Frequency Division Technology in Broadband

Wireless Systems
with Applications to WiMAX and LTE

Jun Zheng

Department of Electrical and Computer Engineering

The University of Texas at Austin

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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Advanced Broadband Wireless Systems

WiMAX (WiMAX Forum & IEEE 802.16 family)

Long Term Evolution (LTE, 3GPP)
Ultra Mobile Broadband (UMB, 3GPP2)

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Frequency Division Technology

Orthogonal Frequency Division Multiplexing (OFDM)

Multiple Access OFDM (OFDMA)
Scalable OFDMA (SOFDMA)
Single Carrier FDMA (SC-FDMA)
Single Carrier - Frequency Domain Equalization (SC-FDE)

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Frequency Division Technology in Standards

OFDM:
WLAN (IEEE 802.11 family)
WiMAX (IEEE 802.15d, xed)
OFDMA (Scalable):
WiMAX (IEEE 802.15 family, mobile)
LTE (downlink)
UMB
SC-FDMA
LTE (uplink)

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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The Idea of OFDM

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OFDM Transmitter

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OFDM Transmitter

Input data stream QAM modulator X [n]

S/P conversion symbol stream X [0], X [1], , X [N 1]
IFFT of X [n] OFDM symbol
N1
1
x[n] = X [k]e j2nk/N , 0 n N 1.
N k=0

Add cyclic prex x[n] = x[], , x[N 1]

P/S conversion D/A conversion upconvert to f0 wireless
channel

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OFDM Receiver

Received signal: y (t) = x(t) h(t) + n(t)

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Orthogonality

QAM symbols: rectangular pulse in time sinc function in frequency

QAM signals on each channel are orthogonal

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Cyclic Prex

Multipath symbol leakage ISI

Solution: cyclic prex

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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Peak-to-average Power Ratio (PAPR)

PAPR:
maxt x(t)2 maxn x[n]2
PAPR , PAPR
Et [x(t)2 ] En [x[n]2 ]

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PAPR

Why PAPR is important: transmit power eciency

PAPR
= max 10 20

Statistical property:

Pr (PAPR ) = 1 (1 e )N

N: number of subcarriers
PAPR grows with N

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Reducing PAPR
Amplitude Clipping

in-band distortion: degrades BER performance

out-of-band leakage: reduces spectral eciency
Peak cancellation with a complementary signal
Special coding

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Synchronization

Timing oset: inter-symbol interference (ISI)

Frequency oset: inter-carrier interference (ICI)

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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Idea of OFDMA

Subchannelization: divide subcarriers into mutually exclusive clusters

Multiple Access: assign clusters to distinct users

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OFDMA system

Resource allocation: subcarrier, bit, and power

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OFDMA Downlink Transmitter

bm,i : ith block of the mth user

CAS: map data blocks onto subcarriers assigned to the user
CAS inserts zeroes to expand bm,i into dm,i
dm,i from dierent users are summed up to form data block di

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OFDMA Downlink Receiver

Frequency and timing oset correction

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OFDMA Uplink Transmitter

Each user deals with its own data

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OFDMA Uplink Receiver

Received signal: sum of signals from M users

M
(R)

r (k) = sm (k) + w (k)
m=1
m 1
L
(R) (T )
sm (k) = e j2m k/N hm (l)sm (k m l)
l=0

m : the frequency error

m = int(m /Ts ): integer timing oset expressed in sampling periods
Timing and frequency synchronization required

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Scalability of OFDMA

Scales the FFT size to the channel bandwidth

Keep sub-carrier frequency spacing constant for dierent channel
bandwidths
Wider channel bandwidth: larger FFT size
Flexibility: minimum modication when deployed in various frequency
band intervals
SOFDMA is the OFDMA mode used in Mobile WiMAX

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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OFDMA in WiMAX

SOFDMA: Nused scales with channel bandwidth Bw , constant

subcarrier spacing f
Fully Used Subchannelization (FUSC):
Uses all the subchannels
Subcarrier permutation to minimize the probability of hits
Partially Used Subchannelization (PUSC):
Subchannels are divided into three segments
Segments are allocated to sectors of the same cell
Subcarrier permutation to minimize the probability of hits

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Downlink-FUSC

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Uplink-PUSC

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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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SC-FDMA System

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Generating SC-FDMA Transmit Symbol

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SC-FDMA Receiver

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Subcarrier Mapping

Localized subcarrier mapping (LFDMA)

Distributed subcarrier mapping (DFDMA)
Interleaved subcarrier mapping (IFDMA)
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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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OFDMA vs. SC-FDMA: System

OFDMA: multicarrier vs. SC-FDMA: single carrier

SC-FDMA: adds DFT at input of transmitter, IDFT at output of
receiver
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OFDMA vs. SC-FDMA: Equalization at Receiver

OFDMA: separate equalization and detection for each subcarrier

SC-FDMA: equalization across the entire channel bandwidth

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PAPR Comparison

No closed-form solution for CCDF of PAPR in SC-FDMA

Upper-bound available
Comparison based on BPSK and raised-cosine pulse shaping
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Outline
1 Introduction

2 OFDM
Fundamentals
Challenges

3 OFDMA
Fundamentals
OFDMA in WiMAX

4 SC-FDMA
Fundamentals
OFDMA vs. SC-FDMA

5 SC-FDE
Fundamentals

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Frequency Domain Equalization

DFT size does not grow linearly with length of channel response
Lower complexity
Similar performance to OFDM for combating frequency selective
fading

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SC-FDE vs. OFDM

OFDM: detection in frequency domain on a per-subcarrier basis

SC-FDE: detection in time domain after additional IDFT
OFDM is more sensitive to a null in the channel spectrum
SC-FDE has better PAPR performance
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Conclusion

Frequency Division Technologies:

OFDM
OFDMA
Scalable OFDMA
SC-FDMA
SC-FDE
Applications: WiMAX, LTE, UMB

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