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08WMN_LTE_2010

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									Mobile and Wireless Networks
         2010-2011

          Chapter 6
Beyond 3G: Long Term Evolution
            (LTE)
3GPP Evolution
•   2G: Started years ago with GSM: Mainly voice
•   2.5G: Adding Packet Services: GPRS, EDGE
•   3G: Adding 3G Air Interface: UMTS
•   3G Architecture:
        Support of 2G/2.5G and 3G Access
        Handover between GSM and UMTS technologies
•   3G Extensions:
        HSDPA/HSUPA
        IP Multi Media Subsystem (IMS)
        Inter-working with WLAN (I-WLAN)
•   Beyond 3G (3.9G):
        Long Term Evolution (LTE)
        System Architecture Evolution (SAE)




                                    Mobile and Wireless Networks
                                 Chapter 4 : Mobile Transport Protocols
LTE Performance Requirements (1)
• Frequency bands:
  700 MHz (USA), 900, 1800 and 2600 MHz (EU), 1800 and 2600 MHz
  (Asia), 1800 MHz (Aus)
• Improved spectrum scalability
     Scalable bandwidth of 20MHz, 15MHz, 10MHz, 5MHz and <5MHz
• Spectrum Flexibility
  Smooth migration into other frequency bands, including those currently
  used for 2G.
• Data Rate:
   Instantaneous downlink peak data rate of 100Mbit/s in a 20MHz
  downlink spectrum (i.e. 5 bit/s/Hz)
   Instantaneous uplink peak data rate of 50Mbit/s in a 20MHz uplink
  spectrum (i.e. 2.5 bit/s/Hz)

                         Mobile and Wireless Networks
                      Chapter 4 : Mobile Transport Protocols
LTE Performance Requirements (2)
• Cell range
   5 km - optimal size
   30km sizes with reasonable performance

     up to 100 km cell sizes supported with acceptable performance
• Cell capacity
  up   to 200 active users per cell (5 MHz) (i.e., 200 active data clients)
• Mobility
     Optimized for low mobility (0-15km/h) but supports high speed
• Latency
   user plane < 5ms
   control plane < 50 ms


                             Mobile and Wireless Networks
                          Chapter 4 : Mobile Transport Protocols
LTE PHY: Modulation/Multiple
Access
• Downlink: OFDMA
      High robustness against frequency selective fading
      Spectrum flexibility: #subcarriers scales with bandwidth
      Large variation in the instantaneous transmit power  low power-
       amplifier efficiency
      In spite of orthogonality of subcarriers, interference may occur; use of
       cyclic prefix: last part of the OFDM symbol is copied and inserted at
       the beginning of the symbol
• Uplink: SC-FDMA (Single Carrier – FDMA)
      Better PAPR (Peak-to-Average Power Ratio) than OFDM (i.e. small
       variations in transmit power)




                            Mobile and Wireless Networks
                         Chapter 4 : Mobile Transport Protocols
    OFDMA in LTE (DL)
•   OFDMA transmits a data stream by using
    several narrow band sub-carriers
    simultaneously
•   In theory, each sub-carrier signal could
    be generated by a separate transmission
    chain hardware block
•   For each sub-carrier a graph which
    shows the frequency on the x-axis and
    the amplitude of each sub-carrier on the
    y-axis can be constructed.
•   Inverse Fast Fourier Transformation
    (IFFT) is applied (from the frequency
    domain to time domain).
•   This represents the same signal as would
    have been generated by the separate
    transmission chains for each sub-carrier
    when summed up



                                  Mobile and Wireless Networks
                               Chapter 4 : Mobile Transport Protocols
    SC-FDMA in LTE (UL)
•   SC-FDMA also transmits data over the
    air interface in many sub-carriers
•   Instead of putting 4 bits together (in e.g.
    16QAM) (as in OFDM) to form the signal
    for one sub-carrier, the information of
    each bit is spread over all the sub-
    carriers
•   The bits are piped into a Fast Fourier
    Transformation (FFT) function first.
•   The output of this process is the basis for
    the creation of the sub-carriers for the
    following IFFT.




                                    Mobile and Wireless Networks
                                 Chapter 4 : Mobile Transport Protocols
LTE PHY: Modulation and Coding

•   Adaptive modulation and coding
       DL modulations: QPSK, 16QAM, and 64QAM
       UL modulations: QPSK and 16QAM




                        Mobile and Wireless Networks
                     Chapter 4 : Mobile Transport Protocols
LTE Frame Structure (1)




One element that is shared by the LTE Downlink and Uplink is the generic frame
structure. The LTE specifications define both FDD and TDD modes of operation. This
generic frame structure is used with FDD. Alternative frame structures are defined for
use with TDD.

LTE frames are 10 msec in duration. They are divided into 10 subframes, each
subframe being 1.0 msec long. Each subframe is further divided into two slots, each
of 0.5 msec duration. Slots consist of either 6 or 7 ODFM symbols, depending on
whether the normal or extended cyclic prefix is employed
                              Mobile and Wireless Networks
                           Chapter 4 : Mobile Transport Protocols
LTE Frame Structure (2)




               Mobile and Wireless Networks
            Chapter 4 : Mobile Transport Protocols
LTE Frame structure (3)
• LTE physical resource: time-frequency grid.
• Time domain:
    time is divided into Transmission Time Intervals (TTI): 1 ms
    TTI consists of 2 slots, each 0.5 ms
• Frequency domain:
    channels of 180 kHz each consisting of 12 subcarriers of Δf = 15kHz
• PRB: Physical Resource Block:
    subchannel x timeslot (180 kHz x 0.5 ms)
    Minimal scheduling unit = 2 PRBs (180 kHz x 1ms)




                          Mobile and Wireless Networks
                       Chapter 4 : Mobile Transport Protocols
LTE Frame Structure (4)
• OFDM symbol duration time
    The cyclic prefix is used to prevent Inter-Symbol Interference between
     information blocks.
    Symbol duration = 1/Δf (= 1/15 μs = 66.67μs) + CP duration.
    CP duration
        • Normal CP : 5.2 μs first symbol in each slot ; 4.69 μs all other symbols
        • Extended CP : 16.67 μs all symbols
      Number of symbols/slot
        • 7 symbols (normal CP)
        • 6 symbols (extended CP)




                                Mobile and Wireless Networks
                             Chapter 4 : Mobile Transport Protocols
Scheduling




Access in both time and frequency domain is shared among the users  need for
scheduler
• Downlink: slots need not to be contiguous
• Uplink: to maintain single-carrier property, only contiguous slots in the frequency
domain are allocated to the same user




                               Mobile and Wireless Networks
                            Chapter 4 : Mobile Transport Protocols
Downlink scheduling (1)

• Channel dependent scheduling:
      The UE measures characteristics of the downlink channels: SNR,
       BER,…
      This leads to a Channel Quality Indication (CQI)
      The UE sends a CQI report to the eNodeB
      The eNodeB makes an adaptation of the modulation scheme, the
       channel coding and/or the power




                           Mobile and Wireless Networks
                        Chapter 4 : Mobile Transport Protocols
Downlink scheduling (2)
• The scheduling algorithm defines in each TTI, the sub-
  channels assignment to packets of the active calls
• The SNR of the channels result in a coding and modulation
  scheme which defines the momentary bit rate supported by
  that channel
• The assignment of sub-channels is determined by
      SNR (  bit rate of sub-channel)
      Delay requirements of call (QoS)
      Fairness requirements




                           Mobile and Wireless Networks
                        Chapter 4 : Mobile Transport Protocols
LTE Network Architecture (1)




                Mobile and Wireless Networks
             Chapter 4 : Mobile Transport Protocols
System Architecture Evolution (SAE)
System Architecture Evolution is the core network architecture of 3GPP's
future LTE wireless communication standard.
SAE is the evolution of the GPRS Core Network, with some differences.
• A common anchor point and gateway (GW) node for all access
technologies
• IP-based protocols on all interfaces;
• Simplified network architecture
• All IP network
• All services are via Packet Switched domain
• Support mobility between heterogeneous RATs, including legacy systems
as GPRS, but also non-3GPP systems (say WiMAX)




                          Mobile and Wireless Networks
                       Chapter 4 : Mobile Transport Protocols
LTE Network Architecture (2)
MME (Mobility Management Entity):
-Manages and stores the UE control plane context, generates
 temporary Id, provides UE authentication, authorization,
 mobility management
UPE (User Plane Entity):
-Manages and stores UE context, ciphering, mobility anchor,
 packet routing and forwarding, initiation of paging
3GPP anchor:
-Mobility anchor between 2G/3G and LTE
SAE anchor:
-Mobility anchor between 3GPP and non 3GPP (I-WLAN, etc)

                      Mobile and Wireless Networks
                   Chapter 4 : Mobile Transport Protocols
LTE Network Architecture (4)

• Reduced number of network nodes
   Reduced cost and complexity (e.g. inter-
    operability testing)
   Reduced call set-up times
   Reduced latency

• GGSN, SGSN, RNC  Access Gateway




                   Mobile and Wireless Networks
                Chapter 4 : Mobile Transport Protocols
LTE Protocol Stack




               Mobile and Wireless Networks
            Chapter 4 : Mobile Transport Protocols
Multiple Antenna Techniques
MIMO (multiple input, multiple output) employs multiple transmit and receive
antennas to substantially enhance the air interface.

The same data stream is mapped onto multiple transmit antennas.

MIMO processing also exploits spatial multiplexing, allowing different data
streams to be transmitted simultaneously from the different transmit
antennas, to increase the end-user data rate and cell capacity.

In addition, when knowledge of the radio channel is available at
the transmitter (e.g. via feedback information from the receiver),
MIMO can also implement beam-forming to further increase
available data rates and spectrum efficiency



                               Mobile and Wireless Networks
                            Chapter 4 : Mobile Transport Protocols

								
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