UIUC ECE 463 Digital Communication Laboratory Fall 2018
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ECE 463 Lab 9: OFDM
1. Introduction
In orthogonal frequency division multiplexing (OFDM), the symbols are considered to start in the
frequency domain. OFDM operates on groups of symbols, the resulting group of symbols being called
an OFDM symbol (or packet). In this lab, we will implement the key features of the OFDM such as
cyclic prefix, null tones, and OFDM channel estimation/correction. However, this lab will not cover
OFDM CFO correction and OFDM phase tracking due to time and resource constraints.
1.1. Contents
1. Introduction
2. OFDM blocks
3. Putting Together in Simulation
4. Putting Together in USRP
1.2. Report
Submit the answers, figures and the discussions on all the questions. The report is due as a hard copy at the
beginning of the next lab.
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2. OFDM Blocks
Figure 1. Overall OFDM block diagram.
The figure shows the overall block diagram of OFDM transmitter and receiver.
We will re-use bit2symbol, frame sync, and symbol2bit blocks. The new OFDM blocks to implement are the
followings:
• S2P
• P2S
• OFDM_insert_null_tones
• OFDM_remove_null_tones
• OFDM_add_cp
• OFDM_remove_cp
• OFDM_preambles
• OFDM_add_preambles
• OFDM_ChEst
• OFDM_ChCor
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2.1. S2P & P2S
S2P block will convert the serial input stream (1D array) into the parallel output (2D array). The number of the
columns of the output array equals to the block size (N-K). If the total length of the input stream is not the integer
multiple of (N-K), we will pad zeros to match the size. For example,
Complete Box1 in the following figure which calculates the number of zeros to pad.
P2S block simply serializes a 2D array into an 1D array. Implement the following figure for P2S block.
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2.2. Insert Null Tones
We will implement a block that insert the null tones into the N-K subcarriers from S2P block. Note that the output
of S2P block is a 2D array whose columns correspond to the subcarriers.
OFDM_insert_null_tones.gvi
Terminal name
Type
Description
Input
Null Tones
Unsigned integer array
(1D)
K-location to
insert the null
tones
Input
Complex double array
(2D)
N-K columns
Output
Output
Complex double array
(2D)
N columns
The following shows an example how this block works.
Complete Box1 in the following figure. Box1 creates a column to be inserted at a given location. The number of
entries of the column should be equal to the number or rows of the input 2D array. Use Array Size to get the
number of rows. Use Initialize Array to set all the entries to zero.
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2.3. Remove Null Tones
We will implement a block that removes the null tones. This block will undo OFDM_insert_null_tones block.
OFDM_remove_null_tones.gvi
Terminal name
Type
Description
Input
Null Tones
Unsigned integer array
(1D)
K-location to
remove the null
tones
Input
Complex double array
(2D)
N columns
Output
Output
Complex double array
(2D)
N-K columns
The following shows an example how this block works.
Complete Box1 in the following figure. Notice that the null tones are sorted in descending order because the
current removal should not affect the index for the next removal. Use a for loop, shift registers, and Delete from
Array block.
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2.4. Add Cyclic Prefix
We will implement a block that adds the cyclic prefix. This block will take first few samples (Lc) and append
them to the end of the samples.
OFDM_add_cp.gvi
Terminal name
Type
Description
Input
Length of CP (Lc)
Unsigned integer
Input
Complex double array
(2D)
N columns
Output
Output
Complex double array
(2D)
N+Lc columns
The following shows an example how this block works.
Complete Box1/Box2 in the following figure. Box1 returns the first Lc columns of the input array (Use Array
Subset). Insert into Array block takes the column index from Box2.
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2.5. Remove Cyclic Prefix
We will implement a block that removes the cyclic prefix. This block will remove the last Lc columns from the
array.
OFDM_remove_cp.gvi
Terminal name
Type
Description
Input
Length of CP (Lc)
Unsigned integer
Input
Complex double array
(2D)
N+Lc columns
Output
Output
Complex double array
(2D)
N columns
The following shows an example how this block works.
Complete Box1 in the following figure to remove the last Lc columns.
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2.6. Generate OFDM Preamble
We will implement a block that generates two OFDM preambles. The block first creates (N-K) random BPSK
symbols (in the frequency domain) and inserts K-null tones. After the insertion, it performs IFFT to transform the
symbols in the time domain.
OFDM_preambles.gvi
Terminal name
Type
Description
Input
Block size (N-K)
Unsigned integer
N-K length
Null Tones
Unsigned integer array
(1D)
K-location to
remove the null
tones
Output
Preambles
Complex double array
(1D)
N length
Complete Box1 in the following figure to create two OFDM preambles. Normalize the IFFT outputs by
multiplying
√
𝑵
.
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2.7. Add OFDM Preambles
We will implement a block that adds two OFDM preambles at the beginning of the OFDM packet. Note that the
packet is serialized by P2S block, and therefore it is 1D array. Before adding the preambles, add the cyclic prefix
at the end of them. Use OFDM_preambles to generate the preambles.
OFDM_add_preambles.gvi
Terminal name
Type
Description
Input
Packet in
Complex double array (1D)
N-K length
Block size (N-K)
Unsigned integer
N-K length
Null Tones
Unsigned integer array (1D)
K-location
to remove
the null
tones
Length of CP (Lc)
Unsigned integer
Output
Packet out
Complex double array (1D)
2N length
2.8. Frame Sync
In this lab, we will re-use our old cross-correlation frame sync block since we already have it. We encourage you
to implement the sliding window method discussed in the lecture, if you are interested in.
You may need some modifications in your old frame sync block. First, make sure it takes CDB-type inputs.
Second, normalize the correlation to make the detection independent with the input signal level. Finally, replace
the old Barker sequence with the OFDM preambles.
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2.9. OFDM Channel Estimation
We will implement a block that estimates the channel in the frequency domain.
OFDM_ChEst.gvi
Terminal name
Type
Description
Input
Block size (N-K)
Unsigned integer
N-K length
Null Tones
Unsigned integer array (1D)
K-location
to remove
the null
tones
Received preambles
Complex double array (1D)
Two
preambles
Output
Channel estimate
Complex double array (1D)
Recall that the channel can be estimated by using the preambles as following:
where
𝑿(𝒇)
is the original transmitted preamble and
𝒀
𝟏
(
𝒇
)
, 𝒀
𝟐
(𝒇)
are the received two preambles.
Complete Box1/Box2 in the following figure. Box1 performs averaging two received preambles and transforms
FFT to convert into the frequency domain. Box2 divides the FFT of the received preambles by the original
preamble. Note that
𝟏/
√
𝑵
is used for the normalization factor for FFT.
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2.10. OFDM Channel Correction
We will implement a block that corrects the channel. This block simply divides the received symbols
𝒀(𝒇)
by the
channel response
𝑯(𝒇)
.
OFDM_ChCor.gvi
Terminal name
Type
Description
Input
Received symbols
Complex double array (2D)
Channel estimate
Complex double array (1D)
N length
Output
Channel corrected symbols
Complex double array (2D)
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3. Putting Together in Simulation
Before proceeding further, make sure all the blocks work in correct way. In this section, we will simulate OFDM
Tx/Rx to verify all the blocks we created work together.
1. Use any modulation scheme (e.g. 4QAM) and do bit-to-symbol mapping.
2. Connect the blocks as shown in Figure1. Make sure all the dimensions are correct each step.
3. FFT/IFFT blocks in Figure1 should process 2D array in row-by-row. Use for loop and auto-indexing to
get a row from 2D array. Use a proper normalization factor for FFT/IFFT.
4. Try a simple case for the block size, CP length, and Null tones. Verify the final output symbols at the Rx
is same as the original symbols at the Tx.
3.1. Question
3.1.1. (Null Tones) We want FFT size N as 128 and K as 8. What is the right index values for Null
tones to avoid the DC bin and the guard bins? Explain why.
3.1.2. (Cyclic Prefix) One advantage of CP is that it preserves orthogonality by allowing some
misalignment. Add an alignment error at the detected peak index from the frame sync block (e.g. if
you detected 100 as a peak, add 1 to make it 101). Use the following configuration.
• Mod type = 4QAM
• Message length = 1000bits
• Block size = 60
• Length of CP = 8
• Null tones = [0,31,32,33]
• Misalignment error = +1
1) Compare the demodulated constellation at the receiver with CP and without CP (length 0). 2)
Increase the misalignment error until the demodulation starts to fail. What value is the
misalignment error?
3.1.3. (Channel Correction) To verify the channel estimation/correction work, convolve the transmitted
symbols with the channel coefficients (time-domain). Use the following configuration.
• Mod type = 4QAM
• Message length = 1000bits
• Block size = 60
• Length of CP = 8
• Null tones = [0,31,32,33]
• Channel coeff. = [0.5+0.3i, 0.01+0.1i, -0.01+0.2i, 0.02-0.05i]
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4. Putting Together in USRP
In this section, we will implement OFDM in USRP.
1. Remove upsampling/pulse shaping in Tx and downsampling/match filter in Rx. In Tx, connect the output
of OFDM_add_preambles to USRP write Tx data block. In Rx, the received samples connects to frame
sync block.
2. In Rx, make sure you capture two OFDM packets (before it was two frames). You need to count all the
overhead (CP, Null tones, preambles).
4.1. Question
Use the following configuration.
• Mod type = 4QAM
• Message length = 1000bits
• Block size = 60
• Length of CP = 8
• Null tones = [0,31,32,33]
• IQ rate = 400kHz
• Carrier = 1GHz
4.1.1. What is the total overhead in one OFDM packet?
4.1.2. What is the data rate?
4.1.3. What is the subcarrier width?
4.1.4. Report the following figures.
• Demodulated constellation on data without channel correction
• Demodulated constellation on data with channel correction
• Demodulated constellation on preamble without channel correction
• Demodulated constellation on preamble with channel correction
4.1.5. Repeat Q.4.1.4 with CP length 30.
