WiMAX and LTE Differences and Similarities Dr. Essam Sourour

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WiMAX and LTEDifferences and Similarities
Dr. Essam Sourour Alexandria University sourour_at_ie
ee.org
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Outline

• Wireless world map
• Historical background
• Why 4G?
• Enabling technologies
• WiMAX and LTE signals
• WiMAX and LTE transmitter and receiver

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Wireless World Map

• Wireless explosion started early 90s
• Many wireless systems serving many aspects of
  life
• Each system optimized for a range of applications
• Distance, data rate, mobility, number of users
• There is an overlap and strong competition
• Some are more successful than others

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Wireless World Map
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Wireless Map
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IEEE and Wireless Systems

• IEEE members produced several wireless standards

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Expected LTE Subscribers
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Cellular Mobile Communications

• Service area is divided into cells
• Frequency channels are re-used at sufficient
  separation
• Calls are handed-off from cell to cell

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Historical Background
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Data Rates of Old Systems
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Why 4G ?
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Technical Requirements

• Increase data rate
• About 100 Mbps downlink and 50 Mbps uplink
• Improve wireless performance
• Better signal reception and better coverage
• Increase spectrum efficiency
• More subscribers and more data transfer in the
  same spectrum
• High flexibility of allocation
• Quickly adjust data rate to subscriber according
  to need

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Technical Requirements

• Spectrum flexibility
• Several bandwidths can be used
• Optimized for low speed
• Best at lt 20 km/hr support up to 350 km/hr
• Add Multi-Cast and Broadcast Services
• To support broadcast services like TV
• Faster call setup

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4G Enabling Technology

• Some key technologies made 4G possible
• Both WiMAX and LTE use
• OFDM, OFDMA and SC-FDMA
• Channel dependent scheduling
• Adaptive coding and modulation (ACM)
• Multiple-In-Multiple-Out (MIMO) antenna
  processing
• Turbo coding and decoding
• Need to fight the fading channel

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Wireless Fading Channels

• The wireless channel is subject to multipath
  (reflections)
• The received signal is affecting by fading

• Fading is a variation in the received signal
  level, with more lows than highs
• Both the time domain and frequency domain are
  affected

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Fading Channels in Time

• Channel gain changes with time
• In most cases the changes are slower than data
  rates
• This is not good since consecutive stream of bits
  may be lost

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Fading Channels in Frequency

• Channel gain varies with frequency
• May cause distortion to the signal spectrum,
  depending on signal bandwidth

• Problem increases with larger bandwidth from 2G
  to 4G

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OFDM Concept

• We have a high rate (hence, large bandwidth)
  stream of modulation symbols Xk (ex. QAM)
• Needs to be transmitted on a frequency selective
  fading channel
• Stream Xk is divided into N low rate parallel
  sub-streams
• Bandwidth of each sub-stream is N times narrower
• Each sub-stream is carried by one subcarrier
• Received must restore each Xk without
  interference from current or previously
  transmitted sub-streams

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OFDM Concept

• Transmitted OFDM Signal
• Received OFDM Signal

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OFDM Concept
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OFDM Concept

• OFDM modulation using IFFT
• Guard time (cyclic prefix) is added to protect
  against inter-symbol interference
• Guard subcarriers to protect against neighbor
  channels at both sides
• Zero subcarrier (dc) not used
• Some subcarriers are used as pilots for channel
  estimation
• After equalization, receiver performs FFT to
  retrieve back the stream Xk

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OFDM Concept
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OFDMA Concept

• In OFDM one user occupies all subcarriers all the
  time (till packet is finished)
• In OFDMA each user occupies few subcarriers for
  few OFDM symbols during a Burst of transmission
• A Burst few subcarriers during few OFDM symbols
• Hence the name Orthogonal Frequency Division
  Multiplex Access

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OFDMA Flexibility

• With OFDMA the user allocation is flexible
• Can change from frame to frame
• Multiple allocations for several applications
• Allocation changes
• In WiMAX every 5 ms
• In LTE every 1 ms

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Single Carrier FDMA (SC-FDMA)

• A major problem with OFDM and OFDMA is high
  peak-to-average power ratio (PAPR)
• Transmitted amplitude with large variation
• Requires a linear amplifier at transmitter
• Linear amplifies consumes high power
• OK at base station
• For mobile station, this consumes battery
• LTE uses a solution for UL SC-FDMA
• Single carrier transmission

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SC-FDMA Process

• After modulation, apply FFT
• Each symbol is on a subcarrier
• Put the subcarriers on selected location and
  apply IFFT
• Back to single carrier transmission
• Now add CP
• Receiver will do the reverse

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PAPR reduction in SC-FDMA
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Channel Dependent Scheduling

• Another benefit for OFDMA
• The Base Station talks to many users at different
  directions
• Their fading channels are independent
• Subcarriers with high attenuation for user 1 may
  have good gain for user 2

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Channel Dependent Scheduling
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Adaptive coding and modulation

• WiMAX and LTE use QPSK, 16QAM and 64QAM
  modulation (m2, 4 and 6)
• Higher m provides higher data rate
• Higher m is subject to more errors
• WiMAX uses Convolutional coding and Turbo coding
• Code rate r between 1/2 and 5/6
• TE use Turbo coding only for user data
• Code rate r between 1/3 and 1

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Adaptive coding and modulation

• Lower m and higher r is effective against fading,
  and vice versa
• But lower m and higher r reduces data rate to and
  from the user
• WiMAX and LTE select the values of m and r to
  match the fading channel for each user

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Adaptive coding and modulation

• Note that each user has a different channel
  condition between the base station and the mobile
  station

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Modulation methods
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Modulation methods
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MIMO

• Signal transmitted from multiple antennas
  (Multiple In)
• Signal received by multiple antennas (Multiple
  Out)

• Receiver combines the received signals and
  optimally combine energy from MxN channels
• Two main types of MIMO
• Transmit Diversity (also called Alamouti)
• Spatial Multiplexing

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MIMO

• Transmit diversity
• Same modulation symbols sent from all Tx M
  antennas
• Receiver combines the signal from N antennas
• Useful to increase performance against fading
• Spatial multiplexing
• Different modulation symbols sent from M Tx
  antennas
• Receiver received the signal from N antennas
• Useful to increase data rate if channel is good
• WiMAX uses up to 2x2. LTE uses up to 4x4

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Conventional Receiver Diversity
Combining two channels strength
Receiver pays the cost of antenna diversity
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MIMO 2X1, Transmit Diversity

• Example M2 and N1 TX Diversity
• Space Two antennas. Time Two intervals
• Cost moved to transmitter (Base Station)

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MIMO 2X2, Transmit Diversity

• Take M2 and N2
• Diversity order 4

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MIMO 2x2, Spatial Multiplexing

• Purpose is to increase data rate (2x2 gives twice
  data rate)
• The 4 gains must be known at receiver
• Simplest way at receiver, matrix inversion

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Turbo Codes

• Turbo codes were proposed by Berrou and Glavieux
  in the 1993 International Conference in
  Communications (ICC 93)
• Break Through performance, much better than
  conventional methods
• Features of turbo codes
• Parallel encoding
• Each encoder is a Systematic encoder
• Interleaving among the encoders
• Iterative decoding

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Turbo Encoder

• Source bits are encoded by first encoder
• Source bits are interleaved in a pseudo-random
  fashion and encoded by second encoder
• Original source bits also transmitted
  (systematic)
• Overall rate is r1/3

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Turbo Encoder
LTE Turbo Encoder
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Turbo Iterative Decoding

• Decoder 1 uses original bits, parity 1
  extrinsic 2
• Decoder 2 uses original bits, parity 2
  extrinsic 1
• Decoder 1 provides extrinsic 1 to improve
  confidence level to input of decoder 2
• Decoder 2 works now better, and feeds back
  extrinsic 2 to improve confidence level to input
  of decoder 1
• Decoder 1 repeats with the better input, provided
  to decoder 2
• Decoder 2 repeats with the better input, feedback
  to decoder 1
• . . . . . Many iterations

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Performance with iterations
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WiMAX Frame Structure

• Frame duration is 5 ms

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LTE Frame Structure

• Frame is 10 ms, divided into 10 sub-frames

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WiMAX User Data Tx
Transmitter Baseband Processing
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WiMAX User Data Rx
Receiver Baseband Processing
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LTE User Data Tx
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LTE User Data Rx
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WiMAX vs LTE parameters
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Conclusions

• WiMAX and LTE employ similar technologies
• Both will achieve very high data rates
• Both will provide new services
• Both use OFDMA, MIMO, TURBO
• LTE has the advantage of large GSM/UMTS customer
  base
• WiMAX has the advantage of being already in
  service in few places in USA