Binary Amplitude Shift Keying Computer Science Essay

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Pseudo Random Binary Sequence consists of band limiting a digital signal before transmitting it over a band limited channel. Digital transmission refers to the transmission of square waves. However this is a problem since real channels are always band limited yet square waves have unlimited frequency content. So, what is done is that the signal is filtered on the transmission side and then sampled on the receiver side. In this way, the square wave becomes band limited while still being possible to retrieve after demodulation. The figure below shows the model used to construct PRBS.

D:\Daniel's Briefcase\Daniel\University\Engineering 3rd Yr\Communications for Engineers\LABS\Comms Labs Pics\Digital Modulation\PRBS\circuit.jpg

Figure - PRBS Model

Figure 2 shows the spectrum of the Square Wave which goes well beyond frequencies of 600Hz. On the other hand, Figure 3 shows the Band limited Square Wave, whose frequencies greatly diminish beyond 600Hz.

Figure - Square Wave Frequency Spectrum

Figure - Band Limited Square Wave Spectrum

Figure - Scope Output

In Figure 4 above; the input random number and the received output from the sample and hold circuit are shown at the top. The band limited signal and the pulses sent to the sample and hold circuit are shown in the bottom plot.

Binary Amplitude Shift Keying (BASK) with Synchronous Demodulation

Binary Amplitude Shift Keying represents a basic digital modulation technique where a carrier signal is turned on and off depending on the binary value of the data being transmitted. This is done by multiplying the binary value with the carrier. However due to discontinuities the resulting signal would have an infinite frequency spectrum, so the data signals is first band limited by using a digital filter. In this way, the bandwidth is greatly reduced as shown in Figure 6 and Figure 7. Figure 5 shows the model used for BASK modulation with Synchronous Demodulation.

Figure - Model of BASK with Synchronous Demodulation

Figure Spectrum Scope 1- Frequency Spectrum of Binary Data Signal

Figure Spectrum Scope 2- Frequency Spectrum of Band limited Binary Data Signal

The BASK modulated signal spectrum obtained after the multiplication is shown in Figure 8.It is quite similair to the frequency spectrum obtained with Double Side Band Large Carrier modulation and so it is very easy to be transmitted. The signal is then multiplied by the same carrier signal and hence and envelope of the signal is obtained. A digital filter is used to retrieve the original binary data signal. Note that carrier frequency should ideally be larger than the bit rate of the data signal so that signal recovery is easier.

Figure - Spectrum Scope 3 - Frequency Spectrum of Modulated BASK signal

Figure - Spectrum Scope 4 - Frequency Spectrum of Demodulated Signal prior to Filtering

Figure - Spectrum Scope 5 - Frequency Spectrum of Demodulated Signal after Filtering

Figure - Scope 1 Output

Figure - Scope 2 Output

Figure - Scope 3 Output

Binary Amplitude Shift Keying (BASK) with Asynchronous Demodulation

The model shown in Figure 14 performs the same modulation technique as that shown above. However, the demodulation is asynchronous instead of synchronous. The difference is that instead of multiplying the modulated signal by the carrier signal, the signal is rectified. Then the envelope signal is obtained and the signal is filtered to obtain the original binary data signal.

Figure - Model of BASK Modulation with Asynchronous Demodulation

Figure Spectrum Scope - Frequency Spectrum of Binary Signal

Figure Spectrum Scope 1- Frequency Spectrum of Band limited Binary Signal

Figure - Spectrum Scope 2 - BASK Modulated Signal

Figure - Spectrum Scope 3-BASK Demodulated Signal before Filtering

Figure - Spectrum Scope 4 - Demodulated Signal after Filtering

Figure - Scope 2 Output

Figure - Scope Output

Figure - Scope 1 Output

Binary Phase Shift Keying (BPSK)

Binary Phase Shift Keying is a modulation technique where the signal is keyed to different parameters to indicate either a one or zero. The parameter this time is the phase of the carrier signal. The square wave neing modulated is first centered such that it goes from -1 to 1 and then the same procedure as for BASK with synchronous modulation is done. Whereby the signal is multiplied by the carrier and filtered. Figure 23 shows the model used for BPSK modulation and demodulation.

Figure -Model of BPSK modulation and demodulation

Figure - Spectrum Scope 1- Frequency Spectrum of Square Wave

Figure - Spectrum Scope 2- Frequency Spectrum of Band limited Square Wave

Figure - Spectrum Scope 3 - Frequency Spectrum of BPSK Modulation

Figure - Spectrum Scope 4 - Demodulated BPSK signal prior to filtering

Figure - Spectrum Scope 5 - Demodulated BPSK signal after filtering

Figure - Scope 1 Output

Figure - Scope 2 Output

Figure - Scope 3 Output

Binary Frequency Shift Keying (BFSK)

In Binary Frequency Shift Keying, frequency is varied according to the binary data being transmitted. The circuit is very similair to BASK but an extra frequency is multiplied with the inverse of the data. Both signals are then added together. Demodulation is then undertaken by seperating these two signals. Each branch filters a particular frequency used at the transmitter, then squares this signal in order to obtain the envelope. Then one signal is subtracted from the other in order to retrieve the original signal centred at zero. Figure 32 below shows the model used for BFSK.

Figure - Model of BFSK Modulation and Demodulation

Figure - Scope Output

Figure - Spectrum Scope 1 - Frequency Spectrum of Modulated Signal

Figure - Spectrum Scope 2 - Frequency Spectrum of Demodulated Signal 1

Figure - Spectrum Scope 3 - Frequency Spectrum of Demodulated Signal 2

Figure - Spectrum Scope 4- Frequency Spectrum of Output Signal 2

16 Quadrature Amplitude Modulation (16-QAM)

Quadrature Amplitude Modulation is different from the other forms of modulation shown above. This is because the keying schemes may only transmit one bit at a time. However, 16-QAM is able to transmit four bits at a time. This can be done because each sequence of bits is given a magnitude and phase.

QAM modulation and synchronous demodulation are done with by using the model below. Data1 consists of the first two bits of data. It is multiplied by a carrier which is in phase. Data2 consists of the other two bits of data. It is multiplied by a carrier which is phase shifted by 90. Both signals are then added together to form the 16-QAM modulated signal. For demodulations The modulated signal is multiplied either by another in phase carrier or a phase shifted one. Then digital filtering is used to retrieve each of the two bits of data.

Figure - 16 QAM Model

Figure -Output of Scope

Figure - Spectrum Scope: 16 QAM Modulated Signal Spectrum

Figure -Spectrum Scope 1: In Phase Demodulated Spectrum

Figure -Spectrum Scope 2: Quadrature Demodulated Spectrum

Figure -Spectrum Scope 3: Filtered In-Phase Spectrum

Figure -Spectrum Scope 4: Filtered Quadrature Spectrum

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