INTRODUCTION TO WICOMM-T Benchmark WiCOMM-T is a wireless digital communication system with a pluggable 70 MHz IF or 2.4 GHz RF module. The system has a transmitt er and a receiver and provides loop-back options at baseband and 70 MHz IF. Using the PC's USB port, it has an interface to MATLAB, which provides maximum flexibility in learning complete digital communication concepts such as digital modulation techniques, baseband equalization, filtering concepts, and the basics of CDMA, GSM, and OFDM and software defined radio, FM radio reception, turbo and LDPC channel coder/decoder, V.32 modem, frequency hopping spread spectrum, and discrete multitone modem — modem — on on their own. The system also enables users to learn concepts such as clock slip and control and timing acquisition. Users can write algorithms for digital communication concepts in MATLAB and validate their code using WiCOMM-T. WiCOMM-T Setup 1. Baseunit Connection
Fig1: WiCOMM-T Base Unit
2. Baseband Connection 2.1 Loopback (Using Single WiCOMM-T) WiCOMM-T)
Connect two BNC-BNC cables (Part # 9932002) to IF Module for connecting baseband loop back as shown in figure below – below –
Fig2: IF module showing baseband loopback
3. RF Connection: 3.1 Working with RF link using two WiCOMM-Ts
Fig3: RF Link using two WiCOMM-Ts
Performing Experiments
1. Open the MATLAB and type WiCOMM_T in command window, WiCOMM_T console window will open. 2. Press ‘RUN’ button from the WiCOMM_T console. This will open the WBU console.WBU console is used to transmit and receive modem samples through WiCOMMT. 3. Choose the desired sampling rate using the sampling rate pull down menu as shown in the figure. Pressing
button will open the configuration window as shown in Figure.
Figure: WBU Console
Figure:WBU_Configuration Window 4. In Configuration window select the WBU driver file, mode of operation, and Tx/Rx operation selections. Tx& Rx mode of operation will be used in loopback mode.‘Tx only’ or ‘Rx only’ mode of operation will be used while two WiCOMM -T setup have to be connected together by keeping one of them to be Tx and the other to be Rx.
5. Now WiCOMM-T is ready to transmit and receive the samples. 6. Press ‘EXPERIMENT’ button in WiCOMM-T WiCOMMT_EXPERIMENT Console (WEC).
console.
This
will
open
Figure:WiCOMMT_EXPERIMENT Console
7. Press ‘GENERATE’ button which will generate the test samples to be transmitted for the WiCOMM-T Installation test. These generated samples have to be transmitted and received through WiCOMM-T using WBU console. 8. Press Start button in WBU console to start transmitting and receiving the test samples through WiCOMM-T. The Tx icon and the Rx icon start blinking in blue indicates that WiCOMM-T is transmitting and receiving properly. This can be ensured by looking at the statistics window. 9.
After sufficient samples, say around 50,000 packets are collected press the Stop button to stop transmitting and receiving the samples.
10. The received samples can be analysed now using the WEC by pressing the ‘ANALYZE’ button
Experiment : QPSK Aim
To simulate QPSK transmitter and receiver taking into account the phase and the frequency offset. Theory Phase shift keying
For binary PSK (BPSK) S0(t) = A cos(wt) represents binary “0” S1(t) = A cos(wt + p) represents binary “1” For M-ary PSK, M different phases are required, a nd every n (where M=2n ) bits of the Binary bit stream are coded as one signal that is transmitted as Asin(wt + qj) j=1,..., M. QuadraturePhase Shift Keying
If we define four signals, each with a phase shift differing by 900 then we have Quadrature phase shift keying (QPSK). The input binary bit stream {dk}, dk = 0,1,2,..... Arrives at the modulator input at a rate 1/T bits/sec and is separated into two data streams dI (t) and dQ (t) containing odd and even bits respectively. dI(t) = d0, d2, d4 ,... dQ(t) = d1, d3, d5 , ... A convenient orthogonal realization of a QPSK waveform , s(t) is achieved by amplitude modulating the in-phase and quadrature data streams onto the cosine and sine functions of a carrier wave as follows: s(t)=1/ 2 dI(t) cos (2pft + p/4) + 1/ 2 dQ(t) sin (2pft + p/4) Using trigonometric identities this can also be written as s(t)=A cos [2pft + p/4 + q(t)]. The pulse stream dI(t) modulates the cosine function with an amplitude of ± 1. This is equivalent to shifting the phase of the cosine function by 0 or p; consequently this produces a BPSK waveform. Similarly the pulse stream dQ(t) modulates the sine function, yielding a BPSK waveform orthogonal to the cosine function. The summation of these two orthogonal waveforms is the QPSK waveform. The values of q(t) = 0, -(p/2), p/2, p represent the four possible combinations of aI(t) and aQ (t).
Each of the four possible phases of carriers represents two bits of data. Thus there are two bits per symbol. Since the symbol rate for QPSK is half the bit rate, twice as much data can be carried in the same amount of channel bandwidth as compared to BPSK. This is possible because the two signals I and Q are orthogonal to each other and can be transmitted without interfering with each other.
In QPSK the carrier phase can change only once every 2T secs. If from one T interval to the next one, neither bit stream changes sign, the carrier phase remains unchanged. If one component aI(t) or aQ (t) changes sign, a phase change of components change sign then a phase shift of
occurs.
/2
occurs. However if both
If a QPSK modulated signal undergoes filtering to reduce the spectral side lobes, the resulting waveform will no longer have a constant envelop and in fact, the occasional 180o shifts in phase will cause the envelope to go to zero momentarily. Offset Quadrature Phase Shift Keying
If the two bit streams I and Q are offset by a 1/2 bit interval, then the amplitude Fluctuations are minimized since the phase never changes by 180o . This modulation scheme, Offset Quadrature Phase shift Keying (OQPSK) is obtained from QPSK by delaying the odd bit stream by half a bit interval with respect to the even bit stream. Thus the range of phase transitions is 0o and 90o (the possibility of a phase shift of 180o is eliminated) and occurs twice as often, but with half the intensity of the QPSK. While amplitude fluctuations still occur in the transmitter and receiver they have smaller magnitude. The bit error rate for QPSK and OQPSK are the same as for BPSK.
When an OQPSK signal undergoes band limiting, the resulting intersymbol interference causes the envelop to drop slightly to the region of 90o phase transition, but since the phase transitions of 180 have been avoided in OQPSK, the envelop will never go to zero as it does in QPSK.
Procedure
1. 2. 3. 4. 5. 6. 7.
Connect WiCOMM-T for baseband loop back. Generate the transmitter modem sample. Transmit and receive the modem sample through WiCOMM-T. Analyze the received modem samples. Observe the various plots generated by MATLAB. Connect WiCOMM-T in IF loop back. Repeat steps 3 to 5 for IF loop back
8. Connect 2 WiCOMM-Ts such that one as transmitter and other as receiver in baseband level. Repeat steps 2 to 5. 9. Connect the 2 WiCOMM-Ts in IF mode and repeat steps 2 to 5.
Transmitter
1. RRC pulse of duration to (where is the bit duration) is generated. The value of is assumed to be large enough that the pulse decays to negligible values within
2. 3. 4. 5. 6.
Random data to be transmitted is generated. Random data is QPSK modulated. The random data is up sampled by 8 in one bit duration. Modulated data is convolved with the RRC pulse to obtain the pulse shaped bits. Frequency offset and noise is added. This is done to show the effect of frequency offset and noise in the received samples using baseband loop back. 7. Pulse shaped bits are given to the WiCOMM-T Tx interface block to send it through WiCOMM-T.
Receiver
Receiver block diagram in MATLAB
1. The samples are received from the WiCOMM-T Rx interface as blocks 2. The coarse frequency offset estimation is done using 4 th - power algorithm on the first block 3. The coarse frequency offset in the received samples are corrected using the estimated offset value
4. Interpolation is done using the polyphase RRC filter and the best sampling instant is found using the non-coherent energy averaging method
5. Phase offset estimation is done using 4 th - power algorithm 6. The phase offset in the received samples is corrected using the estimated value. 7. The early-late interpolators are used in ever y subsequent block of samples to track the best sampling instant.
8. Residual frequency offset is corrected by tracking the residual phase using the LMS algorithm
9. Constellation diagram before frequency offset estimation, constellation diagram after coarse frequency correction, error curve for LMS convergence and the constellation diagram of the filtered signal after clock tracking are plotted.
Observation: Constellation diagram
An ideal constellation diagram shows the discrete symbol representation of a digital modulation scheme in terms of its vector components. In case of an ideal transmission channel free of noise and interference, all symbols are recognized by the demodulator without errors. In this case, they are represented in the constellation diagram as well defined points overlapping in the same area and forming a clear dot. Noise and impairments cause the demodulator to not always read the symbols correctly. In this case the positions of the points disperse and create different shapes.
After timing recovery and MF the constellation points got grouped properly in the 4 quadrants. Because of no AGC action, points are not grouped at ±1. AGC action is also built into the LMS algorithm.
Plot shows the constellation after timing recovery and MF.
The small frequency error
introduced at the transmitter side results in small arcs. Since frequency offset will also results in some amount of phase offset, the arcs are rotated.
Plot shows the constellation after timing recovery and MF. introduced at the transmitter side results in a ring for mation.
The large frequency error
Due to a phase offset of
given at the transmitter side, the constellation plot above shows
the tilted points by approximately the same offset.
