM Evolution (EDGE)
• The next generation of GSM
– Not yet 3G, so called “2.5G” by some
• Three-fold increase in data rate
– Up to 3 bits/symbol for 8-PSK from 1 bit/symbol for GMSK for GSM. – Max data rates per channel up to 22.8 × 3 = 68.4 kbps per channel
– Using all eight channels in a 200 kHz carrier, gross data transmission rates up to 547.2 kbps became possible
• Actual throughput up to 513.6 kbps.
• A later release of EDGE (3GPP Release 7) increased downlink data rates
over 750 kbps and uplink data rates over 600 kbps
• EDGE in Australia: Switched off with the rest of 2G network
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Advantages of CDMA for Cellular systems
• Frequency diversity – frequency-dependent transmission impairments have less effect on signal
• Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation
• Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals
• Graceful degradation – system only gradually degrades as more users access the system
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Drawbacks of CDMA Cellular
• Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized
• Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away
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Mobile Wireless CDMA Design Considerations
• RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them
• Soft Handoff – mobile station temporarily connected to more than one base station simultaneously
– Requires that the mobile acquire a new cell before it relinquishes the old – More complex than hard handoff used in FDMA and TDMA schemes
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THIRD-GENERATION SYSTEMS
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ITU’s initial View of Third-Generation Capabilities (1)
• The ITU’s International Mobile Telecommunications for the year 2000
(IMT-2000) initiative
• Voice quality comparable to the public switched telephone network
• 144 kbps data rate available to users in high-speed motor vehicles over large areas
• 384 kbps available to pedestrians standing or moving slowly over small areas
• Support for 2.048 Mbps for office use – Much higher rates were developed
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ITU’s initial View of Third-Generation Capabilities (2)
• Symmetrical / asymmetrical data transmission rates
• Support for both packet switched and circuit switched data services
• An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic
• More efficient use of the available spectrum in general
• Support for a wide variety of mobile equipment
• Flexibility to allow the introduction of new services and technologies
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Alternative interfaces
• Five alternatives for smooth evolution from 1G and 2G systems
• Two most prevalent
– Wideband CDMA (WCDMA) (Evolved from GSM family) • Used in Australia
– CDMA2000 (Evolved from CDMA family) • Not used in Australia
• Both based on CDMA
• Similar to but incompatible with each other
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CDMA Design Considerations
• Bandwidth – limit channel usage to 5 MHz
• Chip rate – depends on desired data rate, need for error control, and
bandwidth limitations; 3 Mcps or more is reasonable
• Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service
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WCDMA and UMTS
• WCDMA is part of a group of standards from – IMT-2000
– Universal Mobile Telephone System (UMTS)
– Third-Generation Partnership Project (3GPP) industry organization
• 3GPP originally released GSM
– Issued Release 99 in 1999 for WCDMA and UMTS
– Subsequent releases were “Release 4” and onwards
– Many higher layer network functions of GSM were carried over to WCDMA
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WCDMA and UMTS
• 144 kbps to 2 Mbps, depending on mobility
• High Speed Downlink Packet Access (HSDPA)
– Release 5
– 1.8to14.4Mbpsdownlink
– Adaptive modulation and coding, hybrid ARQ, and fast scheduling
• High Speed Uplink Packet Access (HSUPA) – Release 6
– Uplink rates up to 5.76 Mbps
• High Speed Packet Access Plus (HSPA+)
– Release 7 and successively improved in releases through Release 11
– Maximum data rates increased from 21 Mbps up to 336 Mbps
– 64 QAM, 2×2 and 4×4 MIMO, and dual or multi-carrier combinations
• 3GPP Release 8 onwards introduced Long Term Evolution (LTE)
– Pathway to 4G
Apple iPhone 3G (c. 2008)
Image credit: Justin14 – https://en.wikipedia.org/wiki/IPhone_3G#/media/File:IPhone_PSD_White_3G.png (license: CC BY-SA 3.0)
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CDMA2000 and EV-DO
• CDMA2000 first introduced 1xRTT (Radio Transmission Technology) – 1 times the 1.2288 Mcps spreading rate of a 1.25 MHz IS-95 CDMA channel – Not 3G, so considered by some as “2.5G”
• Evolution-Data Only (1×EV-DO)
– Also 1×EV-DV (data/voice) which never succeeded
– 1×EV-DO Release 0
• 2.4Mbpsuplink,153kbpsdownlink
• Only using 1.25 MHz of 5 MHz required of CDMA
– 1×EV-DO Release A
• 3.1Mbpsdownlink,1.8Mbpsuplink,QoS
– 1×EV-DO Release B
• 5 MHz bandwidth, 14.7 Mpbs uplink, 5.4 Mbps downlink
• EV-DO uses only IP, but VoIP can be used for voice
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FOURTH GENERATION SYSTEMS
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4G Technology
• High-speed, universally accessible wireless service capability
• Creating a revolution
– Networking at all locations for tablets, smartphones,
computers, and devices.
– Similar to the revolution caused by Wi-Fi
• LTE and LTE-Advanced will be studied here
– Goals and requirements, complete system architecture, core network (Evolved Packet System), LTE channel and physical layer
– Will first study LTE Release 8, then enhancements from Releases 9-12
Google Pixel 2 (c. 2017)
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Purpose, motivation, and approach to 4G
• Ultra-mobile broadband access – For a variety of mobile devices
• International Telecommunication Union (ITU) 4G directives for IMT-Advanced
– All-IP packet switched network.
– Peak data rates
• Up to 100 Mbps for high-mobility mobile access
• Up to 1 Gbps for low-mobility access
– Dynamically share and use network resources
– Smooth handovers across heterogeneous networks, including 2G and 3G networks, small cells
such as picocells, femtocells, and relays, and WLANs
– High quality of service for multimedia applications
• No support for circuit-switched voice
– Instead providing Voice over LTE (VoLTE)
• Replace spread spectrum with OFDM
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LTE Architecture (1)
• Two candidates for 4G
– IEEE 802.16 WiMax (described in Lecture 8)
• Enhancement of previous fixed wireless standard for mobility
– Long Term Evolution
• Third Generation Partnership Project (3GPP)
• Consortium of Asian, European, and North American telecommunications standards organizations
• Both are similar in use of OFDM and OFDMA
• LTE has become the universal standard for 4G – All major carriers in the United States and Australia
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LTE Architecture (2)
• Some features started in the 3G era for 3GPP
• Initial LTE data rates were similar to 3G
• 3GPP Release 8
– Clean slate approach
– Completely new air interface • OFDM, OFDMA, MIMO
• 3GPP Release 10
– Known as LTE-Advanced
– Further enhanced by Releases 11 and 12
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LTE Architecture (3)
• evolvedNodeB(eNodeB)
– Most devices connect into the network through the eNodeB
• Evolution of the previous 3GPP NodeB
– Now based on OFDMA instead of CDMA
– Has its own control functionality, rather than using the Radio Network Controller (RNC)
• eNodeB supports radio resource control, admission control, and mobility management • Originally the responsibility of the RNC
• Traditionallycircuitswitchedbutnowentirelypacketswitched – Based on IP
– Voice supported using voice over IP (VoIP)
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LTE Radio Access Network (1)
• LTE uses MIMO and OFDM
– OFDMA on the downlink
– SC-OFDM on the uplink, which provides better energy and cost efficiency for battery-operated mobiles
• LTE uses subcarriers 15 kHz apart – Maximum FFT size is 2048
– Basic time unit is
T = 1/(15000×2048) = 1/30,720,000 seconds. s
– Downlink and uplink are organized into radio frames
• Duration 10 ms., which corresponds to 307200T . s
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LTE Radio Access Network (2)
• LTE uses both TDD and FDD
– Both have been widely deployed
– Time Division Duplexing (TDD)
• Uplink and downlink transmit in the same frequency band, but alternating in the time
domain
– Frequency Division Duplexing (FDD)
• Different frequency bands for uplink and downlink
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Spectrum Allocation for FDD and TDD
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Physical transmission
• Release 8 supports up to 4 × 4 MIMO
• The eNodeB uses the Physical Downlink Control Channel (PDCCH) to communicate
– Resource block allocations
– Timing advances for synchronization
• QPSK, 16QAM, and 64QAM modulation based on channel conditions
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LTE-ADVANCED
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LTE-Advanced
• So far we have studied 3GPP Release 8 – Releases 9-12 have been issued
• Release 10 meets the ITU 4G guidelines -> Full 4G – Took on the name LTE-Advanced
• Key improvements – Carrier aggregation
– MIMO enhancements to support higher dimensional MIMO
– Relay nodes
– Heterogeneous networks involving small cells such as femtocells, picocells, and relays – Cooperative multipoint transmission and enhanced intercell interference coordination – Voice over LTE
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Carrier Aggregation
• Ultimate goal of LTE-Advanced is 100 MHz bandwidth
– Combine up to 5 component carriers (CCs)
– Each CC can be 1.4, 3, 5, 10, 15, or 20 MHz
– Up to 100 MHz
• Three approaches to combine CCs
– Intra-band Contiguous: carriers adjacent to each other
– Intra-band noncontiguous: Multiple CCs belonging to the same band are used in a noncontiguous manner
– Inter-band noncontiguous: Use different bands
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Carrier Aggregation
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Enhanced MIMO
• Expanded to 8 × 8 for 8 parallel layers
• Or multi-user MIMO can allow up to 4 mobiles to receive signals simultaneously
– eNodeB can switch between single user and multi-user every subframe
• Downlink reference signals to measure channels are key to MIMO
functionality
– UEs recommend MIMO, precoding, modulation, and coding schemes
– Reference signals sent on dynamically assigned subframes and resource blocks
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Relaying
• Relay nodes (RNs) extend the coverage area of an eNodeB – Receive, demodulate and decode the data from a UE
– Apply error correction as needed
– Then transmit a new signal to the base station
• An RN functions as a new base station with smaller cell radius
• RNs can use out-of-band or inband frequencies
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Relay Nodes
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Heterogeneous networks (1)
• It is increasingly difficult to meet data transmission demands in densely populated areas
• Small cells provide low-powered access nodes – Operate in licensed or unlicensed spectrum
– Range of 10 m to several hundred meters indoors or outdoors – Best for low speed or stationary users
• Macro cells provide typical cellular coverage – Range of several kilometers
– Best for highly mobile users
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• Femtocell
Heterogeneous networks (2)
– Low-power, short-range self-contained base station
– In residential homes, easily deployed and use the home’s broadband for backhaul
– Also in enterprise or metropolitan locations
• Network densification is the process of using small cells
– Issues: Handovers, frequency reuse, QoS, security
• A network of large and small cells is called a heterogeneous network (HetNet)
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Other Enhancements in LTE-Advanced (1)
• Traffic offload techniques to divert traffic onto non-LTE networks
• Adjustablecapacityandinterferencecoordination
• Enhancements for machine-type communications
• Support for dynamic adaptation of TDD configuration so traffic fluctuations can be accommodated
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Other Enhancements in LTE-Advanced (2)
• Release 12 also conducted studies
– Enhancements to small cells and heterogeneous networks, higher order modulation like 256-QAM, a new mobile-specific reference signal, dual connectivity (for example, simultaneous connection with a macro cell and a small cell)
– Two-dimensional arrays that could create beams on a horizontal plane and also at different elevations for user-specific elevation beamforming into tall buildings.
• Would be supported by massive MIMO or full dimension MIMO
• Arrays with many more antenna elements than previous deployments.
• Possible to still have small physical footprints when using higher frequencies like millimeter waves
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FIFTH GENERATION SYSTEMS
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Evolution of Cellular Wireless Systems (1)
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Evolution of Cellular Wireless Systems (2)
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What’s Next?
• 5G systems development targets a exponential growth in mobile bandwidth consumption
• Three fundamental requirements are:
– Capabilities for supporting massive capacity and massive connectivity
– Support for an increasingly diverse set of services, application and users – all with extremely diverging requirements for work and life
– Flexible and efficient use of all available non-contiguous spectrum for wildly different network deployment scenarios
• Target for deployment: 2020
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5G Performance Targets
• Bandwidth: up to 20GB/s peak per cell (divided among users)
• Density: 1 million devices/square kilometer
– To support IoT
• Mobility: support for high speed vehicular access (up to 500Km/h)
• Latency: < 4ms
• 10ms switching between different radio access technologies
• Energy efficiency: energy consumption per bit 1000X smaller than in 4G
• Use of non-contiguous spectrum
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Unless otherwise stated:
Sources for this lecture
Cory Beard, William Stallings. Wireless Communication Networks and Systems, 1st edition. Pearson Higher Education, 2016
(Chapters 13 and 14)
All material copyright 2016
Cory Beard and William Stallings, All rights reserved
5G:
• ITU, Draft new Report ITU-R M.[IMT-2020.TECH PERF REQ] - Minimum requirements related to technical performance for IMT-2020 radio interface(s), released on 23-Feb-2017, available at
https://www.itu.int/dms_pub/itu-r/md/15/sg05/c/R15-SG05-C-0040!!MSW-E.docx
• Huawei, 5G Network Architecture: A high-level perspective, whitepaper, available at http://www.huawei.com/minisite/5g/img/5G_Network_Architecture_A_High-Level_Perspective_en.pdf
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