Question Assignment

Q 1. i. This part looks at using MATLAB to simulate direct sequence spread spectrum.

Investigate the performance of direct sequence spread spectrum by looking at the manipulation of data as opposed to signals. The data is represented using the format -1,1 and multiplication will be the method used. The spreading codes are based on the Hadamard matrix.

Now explore the technique using a signal based version of the data using bi-polar non return to zero signalling. Save your file for this part as, your_nameQ1i for downloading onto Blackboard.

ii. Consider the following block diagram;

The data signal is rectangular with a height of one with a bit period of 10 µs and is spread using a spreading code that uses 1 and -1 for the binary representation and is four bits long. The spreading signal is also rectangular with a height of one. The carrier is a sinusoid with amplitude one and frequency 1.6 MHz. Explain using accurately labelled sketches the working of this and the effect of an extra signal entering multiplier 1 that is a sinusoid with amplitude 1/10 and frequency 1.6 MHz and random phase. iii.

A spread spectrum system utilises a method whereby a rectangular data signal of height five width 0.1s and data rate 10 Mbit/s is spread using a spreading factor of four. The spreading signal is also rectangular with a height of one and is non return to zero. Sketch, based on a, time based and frequency based, baseband description; the data signal, the spreading signal and the spreading of a single data bit. The data bit has an amplitude of 1 and the format of the spreading code is 1, -1, 1 and -1. [35 marks] 2.

i. A basic multicarrier modulation method has the following specifications. Symbol rate 1 symbol/s.

Channel number Carrier frequency/Hz Modulation Data (one symbol) 1 32 BPSK 1

2 33 BPSK -1 3 34 BPSK 1

4 35 BPSK -1 Simulate in MATLAB modulation of the four carriers individually with the relevant data and demodulate using the relevant carrier. Now look at simulating. Symbol rate 1 symbol/s. Channel number Carrier frequency/Hz Modulation Data (one symbol) 1 64 BPSK 1 2 66 BPSK -1 3 68 BPSK 1

4 70 BPSK -1

Save your file for this part as, your_nameQ2i for downloading onto Blackboard. Analyse each method and compare with the use of a single carrier method in mobile communications.

ii). Assuming the spectrum of each modulated carrier can be represented by a sinc function, plot the spectrum over a range of 30 Hz to 37 Hz for the first method in part i). State any assumptions required for this to accurately represent the spectrum.

iii). A multicarrier modulation method uses a symbol period of 10 µs. and a carrier frequency of 0.4 MHz. During propagation the received signal is comprised of a direct and reflected ray. The reflected ray incurs a delay of 0.00625µs. Using time based signals show the effect of this. Now look at adding an extra time period of 0.0125µs before the onset of the symbol and describe the effect of this. [35 marks]

3. a). A transmitter of height 45 m is located at a distance of 45 m from a 75 m high diffraction screen. A receiver of height 45m is 85 m from the screen. A signal of frequency 900 MHz and power 15 W is transmitted.

Determine: The complex diffraction coefficient. and diffraction angle. The amplitude attenuation due to diffraction (magnitude of complex diffraction coefficient). The received power due to diffraction.

Note use your best estimate when reading the real and imaginary components of the diffraction coefficient Ad

Demonstrate how the Fresnel integral given in the lecture notes would be used to determine a value for v = 12?

b). Figure 3 depicts two antennas of a radio based communication system. A signal of frequency 750 MHz is transmitted between the antennas. Explain if the path loss between the two antennas can be described by the following expression and discuss the use of this. With reference to appropriate wave propagation concepts give reasons for your explanation.

Figure 3

c). The following questions refer to the equation where x represents a Gaussian distribution. i) Show that  is a lognormal distribution.

ii). Determine the value of  if the associated Gaussian distribution has a mean of 0 and a standard deviation of 1 where the value of  is when the Gaussian mean occurs.

iii) Using information from the previous answers can you identify the following plots and how they have been produced? Note plot1 has been generated using a formula and plots 2 and 3 have been generated using data from one of the other plots.

v). Consider the following expression that describes the attenuation of a radio based signal x(z) as it propagates through a material described by a factor .

where x(0) is the signal incident on the material. Discuss the effect of this and consider a mobile signal that encounters a large number of objects over the area of reception, each having the same value of . d). The spread sheet assignment.xls - data 1 loaded on Blackboard contains signal levels received (column B) when an impulse was transmitted, over a mobile communications channel, against a time scale (column A) in seconds with the transmitter and receiver fixed in position over the time scale considered. i). Plot this data and comment on the shape of this graph and the reasons behind the shape. Include the plot in your assignment. ii). Determine a suitable value for the delay imposed by this channel.

iii). Explain what extra information is needed to describe a mobile channel and how this would be used.

iv). Plot the data in assignment.xls - data 2 and explain the shape based on the labels given for each column. Include the plot in your assignment.