LTE resource PCI learning

OFDMA LTE Air Interface Course

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

Air Interface Main Issues Air Interface UE

eNodeB

eNodeB

eNodeB UE 3

UE 1

1. Duplex Transmission

UE 2

2. Multiple Access

LTE FDD and TDD Modes t

Bandwidth

Bandwidth

up to 20MHz

up to 20MHz

t

Bandwidth up to 20MHz Uplink

Uplink

Downlink Guard Period

f Duplex Frequency

Downlink

f

TDD vs. FDD (1/2) In FDD, DL & UL use different bands with the same bandwidth

• => DL throughput = UL throughput • What happens if throughput requirements are different for DL and UL? • Potential solution: Use different bandwidth for DL & UL? • Hard to manage frequency bands in this case • Simpler solution • DL & UL are duplexed in time rather than in frequency => TDD (Time Division Duplexing)

• DL & UL share the same bandwidth • DL and UL are active in different subframes

TDD vs. FDD (2/2)

Frequency

TDD

FDD

Downlink

Downlink

Uplink

Uplink

Downlink throughput is also affected

Wasted Throughput DL

UL

Only this is needed

Time

We get what we need DL

UL

RF FDD architecture TX Duplex Filter TX

Power amplifier

RX Duplex Filter RX

Low-Noise amplifier

Duplex filters for each Tx and Rx path Circulator has the role of separating DL & UL waves

• It must exhibit great isolation properties, so that Tx signal does not leak into Rx path

RF TDD architecture Channel Filter TX

Power amplifier TX Duplexer RX

Channel Filter RX

Low-Noise amplifier

Duplexer must switch between Tx and Rx paths

• Switching driving signal must be accurate • Good timing control of the signal

FDD and TDD Modes Comparison FDD and TDD modes Harmonisation (commonalities)

FDD and TDD mode included together in the same specification Same radio interface schemes for both uplink and downlink (OFDM and SC-FDMA)

FDD and TDD modes differences regarding the air interface

1. Spectrum Allocation: TDD is using the same frequency bands for both UL and DL → FDD requires a paired spectrum with duplex separation in frequency →TDD requires an unpaired spectrum with some guard bands in time to separate UL and DL

Same subframe formats Same network architecture Same air interface protocols Same physical channels procedures

In LTE there is a high degree of harmonisation between FDD and TDD modes

2. UE complexity: In FDD the UE is requiring an duplex filter (for UL – DL separation) In TDD the filter is not needed → Lower complexity for TDD terminals

Multiple Access

1 UE 1

2 UE 2

3 UE 3

4 UE 4

  r   e   w   o    P 1 TDMA Time Division Multiple Access, 2G e.g. GSM, PDC

2 4

3

4

5

5 UE 5

FDMA Frequency Division Multiple Access 5 1G e.g. AMPS, NMT, TACS

3 2 12

OFDMA Orthogonal Frequency Division Multiple Access e.g. LTE

4

3

5 4

2

1 3

Code Division Multiple Access 3G e.g. UMTS, CDMA2000

1

3 5

1

2

5 4 3 2 1 Frequency

Multiple Access • In LTE OFDMA = Orthogonal Frequency Division Multiple Access it is used in the Downlink

• In the UL SC-FDMA = Single Carrier Frequency Division Multiple Access  Access it is used

•

OFDMA and SC-FDMA will be used for both FDD and TDD Modes!

•  Approach for the explanation: • First OFDM as technology will be explained (for single user case) • Second it is shown how OFDM could be used to separate users

• UL SC-FDMA will be explained in the next chapter

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

Challenges for the Air Interface Design For the LTE Air Interface design it should be considered a trade-off between the following factors (based on the LTE requirements): 1. What should be the required radio spectrum ? 2. Speed of data transmission (bit rate as high as possible) 3. Complexity of implementation (as small as possible) → How could it be realised ?

Solution: use the r e c t a n g u l a r p u l s e s h a p e (see next slide)

The Rectangular Pulse Fourier Transform Time Domain

  e    d   u    t    i    l   p   m   a

  f   s 

Ts

1

T  s

time

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

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.

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.

Pulse Form and Spectrum in WCDMA Time Domain

Tc

W  

1 T c

Fourier Transform

Frequency Domain 1.3 * W

Inverse Fourier Transform

As a counter example look at the root raised cosine roll off pulse that is used in WCDMA. As one can see this pulse is not clearly located in the time domain. So if we put two such pulses one after another, there will be always some interference from the first to the second. On the other hand the spectrum of these pulses is concentrated in a clearly defined

Fc

OFDM Basics • 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

The OFDM Signal

Challenges for the Air Interface Design The usage of the pulse leads to other challenges to be solved: 1. ISI = Intersymbol Interference Due to multipath propagation 2. ACI = Adjacent Carrier Interference Due to the fact that FDM = frequency division multiplexing will be used 3. ICI = Intercarrier Interference Losing orthogonality between subcarriers because of effects like e.g. Doppler → What should be the solutions to these challenges?

(see next slides)

1. Multi-Path Propagation and Inter-Symbol Interference

+ Tt

BTS Time 0

Ts Time 0 Tt

Ts+Tt

1. Inter Symbol Interference

Multi-Path Propagation and the Guard Period 2 1 3

Time Domain

Tg

TSYMBOL

1

Guard Period (GP) time

TSYMBOL 2

Guard Period (GP) TSYMBOL

3

time Guard Period (GP) time

Propagation Delay Exceeding the Guard Period 2 1 3

Time Domain T SYMBOL

4

Tg

1 Obviously when the delay spread of the multi-path environment is greater than the guard period duration (Tg), then we encounter inter-symbol interference (ISI)

time 2 time 3 time 4 time

Cyclic Prefix

2 1 3 Tcp

Tsymb

CP

symbol

1 CP

symbol

CP

symbol time

2 CP

symbol

CP

symbol

3 CP

symbol

CP

symbol

CP

CP

symbol

symbol

Cyclic Prefix In LTE the slot of 500 µs is subdivided in the (useful part of the) symbol (grey) and CPs as follows:

T [TS]

160 2048

T [µs]

5,2

max. delay [km]

1,6

66,7

144 2048

144 2048

144 2048

144 2048

144 2048

144 2048

4,7

4,7

4,7

4,7

4,7

4,7

66,7

1,4

66,7

1,4

66,7

1,4

66,7

1,4

66,7

1,4

66,7

1,4

For the extended CP slot structure the overall 500 µs is kept but the number of symbols is reduced in order to extent the cyclic prefix durations:

T [TS]

512 2048

512 2048

512 2048

512 2048

512 2048

512 2048

T [µs]

16,7

16,7

16,7

16,7

16,7

16,7

max. delay [km]

5,0

66,7

5,0

66,7

5,0

66,7

5,0

66,7

5,0

66,7

5,0

66,7

Challenges for the Air Interface Design The usage of the pulse leads to other challenges to be solved: 1. ISI = Intersymbol Interference Due to multipath propagation → solution: use cyclic prefix

2. ACI = Adjacent Carrier Interference Due to the fact that FDM = frequency division multiplexing will be used 3. ICI = Intercarrier Interference Losing orthogonality between subcarriers because of effects like e.g. Doppler → What should be the solutions to these challenges?

(see next slides)

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

2. ACI = Adjacent Carrier Interference

f N-2

f N-1

frequency

OFDM: Orthogonal Frequency Division 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

Challenges for the Air Interface Design The usage of the pulse leads to other challenges to be solved: 1. ISI = Intersymbol Interference Due to multipath propagation → solution: use cyclic prefix

2. ACI = Adjacent Carrier Interference Due to the fact that FDM = frequency division multiplexing will be used → solution: orthogonal subcarriers

3. ICI = Intercarrier Interference Losing orthogonality between subcarriers because of effects like e.g. Doppler → What should be the solutions to these challenges?

(see next slides)

Inter-Carrier Interference (ICI) in OFDM •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).

Leakage Effect due to Frequency Drift: ICI

Two effects begin to work: 1.-Subcarrier 2 has no longer its power density maximum here - so we loose some signal energy.

∆P

2.-The rest of subcarriers (0, 1, 3 and 4) 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  .    3

Challenges for the Air Interface Design The usage of the pulse leads to other challenges to be solved: 1. ISI = Intersymbol Interference Due to multipath propagation → solution: use cyclic prefix

2. ACI = Adjacent Carrier Interference Due to the fact that FDM = frequency division multiplexing will be used → solution: orthogonal subcarriers

3. ICI = Intercarrier Interference Losing orthogonality between subcarriers because of effects like e.g. Doppler → solution: use reference signals – will be explained in chapter 7

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDM Transmitter

xN-1

Frequency Domain Signal: (Collection of Sinusoids) s0 s1

s2 …

f 0 f 1 f 2

x0 x1 x2

sN-1

t0 t1 t2

f N-1 freq.

… tN-1 time

Time Domain Signal

b10 ,b11,… Modulation s0 Mapper

Binary Coded Data

b20 ,b21,… Modulation s1   n    i Mapper   a   m Serial to   o    D Parallel   y   c Converter .   n   e (Bit .   u   q Distrib.) .   e   r    F

bN-1 0 …

Modulation sN-1 Mapper

cos(2πf ct) D

IFFT

x0, x1, …, x N-1 Time Domain

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

I A

IQ Split Q

D A

Low I Pass RF Low Q Pass

•Each entry to the IFFT module corresponds to a different subcarrier •Each sub-carrier is modulated independently •Modulation Schemes:

-sin(2πf ct)

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDM Receiver

s0 yN-1

s1

s2 …

y0 y1

f 0 f 1 f 2

…

x2 t0 t1 t2

tN-1 time

RF

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

s’0

LNA gain

Derotator

A D A D

 j

AGC Automatic Gain Control

  h   t  g  n  e  r   t  s   l  a  n  g   i  s

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

f N-1 freq.

Frequency Domain

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

sN-1

 n  o   i   t  a  e  r  r  o  c  o   t  u  a   l  a  n  g   i  s

s’1    +   g   n    i   w   T   o    F    d    F   n    i    W

s0   n    i   a   m   o    D .   y .   c   n .   e   u   q   e   r    F

s’N-1  e  t reference  e  s  u   j (pilot)  m   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

s1

. . .

sN-1   l  e  e  s  n  n  n  o  a  p   h  s  c  e  r

Channel Estimation

Bit Mapping Bit Mapping

B10 ,B11,… B20 ,B21,…

. . .

. . .

BN-1 0 …

Bit Mapping 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

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDM Key Parameters 1. Variable Bandwidth (BW)

Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz

A higher Bandwidth is better because a higher peak data rate could be achived and also bigger capacity. Also the physical layer overhead is lower for higher bandwidth Δf 

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

Frequency

Power density

A compromise needed between: → Δf as small as possibile so that the symbol time Tsymbol is as large as possibile.

Frequency Amplitude

This is beneficial to solve Intersymbol Interference in time domain

TCP

TSYMBOL

→ A too small subcarrier spacing it is increasing the ICI = Intercarrier Interference due to Doppler effect

CP

T SYMBOL Time

OFDM Key Parameters 3. The number of Subcarriers Nc → Nc x Δf = BW In LTE not all the available channel bandwidth (e.g. 20 MHz) will be used. For the transmission bandwidth typically 10% guard band is considered (to avoid the out band emissions). 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]

 C  h   a n n  e l    e  d   g  e

Transmission Bandwidth [RB] R  e  s  o  u r   c  e  b  l    o  c k 

 C  h   a n n  e l    e  d   g  e

OFDM Key Parameters 4. FFT (Fast Fourier Transform) size Nfft Nfft should be chosen so that: 1. Nfft > Nc number of subcarriers (sampling theorem) 2. Should be a power of 2 (to speed-up the FFT operation) Therefore 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!

Resource Block and Resource Element 6. Physical Resource Block or Resource Block (PRB or RB)

 – 12 subcarriers in frequency domain x 1 slot period in time domain. • Capacity allocation is based on Subcarrier 1

  z    H    K    0    8    1

Subcarrier 12

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

0 1 2

3 4 5 6 0 1 2 3 4 5 6

1 slot

1 slot

1 ms subframe

Resource Blocks

• Resource Element ( RE):  – 1 subcarrier x 1 symbol period  – Theoretical minimum capacity allocation unit.

 – 1 RE is the equivalent of 1 modulation symbol on a subcarrier, i.e. 2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM.

Resource Element

OFDM Key Parameters for FDD and TDD Modes Bandwidth (NC×Δf)

1.4 MH

Subcarrier Spacing (Δf)

3 MHz

5 MHz

10 MHz

15 MHz

20 MHz

Fixed to 15 kHz (7.5kHz defined for MBMS)

Symbol duration

Tsymbol = 1/Δf = 1/15kHz = 66.67μs

Sampling rate, f S (MHz)

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

Number of Resource Blocks

6

15

25

50

75

100

Symbols/slot

CP length

Normal CP=7; extended CP=6

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

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

Data Rate Calculation 1. Maximum channel data rate The maximum channel data rate is calculated taking into account the total number of the available resource blocks in 1 TTI = 1ms Max Data Rate = Number of Resource Blocks x 12 subcarriers x (14 symbols/ 1ms) = Number of Resouce Blocks x (168 symbols/1ms)

2. Impact of the Channel Bandwith: 5, 10, 20 MHz For BW = 5MHz -> there are 25 Resource Blocks -> Max Data Rate = 25 x (168 symbols/1ms) = 4,2 * Msymbols/s BW = 10MHz -> 50 Resource Blocks -> Max Data Rate = 8,4 Msymbols/s BW = 20MHz -> 100 Resource Blocks -> Max Data Rate =16,8 Msymbols/s

3. Impact of the Modulation: QPSK, 16QAM, 64QAM For QPSK – 2bits/symbol; 16QAM – 4bits/symbol; 64QAM – 6 bits/symbol QPSK: Max Data Rate = 16,4 Msymbols/s * 2bits/symbol = 32,8 Mbits/s (bandwith of 20 MHz) 16QAM: Max Data Rate = 16,4 Msymbols/s * 4 bits/symbols = 65,6 Mbits/s 64QAM: Max Data Rate = 16,4 Msymbols/s * 6 bits/symbols = 98,4 Mbits/s

Data Rate Calculation 4. Impact of the Channel Coding Channel Coding will be discussed in chapter 6. In LTE Turbo coding of rate 1/3 will be used. The effective coding rate is dependent on the Modulation and Coding Scheme selected by the scheduler in the eNodeB. In practice several coding rates can be obtained. Here it is considered 1/2 and 3/4 1/2 coding rate: Max Data rate = 98,4 Mbits/s * 0,5 = 49,2 Mbits/s 3/4 coding rate: Max Data rate = 98,4 Mbits/s * 0,75 = 73,8 Mbits/s

5. Impact of MIMO = Multiple Input Multiple Output MIMO is discussed in chapter 9. If spatial diversity it is used (2x2 MIMO) then the data rate will be doubled since the data is sent in parallel in 2 different streams using 2 different antennas 2x2 MIMO: Max Data Rate = 73,8 Mbit/s * 2 = 147,6 Mbits/s

6. Impact of physical layer overhead and higher layers overhead The real data rate of the user will be further reduced if the physical layer overhead is considered. Also the higher layers may introduce overhead as shown in chapter number 2. For example IP , PDCP , RLC and MAC are introducing their own headers. This type of overheads are not discussed here

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDM Multiple Access Up to here we have only discussed simple point-to-point or broadcast OFDM. Now we have to analyze how to handle access of multiple users simultaneously to the system, each one using OFDM. OFDM can be combined with several different methods to handle multi-user systems: 1.-Plain OFDM 2.-Time Division Multiple Access via OFDM 3.-Orthogonal Frequency Division Multiple Access OFDMA®

OFDM Plain OFDM

•OFDM stands for Orthogonal Frequency Division

Multicarrier •OFDM: Plain or Normal OFDM has no built-in multiple-access mechanism. • This is suitable for broadcast systems like DVB-T/H which transmit only broadcast and multicast signals and do not really need an uplink feedback channel (although such systems exist too).

•Now we have to analyze how to handle access of

time ...

  r   e    i   r   r   a   c    b   u   s

... ... ... . . .

multiple users simultaneously to the system, each one using OFDM.

. . .

. . .

. . .

. . .

... ... ... ... ...

1 UE 1

2 UE 2

3 UE 3

common info (may be addressed via Higher Layers)

OFDMA® •OFDMA® stands for  Orthogonal  Orthogonal Frequency Division

Multiple Access trademark by Runcom Ltd. •It is a registered trademark •The basic idea is to assign subcarriers to users based on their bit rate services. With this approach it is quite easy to handle high and low bit rate users simultaneously in a single system. efficiently. •But still it is difficult to run highly variable traffic efficiently. •The solution to this problem is to assign to a single users so called resource blocks or scheduling blocks. •Such block is simply a set of some subcarriers over some time. one or more Resource blocks. blocks. • A single user can then use one

Orthogonal Frequency Multiple Access OFDMA® time ...

  r   e    i   r   r   a   c    b   u   s

2 ... 2 ...

1

1

1

2

1

1

1

2

1 . . 1.

1 . . 1.

1 . . 1.

2 2 ... . . . . . . ...

1

1

1

...

3

3

3

3

3 ...

3

3

3

3

3 ...

3

3

3

3

3 ...

Resource Block (RB) 1 UE 1

2 UE 2

3 UE 3

common info (may be addressed via

OFDMA FDD and TDD Modes Basics of OFDM OFDM Transmitter OFDM Receiver OFDM Key Parameters for FDD and TDD Modes Data Rate Calculation OFDMA OFDM Transmitter Simulation

OFDM Transmitter Simulation – Assumptions subcarriers subcarriers subcarriers are transmitted (assuming (assuming that the system •  All 1200 subcarriers bandwidth is 20 MHz)

• Transmit only one OFDM symbol (66.7 us) • No difference between the subcarriers used for physical layer overhead and the subcarriers used for transmission of user data  – No difference between different physical channels like e.g. PBCH (Physical Broadcast Channel). The difference could be seen in parameters like e.g. modulation

• The serial to parallel convertor is not considered (because it assumed to transmit only one OFDM symbol)

• Cyclic prefix insertion neglected (less relevant for simulation  – impact on symbol duration only)

Data Generation b10 b20 Binary Coded Data

Serial to Parallel Converter (Bit Distrib.)

bN-1



A random string is generated with N=1200 integers numbers from 0 to 3 that needs to be transmitted;

 For 

simplicity only first 40 integers are plotted (the same is true for the rest of the simulation)

One can look at this sequence vertically, as being the output of the serial to parallel block (only one OFDM symbol is transmitted )

OFDM Transmitter

b10 ,b11,…

Modulation s0 Mapper

b20 ,b21,…

Modulation s1 Mapper

Binary

Serial to

Coded

Parallel

Data

Converter

.

(Bit

.

Distrib.)

.

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

cos(2πf ct)

D x0, x1, …, xN-1

IFFT Time Domain

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

Low

I

A

IQ Split Q

D

Pass RF

Low A

I

Q

Pass -sin(2πf ct)

bN-1 0 …

Modulation sN-1 Mapper

• QPSK modulation assumed (16QAM or 64QAM also possibile)

QPSK Modulation Our Tx

Bit 1

Bit 0

I

Q

0

0

0

+1

+1

1

0

1

-1

+1

2

1

0

-1

-1

3

1

1

+1

-1 Step 1 of QPSK modulation: map the input bits to the symbols using the constelation diagram I + jQ (complex = inphase + quadrature)

Step 2 of the QPSK modulation : in LTE the complex symbols are input for the IFFT !

Modulation

s0

Mapper

Modulation

s1

Mapper

. . .

Modulation

sN-1

Mapper



Note that the sequence s0 … sN-1 is a complex sequence = I + jQ (Inphase and Quadrature)

OFDM Transmitter

b10 ,b11,…

Modulation s0 Mapper

b20 ,b21,…

Modulation s1 Mapper

Binary

Serial to

Coded

Parallel

Data

Converter

.

(Bit

.

Distrib.)

.

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

cos(2πf ct)

D x0, x1, …, xN-1

IFFT Time Domain

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

Low

I

A

IQ Split Q

D

Pass RF

Low A

I

Q

Pass -sin(2πf ct)

bN-1 0 …

Modulation sN-1 Mapper

• IFFT = Inverse Fast Fourier Transformation

IFFT Result –> Time Domain

x0, x1, …, xN-1

IFFT

Time Domain

Result interpretation: 1. The signal is complex = I+jQ 2. The signal is almost white noise (1200 subcarriers

IFFT Result -> Frequency Domain The spectrum is splitted in 2 parts because of the zero padding in the middle of the sequence

Low pass filtering required to achieve a compact spectrum

Zero padded subcarriers 2048-1200 = 848

First 600 subcarriers BW=600*15kHz=9MHz

Last 600 subcarriers

Total BW=18MHz

BW=600*15kHz=9MHz

OFDM Transmitter

b10 ,b11,…

Modulation s0 Mapper

b20 ,b21,…

Modulation s1 Mapper

Binary

Serial to

Coded

Parallel

Data

Converter

.

(Bit

.

Distrib.)

.

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

cos(2πf ct)

D x0, x1, …, xN-1

IFFT Time Domain

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

Low

I

A

IQ Split Q

D

Pass RF

Low A

I

Q

Pass -sin(2πf ct)

bN-1 0 …

Modulation sN-1 Mapper

•Digital to Analog Conversion and Low Pass Filtering

D

Low A

D

Pass

Low A



I

Q

Pass

Note the delay produced by the filtering process (low pass filtering)

OFDM Transmitter

b10 ,b11,…

Modulation s0 Mapper

b20 ,b21,…

Modulation s1 Mapper

Binary

Serial to

Coded

Parallel

Data

Converter

.

(Bit

.

Distrib.)

.

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

cos(2πf ct)

D x0, x1, …, xN-1

IFFT Time Domain

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

Low

I

A

IQ Split Q

D

Pass RF

Low A

I

Q

Pass -sin(2πf ct)

bN-1 0 …

Modulation sN-1 Mapper

•Up - Conversion

Up-conversion -> Time Domain Result

 This   It

is the signal transmitted over the air interface

can be observed the large value of the PAR (peak to average ratio) in the time response

Up-conversion -> Frequency Domain Result