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Case Study Competition 2012 Mobile Radio Testers

Station: Data transfer in mobile networks

The success of smartphones and Internet

services in general has increased the

need for high-performance data transfer

over mobile networks.

At this station you will investigate how the

two radio access network standards,

WCDMA and LTE, perform while

transferring data.

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Case Study Competition Final 2012

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1 Introduction The launch of the Global System for Mobile Communications (GSM) standard in the 1990s was

the beginning of digital mobile communications. GSM has been optimized to support voice

communications that were already handled by landline voice connections, e.g. ISDN.

The emerging success of the Internet in general and its services such as social media, in

combination with the success of smartphones, has increased the demand for high-performance

mobile data connections.

The first release of the WCDMA standard supports up to 386 kbit/s, and its extension HSPA

supports impressive data rates of up to 42 Mbit/s.

Long Term Evolution (LTE), the successor standard to GSM, offers, besides data rates above

100 Mbit/s, even more:

- All IP network

- Scalable bandwidth

- High spectral efficiency

Along with the availability of higher data rates, the demand for shorter round-trip times, as well as

lower jitter for packet data transmission has increased. A typical mobile data session rarely

exhibits single large downloads, but has a highly interactive character, where both high bandwidth

as well as low latency play an important role.

In the developmental and evolutionary path from GSM toward LTE, the two key user experience

parameters, bandwidth and latency, have changed dramatically, and now provide the instant-on

feeling of a wired connection in a mobile environment.

Alongside the measurement of modulation quality and transmitted power, increasingly the

importance of higher-layer measurements has risen, for example, to reliably and repeatably check

the TCP/IP performance of a mobile device. Therefore, a modern mobile radio tester also needs to

support this type of measurement, and provide access to these measurements in a single box.

The following exercise will use the built-in functionality of a state-of-the-art mobile radio tester to

compare the user-experienced performance of two different radio access technologies.

CMW500

Radio Communication Tester

LTE

WCDMA

or

IP connection IMS VoIP client IMS VoIP server

Fig. 1: User experience test setup.


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R&S CMW500 application Purpose

WCDMA Signaling

Controls and configures the WCDMA base

station simulation.

Press Tasks button,

then WCDMA Signaling:

LTE Signaling

Controls and configures the LTE base station

simulation.

Press Tasks button,

then LTE Signaling:

Data Application Measurement

Supports IP-based measurements such as

Throughput, IP logging and Ping.

Furthermore, it can be used to impair the IP

communications between the UE and the

R&S CMW500 server.

Press Tasks button,

then Data 1 Meas:


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Data Application Control

Controls and configures the R&S CMW500

IP servers and services, such as the IMS

VoIP server.

Go to Data Application Measurements

(shown on preceding page),

then press Configure Services:

ON/OFF button

Starts or stops a network simulation, a server

or a measurement.

Wireshark

Captures and decodes e.g. IP traffic and

shows the messages and content.

www.wireshark.org

In Data Meas 1, the tool can be started from

the IP logging tab.

On the laptop, it can be started from the

Desktop.

Mercuro IMS VoIP Client

IMS VoIP client for Windows operating

systems.

To set up a VoIP call, type e.g. 123 as phone

number:

123

http://www.wireshark.org/


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2 Abbreviations 3GPP Third Generation Partnership Project

DL Downlink (signal from base station to mobile station)

DUT Device under test (see UE)

IMS IP multimedia subsystem

IP Internet protocol

LTE Long Term Evolution (3.9G, 4G)

MAC Medium access control

OS Operating system

PDCP Packet data convergence protocol

PDU Protocol data unit

RAT Radio access technology

RF Radio frequency

RTP Realtime transport protocol

TX Transmission

UDP User datagram protocol

UE User equipment (mobile phone, data dongle, smartphone, terminal, device under test)

UL Uplink (signal from mobile station to base station)

UMTS Universal Mobile Telecommunications Service (3G)

USB Universal serial bus

VoIP Voice over Internet protocol

VoLTE Voice over LTE

WAN Wide area network

WCDMA Wideband Code Division Multiple Access (see UMTS)


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3 Exercise The improvements in the data transport domain of mobile networks is not only important for increased data throughput, it also provides the performance to achieve an adequate user experience for multimedia services such as voice. The aim of this exercise is to compare the user experience of a voice session over IP connections based on WCDMA and LTE. The test setup in Fig. 1 shows the R&S CMW500 in combination with a PC and a WCDMA and LTE capable data card (USB stick). Hint: Take screenshots of your measurement results

using the Print button on the left of the R&S CMW front panel.

a. VoIP loopback connection over a WCDMA network simulation

1. Start a WCDMA network simulation on the R&S CMW500 by pressing the ON/OFF button. 2. Start the IP logging functionality and the throughput measurement on the R&S CMW500

by pressing the ON/OFF button. 3. Plug-in the USB stick to a USB port of the laptop and wait till an IP connection has been

established. 4. Start the IP logging functionality on the VoIP client (laptop) by starting Wireshark and

selecting the correct network interface. 5. Verify the IP connectivity and the RTT of this IP connection. 6. Start the Mercuro VoIP client on the laptop and press the Connect button. 7. Dial any number (e.g. 123) and establish a VoIP connection. 8. Verify that a VoIP session has been established. Is the delay audible? 9. Hang up the VoIP call and close the Mercuro VoIP client. 10. Stop the IP logging on the R&S CMW500 and the laptop.

b. VoIP loopback connection over LTE connection

1. Start an LTE cell simulation on the R&S CMW500 by pressing the ON/OFF button. 2. Repeat steps 2 to 10 from measurement a.

a. Bonus: To simulate an IP connection over a GSM/GPRS connection, try to use the network impairment functionality. A GSM network has an RTT of ~800 ms.

Exercise App-1: Compare the IP logs from the R&S CMW and the laptop: How long does a packet need for the transmission in WCDMA and LTE? Exercise App-2: What is the interval of the VoIP audio packets sent by the Mercuro VoIP client? Exercise App-3: At what data rate does the VoIP client send the audio stream? Exercise App-4: Name three key measures that contributed to the latency reduction in LTE compared with older radio access technologies.


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4 Feedback

Rohde & Schwarz GmbH & Co. KG

Mobile Radio Testers

www.rohde-schwarz.com

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Station: LTE physical layer

Configuration of LTE parameters that

influence data throughput

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1 Introduction The physical layer (L1) is used in the R&S CMW radio communication tester and in callbox products.

Physical layer implementations exist for LTE, WCDMA/HSPA, GSM and other standards. This exercise

focuses on the LTE physical layer.

In LTE, the transmit time interval (TTI) is 1 ms. This means that transport blocks in the uplink and

downlink must be processed every 1 ms. Additionally, the transmit and receive parameters such as

modulation and coding rate may change every TTI.

Payload data is sent between the medium access control (MAC) and physical layers as transport blocks.

Control data sent from the physical to the MAC layer includes channel status information such as

channel quality indicator (CQI), precoding matrix indicator (PMI), rank indication (RI), as well as

Ack/Nack information for hybrid automatic repeat request (HARQ).

The aim of this exercise is to find the settings that allow the maximum data throughput in the downlink.

Fig. 1: LTE physical layer test setup.

CMW500

Radio Communication Tester

LTE

IP connection

Power

RF

2x2 MIMO


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LTE signaling

Controls and configures the LTE base station

simulation.

Press Tasks button,

then LTE Signaling:

LTE extended BLER

Measures the possible throughput and BLER

for a device under test.

Press Tasks button,

then LTE Ext. BLER:

Change the connection setup:

Switch between Throughput and BLER:

and


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2 Abbreviations

3GPP Third Generation Partnership Project

ADC Analog/digital converter

AWGN Additive white Gaussian noise BLER Block error rate CQI Channel quality indicator

DL Downlink (signal from base station to mobile station)

DUT Device under test (see UE)

HARQ Hybrid automatic repeat request

LTE Long Term Evolution (3.9G, 4G)

MAC Medium access control

MIMO Multiple input multiple output

PHY Physical

RAT Radio access technology

RF Radio frequency

SNR Signal-to-noise ratio

SS System simulator (network simulator, R&S CMW500, LTE protocol test equipment)

TBS Transport block size

TX Transmission

UE User equipment (mobile phone, data dongle, smartphone, terminal, device under test)

UL Uplink (signal from mobile station to base station)


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3 Exercise

3.1 Description

The R&S CMW500 radio communication tester simulates an LTE base station. As soon as the UE is

powered up, it searches for an LTE cell and, if found, will register and attach to it.

In this exercise the R&S CMW500 simulates a special LTE cell in a configuration called Testmode. This

Testmode is meant to measure the performance of the UE on the PHY and MAC layers, by sending

generated data to the UE and verifying if this data has been correctly received.

The Testmode does not support a real IP connection. Therefore, no laptop behind the UE is required.

In the test setup, two antennas of the UE are connected to the R&S CMW, in order to allow a MIMO 2x2

connection with transmission mode 4 (closed loop spatial multiplexing).

The R&S CMW generates a downlink signal for each of the two antennas with a power of –85 dBm per

subcarrier.

To simulate a real-world scenario, AWGN is generated in the R&S CMW and added to the signal. The

SNR is fixed during the exercise (+15 dB).

Fig. 2: LTE signal – power and noise level.

The R&S CMW sends two transport blocks of data in every subframe (1 ms). The UE tries to decode the

transport blocks and checks the decoding success with a cyclic redundancy check. If the decoding is

successful, the UE will send an Ack per transport block to the R&S CMW. If the decoding fails, it will

send a Nack.

The “Extended BLER” measurement measures the throughput and BLER over a user-defined period

(here set to 10000 subframes = 10 s).


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The transport block size for the two MIMO streams depends on the modulation and on the transport

block size index (TBS Idx). Fig. 3 shows the transport block size and overall theoretical throughput

(circled in red) displayed on the R&S CMW.

Fig. 3: LTE connection setup – theoretical throughput.


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3.2 Exercise

Hint: Take screenshots of your measurement results

using the Print button on the left of the R&S CMW front panel.

1. Connect the UE to the active RF connectors on the R&S CMW. The UE will start searching for

an LTE network and attach to it as soon as it has been connected to the USB power adapter.

2. Start a data connection (“Connect” button). All 100 resource blocks of the cell should be used in

this exercise.

a. Go to the “Extended BLER” measurement and measure the BLER and throughput for

different DL data rates.

b. Start with TBS Index = 5 and QPSK and increase the TBS and modulation. Note the

BLER and average throughput for each step. They are circled in red in the screenshot.

c. Restart the measurement after every change of the transport block size.

d. Make a screenshot of the BLER measurement with the highest throughput.

Fig. 4: Throughput and BLER measurement.

Exercise PHY-1: Draw diagrams for BLER vs. transport block size and throughput vs. transport

block size and present them to the jury. Explain your findings.


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4 Feedback




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Station: Protocol testing

LTE / WCDMA handover and data

throughput measurements

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1 Introduction In this hands-on lab test you will use the R&S CMW500 radio communication tester to perform

data throughput measurements using an LTE / WCDMA USB data stick.

Firstly, you will measure the real end-to-end throughput that can be achieved in an

WCDMA network on IP level by downloading a file from an FTP server and compare the

results with the maximum data rate in the LTE network.

Secondly, you will perform two different types of LTE / WCDMA mobility procedures and

analyze their impact on a real end-to-end data connection.

The above tests will be performed with the R&S CMW500 radio communication tester and a

selection of R&S CMW500 applications.

Test setup

The following figure shows the test setup at this station:

Laptop USB Stick CMW500 Protocol Tester

LTE

WCDMA

FTP Client

RF

RF

USB

FTP ServerIP connection

Hand

over

Test procedure

The R&S CMW500 radio communication tester creates two independent cells, one for LTE and

the other for WCDMA. The device under test is a USB data stick that is controlled by a laptop

and connected to the R&S CMW500 with an RF cable. The R&S CMW500 hosts an FTP server

that acts as a data source, whereas the FTP client resides on the laptop.

The following applications are provided to control the R&S CMW500,in order to measure the

data throughput and to analyze signaling procedures between the device under test and the

R&S CMW500. All applications run on the Windows PC integrated in the R&S CMW500.


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Project Explorer (PE)

Controls and configures

R&S CMW500 radio

communication tester

- Start test script to setup LTE

and WCDMA cell

- Perform handover between

LTE and WCDMA

Protocol Testing Monitor (PTM)

Measures data throughput in

LTE and WCDMA network in

different protocol states


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Message Analyzer (MA)

Analyzes signaling procedures during handover

To open a message log in Message Analyzer, right-click on a test case result in the "Test Project Results" tab of Project Explorer and select "Open message log”:

Taking screenshots

Screenshots can be taken by

pressing the PrintScreen key.

The screen contents are

stored in the clipboard and

can be pasted to paint.


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2 Exercise

2.1 Test 1 – comparison of throughput in LTE and WCDMA

In this exercise the data rates achieved in LTE and WCDMA shall be measured and compared.

2.1.1 Work steps

The task is to execute a test case and take screenshots of the throughput graphs for WCDMA

and LTE displayed in the Protocol Testing Monitor (PTM) when requested by the test application.

1. Run test case “Test 1” from Project Explorer

2. Follow the instructions given in the dialog boxes Exercise Stack-1: Compare the achieved maximum throughput. Explain the difference.

2.2 Test 2 – inter-RAT mobility procedures

In this exercise two variants of moving the UE from LTE to WCDMA shall be compared.

2.2.1 Work steps

1. Run test case “Test 2A” and follow the instructions in the dialog boxes. One screenshot shall

be taken showing the transition from LTE to WCDMA.

2. Run test case “Test 2B”. Again a screenshot shall be taken showing the throughput while

going from LTE to WCDMA.

Exercise Stack-2: Compare the throughput charts of test 2A and test 2B. Describe the

difference.

Exercise Stack-3: Open the R&S CMW500 message logs for both test scenarios. Compare the

layer 3 signaling. Which procedure is used to bring the UE from LTE to WCDMA in each test

scenario? What effect does the chosen procedure type have on the data transfer?


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MMI command dialogs

Perform the requested action first and then confirm e.g. the “Please switch on the UE”

command by clicking the Send button.


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3 Feedback




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Station: Signals in the air

Measure the signals between base

stations and mobile stations

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1 Introduction

Our world is full of radio signals. A mobile phone communicates with a base station, the entry gate to the

core mobile network. A spectrum analyzer in combination with an antenna can be used to analyze the

RF signals and determine whether communications function as expected.


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2 Exercise

2.1 Test setup

Required equipment: - R&S FSH8 spectrum analyzer - Antenna

The following measurements should be performed inside the building for the available Rohde & Schwarz WLAN. For mobile phone signals, they should be performed outside the building. Connect the antenna to the spectrum analyzer (SA) and switch on the SA. Press the green PRESET button on the SA.

2.2 Test procedure

1) Find which signals are available in the air. Search especially for WLAN signals. Measure the frequencies, power levels, bandwidths. What does the spectrum look like? What is the time domain? Measure whether the signals are continuous or burst. What duration do burst signal have? 2) Please classify the signal. To which networks do they belong? WLAN IEEE 802.11x? Others? Uplink/downlink? 3) Please present your results.


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3 Feedback




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Mobile channel and MISO systems

Understand and exploit the properties of

mobile radio channels

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1 Introduction Due to effects such as multipath propagation and shadowing, the mobile channel imposes severe

constraints on communications receivers. In this exercise you will study the properties of mobile

channels and analyze a transmission scheme that is designed to exploit such channels.


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2 Exercise

Mobile channel

Time dispersion

The mobile channel at a given time instant can be modeled as a finite impulse response (FIR) filter, h(t).

Tx Rx

h(t)x(t) y(t)

Fig. 1: Time dispersion in mobile channels.

Assume that signal x(t) is transmitted at transmitter antenna Tx. If there is no noise and no other interfering signals, signal y(t) received at receiver antenna Rx is given as linear convolution of x(t) and h(t):

d)t(h)(x)t(h)t(x)t(y

In Fig. 2 a typical channel h(t), transmitted signal x(t) (three symbols) and received signal y(t) are visualized.

y(t)

t

t

h(t)

1

x(t)

symbol 1 symbol 2

t

symbol 3

Tsymb

Reception Window

Tsymb

Taps

Fig. 2: Typical transmission scenario for mobile channel.


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The received signal y(t) consists of several scaled and delayed versions of the transmitted signal x(t) (tapped delay line model). As can be observed, it is not possible to choose a reception window (duration = Tsymb = 1 symbol period) such that e.g. symbol 2 can be received without interference from either symbol 1 or symbol 2 or even both. This interference is called intersymbol interference (ISI). In order to overcome that problem, a cyclic prefix (CP) can be added to the signal. This means that the end of the symbol is repeated at its beginning (see Fig. 3).

Tsymb TsymbTcp

Fig. 3: Cyclic prefix is added. In Fig. 4 you can observe that by introducing a CP, it is possible to synchronize the receiver (find a reception window) such that ISI is avoided.

symbol 1 symbol 2

Tsymb

x(t)

Tcp

symbol 3

t

h(t)

1

Reception Window

Tsymb

t

t

Fig. 4: Typical transmission scenario for mobile channel. Consider the following channel impulse response with four channel taps:

Fig. 5: Channel impulse response.


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Exercise 2-1: (2 points)

What is the relationship between Tcp and max that has to be maintained in order to avoid ISI?

Consider an LTE (Long Term Evolution, state-of-the-art mobile communications standard) symbol given in Fig. 6

x(t)

t

Tcp

4,69 us

Tsymb

66,67 us

Fig. 6: LTE symbol.

and the channel models defined in the LTE test specification (Table 1 through Table 3). These tables define the tap delays and attenuations.

Excess tap delay [ns]

Relative power [dB]

0 0.0

30 –1.0

70 –2.0

90 –3.0

110 –8.0

190 –17.2

410 –20.8

Table 1: Extended pedestrian A (EPA) model.

Excess tap delay

[ns] Relative power

[dB]

0 0.0

30 –1.5

150 –1.4

310 –3.6

370 –0.6

710 –9.1

1090 –7.0

1730 –12.0

2510 –16.9

Table 2: Extended vehicular A (EVA) model.


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Excess tap delay [ns]

Relative power [dB]

0 –1.0

50 –1.0

120 –1.0

200 0.0

230 0.0

500 0.0

1600 –3.0

2300 –5.0

5000 –7.0

Table 3: Extended typical urban (ETU) model.


Given these channels, is it possible to receive an LTE signal (see Fig. 6) without ISI? Why?


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Frequency dispersion

Now consider the channel model in Fig. 7:

max

])[(h 2

1

Fig. 7: Channel model.

The Rayleigh fading model assumes that each of the four taps can be considered as a complex random process, where real and imaginary parts are independent, zero mean Gaussian distributed random

variables with a variance of 2

2

.

)t(ib)t(a)t(h , 2

,0N)t(b),t(a , max321 ,,,0

The power spectrum of each tap can be modeled by the well known Clarke’s model:

d

d2

d

dh

ff,0

ff,

f

f1f)f(S

where fd is the maximum Doppler frequency. An important parameter of a random process is its coherence time Tc. Tc is a measure of how fast the random process varies in time.

Assuming that )t(h is a wide sense stationary (WSS) process for each , the correlation between

)t(h 1 and )t(h 2 only depends on the time difference 12 ttt :

2t

)t(hE

)tt(h)t(hE

where E{} denotes the expected value.

The X% coherence time Tc is defined as that value of t such that

100

Xt


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The coherence time of )t(h is closely related to the maximum Doppler frequency. Assuming Clarke’s

model, the 50% coherence time is given as

d

cf16

9T

In order to reliably demodulate a signal received from a mobile phone (UE), a base station (BS) needs an accurate estimate of the channel impulse response. Consider the LTE uplink (transmitted from UE) subframe in Fig. 8:

symb0 symb1 symb2 DMRS symb4 symb5 symb6 symb0 symb1 symb2 symb4 symb5 symb6DMRS

slot0 (500us) slot1 (500us)

subframe (1ms)

Fig. 8: LTE subframe (uplink).

In the middle of each slot the demodulation reference signal (DMRS) is transmitted from the UE. The DMRS is a signal sequence that is known from the BS. The BS can therefore determine a channel estimate from the DMRS.


What is the maximum Doppler frequency that can be supported in the LTE uplink? Why? (Assume that a random process can be regarded constant during Tc.)


Assume that the UE transmits at a carrier frequency fc = 1.92 GHz. What is the maximum velocity allowed for the UE such that the BS can reliably demodulate its signal?

MISO systems

One of the main problems introduced by a mobile channel are the power fluctuations of the received signal. Consider the non-frequency-selective (only one tap) fading channel in Fig. 9:

n(i)

y(i)d(i)

h

rd(i)

Fig. 9: Non-frequency-selective fading channel.


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Assuming that d has a power of 1)i(dE 22

d and h is a Rayleigh fading channel with a maximum

Doppler frequency of 5 Hz and 1hE 22

h , the instantaneous received signal power could look like

the display shown in Fig. 10:

Fig. 10: Received signal power (Rayleigh fading, maximum Doppler frequency 5 Hz).


Why are degradations of the received signal power (see Fig. 10, indicating degradations of up to almost 35 dB in the observed time interval!) a problem for the receiver? To combat those power fluctuations, a common approach is to use multiple antennas at the transmitter side.

S/P

d(i)

d(i)

d(i)

w1

wNt

h1

hNt

s1(i)

sNt(i)

n(i)

r(i) y(i)

|h|

Fig. 11: Nt x 1 MISO system.


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It can be shown that the weight vector w = T

Nt1 w,,w has to be chosen as h

hw

* , where

h = T

Nt1 h,,h in order to maximize the signal-to-noise ratio (SNR) at the receiver.

After renormalizing with h at the receiver, we have:

y(i) = h ( hTs(i) + n(i) ) =

Nt

1v

2

vh)i(d + h n(i)


Give an expression for y(i) for the transmission scheme in Fig. 9.


Define SNR’ = )i(nE

)i(dE2

2

. In addition, define SNR(y) =

N

S

P

P , where PS is the signal power in y(i) and PN

is the noise power in y(i). Express SNR(y) in terms of SNR’ and h for the transmission schemes in Fig. 9

and Fig. 11. Assume that h

hw

*.


Under what conditions on hv for v = 1,…,Nt is the transmission scheme in Fig. 11 advantageous as compared with the transmission scheme in Fig. 9? Why? Explain what the term transmit diversity could mean in this respect. Now consider the transmission scheme with two transmit antennas given in Fig. 12:

S/P

d(i’)

d1

d2

h1

h2

s1(i)

s2(i)

n(i)

Co

de

r

d1

d2

-d2*

d1*

2

1

2

1

S/P

De

co

de

r

*

r1

r2*y(i)

r(i)

2i 2i+1

Fig. 12: 2x1 transmission scheme.

Here we define d = )1'i2(d

)'i2(d

d

d

2

1 and n =

)1i2(n

)i2(n

n

n

2

1.


Derive expressions for r1 = r(2i) and r2 = r(2i+1).


Responsible

Braun/1CM9

State

Draft

Version

1

Page

11 of 12


Define r =

2

1

r

r. r can be expressed as: r = Hd + n. Determine H.


Assume that the decoder in Fig. 12 performs a matrix multiplication according to y = H

Hr. Compute y in terms of d and n.


Assume that the power )i(dE 22

D = 1 in both Fig. 11 and Fig. 12 and h

hw

*.

Compute the total transmitted power for the transmission schemes in Fig. 11 and Fig. 12, P1 and P2 respectively.


Compare the expressions for y and y(i) derived in exercises 2-11 and 2-6. What is the drawback of the transmission scheme in Fig. 12 as compared with the transmission scheme in Fig. 11 in terms of SNR?


What is the advantage of the transmission scheme in Fig. 12 as compared with the one in Fig. 11?


Responsible

Braun/1CM9

State

Draft

Version

1

Page

12 of 12

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