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Wireless communications course

OFDMA Receiver in the LTE System

(with the use of LTE PHY LabTM)

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IS-Wireless (brand of Innovative Solutions)

ul. Postepu 15c, 3rd floor

02-676 Warsaw, Poland

Tel: +48 22 213 8297

Fax: +48 22 213 8298

Email: info@is-wireless.com

www.is-wireless.com

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IS-Wireless has been advised of the possibility of such damages, or claim by any third party.

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1. Exercise target

The aim of this exercise is to familiarize students with the impacts of the

following distortions on the OFDMA signal, that can be seen at the receiver:

a. Impact of the multipath channel on the received signal

b. Influence of the pulse shaping filtering on the received signal

c. The reception of the signal in the presence of co-channel interference

d. Distortion of the signal in the presence of unsynchronized transmitter and

receiver (unsynchronized time, unsynchronized frequency, unsynchronized

sampling frequency, unsynchronized phase)

After this exercise student shall be able to recognize the distortion that he or she

could see on the scatterplot or de-mapper measuring the real signal.

Note! This exercise shall last approximately 3hrs in order to fulfill all the included

tasks. The exercise can be done by a single student or a group of students.

2. Required background

Prior to laboratory exercises, students should prepare and get familiar with the

necessary range of material including:

Principles and parameters of OFDMA system (including transmit and receive

processing chains and block diagrams)

Basics of radio-communication systems

Principles and basics of LTE system and main LTE physical layer parameters

Basics of multipath channels and possible impacts of the channel on the

received signal

Possible types of synchronization errors present in the OFDMA systems

Basics of pulse shaping filtering and co-channel interference

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3. Theoretical introduction

3.1. Theoretical background for the OFDMA system behavior under certain distortions

Let’s assume the following nomenclature:

x(t) and y(t) – accordingly transmitted and received signal in the time

domain

X(f) and Y(f) - accordingly transmitted and received signal in the frequency

domain

h(t) - multipath channel impulse response in the time domain

H(f) - multipath channel frequency response in the frequency domain

n(t) – AWGN

Influence of multipath channel on the OFDMA signal

Multipath channel influences the transmitted signal in two ways, that are seen

from different perspectives: inter-symbol interference in time domain resulting from

the time spread and frequency selectivity in the frequency domain.

Received signal in the time domain results from the convolution of the

transmitted signal with the channel impulse response:

y(t) = x(t) * h(t)

which means, that if the channel consists of multiple taps, the signal will be

spread in time distorting following symbols (see figure 5). In the OFDMA symbols this

effect is neglected by using cyclic prefix that consumes the time spread.

In the frequency domain the received signal results from the multiplication of the

transmitted signal with the channel frequency response:

Y(f) = X(f) ∙ H(f)

which means, that if the channel coherence bandwidth is smaller than the signal

bandwidth, the signal is frequency selective (some frequencies are attenuated and

some amplified – see figure 5).

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Figure 1 Impact of multipath channel on the transmitted signal

Influence of pulse shaping filtering on the OFDMA signal

The OFDMA signal consisting of multiple subcarriers in the frequency domain

results in out-of band emissions providing interference to neighboring systems /

carriers. Therefore it is required to cut-off the out-of band signal by shaping the pulse

which is normally done in the time domain. By doing so, the undesired power is

lowered, but the filter also distorts the in-band part of the signal. Figure 6 shows the

influence of the pulse shaping filter in the time and frequency domains.

Figure 2 Pulse shaping filtering in the time and frequency domain

t

ISI Original symbol length

First symbol second symbol

Multipath channel influence in the time domain

Multipath channel influencein the frequency domain

f

Y(f)

System bandwidth

0dB

X(f) [dB]

t f

Pulse shaping filtering

in the time domain Frequency domain result

of the pulse shaping

Original OFDMA

symbol Filtered OFDMA symbol

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Influence of Co-Channel Interference on the OFDMA signal

Co-channel interference is an interference coming from the undesired signals

transmitted at the same carrier frequency as the desired signal (e.g. signals from

neighboring base stations transmitting at the same frequency seen by the UE

receiver). The co-channel interference in the OFDMA signal can be recognized per

subcarrier basis as the low-power vector corresponding to the constellation point

from the interferer imposed on the desired vector corresponding to the constellation

point transmitted by the serving base station (see figure 7).

Figure 3 Influence of the co-channel interference on the OFDMA subcarriers

Influence of synchronization errors on the OFDMA signal

The four synchronization errors that can be present in the OFDMA system

(resulting from synchronization mismatch between transmitter and receiver), are:

carrier phase offset (CPO), carrier frequency offset (CFO), symbol time offset (STO)

and sampling frequency offset (SFO).

Carrier phase offset is seen as equal phase shift at all subcarriers (in the

frequency domain) and can be described by the following formula:

( ) ( )

where ϕCPO is the phase shift in degrees.

Carrier frequency offset (CFO) is seen as phase shift of the time domain samples

increasing from sample number 1 up to last sample in the symbol and can be

described by the following formula:

( ) ( )

where ϕCFO is the carrier frequency offset expressed the % of subcarrier spacing.

Subcarrier

index

Subcarrier power

Original subcarrier

Imposed interfering subcarrier

I

Q

Vector of QPSK signal

(single subcarrier)

Demapped subcarriers

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Symbol time offset (STO) is seen as the phase offset in the frequency domain,

that is increasing from DC up to the last lower band subcarrier and from DC up to the

last upper band subcarrier. It can be described by the following formula:

( ) ( )

where ϕSTO is the symbol time offset expressed in the number of samples, n is the

subcarrier index, N_FFT is the FFT size.

Sampling frequency offset (SFO) is seen as the phase offset in the frequency

domain, increasing over subcarriers, and also over OFDMA symbols. SFO could result

also in inter-carrier interference (similar to CFO). The offset results from the fact that

the analog waveform is sampled too fast or too slow at the receiver compared to the

sampling frequency at the transmitter. The effect is shown in the figure 8. The SFO is

typically expressed in the ratio between receive and transmit sampling frequency.

Figure 4 Sampling frequency offset effect

3.2. System description for the exercise

The course exercises are based on the LTE PHY LabTM. LTE PHY LabTM is a

comprehensive implementation of the 3GPP Release 8 E-UTRA physical layer. It has a

form of a MATLABTM Toolbox. It includes both the downlink and the uplink processing

chains covering all the PHY steps such as FEC, modulation, MIMO processing,

resource mapping, OFDMA and SCFDMA signal generation. Due to that structure

baseband models of both the eNB and UE can be created. For the lab course

exercises it is important to evaluate the role of every single component block, or the

whole transmitter and receiver.

Transmitted Samples

Received Samples

t

Ts

(1 - 1/SFO)Ts

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For the purpose of this exercise, the blocks of LTE PHY LabTM that are required for

evaluation are:

LTE Downlink PHY transmitter (realizing transmission of OFDMA signal)

AWGN and multipath channel

LTE Downlink PHY receiver (realizing reception of OFDMA signal)

The block diagrams, that are useful for this exercise are shown below:

a) For the experiments showing the influence of multipath channel the received

signal:

Figure 5 Block diagram for experiments a)

b) For the experiments showing the impact of pulse shaping filtering on the

received signal:

Figure 6 Block diagram for experiments b)

LTE PHY

Transmitter

LTE PHY

Receiver Multipath

channel

Signal in

time domain

Scatterplot &

subcarriers

LTE PHY

Transmitter

LTE PHY

Receiver

Signal in

time domain

Scatterplot

Pulse shaping

filter

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c) For the experiments showing the influence of co-channel interference on the

received signal:

Figure 7 Block diagram for experiments c)

d) For the experiments showing the influence of synchronization errors between

transmitter and receiver on the received signal:

Figure 8 Block diagram for experiments d)

LTE PHY

Transmitter

LTE PHY

Receiver

Desired Signal in

time domain

Scatterplot LTE PHY

Transmitter

LTE PHY

Transmitter

Undesired multiple signals

in time domain

LTE PHY

Transmitter

LTE PHY

Receiver

Signal in

time domain

Scatterplot &

subcarriers

Synchronization

error

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The important parameters that are related to the LTE system for this exercise are

gathered in table 1.

Table 1 LTE PHY Lab parameters used in exercise Parameter name Parameter value Comment

System bandwidth 3MHz

FFT size 256

Transmission direction Downlink eNB Transmitter and UE receiver

Measured PHY Channel PDCCH It occupies all useful subcarriers in OFDMA symbol

Modulation order QPSK

Multipath channels SUI and EUTRA 3GPP TS 36.942

Subcarrier spacing 15kHz

Normal CP length 4.6 μs

3.3. References

1) 3GPP TS 36.300 “E-UTRAN overall description”

2) 3GPP TS 36.201 “LTE physical layer, general description”

3) 3GPP TS 36.211 “Physical channels and modulation”

4) www.3gpp.org

5) “Fundamentals of wireless communication”, D. Tse, P. Viswanath, Cambridge

University Press 2005

6) “OFDM for Wireless Communications Systems”, R. Prasad, Artech House 2004

7) “3G Evolution, HSPA and LTE for Mobile Broadband”, E. Dahlman, S. Parkvall, J.

Skold, P. Beming, Elsevier Ltd. 2007

8) “Fundamentals of WiMAX, Underdstanding Broadband Wireless Networking”, J.

Andrews, A. Ghosh, R. Muhamed, Prentice Hall 2007

9) T. Chiueh and P. Tsai, “OFDM Baseband Receiver Design for Wireless

Communications”, John Wiley & Sons, 2007

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4. Setting up laboratory environment

Before starting the exercise:

Power on the computer, insert the hardware key into the USB port (and let the

installation wizard to download and configure the drivers – if not yet done). Then

please follow the instructions below:

1) Extract the LTE_PHY_LAB_Exercises.zip file into a desired directory, e.g. “C:\LTE_PHY_LAB_Exercises” (if not yet extracted).

For the use of LTE PHY LabTM only:

2) Launch MATLABTM. In the work directory window (upper right corner) browse to your extracted folder or insert the path manually (refer to the figure 9).

3) Enter the function “LTE paths” in the command line, in order to add all the directories including LTE PHY LabTM functions (refer also to the figure 9). Each time you restart MATLABTM you have to perform steps 2 and 3 to properly attach all the LTE PHY LabTM directories.

Figure 9 LTE PHY LabTM

Installation Steps (LTE PHY LabTM

only)

For the use of one of the folders with experiments for exercise:

Perform the same steps (2 and 3) as in the case of LTE PHY LabTM only, but

instead of browsing to the LTE_PHY_Lab directory, browse to one of the folders with

simulation cases and use “LTEpaths” from it to add the paths (refer to the figure 10).

Figure 10 LTE PHY LabTM

Installation Steps (LTE PHY LabTM

for the use of lab exercise folders)

NOTE! In order to install the USB hardware key drivers the computer must have internet access.

NOTE! In case you remove the hardware key out of the USB port, you need to restart Matlab for proper operation or put the key back.

NOTE! In case you would like to move from one directory with experiments to another, please repeat the above.

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5. “Warm up exercises”

In this laboratory exercise each of the impairment, described in the chapter 3 will

be considered separately, to show the different influences it has on the received

signal. However in real life OFDMA system, they might be present altogether.

5.1. Examination of the impact of the multipath channel on the received signal

(MultipathChannel.m)

1) Open the script and examine source code to identify: baseband transmit

signal creation, transmission over wireless channel, receive signal reception

and subcarriers demapping

2) Use help command to get description of the script (help MultipathChannel)

3) Run the script with the first predefined channel

4) Run the script with the second predefined channel

5) Describe the similarities and differences between these two channels’

influence on the received signal – e.g. if it is necessary to perform channel

estimation and equalization at both?

6) Use another channels for script running (the description of the range of

possible channels is present in LTE_PHY_Lab/Channels and answer question

at point 5)

5.2. Examination of the impact of the pulse shaping filtering on the received signal

(TimeFiltering.m)

1) Open the script and identify: signal creation, filter function description,

filtering operation, signal reception and plotting

2) Use help command to get description of the script (help TimeFiltering)

3) Run the script with the predefined settings

4) Examine the impact of the two predefined filters on the PSD of the OFDMA

signal

5) Answer for the questions:

a. Which filter is better and why?

b. What is the maximum out of band power of the original signal and

filtered signal?

c. What is the difference between the impacts of the filters at the PSD?

d. What is the difference between the impacts of the filters seen at the

scatterplot?

e. What other impairments (that you are familiar with) are similar to the

effects, that the filtering has on the received signal?

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5.3. Examination of influence of the co-channel interference on the received signal

(CoChannelInterference.m)

1) Open the script and identify: desired signal creation, interference signals

creations, summing of the signals, signal reception and plotting

2) Use help command to get description of the script usage (help

CoChannelInterference)

3) Run the script with the predefined settings

4) Examine the differences and similarities between the 4 different examples: no

impairment, AWGN, multiple low power uncorrelated interferers, single

dominating interferer

5) Answer for the questions:

a. What are the possible system settings / scenario requirements for

each of the examples from point 4 (e.g. typical urban scenario,

laboratory scenario, rural scenario, ideal scenario, single BS scenario,

etc.) and why do you think so?

b. What are the differences and similarities between these 4 different

examples?

c. To which scenario (from the 4) would you apply the following two

reception concepts: maximum ratio combining, interference rejection

combining, and why?

6) Open the script and modify the source code, to obtain yet another different

scenarios:

a. In the example where there are multiple non dominating interferers,

adjust the power of one of them to be dominating one, leaving the

others without any change. Observe the difference compared to base

situation, copy the picture and describe the difference.

b. Modify the source code such that both AWGN and multiple interferers

will be present simultaneously. Observe the difference compared to

base situation, copy the plot and describe the difference.

c. Modify the source code such that both AWGN and single dominating

interferer will be present simultaneously. Observe the difference

compared to the base situation, copy the plot and describe the

difference.

d. For each of these topics answer for questions a-c from point 5).

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5.4. Examination of influence of the synchronization errors on the received signal

(Offsets.m)

1) Open the script and identify: signal creation, reception of the original signal,

CPO application, signal reception and plotting, CFO application, signal

reception and plotting, STO signal reception and plotting and SFO application,

signal reception and plotting

2) Use help command to get description of the script usage (help Offsets)

3) Run the script with the predefined settings

4) Examine each of the impact of the synchronization error separately on the

demapped subcarriers and scatterplots as well as on the 3D view. Recognize

the effect’s influence on the signal (according to the information from chapter

3) and describe the influence each of the sync error, that it has on the power

of subcarriers and on the scatterplot.

5) Use different settings for each of the particular error (smaller and higher

offsets) and notify the differences on the scatterplots and at the resource

demappers. Copy the plots with information of what settings have you used

and describe the differences to the base case.

6) Open the script and modify the source code to obtain yet another scenarios:

a. Apply both CPO and CFO simultaneously with the predefined settings

b. Apply both CFO and STO simultaneously with the predefined settings

c. Apply both CFO and SFO simultaneously with the predefined settings

d. Apply both STO and SFO simultaneously with the predefined settings

e. Apply all of the offsets simultaneously with the predefined settings on

the same signal

7) Copy the plots of the results and describe the influence that the impairments

have on the signal from sections a – e from point 6.

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6. Main exercise tasks

After the student gets familiar with all the impairments, that can be present in

the wireless OFDMA system by following the tasks from point 5, student shall write

his own files realizing the following tasks

Note! These tasks can be done in the class or at home or used as a separate

project.

6.1. Examination of the impact of the pulse shaping filter on the received signal

1) Using as the example source code from file TimeFiltering write a filter that has

different edges to obtain tradeoff between out-of band emission decrease (seen

on the PDF) and in-band impairments (seen on scatterplots). Copy the source

code, the plots and write the conclusions.

2) Write the script realizing measurements of EVM (error vector magnitude) of the

frequency domain samples.

EVM is defined as a ratio of the difference between RMS of the error (in the

constellation point between received signal and transmitted signal) and RMS of

the transmitted signal.

Notifying: Y(n) – as I/Q symbol on the subcarrier n at the receiver, and X(n) as the

I/Q symbol from the original constellation (e.g. QPSK) at subcarrier n at the

transmitter, RMS of the error at subcarrier n is equal to:

( ) √( ( ) ( ) ) ( ( ) ( ) )

the RMS of the transmitted signal at subcarrier n is equal to:

( ) √( ( ) ) ( ( ) )

and EVM at subcarrier n is equal to:

( )[ ] (

)

The script shall also calculate the average EVM value (over all subcarriers).

After writing the script, measure the EVM of the two defined filters (from

TimeFiltering.m) and the filter that you have implemented in point 1).

Copy the source code, results and write conclusions on the EVM measurements.

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3) Write a script plotting the EVM together with the original and filtered signal (on

one plot: i.e. on the top – the original subcarriers, in the center – the filtered

subcarriers and at the bottom – the EVM measurement). Copy the plots and write

conclusions.

6.2. Examination of the impact of the synchronization errors on the received signal

1) Using as the example source code from file Offsets.m, write a script that will

realize incrementally increasing each offset sepearately (from ideal

synchronization up to the defined value). Estimate the maximum synchronization

error that can be accepted (for each error separately). The acceptance shall be

subjective. Write your own feelings about the results (including value of the offset

and conclusions).

2) Use the EVM measurement script that you wrote in the previous section and

measure EVM of all of the different synchronization errors from the default

settings. Copy the results and write conclusions.

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7. Test questions

After all the tasks are filled, the student shall answer for these questions in the

report.

1) What in your opinion are the most dangerous distortions for OFDMA signal

transmission and why?

2) Draw a block diagram including: OFDMA transmitter (mapper S/P, IFFT, CP,

D/A), channel, OFDMA receiver (A/D, CP, FFT, P/S, demapper) and point the

places where each of the discussed distortion is taking place. Then draw and

connect additional blocks that should be used for reducing these effects (e.g.

channel estimator and corrector, time synchronizer, etc.)

3) What typical methods do you know, that are used for:

a. Channel estimation

b. Channel equalization

c. Time and frequency synchronization

d. Co-channel interference reduction

4) What is the maximum CFO that can be accepted, according to typical OFDMA

system’s requirements (expressed in the percent of the subcarrier spacing)?

What is the maximum CFO that can be accepted in the LTE system?

5) What is the maximum STO that can be accepted, according to the OFDMA

signal characteristics? What is the maximum STO that can be accepted in LTE

system?

6) Name the typical pulse shaping filter characteristics.

7) What shall be the relation between channel coherence bandwidth and

subcarrier spacing in order to have flat fading subchannels? What is the

assumed channel coherence bandwidth for the LTE system (according to the

specified subcarrier spacing)?

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8. Report content

The report shall include:

Answers for the questions from chapter 5 and 6.

Required descriptions from chapter 5 and 6.

Required plots / simulation results and source codes from chapter 5 and

6.

Answers for the questions from chapter 7.

Required block diagrams from chapter 7.

Related conclusions and observations from the exercise.

The report header should look like this presented below.

Wireless communication course

Exercise Title: “OFDMA Receiver in the LTE system”

Group ID: Students’ names: 1. 2. 3. 4.

Exercise time and date:

Student providing report:

Date of the report: Grade and lecturer signature:

Note! This is the example report header that can be used.