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B 3 I G M atlab Code

Function call

To generate the received signal, the B 3 I G Transmitter is called by

[r, s]=BigTransmitter(m, frameParams...              ..., rfParams, chanParams);

where m is the message (or messages, in the case of multiple users) to be transmitted, frameParams is a structure containing the parameters relating to the frame structure, rfParams contains the analog/RF related parameters, and chanParams contains the channel parameters. There are two output arguments: r is the received sampled signal and s is the 4-PAM symbols which were transmitted. Thus, the length of s is approximately equal to the length of r times the effective oversampling factor.

The input parameters

In order to control all these features of the B 3 I G , a number of parameters are needed, as summarized in [link] .

B^3IG Parameters passed via the stuctures frameParams , rfParams , and chanParams .
B 3 I G Parameters passed via the stuctures frameParams ,rfParams , and chanParams.

A more detailed description of the parameters follows:

  • m : A character string matrix, with dimensions equal to the number of users by the message length, containing the ASCII messages to be transmitted. If there is only a single user, this is a single string (row vector).
  • frameParams.userDataLength : The number of text characters per user per frame which (together with frameParams.preamble ) effectively determines the length of a frame. See [link] .
  • frameParams.preamble : A character string (i.e. a row vector) containing the preamble text which is presumed known at all receivers.
  • frameParams.chanCodingFlag : This parameter can be set to either 0 or 1. If set to 0, thetransmitter sends uncoded bits. If set to 1, the transmitter encodes the data using the binary block (5,2) channel code described in [link] .
  • frameParams.bitEncodingFlag : This parameter can be set to either 0 or 1, and controls how thetransmitter translates text into bits. If set to 0, the transmitter uses an 8-bit representation of the ASCII code, and employs letters2pam.m . If set to 1, the transmitter uses the 7-bit representation of the ASCII code, and employs text2bin.m . See Examples [link] and [link] for details.
  • chanParams.c1 : The impulse response of the time-varying channel at the start of the transmission is specified as a row vector.
  • chanParams.c2 : The impulse response of the time-varying channel at the end of the transmission is also a row vector. The actual channel varies linearly over the course of the transmission from the impulse response in chanParams.c1 to that in chanParams.c2 . Hence, if chanParams.c1 = chanParams.c2 and chanParams.randomWalkVariance =0, the channel will be time invariant.
  • chanParams.randomWalkVariance : In addition being able to control the evolution of the channel using chanParams.c1 and chanParams.c2 , this parameter specifies a random variation in the channel taps using a random walk. The parameter controls the variance of the random walk process.
  • chanParams.SNR : The signal to noise ratio of the signal. Note that it only controls the amount of AWGN present in the band of the transmitted signal.
  • chanParams.adjacentUser1Power : The relative power of the interference caused by the first adjacent user, specified in dB. Thus, when set to 0 dB the interferer will have the same power as the transmitter. When set to -Inf, the interferer is effectively disabled.
  • chanParams.adjacentUser1f_if : The carrier frequency of the first interfering user after conversion to IF.
  • chanParams.adjacentUser1Chan : The time-invariant channel of the first interfering user.
  • chanParams.adjacentUser2Power : The relative power of the second interfering user, specified in dB.
  • chanParams.adjacentUser2f_if : The carrier frequency of the second interfering user after conversion to IF.
  • chanParams.adjacentUser2Chan : The time-invariant channel of the second interfering user.
  • chanParams.NBIfreq : The frequency of a narrowband inteferer, after conversion to IF.
  • chanParams.NBIPower : The relative power of the narrowband interferer, specified in dB.
  • rfParams.f_s : The sampler frequency.
  • rfParams.T_t : The nominal (i.e. expected) symbol period.
  • rfParams.T_t_err : The percent error in the transmitter's symbol period, which is presumed unknown at the receiver.
  • rfParams.f_if : The assigned IF carrier frequency (or frequencies).
  • rfParams.f_if_err : The percent error in the transmitter's local oscillator, which is presumed unknown at the receiver.
  • rfParams.phaseNoiseVariance : The variance of the additive noise used to model phase noise in the transmitter.
  • rfParams.SRRCLength : The length of the square-root raised cosine filter.
  • rfParams.SRRCrolloff : The rolloff factor of the square-root raised cosine filter.

Overview of the BigTransmitter.m Code

While the B 3 I G Transmitter is a bit more sophisticated than the simple communications systems of [link] , it still consists of the same basic operations. This section provides a high-level outline of the code in BigTransmitter.m , which can be compared with idsys.m of [link] . Peeking inside the B 3 I G Transmitter code, you will find many familiar operations. In addition, acquainting yourself with the inner workings of code will help prepare you for the receiver design challenges that await. Line numbers of BigTransmitter.m are indicated in parentheses:

  • (3-13) Parse message, convert to bits, perform coding if requested, insert preamble, and generate 4-PAM sequence
  • (17-40) Create adjacent user signals
  • (41-44) Add in main transmission, narrowband interference, adjacent users, and AWGN
  • (3-6) Calculate the transmitter's “erroneous” sample rate and IF frequency
  • (17-19) Upsample signal, perform pulse-shaping
  • (20-22) Generate phase noise process
  • (23-26) Mix signal to IF, while incorporating phase noise
  • (27-29) Pass signal through channel (time invariant case)
  • (30-43) Pass signal through channel (time-varying case)
  • (45-47) Perform downsampling and normalization (i.e. AGC)

[link] shows a typical received signal and its spectrum as generated by B 3 I G . The desired message is centered at 10 MHz and has a 1 MHz absolute bandwidth. As seen in the figure, there is an interfering adjacent user centered at baseband with a 1 MHz absolute bandwidth. Assume that the desired user's signal was transmitted at an RF frequency of f R F = 300 MHz and that the local oscillator shown in [link] operates at f L O = 90 MHz. Assume the Preselect and IF filters are ideal bandpass filters from 250 - 350 MHz and 200 - 260 MHz respectively. Use the transmitter script in BigEx2.m to generate a received signal identical to [link] . Note that three parameters have been left incomplete, as indicated by the question marks in BigEx2.m . Determine the IF frequencies of the desired user and adjacent interfering user, and the sampling frequency f s in order to generate the signal in [link] . Plot the spectrum of the signal you generate.

Magnitude Spectra of Received Signal r[l]
Magnitude Spectra of Received Signal r [ l ]

The script in BigEx3.m generates an ideal received signal much like idsys.m from Chapter [link] . This time, however, the transmitter sends signals for two users using TDMA, where frameParams.userDataLength=5 . Mimic the receiver code in BigIdeal.m to accurately decode the signal for each user. Write your receiver code so that it stores the message for the first user in a variable called decoded1 , and the message for the second user in a variable called decoded2 .

Notes on debugging and signal measurement

In experimenting with the B 3 I G Transmitter, you may find that it will be very useful to look closely at intermediate signals the are inside the transmitter function. There are a variety of ways this can be accomplished. Some possibilities are:

  • Add more output arguments to Tx_tdma so that the variables are available in the M atlab workspace.
  • Use the debugging capabilities of M atlab , including breakpoints, step execution, etc.
  • Add plotting commands within the transmitter code.

Questions & Answers

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Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
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