03_tm51173en02gla01_ofdma

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OFDMA

Contents

1 FDD and TDD Modes 3

2

Basics of OFDM 14

2.1 Pulse shaping and spectrum 15

2.2 OFDM Signal 19

2.3 Challenges for the Air Interface Design 24

3 OFDM Transmitter 40

4 OFDM Receiver 43

5 OFDM Key Parameters for FDD and TDD Modes 46

6 Data Rate Calculation 53

7 OFDMA 57

8 OFDM Transmitter Simulation 61

TM5117 LTE AIR INTERFACE 2010 Nokia Siemens Networks

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1 FDD and TDD Modes


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Air Interface Main Issues

UL

DL

UE 1

UE 2

UE 3

Air Interface

UE

eNodeB

1. Duplex

Transmission2. Multiple

Access

eNodeB

eNodeB

Fig. 1Air Interface Main Issues

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LTE FDD and TDD Modes

Uplink Downlink

Bandwidth

up to 20MHz

Duplex Frequency

f

t Bandwidth

up to 20MHz

Guard

Period

f

t

Uplink

Downlink

Bandwidth

up to 20MHz

Fig. 2LTE FDD and TDD Modes

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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 DivisionDuplexing)

DL & UL share the same bandwidth

DL and UL are active in different subframes

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TDD vs. FDD (2/2)

Downlink Downlink

Uplink

Uplink

FDD TDD

Time

Frequency

Throughpu

t

DL DLUL UL

Only this is

needed

Wasted

We get what we

need

Downlink

throughput is also

affected

Fig. 3TDD vs. FDD

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RF FDD architecture

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

Power

amplifier

Low-

Noise

amplifier

TX

RX

TX Duplex Filter

RX Duplex Filter

Fig. 4RF FDD architecture

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RF TDD architecture

Duplexer must switch between Tx and Rx paths

Switching driving signal must be accurate

Good timing control of the signal

Power

amplifier

Low-

Noise

amplifier

TX

RX

Channel Filter

Channel Filter

TX

RX

Duplexe

r

Fig. 5RF TDD architecture

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The basic principle for TDD is to use the same frequency band for transmission andreception but to alternate the transmission direction time (UL or DL). Like FDD, TDDsupports bandwidths from 1.4MHz up to 20 MHz but depending on the frequency

band, the number of supported bandwidths may be less than the full range.Since the bandwidth is shared between UL and DL and the maximum bandwidth is20MHz the maximum data rates are lower in TDD than in FDD mode.

The TDD system could be implemented on an unpaired band while the FDD systemalways requires a pair of bands with some separation between UL and Dl for theduplex separation.

In FDD UE implementation requires a duplex filter for the separation of UL and DL.The filter is not required for the TDD mode. The complexity of the duplex filter isincreasing when the UL and DL frequency bands are in close proximity.

In TDD mode since the UL and DL share the same frequency band the signals in

these 2 transmission directions can interfere to each other. For uncoordinateddeployment (not synchronized) on the same frequency band, the devices connectedto cells with different timing and/or different UL/DL allocation may cause blocking ofother users. In TDD Mode the base stations need to be synchronized to each other atframe level in the same coverage area to avoid this interference.

In FDD mode there is no need for base station synchronization.

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FDD and TDD Modes Comparison

FDD and TDD mode included

together in the same

specification

Same radio interface schemes

for both uplink and downlink

(OFDM and SC-FDMA)

Same subframe formats

Same network architecture

Same air interface protocols

Same physical channels

procedures

FDD and TDD modes Harmonisation

(commonalities)

In LTE there is a high

degree of harmonisation

between FDD and TDD

modes

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

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

FDD and TDD modes

differences regarding the air

interface

Fig. 6FDD and TDD Modes Comparison

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Time

1 2 3 4 5

2

12345

4 2

1

23

45

31

1

5

53

3

24

1

Power

Frequency

TDMA

Time Division

Multiple

Access,

2G e.g. GSM,

PDC

FDMA

Frequency

Division

Multiple Access

1G e.g. AMPS,

NMT, TACS

CDMACode Division

Multiple Access

3G e.g. UMTS,

CDMA2000

1 2 3UE 1 UE 2 UE 3 4 UE 4 UE 55

OFDMA

OrthogonalFrequency

Division

Multiple Access

e.g. LTE

Fig. 7Multiple Access

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In LTE OFDMA = Orthogonal Frequency Division Multiple Access it is used in theDownlink

In the UL SC-FDMA = Single Carrier Frequency Division Multiple Access Access it isused

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


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2 Basics of OFDM

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2.1 Pulse shaping and spectrum

RF engineering is a trade off between: required radio spectrum (bandwidth), speed ofdata transmission (bit rates) and complexity of implementation. The pulse form usedto modulate complex data symbols to the radio carrier frequency is the major elementof this story. Over the years several pulse forms and their associated pulse shapingfilters have been studied and used in private and commercial radio systems. GSM forinstance uses GMSK (Gaussian Minimum Shift Keying) filter that produces pulsesthat are close to sin/cosine waveforms with a Gaussian curve as amplitude, WCDMAuses root raised cosine roll off pulse shaping filters.

Two characteristics are important for a pulse: the time domain presentation and thefrequency domain presentation. In the time domain one can recognize how long the

symbol pulses on air will be and in the frequency domain the required spectrum interms of bandwidth can be studied. One of the most simple time-domain pulses is therectangular pulse. It simply jumps at time t=0 to its maximum amplitude and after thepulse duration TS it jumps back to 0. This pulse form has two major advantages. Firstit is simple to implement, there is no complex filter system required to detect suchpulses and to generate them. Second the pulse has a clearly defined duration. AfterTS the signal amplitude is zero, this is a major advantage in case of multi-pathpropagation environments as it simplifies handling of inter-symbol interference.Furthermore if the next symbol starts after the first pulse finished, there will be nointer-symbol interference in a non-multi-path environment. For a receiver this means,that there are no complicated and expensive inter-symbol interference cancellationmechanisms required. A disadvantage of the rectangular pulse is, that it allocates aquite huge spectrum.


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The Rectangular Pulse

Advantages:

+ Simple to implement: there is no complexfilter system required to detect such pulsesand to generate them.

+ The pulse has a clearly defined duration.This is a major advantage in case of multi-path propagation environments as it simplifieshandling of inter-symbol interference.

Disadvantage:

- it allocates a quite huge spectrum. Howeverthe spectral power density has null pointsexactly at multiples of the frequency fs = 1/Ts.This will be important in OFDM.

time

amplitude

Ts f

s

1

Ts

Time Domain

frequency f/fs

spectralpowerdensity Frequency Domain

fs

Fourier

Transform

Inverse

Fourier

Transform

Fig. 8The Rectangular Pulse

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As a counter example look at the root raised cosine roll off pulse that is used inWCDMA. 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 someinterference from the first to the second. On the other hand the spectrum of thesepulses is concentrated in a clearly defined frequency band.


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Fourier Transform

InverseFourier Transform

Time Domain

Frequency Domain

W 1

Tc

Tc

Fc

1.3 * W

Fig. 9: Pulse form and spectrum of root raised cosine roll off filters used in WCDMA.

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2.2 OFDM Signal


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The basic idea for the OFDM Signal is to transmits hundreds or even thousands ofseparately modulated radio signals using orthogonal subcarriers spread across awideband channel


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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

frequency

bandwidth

Fig. 11 OFDM Signal

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The OFDM Signal

Fig. 12 The OFDM Signal

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2.3 Challenges for the Air Interface Design

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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?


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2.3.1 ISI = Intersymbol Interference

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1. Multi-Path Propagation and Inter-Symbol Interference

1. Inter Symbol Interference

BTSBTSTime 0 Ts

+

d1(Directpath)

d3

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Multi-Path Propagation and the Guard Period

2

time

TSYMBOL

Time Domain

1

3

time

TSYMBOL

time

TSYMBOL

Tg

1

2

3

Guard Period (GP)

Guard Period (GP)

Guard Period (GP)

(Direct path)

Fig. 14Multi-Path Propagation and the Guard Period

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Obviously when

the delay spread

of the multi-path

environment is

greater than theguard period

duration (Tg),

then we

encounter inter-

symbol

interference (ISI)

Propagation Delay Exceeding the Guard Period

1

2

3

4

time

TSYMBOLTime Domain

time

time

Tg

1

2

3

time

4

Fig. 15Propagation Delay Exceeding the Guard Period

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The Cyclic Prefix

The guard period after each rectangular pulse carrying the modulated data symbol isa simple and efficient method to deal with multi-path reception.

The cyclic prefix (CP) simply consists of the last part of the following symbol. Thesize of the cyclic prefix field depends on the system and can even vary within onesystem. Cyclic prefixes are used by all modern OFDM systems and their sizes rangefrom 1/4 to 1/32 of a symbol period. Most receiver structures use the cyclic prefix tomake an initial estimation of time and frequency synchronization (pre-FFTsynchronization, non-data assisted synchronization).

A receiver typically uses the high correlation between the cyclic prefix and the lastpart of the following symbol to locate the start of the symbol and begin then with

decoding.

In multi-path propagation environments the delayed versions of the signal arrive witha time offset, so that the start of the symbol of the earliest path falls in the cyclicprefixes of the delayed symbols. As the CP is simply a repetition of the end of thesymbol this is not an inter-symbol interference and can be easily compensated by thefollowing decoding based on discrete Fourier transform.

Of course cyclic prefixes reduce the number of symbols one can transmit during atime interval. This method to deal with inter-symbol interference from multi-path

propagation is theoretically sub-optimal. CDMA with RAKE receiver for instanceprovides a much better efficiency. On the other hand non-ideal implementations ofRAKE receivers also degrade system performance drastically but still require a lot ofhardware capacity for the basic implementation. The rectangular pulse with cyclicprefix requires far less hardware, so the free capacity can be used to implement otherperformance optimization techniques like MIMO.

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Cyclic Prefix

symbolCP

time

Tsymb

1

2

3

1

2

3

Tcp

symbolCP symbolCP

symbolCP symbolCP symbolCP

symbolCP symbolCP symbolCP

Fig. 16Cyclic Prefix

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Cyclic Prefix

T [TS] 160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048

T [s] 5,2 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7

max. delay [km] 1,6 1,4 1,4 1,4 1,4 1,4 1,4

T [TS] 512 2048 512 2048 512 2048 512 2048 512 2048 512 2048

T [s] 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7

max. delay [km] 5,0 5,0 5,0 5,0 5,0 5,0

In LTE the slot of 500 s is subdivided in the (useful part of the)

symbol (grey) and CPs as follows:

For the extended CP slot structure the overall 500 s is kept but thenumber of symbols is reduced in order to extent the cyclic prefix

durations:

Fig. 17Cyclic Prefix

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2.3.2 ACI = Adjacent Carrier Interference

Conventional multi-carrier operation as it is used for FDM works simply by selecting a

number of center frequencies - one for each carrier to be used.

The center frequencies must be spaced. In fact there is a trade-off betweenminimizing interference between different carriers and using the total bandwidthefficiently.

In other words each carrier uses an upper and lower guard band to protect itself fromits adjacent carriers. Nevertheless, there will always be some interference betweenthe adjacent carriers - known as Adjacent Carrier Interference (ACI)

Especially for rectangular pulses the guard bands must be quite big, as therectangular pulse has a huge spectrum.

Otherwise we would have to apply a pulse shaping filter, but this would destroy therectangular form of our pulse and thus complicate inter-symbol interference handling.

For the rectangular pulse there is a better option possible and it is even easier toimplement.

The spectrum of a rectangular pulses shows null points exactly at integer multiples ofthe frequency given by the symbol duration. Orthogonally avoids ACI to some extent.

Thus OFDM simply places the next carrier exactly in the first null point of the previousone.

With this we dont need any pulse-shaping and between OFDM carriers using thesame symbol duration TS and the same grid of center frequencies no guard bandsare required.

This allows a tight packing of small carrier -called the sub-carriers or tones- into abigger frequency band. Of course at the edges of this bigger band there might besome guard bands required to protect systems on adjacent bands from out-of-spectrum emissions by the OFDM system.


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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.

frequency

fsubcarrier

f0 f1 f2 fN-1fN-2

fsub-used


Fig. 18Multi-Carrier Modulation

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OFDM: Orthogonal Frequency Division Multi-Carrier

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

No ACI (Adjacent Carrier Interference) in OFDM

due to the orthogonal subcarriers !

Pow

erDensity

Pow

erDensity

Frequency (f/fs) Frequency (f/fs)

Saved

Bandwidth

Fig. 19OFDM: Orthogonal Frequency Division Multi-Carrier

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2.3.3 ICI = Intercarrier Interference

The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency

errors. If the receivers frequency is some fractions of the subcarrier spacing(subcarrier bandwidth) then we encounter not only interference between adjacentcarriers, but in principle between all carriers. This is known as Inter-CarrierInterference (ICI) and sometimes also referred to as Leakage Effect in the theory ofdiscrete Fourier transform.

The effect is illustrated on the following figure. It shows the spectral power density ofan OFDM system with five subcarriers. If we have an exact match between receiverand transmitter frequency and we would like to get the symbol transmitted insubcarrier 2, then there is no interference from the other subcarriers. This is due tothe fact, that at the center frequency of subcarrier 2 all other subcarriers have a null

point of their power spectrum.But if we have a little frequency drift between transmitter and receiver, then wedecode the symbol of subcarrier 2 a little bit offset from its true center frequency. Butnow two effects begin to work. First subcarrier 2 has no longer its power densitymaximum here - so we loose some signal energy. Second the other subcarriers 0, 1,3 and 4 have no longer a null point here. So we get some noise from each othersubcarrier. The result is a lower signal to noise ratio by a decreased signal level andan increased noise level. This is the inter-carrier interference effect for OFDM. Asone can see this strongly depends on the ratio between absolute frequency offsetbetween transmitter and receiver and the subcarrier spacing.

To limit the influence of the ICI on OFDM systems completely by hardware we wouldhave to have receivers and transmitters with under 0.1 ppm frequency stability. Thiswould drastically increase the cost and complexity of hardware. Thus quite a big partof the OFDM software in the receiver deals with frequency correction using the cyclicprefix, but also reference or pilot signals sent with the signal.

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Inter-Carrier Interference (ICI) in OFDM

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

Fig. 20 Inter-Carrier Interference (ICI) in OFDM

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f0 f1 f2 f3 f4

P

I3

I1I4I0

3.ICI=Inter-CarrierInterference

Leakage Effect due to Frequency Drift: ICI

Two effects begin to work:

1.-Subcarrier 2 has no longer itspower density maximum here -so we loose some signal energy.

2.-The rest of subcarriers (0, 1, 3and 4) have no longer a nullpoint here. So we get somenoise from the other subcarrier.

Fig. 21 Leakage Effect due to Frequency Drift: ICI

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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


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

Fig. 22 Challenges for the Air Interface Design

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3 OFDM Transmitter

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A typical OFDM transmitter is shown on the following figure. To reduce the amount ofRF hardware required for OFDM the modulation process is split into two parts. A first

part uses the inverse discrete Fourier transform (IDFT) or one of its more efficient butequivalent implementations known as Inverse Fast Fourier Transformto modulateall the OFDM subcarriers in the baseband around the center frequency 0. In thesecond step the signal is then modulated to higher frequencies for transmission overair.

The binary data sequence is put into the bit distribution where each bit is assigned toa subcarrier. This function is highly specific to the system using OFDM. In EUTRANfor instance the scheduler has great influence to this step. For each subcarrier amodulation mapper takes a number of bits from the assigned stream and maps themto a single complex valued data symbol. How many bits will be mapped in one

symbol period depends on the selected modulation scheme (e.g. 1 bit of OOK, BPSK;2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM). Note that each subcarriercan use a different modulation scheme at the same time.

Then the complex valued data symbols from the modulation mappers are interpretedas frequency domain signal for one symbol period. They are fed into the IFFTalgorithm which transforms the frequency domain vector into the corresponding timesequence. The number of time symbols (also complex of course) is typically equal tonumber of carriers. Note also that some subcarriers before the IFFT step beginsmight be inserted without data symbol (so called virtual subcarriers). They are usuallyused as guard bands to protect from interference of adjacent radio systems.

The time sequence of complex valued samples is next brought to the OFDM symbolgenerator, which inserts cyclic prefix and if required cyclic suffix. This is simply donebe taking some bits from the end of the symbol and placing them as cyclic prefix infront of the symbol. Similar is the mechanism for cyclic suffixes. This step isequivalent to the insertion of cyclic prefix and suffix for each subcarrier, but it requireslower number of arithmetical operations.

Optionally an up-conversion unit can increase the sampling rate now before we go tothe DAC. The up-conversion can be used to reduce the amount of hardware requiredfor the anti-aliasing filter after the DAC which translates the signal into an analogwaveform such that the digital sampling values before corresponds to voltage orcurrent afterwards. Because a DAC generates a signal that contains the original

spectrum again in mirrored versions in higher bands, a low pass (anti-aliasing filter)filter is required to suppress the unwanted spectrum.

The last step is to modulate the signal onto the radio carrier. This is done using aclassical I/Q modulator where the real part of the complex samples goes to thecosine and the imaginary part of the complex samples goes on the sine of the carrierfrequency. Then we fed the signal to some spectral filter (to suppress out-of-bandemissions) and to the RF amplifier.


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LowPass

LowPass

cos(2fct)

-sin(2fct)

I

Q

ModulationMapper

ModulationMapper

IFFTIFFT

a0

ModulationMapper

ModulationMapper

a1

ModulationMapper

Modulation

Mapper

aN-1

b10 ,b11,

BitDistrib.

BitDistrib.

b20 ,b21,

bN-1 0

BinaryCoded

Data.

.

.

D

A

D

As0, s1, , sT-1

Up-conversion

Up-conversion

IQSplit

IQSplit

LowPass

LowPass

D

A

D

A

RF

OFDM Transmitter

freq.f1 f2f0 fN-1

complex

a0

a1 aN-1a2

FrequencyDomain

b0

BPSK

b0 b1

QPSK

Im

Re01

00

11

10

b0 b1b2b3

16QAM

Im

Re

0000

1111

Im

Re

64QAM

b0 b1b2b3 b4 b5

timet1 t2t0 tT-1

complex

s0s1

sT-1

s2

Time

DomainCP/Guard

Generation

CP/Guard

Generation

Symbols0,..sN-1

CPsl,,sN-1

time

I

Q

Im

Re

0

1

Fig. 23: Basic functional architecture of an OFDM transmitter.

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4 OFDM Receiver


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The receiver is like in any other radio system the more complicated part. In radiosystems and of course also OFDM there are two special points a receiver has to pay

attention to: time/phase and frequency synchronization. Both are crucial for theperformance of the receiver.

A receiver gets its input from the antenna (or antennas) and the attached low noiseamplifier. A band pass suppresses signals out of the spectrum. The demodulatorconverts the signal back into the baseband and with this recovers the complex valueddata signal. At this step we have the time domain representation of the signal.

The time signal is now given to the De-rotator which applies to each time sample aphase offset to compensate frequency drifts and global phase offsets. A special unitin the receiver is responsible to determine and track the frequency and phase driftsand calculate the associated correction value for each sample. This is a quite critical

task, as errors made here, apply as additional (receiver intrinsic) noise to all datasymbols. The frequency and time synchronization unit uses typically as input theautocorrelation of the input time sequence (especially cyclic prefix) and reference (orpilot) symbol interleaved with the data at predefined positions.

The corrected signal is now fed into the Fast Fourier Transform (FFT)whichimplements a fast and efficient algorithm for the discrete Fourier transform to bringthe signal back into the frequency domain representation. In other words the FFTdecodes the complex valued data symbols for each subcarrier. Of course before theFFT is applied, the cyclic prefix has to be removed.

The recovered subcarrier data symbols are not useful yet, as there might be stilldistortion from phase offsets and from the channel propagation (multi-pathpropagation) on it. Thus the next step is to correct the data according to the knownchannel response. The channel estimation uses the pilot and reference signals thatare interleaved with the normal data at predefined positions to estimate andpermanently correct the channel state information. A nice thing of the frequencydomain representation is, that a distortion coming from channel propagation and timeoffset are in first order simple correction factors to each subcarrier, so that nocomplex filtering is required here.

After we have corrected our data symbols for each subcarrier, the symbol de-mapping can take place. Here we recover the original bit sequence either as harddecided bits or as soft decided bits. (Soft bits have some advantages in the further

processing, namely in the channel decoding.)

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ChannelCorrection

ChannelCorrection

Demodulator

Demodulator

(Soft)

Bit Mapping

(Soft)

Bit Mapping

j

I

Q

A

D

A

D

ChannelEstimation

ChannelEstimation

RF

Low

NoiseAmp.

+Bandpass

Low

NoiseAmp.

+Bandpass

A

D

A

D

AGCAutomatic

Gain Control

AGCAutomatic

Gain Control

De-

rotator

signalstrength

LNA gain

Windowing

+FFT

Windowing

+FFT

Frequency And Timing SyncFrequency And Timing Sync

signalautocorr

eation

phasecorrection

timee

adjust

.

.

.

a0

a1

aN-1

reference(pilot)

channel

response

a0

(Soft)Bit Mapping

(Soft)Bit Mapping

a1

(Soft)Bit Mapping

(Soft)Bit Mapping

aN-1

.

.

.

.

.

.

.

.

.

B10 ,B11,

B20 ,B21,

BN-1 0

BitDistribution

BitDistribution

Soft BitCoded

Data

OFDM Receiver (Principle Architecture Concept)

freq.f1 f2f0 fN-1

complex a0

a1 aN-1a2

Frequency Domain

Time Domain

timet1 t2t0 tT-1

complex

y0y1

yT-1

x2

QPSK

Im

Re

01

00

11

10

skd11

d10

Fig. 24 Basic functional architecture of an OFDM receiver.

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5 OFDM Key Parameters for FDD and TDD

Modes

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OFDM Key Parameters

2. Subcarrier Spacing (

f = 15 KHz)

The Symbol time is

Tsymbol = 1/

f = 66,7

s

f

A compromise needed between:

f as small as possibile so that

the symbol time Tsymbol is as large

as possibile.

This is beneficial to solve

Intersymbol Interference in time

domain

A too small subcarrier spacing it

is increasing the ICI = Intercarrier

Interference due to Doppler effect

TSYMBOL

TCP SYMBOL

TCP

TS

Frequency

Time

Power

density

Amplitude

1. Variable Bandwidth (BW)Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHzBandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz

Frequency

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

Fig. 25 OFDM Key Parameters

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OFDM Key Parameters

3. The number of Subcarriers Nc Nc x f = 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

Transmission

Bandwidth [RB]

Transmission Bandwidth Configuration [RB]

Channel Bandwidth [MHz]

Resourceblock

Channeledge

Channeledge

DC carrier (downlink only)Active Resource Blocks


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4. Fast Fourier Transform Size Nfft

The FFT/ IFFT (Inverse Fast Fourier Transform) it is used for the generation of thesubcarriers.

Input for the FFT/ IFFT are the modulation symbols.

FFT/ IFFT could be seen as a kind of operation acting on a Nfft discrete points of theinput signal

Therefore the terminology is naming the FFT/ IFFT sampling.

Nfft size:

The number of samples Nfft on which FFT/ IFFT is applied should be big enoughto satisfy the sampling theorem (giving the minimum number of samples)

From this: Nfft > Nc number of the input subcarriers

FFT/IFFT operation requires that input length must be a power of 2. This isbecause in this way the operation is much faster than ordinary DFT (Discrete FourierTransform).

Example:

For a bandwidth BW = 20 MHz there are 1200 subcarriers -> the length of the IFFTinput is a signal with 1200 symbols

1200 is not a power of 2 so that the IFFT operation requires zero padding-> Nextpower of 2 is 2048

The rest of input: 2048 - 1200 = 848 will padded with zeros


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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 f

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!


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Resource Block and Resource Element

12 subcarriers in frequency domain x 1 slot period in time

domain.

0 1 2 3 4 5 6 0 1 2 3 4 5 6Subcarrier1

Subcarrier12

18

0KHz

1 slot 1 slot

1 mssubframe

R

B

Capacity allocation is basedon Resource Blocks

Resource Element ( RE):

1 subcarrier x 1 symbolperiod

Theoretical minimum

capacity allocation unit. 1 RE is the equivalent of 1

modulation symbol on asubcarrier, i.e. 2 bits forQPSK, 4 bits for 16QAM and6 bits for 64QAM.

ResourceElement

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

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

Fig. 28 Resource Block and Resource Element

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OFDM Key Parameters for FDD and TDD Modes

Fig. 29 OFDM Key Parameters for FDD and TDD Modes

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6 Data Rate Calculation


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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 = 8,2 Msymbols/sBW = 20MHz -> 100 Resource Blocks -> Max Data Rate =16,4 Msymbols/s

3. Impact

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