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Lecture 13LTE
Long Term Evolution (LTE)
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• Material Related to LTE comes from• “3GPP LTE: System Overview, Product
Development and Test Challenges,” Agilent Technologies Application Note, 2008.
• IEEE Communications Magazine, February 2009
• IEEE Communications Magazine, April 2009
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LTE – Long Term Evolution
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• Highlights▫ Different network architecture to support packet based access▫ All IP core network that can interface better with technologies
such as WiFi and WiMax▫ Use of OFDMA as the medium access/modulation scheme▫ Flexibility to deploy it in as little spectrum as 1.4 MHz and as
much as 20 MHz of spectrum▫ Support for true “broadband”
FDD Downlink Peak Data Rates Using 64 QAM
Antenna Configuration SISO 2 × 2 MIMO 4 × 4 MIMO
Data Rate (Mbps) 100 172.8 326.4
Expected Downlink Data Rates in LTE
History
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• UMTS migration towards LTE was proposed by 3GPP• Release-99▫ Commercial deployment in 2003▫ Provided 3.84 Mcps – we looked at this
• Release-5▫ Deployed in 2006 introduced HSDPA
• Release 6• Release-6▫ Deployed in 2007 introduced HSUPA
• Release-7▫ Currently in deployment introduced HSPA+▫ Also includes study of LTE’s system architecture evolution
(SAE)
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3GPP Release 8 and beyond
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• Expected Deployment▫ HSPA+ this year▫ LTE in 2010 and beyond
• New IP Core Network• OFDMA Air Interface▫ Support for both FDD and TDDSupport for both FDD and TDD
• In releases beyond release 8▫ Multimedia broadcast multicast service (MBMS)▫ IMT-Advanced Considered true 4G
LTE Network Architecture
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• Evolved Packet System (EPS) consists of two parts▫ E-UTRAN – Evolved UMTS Terrestrial Radio Access
Network▫ EPC – Evolved Packet Core
• E-UTRAN▫ Consists of only one kind of node: eNode-B▫ Compare with UMTS
• EPC• EPC▫ Fully based on IP – consists of elements MME – Mobility Management Entity (like SGSN) S-GW & PDN-GW: Serving and Packet Data Network Gateways Home subscriber server (HSS)
▫ Voice and real-time applications will make use of the IP Multimedia Subsystem (IMS)
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LTE Network Architecture
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Functional Changes
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• E-NodeB▫ Does a lot more now! (no RNC or BSC)▫ Selection of MME, RRM functions, Handling Mobility
• MME▫ Sends pages to e-NodeBs▫ Security▫ Security▫ Idle state mobility
• S-GW▫ Termination of user plane▫ Switching of user plane (mobility)
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Channel Bandwidths
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• Compare with AMPS, GSM, IS-95, UMTS and WiMax• Can vary from 1.4 MHz to 20 MHz• Resource Block (RB)▫ 180 kHz wide and 0.5ms long▫ 12 subcarriers spaced at 15 kHz (24 at 7.5 kHz possible
later)• Data rate limited by User Equipment (UE) categories• Data rate limited by User Equipment (UE) categories
Channel BW (MHz) 1.4 3.0 5 10 15 20
Resource Blocks 6 15 25 50 75 100
Downlink Multiple Access: OFDMA
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Source: Agilent Application Note
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Uplink Multiple Access: Single Carrier FDMA
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Source: Agilent Application Note
Why SC-FDMA
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Source: Agilent Application Note
• Avoid high peak-to-average power ratio (PAPR) in MS
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LTE Physical Signals
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Source: Agilent Application Note
LTE Physical Channels
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Source: Agilent Application Note
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Transport Channels in LTE
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Source: Agilent Application Note
Compare withTransportChannelsIn UMTS
Mapping from Transport to Physical Channels
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Source: Agilent Application Note
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Random Accessl k ll b h l
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• Uplink transmissions in a cell must be orthogonal▫ They are aligned with frame timing of an e-NodeB
• When a MS powers up or after a long period of inactivity, this alignment is lost
• RA Procedure▫ MS sends one of several preambles (shared) with a
guard periodg p▫ E-NodeB detects preamble, estimates MS’s timing,
and responds with a correct timing advance and uplink resource
▫ MS sends its identity in this allocated resource + some data
▫ E-NodeB echos the MS identity
More on RA Procedure
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• RA procedure must be repeated if the echoed identity is not correct
• Backoff indication from e-NodeB can be used to reduce contention
• After an inter-eNodeB handoff, an RA procedure is imminent▫ Contention free RA procedure is possible▫ Unique preamble is assigned to the MS
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Obtaining Uplink Resources
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• If a MS has data to send, it can send a single “scheduling request” (SR) bit using ▫ The RA Procedure▫ Dedicated SR on the PUCCH
• In the first allocated resource, the MS sends a buffer status report that has more information buffer status report that has more information about how much data it wants to send
• E-NodeB allocates resources on a per sub-frame basis (every ms!)▫ Scheduler is responsible for handling QoS
Frame Structure (FDD)
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Source: Agilent Application Note
• Referenced to Ts which is the shortest time interval in the system
• Defined as Ts = 1/(15000 × 2048) = 32.552 ns• OFDM symbol length = 66.7 μs
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Detailed Downlink Frame Structure (FDD)
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Source: Agilent Application Note
Miscellaneous - 1• Discontinuous Reception
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• Discontinuous Reception▫ MSs can save battery by checking the paging channel
once every DRX cycle• Handoff▫ E-NodeB’s are connected over an X2 interface▫ They can negotiate whether or not a MS must handoff
• Hybrid ARQ▫ Similar to HSPA
• Power control▫ It is specified, but not as crucial as in CDMA systems
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Miscellaneous - 2
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• Frequency reuse of 1 is possible in the center of the cell
• Some frequency i management is
essential at the cell edges
