OFDM and LTE implementation in Simulink

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Lab 4 LTE Frames, Slots and Resource Grid

Objectives

1. To understand LTE frames, slots and resource grid

2. To understand and apply the principles of frequency and time coordinate system

3. To calculate data rate based on resource allocation

4. To implement OFDM transmission with specific LTE resource grid

Introduction

In lab 1, you have explored and worked on various aspects of OFDM transmission through the

predefined Simulink model. In the model, there is a Symbol Generator block that randomly

generates a series of 16-QAM symbols. In LTE framework, prior to OFDM modulation, physical

channels and signals in LTE are assigned to different portion of the resource grid. In this lab, you

will explore and apply some important concepts in LTE physical layers, i.e LTE Frames, Slots and

Resource Grid.

Experiment 1: Understanding LTE Frequency and Time Representation

First, we look at the time domain of LTE transmission. There are some terminologies that you need

to get familiar with. They are radio frame, subframe, slot and OFDM symbols, as illustrated in

Figure 1. In the time domain, each radio frame, that forms the basic sequence of transmission, has

the length of 10ms. Each frame consists of 10 subframes (1ms length each). Each subframe is

further composed of two slots (0.5ms length each). Each slot consists of either seven or six OFDM

symbols based on a normal or an extended cyclic prefix.

Figure 1 LTE frame structure [1]

In the frequency domain, OFDM is regarded as a multicarrier transmission system and the spacing

between two consecutive carriers is defined as 15 kHz in LTE. The bandwidth of the transmission

corresponds directly to the number of the carriers.

As shown in Figure 2, the frequency and time representation of placing symbols is the key to

understand the operation of LTE. The transmitted symbols are explicitly mapped to different

portion of the two-dimension representation, which consists of a subcarrier axis (frequency) and a

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OFDM symbol axis (time). This coordinate system is called resource grid. Some additional

terminologies that you need to understand are resource element and resource block. A resource

element is one transmitted symbol placed at the intersection of an OFDM symbol and a subcarrier.

A resource block contains data symbols placed within 12 subcarriers in frequency axis and one

0.5ms slot in the time axis.

Figure 2 LTE Time-frequency coordinate [1]

Start Matlab, and in the command window, type the following commands to display a typical

transmitted resource grid

enb = lteTestModel('3.2','1.4MHz'); [txwave,txgrid,info]

= lteTestModelTool(enb); figure('Color','w'); helperPlotTransmitResourceGrid(enb,txgrid);

Task1: Based on the resource grid displayed and assuming normal cyclic prefix, answer the

following questions and describe how you arrived at the solutions:

• What is the transmission time of this LTE signal?

• What is the minimum bandwidth required to transmit this LTE signal?

• How many slots and radio frames displayed in the figure?

• How many resource elements and resource blocks displayed in the figure? How

many resource blocks per 0.5m slot displayed in the figure?

Task2: In the command window, type the following commands (5MHz physical channel parameter)

to display a typical transmitted resource grid and answer the above questions again.

enb = lteTestModel('3.2','5MHz');

[txwave,txgrid,info] = lteTestModelTool(enb); figure('Color','w'); helperPlotTransmitResourceGrid(enb,txgrid);

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Experiment 2: Data Rate Estimation

Task1: Referring to Table 1 and assuming 64-QAM modulation and one spatial stream, calculate

the maximum data rate for the channel parameters of 1.4MHz. Describe how the calculation is

performed. Note that not all symbols placed on the resource grid are data, there are also control

signals. However for simplicity of illustrating the concept, in this exercise, we consider that

resource grid contains data symbols only. Two intermediate steps to consider is as follows

• How many bits per data symbol?

• How many symbols (or resource element) per second?

Task2: In addition, calculate the maximum data rate for the channel parameters of 5MHz. Describe

how the calculation is performed.

Channel

Bandwidth

(MHz)

Sample

Frequency

(MHz)

FFT Size Subcarriers (excl.

DC sub-carrier)

# of resource

block (per slot)

1.4 1.92 128 72 6

3 3.84 256 144 12

5 7.68 512 300 25

10 15.36 1024 600 50

15 23.04 1536 900 75

20 30.72 2048 1200 100

Table 1 LTE downlink physical layer parameters

Experiment 3: Implementing Resource Grid with OFDM Transmission with

Normal Cyclic Prefix

In this exercise, you will implement OFDM transmission with LTE specific resource grid. The

Simulink file is Simple_Resource_Grid_Start_2016.slx. The Simulink blocks that you need

implement/modify are the Symbol Generator, Channel, Add Cyclic Prefix and Remove Cyclic

Prefix. Sample codes for three blocks have been given to you for your reference.

Task1:

• In Channel block, enter 6 (LTE fading channel) to the parameter Channel Selection and

enter relevant sampling frequency in reference with Table 1.

• To implement resource grid which has 1 radio frame, LTE 1.4MHz parameters (FFT length

and sample frequency according to Table 1) and 64 QAM.

o In the Bernoulli Binary Generator Block within Symbol Generator Block, verify

that

▪ Samples per frame is entered as nOFDMSym*nSubCar*6 o In the

Rectangular QAM Block within Symbol Generator Block Block, verify that

▪ M-ary number is entered as 64

▪ Click View Constellation button to observe the transmitting constellation

diagram o What are nOFDMSym and nSubCar based on the specifications? Edit the

variables nOFDMSym and nSubCar in the Matlab Workspace accordingly.

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Figure 3 Bernoulli Binary Generator Parameters

Figure 4 Rectangular QAM Block Parameters

• Verify the FFT length of the IFFT block o FFT Length is entered as 128

• Run the simulation model in which Cyclic Prefix functions are not implemented, and record

the results for future comparisons.

• To implement Add Cyclic Prefix function accordingly.

o How many CP samples are needed? And note that in each slot, 1st CP and

remaining CP have different lengths in time in reference to Figure 1.

o How many samples of the entire radio frame are required? To

implement Remove Cyclic Prefix block accordingly.

Run the Simulink model and you shall observe first 12 subcarrier plots. And you need to have key

results, Matlab codes and figures presented in your lab report.

Task2:

• To implement resource grid which has 2 radio frame, LTE 5MHz parameters (FFT length

and sample frequency according to Table 1) and 64 QAM.

• To modify FFT length of the IFFT block.

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• To modify Sample Rate of the Channel block.

• To implement Add Cyclic Prefix function accordingly.

• To implement Remove Cyclic Prefix block accordingly.

Run the Simulink model and you shall observe first 12 subcarrier plots. And you need to have key

results, Matlab codes and figures presented in your lab report.