4) Ofdma and Scfdma

MobileComm MobileComm Professionals, Professionals, Inc. Your Partner for Wireless Engineering Solutions

Objective 

Understand LTE LTE Duplexing Dupl exing

• Single Transmitter • FDMA Principle • Multi carrier principle

OFDMA and SC FDMA Principle Propagation  Multipath Propagation Cyclic Prefix  OFDMA and SC FDMA 

• Transmitter • Receiver  OFDM

and SC FDMA Key Parameters Resource Block

Duplexing and Multiple Access

Legacy- Single Transmitter

FDMA Principle

LTE: Multi-Carrier Principle

The Rectangular Pulse Fourier Transform Time Domain

  e    d   u    t    i    l   p   m   a

  f   s 

Ts

1

T  s

time

 Advantages:  Simple to implement: there is no complex filter system required to detect such pulses and to generate them.  The pulse has a clearly defined duration. This is a major advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference.

Inverse Fourier Transform

  y    t    i   s   n   e    d   r   e   w   o   p    l   a   r    t   c   e   p   s

Frequency Domain

f s

frequency f/f s

Disadvantage:  It allocates a quite huge spectrum  However the spectral power density has null points exactly at multiples of the frequency fs = 1/Ts.  This will be important in OFDM.

OFDMA Principle Transmits hundreds or even thousands of separately modulated radio signals using orthogonal subcarriers spread across a wideband channel Total transmission bandwidth 15 kHz in LTE: fixed

Orthogonality:

The peak (centre frequency) of one subcarrier …

…intercepts the  ‘nulls’ of the neighbouring subcarriers

OFDM Basics 

Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission



The throughput is the sum of the data rates of each individual (or used) subcarriers while the power is distributed to all used subcarriers



FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined by the FFT size ( by the bandwidth)

Power

Bandwidth

Frequency

OFDM Signal

OFDM: Nutshell

Frequency-Time Representation

FFT/IFFT It can be shown that the OFDM signal may be obtained by transforming L data symbols by the IFFT, where L is the number of subcarriers.  Therefore, OFDM transmitter and receiver are implemented using IFFT and FFT respectively. 

Time-domain (to be transmitted) d 1 d 2

IFFT d L



FFT

d 1 d 2

d L

The size of the FFT should be chosen carefully as a balance between protection against multipath (i.e. ISI), temporal variations (i.e. ICI), and design cost/complexity.  LTE FFT period is 66.67 usec, corresponding to the 15 KHz subcarrier separation.

Motivation for OFDMA



Good performance in frequency selective fading channels

  Low

complexity of base-band receiver

  Good

spectral properties and handling of multiple bandwidths

  Link

adaptation

  Frequency

domain scheduling

  Compatibility with advanced receiver and

technologies.

antenna

Challenges

1) ISI

Solution: CP

2) Multi-Carrier Modulation 

The center frequencies must be spaced so that interference between different carriers, known as Adjacent Carrier Interference ACI, is minimized; but not too much spaced as the total bandwidth will be wasted.



Each carrier uses an upper and lower guard band to protect itself from its adjacent carriers. Nevertheless, there will always be some interference between the adjacent carriers.  ∆f subcarrier  ∆f sub-used

f 0

f 1

f 2

f N-2

 ACI = Adjacent Carrier Interference

f N-1

frequency

Solution: OFDM Multi-Carrier 

OFDM allows a tight packing of small carrier  – called the subcarriers - into a given frequency band.

  y    t    i   s   n   e    D   r   e   w   o    P

  y    t    i   s   n   e    D   r   e   w   o    P

Frequency (f/fs)

Saved Bandwidth

Frequency (f/fs)

No ACI (Adjacent Carrier Interference) in OFDM due to the orthogonal subcarriers !

3)Inter-Carrier Interference (ICI) 

The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.  If the receiver’s frequency slips some fractions from the subcarriers center frequencies, then we encounter not only interference between adjacent carriers, but in principle between all carriers.  This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform.   One possible cause that introduces frequency errors is a fast moving Transmitter or Receiver (Doppler effect).

Frequency Drift Two effects begin to work:  Subcarrier has no longer its power density maximum- so loose of signal energy.  ∆P 

The rest of subcarriers have no longer a null point here. So we get some noise from the other subcarrier.

I3 I1 I4 I0 f 0

f 1

f 2

f 3

f 4

  e   c   n   e   r   e    f   r   e    t   n    I   r   e    i   r   r   a    C     r   e    t   n    I   =    I    C    I

OFDM Transmitter Frequency Domain Signal: (Collection of Sinusoids)

Binary Coded Data

s0 s1

s2 …

t0 t1 tx22

f N-1 freq.

Modulation Mapper

s0

b20 ,b21,…

Modulation Mapper

s1

. . .

x0 x1

sN-1

f 0 f 1 f 2

b10 ,b11,…

Serial to Parallel Converter (Bit Distrib.)

xN-1

  n    i   a   m   o    D   y   c   n   e   u   q   e   r    F

…

tN-1 time

Time Domain Signal

cos(2πf ct)

IFFT

x0, x1, …, xN-1 Time Domain

  n    d   o   r   i   a   t   u  a   r    G    /   e    P   n   e    C   G

I

D  A

IQ Split Q

D  A

Low Pass

I R 

Low Pass

Q

-sin(2πf ct) bN-1 0 …

Modulation Mapper

sN-1

Each entry to the IFFT module corresponds to a different subcarrier  Each sub-carrier is modulated independently by Modulation Schemes: BPSK,QPSK, 16QAM, 64QAM 

OFDM Receiver s0

yN-1

y0 y1 x2

tN-1 time

Derotator

 A

  r I   o    t   a D    l   u    d   o   m   e Q  A    D

LNA gain

D

 j

 AGC  Automatic Gain Control

sN-1

…

Frequency Domain

Time Domain

RF

s2

…

t0 t1 t2

 .   p   m  s    A  s   a   p   e    d   s    i   n   o  a    N   B   w  +   o    L

s1

   h    t   g   n   e   n   r    i   o    t   s    t   c    l   a   e   n   r   g   r   o    i   s   c   e   s   a    h   p

  +   g   n    i    T   w   o   F    F    d   n    i    W

s’ 0

s0

s’ 1

s1

  n    i   a   m   o .   D .   y   c .   n   e   u   q   e   r    F

  n   o    i    t   a   e   r   r   o   c s’ N-1   o    t   u   a    l   a   n   e   t reference   g   e  s    i   u (pilot)   s   m    j    i    t   d   a

Frequency And Timing Sync

  n   o    i    t   c   e   r   r   o    C    l   e   n   n   a    h    C

f 0 f 1 f 2

Bit Mapping Bit Mapping

. . .

sN-1

   l   e   e  s   n  n   n  o   a  p   s    h   c   e   r Channel Estimation

f N-1 freq. B10 ,B11,… B20 ,B21,…

. . .

. . .

Bit Mapping

BN-1 0 …

QPSK Im 11

01 sk 

d11 Re d10

00

10

  n   o    i    t   u    b    i   r    t   s    i    D    t    i    B

Soft Bit Coded Data

OFDM Key Parameters 1) Variable Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz

Frequency  Δf  Power density

2) Subcarrier Spacing ( Δ f = 15 KHz) → The Symbol time is Tsymbol = 1/ Δ f = 66,7μs Frequency

 Amplitude TCP TSYMBOL

CP

T SYMBOL TS

Time

OFDM Key Parameters 3) The number of Subcarriers Nc If BW = 20MHz → Transmission BW = 20MHz – 2MHz = 18 MHz → the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers

Channel Bandwidth [MHz] Transmission Bandwidth Configuration [RB] Transmission Bandwidth [RB]  C  h   a  n n  e  l    e   d   g   e 

 C  h   a  n n  e  l    e   d   g   e 

R   e   s   o   u r   c   e   b  l    o   c  k 

 Active Resource Blocks

DC carrier (downlink only)

OFDM Key Parameters 4) FFT (Fast Fourier Transform) size Nfft For a bandwidth BW = 20 MHz Nc = 1200 subcarriers not a power of 2 → The next power of 2 is 2048 → the rest 2048 -1200 848 padded with zeros 

5. Sampling rate fs This parameter indicates what is the sampling frequency: → fs = Nfft x Δf  Example: for a bandwidth BW = 5 MHz (with 10% guard band) The number of subcarriers Nc = 4.5 MHz/ 15 KHz = 300 300 is not a power of 2 → next power of 2 is 512 → Nfft = 512 Fs = 512 x 15 KHz = 7,68 MHz → fs = 2 x 3,84 MHz which is the chip rate in UMTS!!

The sampling rate is a multiple of the chip rate from UMTS/ HSPA. This was acomplished because the subcarriers spacing is 15 KHz. This means UMTS and LTE have the same clock timing!

OFDM Recap Bandwidth (NC×Δf)

1.4 MHz

Subcarrier

Fixed to 15 kHz Spacing (Δf)

Symbol duration Sampling rate, f S (MHz)

3 MHz

5 MHz

10 MHz

15 MHz

20 MHz

Tsymbol = 1/Δf = 1/15kHz = 66.67μs 1.92

3.84

7.68

15.36

23.04

30.72

Data Subcarriers (NC)

72

180

300

600

900

1200

NIFFT (IFFT Length)

128

320

512

1024

1536

2048

6

15

25

50

75

100

Number of Resource Blocks Symbols/slot

CP length

Normal CP=7; extended CP=6

Normal CP=4.69/5.12μsec., Extended CP= 16.67μsec

OFDMA Challenges ICI

1) Tolerance to frequency offset (Inter carrier Interference-ICI)

•Frequency

2) High Peak-to-Average Power Ratio (PAPR)

SC FDMA

SC-FDMA 

Single Carrier Frequency Division Multiple Access is another variant of OFDMA used to reduce the PAPR for lower RF hardware requirements.



SC-FDMA is a new hybrid modulation scheme that cleverly combines the low PAR of singlecarrier systems with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM.



This mechanism can reduce the PAPR of 6..9 dB compared to normal OFDMA.



 SC-FDMA is one option in WiMAX (802.16d) and it is the method selected for EUTRAN in the uplink direction.

 S   C  F  D  M A 

 O  F  D  M A 

SC-FDMA and OFDMA 

OFDMA transmits data in parallel across multiple subcarriers



SC-FDMA transmits data in series employing multiple subcarriers



In the example:



OFDMA: 6 modulation symbols ( 01,10,11,01,10 and 10) are transmitted per OFDMA symbol, one on each subcarrier



SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA symbol using all subcarriers. The duration of each modulation symbol is 1/6th of the modulation symbol in OFDMA

OFDMA

SC-FDMA

SC-FDMA and OFDMA Difference in transmission: for SC-FDMA there is an extra block on the transmission chain: the FFT block which should “spread” the input modulation symbols over all the allocated subcarriers

OFDM

SC-FDMA

OFDMA vs SC-FDMA: QPSK

From: TS

36.211.

SC-FDMA Principles PAPR is the same as that used for the input modulation symbols 

  

This could be achieved by transmitting N modulation symbols in series at N times the rate. One can see that the SC-FDMA symbol which is having 66.66µs is containing N “sub-symbols” N = 6 in the example shown In Time domain only one modulation symbol is transmitted at a time.

The number of subcarriers which could be allocated for transmission should be multiple of 2,3 and/or 5 



This limitation is imposed by the input of the FFT block which is before the IFFT. This enables efficient implementation of the FFT. Note that also the number of Resource Blocks should be multiple of 2,3 or/and 5

SC-FDMA Principles The FFT output size is always smaller than the IFFT input size 

This is because a typical cell’s uplink capacity will be greater  than 180kHz

Other UEs will be assigned o t h e r   groups of   subcarriers to use across the uplink channel bandwidth.  No two UEs will be assigned the same 180KHz block to use simultaneously.   As not all sub-carriers are used by the mobile station, many of them are set to zero in the diagram 

Note that if the output size of the FFT is equal to the size of the IFFT input then the overall effect is null since the two operations (FFT and IFFT are complementary)

FFT

…

Subcarriers allocated for one UE

Subcarriers allocated to other users or set to zero

. . .

IFFT

SC-FDMA Principles  Adjusting the data rate in SC-FDMA 



If the data rate increases more bandwidth is needed to transmit more modulation symbols (when data rate is doubled the resource allocation in the frequency domain is also doubled). The individual transmission is now shorter in time but wider in the frequency domain. For double data rate the amount of inputs in transmitter doubles and the “sub symbol” duration (Time) is halved. Note that the SC-FDMA is still 67 µs

Double the data rate SC-FDMA “subsymbol” duration

Halved SC-FDMA “sub-symbol” duration

Doubled bandwidth

Initial bandwidth

SC-FDMA symbol 67µs

SC-FDMA symbol 67µs

In the example 6 modulation symbols are sent initially and 12 modulations for double data rate

 SC-FDMA: Multiplexing  One user always continuous in frequency Smallest uplink bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in downlink) Largest uplink bandwidth: 20 MHz (same for OFDMA in downlink)

 In time domain the granularity for resource allocation is 1 ms for one user (same for OFDMA in downlink)

Receiver User 1

f

User 1 User 2

f

User 2

f

Bandwidth Distribution Carrier Number of Bandwidth SubCarriers (MHz)

1.4

72

3

198

5

330

10

660

15

990

20

1320

Resource: Element, Block, Grid

LTE Reference Signals (R)are Interspersed Among Resource Elements [source: 3GPP TR 25.814]

The Usage of RE One subframe (1ms)

Resource elements reserved for reference symbols

  y   c   n   e   u   q   e   r    F

  s   r   e    i   r   r   a   c    b   u   s    2    1

Control Channel Region (1-3 OFDM symbols)

Data Region

Time

Duplexing – FDD/TDD

FDD

..

Frequency band 1..

..

Frequency band 2..

TDD

..

Single frequency band

Downlink

..

Uplink

Frame Structure: Generic

Radio Frame Type 1 - FDD subframe 1 msec

Type 1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

radio frame 10 msec

0

1 2 3 4 5 6 7 OFDM symbols (short CP)

Radio Frame Type 2 - TDD radio frame 10 ms f UL/DL carrier

Slot

   S    T    P    P   w    G    D

   S    S    T    T    P    P    P    G   p   w    U    D

   S    T    P   p

   U

subframe 0 subframe 1 subframe 2subframe 3 subframe 4 subframe 5 subframe 6 subframe 7subframe 8 subframe 9

half frame

half frame

time radio frame 10 ms f UL/DL carrier

Slot

   S    T    P    P   w    G    D

   S    T    P   p

   U

subframe 0 subframe 1 subframe 2subframe 3 subframe 4 subframe 5 subframe 6 subframe 7subframe 8 subframe 9

half frame

half frame

time Downlink Slot

Uplink Slot

Uplink or Downlink

Special Slot

Special Subframe DwPTS (Downlink Pilot Timeslot Channel)     

Can contain synchronization, PDSCH and PDCCH. The DwPTS is used for downlink synchronization. Primary synchronization signal transmitted in the first OFDM symbol of the DwPTS. Secondary synchronization signal transmitted in the last OFDM symbol of subframe 0 (immediately preceding to the DwPTS). Resources not used for synchronization signals can be used for data, reference signals and control signaling.

UpPTS (Uplink Pilot Timeslot Channel) Used by eNB to determine the received power level and the received timing from the UE.  Resources not used for reference signals(sounding and/or demodulation reference signals) can be used for random access.  No PUCCH is transmitted in UpPTS. 

GP (Guard Period) 

The guard period between DwPTS and UpPTS determines the maximum cell size.

TDD Frame Configurations Configuration1

DL:UL=2:2 (or 3:2)

Configuration2

DL:UL=3:1 (or 4:1)

Uplinkdownlink

Downlink-to-Uplink

Subframe number

configuration

Switch-point periodicity

0

1

2

3

4

5

6

7

8

9

0

5 ms

D

S

U

U

U

D

S

U

U

U

1

5 ms

D

S

U

U

D

D

S

U

U

D

2

5 ms

D

S

U

D

D

D

S

U

D

D

3

10 ms

D

S

U

U

U

D

D

D

D

D

4

10 ms

D

S

U

U

D

D

D

D

D

D

5

10 ms

D

S

U

D

D

D

D

D

D

D

6

5 ms

D

S

U

U

U

D

S

U

U

D

D

Downlink

S

Special

U

Uplink

Different Methods for OFDMA Plain OFDM

Time Division Multiple Access on OFDM

time

time

. . .

. . .

. . .

...

...

...

1

2

3

1

2

...

1

1

1

1

1

...

1

1

1

2

2

...

...

1

2

3

1

2

...

2

2

2

2

2

...

1

1

1

2

2

...

1 . . .

2 . . .

3 . . .

1

2

...

2 . . .

2 . . .

2 . . .

2

2

...

1 . . .

1 . . .

1 . . .

2

2

...

1

1

1

... ...

. . . ...

  r   e    i   r   r   a   c    b   u   s

. . .

. . . ...

...

1

time

...

... . . .

Orthogonal Frequency Multiple Access OFDMA®

time

...

  r   e    i   r   r   a   c    b   u   s

Plain Orthogonal Frequency Multiple Access OFDMA®

  r   e    i   r   r   a   c    b   u   s

. . .

. . . ...

  r   e    i   r   r   a   c    b   u   s

...

1

1

1

1

1

...

1

1

1

. . .

. . .

...

1

2

3

1

2

...

3

3

3

3

3

...

3

3

3

3

3

...

...

1

2

3

1

2

...

1

1

1

1

1

...

3

3

3

3

3

...

...

1

2

3

1

2

...

3

3

3

3

3

...

3

3

3

3

3

...

UE 1

2

UE 2

3

UE 3

OFDMA® is registered trademark of Runcom Technologies Ltd.

common info (may be addressed via HL)

Resource Block (RB)

Summary 

Understand LTE Duplexing

• Single Transmitter • FDMA Principle • Multi carrier principle

OFDMA and SC FDMA Principle  Multipath Propagation Cyclic Prefix  OFDMA and SC FDMA 

• Transmitter • Receiver  OFDM

and SC FDMA Key Parameters Resource Block