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This report organized into five chapters. Chapter one is the introductory chapter contain a simple overview on the code division multiple access technique, project objective, and report organization. Chapter two studied the pulse modulation techniques and focus toward the pulse position modulation technique as a good modulation technique for the use of optical fiber as a transmission channel. Chapter three contain an overview on the optical link components, such as: optical sources, types and there characteristics. Optical fiber cables, types and limitations, and photodetectors types and characteristics. In chapter four focuses oriented toward the CDMA system basic principles and futures. Also the channels used in CDMA systems will be studied in this chapter. Finally the codes used in CDMA system are studied and make some simple examples to show how the basic principle of codes design and orthognality check. Chapter five is the system design; the design will be done using the ORCAD tools. Such as: driver circuit of optical source, pre-amplifier of optical receiver, PPM circuit design, CDMA codes (i.e short and long codes), CDMA transmitter and CDMA receiver.
In continuous-wave (CW) modulation, some parameter of a sinusoidal carrier wave is varied continuously in accordance with the message signal. On the other hand, in pulse modulation, some parameter of a pulse train is varied in accordance with the message signal. We may distinguish two families of pulse modulation: analog pulse modulation and digital pulse modulation. In analog pulse modulation, a periodic pulse train is used as the carrier wave, and some characteristic feature of each pulse (e.g., amplitude, duration, or position) is varied in a continuous manner in accordance with the corresponding sample value of the message signal. Thus in analog pulse modulation, information is transmitted basically in analog form, but the transmission takes place at discrete times. In digital pulse modulation, on the other hand, the message signal is represented in a form that is discrete in both time and amplitude, thereby permitting its transmission in digital form as a sequence of coded pulses; this form of signal transmission has no CW counterpart.
The use of coded pulses for the transmission of analog information- bearing signals represents a basic ingredient in the application of digital communication. This chapter may therefore be viewed as a transition from analog to digital communications in our sampling process, which is basic to all pulse modulation systems, whether they are analog or digital.
2.2 forms of pulse modulation
1- pulse amplitude modulation
2- pulse width modulation
3- pulse position modulation
2.3 Pulse- Amplitude Modulation:
Now that we understand the essence of the sampling process, we are ready to formally define pulse-amplitude modulation, which is the simplest and most basic form of analog pulse modulation. In pulse-amplitude modulation (PAM), the amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of a continuous message signal; the pulses can be of a rectangular form or some other appropriate shape. Pulse-amplitude modulation as defined here is somewhat similar to natural sampling, where the message signal is multiplied by a periodic train of rectangular pulses. However, in natural sampling the top of each modulated rectangular pulse varies with the message signal, whereas in PAM it is maintained flat; natural sampling is explored further in Problem 2.2.
The waveform of a PAM signal is illustrated in Figure 2.1. The dashed curve in this figure depicts the waveform of a message signal m(t), and the sequence of amplitude modulated rectangular pulses shown as solid lines represents the corresponding PAM signal s(t). There are two operations involved in the generation of the PAM signal:
1- Instantaneous sampling of the message signal m(t) every Ts seconds, where the sampling rate fs=1/Ts is chosen in accordance with the sampling theorem.
2- Lengthening the duration of each sample so obtained to some constant value T.
In digital circuit technology, these two operations are jointly referred to as ï¿½sample and hold.ï¿½ One important reason for intentionally lengthening the duration of each sample is to avoid the use of an excessive channel bandwidth, since bandwidth is inversely proportional to pulse duration. However, care has to be exercised in how long we make the sample duration T, as the following analysis reveals.
Let s(t) denote the sequence of flat-top pulses generated in the manner described in Figure 2.1. We may express the PAM signal as :
where Ts is the sampling period and m(nTs) is the sample value of m(t) obtained at time t=nTs. The h(t) is a standard rectangular pulse of unit amplitude and duration T, defined as follows (see Figure 2.2a) :
By definition, the instantaneously sampled version of m (t) is given by
where (t-nTs) is a time-shifted delta function. Therefore, convolving m (t) with the pulse h(t), we get :
Using the shifting property of the delta function, we thus obtain
From Equations (2.1) and (2.5) it follows that the PAM signal s(t) is mathematically equivalent to the convolution of m(t), and the instantaneously sampled version of m(t), the pulse h(t), as shown by :
Taking the Fourier transform of both sides of Equation (2.6) and recognizing that the convolution of two time functions is transformed into the multiplication of their respective Fourier transforms, we get :
where S(f) = F[s(t)], = , and H(f) = F[h(t)]. Adapting Equation (2.2) the problem at hand, we note that the Fourier transform is related to the F transform M(f) of the original message signal m(t) as follows :
Where f. is the sampling rate. Therefore, substitution of Equation (2.8) into (2.7) yields :
Given a PAM signal s(t) whose Fourier transform S(f) is as defined in Equation (2.9), how do we recover the original message signal m(t)? As a first step in this reconstruction, we may pass s(t) through a low-pass filter whose frequency response is defined in Figure 2.2c; here it is assumed that the message is limited to bandwidth W and the sampling rate fs is larger than the Nyquist rate 2W. Then, from Equation (2.9) we find that the spectrum of the resulting filter output is equal to M(f)H(f). This output is equivalent to passing the original message signal m(t) through another low-pass filter of frequency response H(f).
From Equation (2.2) we note that the Fourier transform of the rectangular pulse h(t) is given by
which is plotted in Figure 2.2b. We see therefore that by using flat-top samples to generate a PAM signal, we have introduced amplitude distortion as well as a delay of T/2. This effect is rather similar to the variation in transmission with frequency that is caused by the finite size of the scanning aperture in television. Accordingly, the distortion caused by the use of pulse-amplitude modulation to transmit an analog information-bearing signal is referred to as the aperture effect.
This distortion may be corrected by connecting an equalizer in cascade with the low- pass reconstruction filter, as shown in Figure 2.3. The equalizer has the effect of decreasing the in-band loss of the reconstruction filter as the frequency increases in such a manner as to compensate for the aperture effect. Ideally, the magnitude response of the equalizer is given by
The transmission of a PAM signal imposes rather stringent requirements on the magnitude and phase responses of the channel, because of the relatively short duration of the transmitted pulses. Furthermore, the noise performance of a PAM system can never be better than baseband-signal transmission. Accordingly, we find that for transmission over long distances, PAM would be used only as a means of message processing for time- division multiplexing
2.4 PULSE MODULATION
The basis of a pulse modulation system is the sampling process whereby a continuous time can be converted into a corresponding sequence of samples that may or may not be uniformly spaced in time. The converted signal is called a discrete time signal. In practice however, the samples are taken to be uniformly spaced in time. This is called uniform sampling. Uniform sampling offers two advantages .It leads to simpler system design and simpler algorithms for discrete time signal processing. its customary to represent the experimental data and mathematical function in the form of continuous curves. These curves are usually obtained by using a finite number of discrete points or samples .if the samples points are sufficiently close, itï¿½s possible to draw a smooth curve and the intermediate values can be interpolated to any degree of accuracy. This means that a continuous curve can be adequately described by sampling points. In a similar manner, an electrical signal satisfying certain requirements can be reproduced entirely from an appropriate set of sample values. since a signal can be determined from the sampled values, we can transmit only the sample values as they occur instead of sending the signal continuously. This is the essence of sampling that is used in pulse modulation.
2.5 Classification of pulse modulation
The major difference between the continuous and pulse modulation techniques that in continuous techniques the parameters of the carrier signal changes according to the input signal (message).on the other hand, in pulse modulation, some parameter of each pulse is modulated by a particular sample value of the message .here we should distinguish between two types of pulse modulation, pulse analog modulation PAM and pulse digital modulation which is popularly known as pulse code modulation PCM. A comparison of the two types of pulse modulation reveals that pulse analog modulation is discrete in time because of sampling but some characteristics feature of each pulse, amplitude ,duration or position is varied in a continuous manner in accordance with the with the pertinent value of the message .this is in contrast to pulse code modulation in which a discrete time, discrete amplitude representation is used for the signal .this is achieved through sampling, quantization and coding of analog signal
2.6 Analogy pulse modulation
If a signal say a message signal is adequately described by its sample values it can be transmitted via analog pulse modulation, where the sample values directly modulate a periodic pulse train with one pulse for each sample. There are three types of analog pulse techniques; pulse amplitude modulation (PAM), pulse duration modulation (PDM) and pulse position modulation (PPM). PDM is also called pulse width modulation PWM. PDM or PWM and PPM are sometimes lumped together under the general category of pulse time modulation PTM. The pulse modulation should not be considered as modulation in the usual sense .The following points is important for designing pulse modulation systems.
(1) Pulse modulated waves are rich in D.C. and low frequency components. Direct transmission of pulse modulated signal is very difficult.
(2) Care must be taken to prevent the over lapping of pulses during transmission of pulse modulated wave
(3) Pulse modulated waves need reconstruction of the signal through the extraction of sample values and low passes filtration.
2.7 Pulse-Width-Modulation (PWM)
The pulse width modulation is well known in many electrical communication references as pulse-length-modulation (PLM) or pulse-duration-modulation (PDM), eventually both names are categorized under pulse-time-modulation techniques. The general idea behind behind PWM technique is to change pulse using the samples taken from the input signal (message). The pulse width may be varied by varying the time occurrence of the leading edge the trailing edge or both edges of the pulse on accordance with the sampled value of the modulating wave as it is shown in Fig.1.
A simple procedure for generating PWM wave is illustrated in Fig.2 in which the trailing edges of the pulses are modulated. The message signal and a saw-tooth wave are added and the combination is applied to a slicer.
An ideal slicer has the property that its output is zero whenever the input is below the slicing level and is constant whenever the input exceeds this level. It can be easily seen from Fig.2 that the duration of each pulse is dictated by the value of the message wave at the time of occurrence of the trailing edge.PWM can also be generated by using emitter coupled monostable multivibrator.
A typical arrangement is shown in Fig.3. The emitter follower monostable multivibrator is an excellent voltage-to-time converter since its gate width is dependent on the voltage to which the capacitor is charged. If this voltage can be varied in accordance with a signal voltage a series of rectangular pulses can be obtained with width varying as required. The circuit shown in Fig.3 does both the jobs of sampling the message signal and converting the samples to PWM. The demodulation of PWM is quit simple.PWM is passed through a low-pass filter for this purpose .the reconstruction is how ever associated with a certain amount of distortion caused by the cross modulation products that fall in the signal band.
2.8 Pulse-Position Modulation (PPM)
PPM may be considered as a modified version of PDM. In PDM long pulses expend considerable amount of power during the pulse while bearing no additional information .if an arrangement is made so that the unused power could be subtracted from the PDM we get a more efficient type of pulse modulation. In pulse position modulation technique each pulse is changed according to the input signal (message) without neglecting the fact that each pulse is related its time of occurrence. Pulse position modulation may be obtained from PWM as shown in Fig.4. In the PWM shown here the location of the leading edges of the pulses are kept fixed whereas those of the trailing edges are made to vary in accordance to with the message signal. The position of the trailing edges of PWM pulses are in fact position modulated. The method of obtaining PPM from PWM is thus accomplished by getting rid of the leading edges and bodies of the PWM pulses.
Figure.4 shows the message signal and shows the PWM signal. If the PWM signal shown in Fig.4, is differentiated the pulse train is obtained. Both the leading and trailing edges will define the positive and negative narrowed pulses. If the position corresponding to the trailing edge of an unmodulated pulse is counted as zero displacement then the other trailing edges will arrive earlier or later. Thus these pulses will have time displacement proportional to the instantaneous value of the signal voltage .The differentiate pulses corresponding to the leading edges can be removed by diode clipper or rectifier. The remaining pulses as shown are position modulated.
The simplest method of generating PPM wave from PWM is to use a monostable multivibrator which has one stable-state and one quasi-stable state. The monostable multivibrator can be triggered from stable to quasi-stable state by externally applied pulses. The timing circuit is designed to determine specifically the time where the monostable multivibrator will be in the state of quasi-stable. If a PWM signal is applied at the input. The output will be obviously a pulse position modulated signal whose duration will be determined by the timing circuit of the multivibrator.
For demodulation of PPM it is first converted into PWM with help of a flip-flop or bistable multivibrator. One input of the multivibrator receives trigger pulses from a local generator which is synchronized by trigger pulses received from the transmitter. These triggers are used to switch off one of the stages of the flip-flop. The PPM pulses are fed to the other base of the flip-flop and switch that state ON. The period of time during which this particular stage is OFF depends on the time difference between the two triggers so that the resulting pulse has a width that depends on the time displacement of each individual PPM pulse. The resulting PWM signal is then demodulated as already discussed.
The increased bandwidth by a pulse modulation system can be used to improve noise performance which is considered a great advantage. This can only be achieved when the message signal is used to vary some property of the pulse other than amplitude. Thus PTM is much more superior from this angle to PAM. The generation of PTM signals has become extremely simplified with availability of linear integrated circuit (LIC). Fig.5 shows an arrangement using 565PLL linear integrated circuit for generation of PWM and PPM signals. The voltage controlled oscillator (VCO) output is actually a PPM signals which has been converted to PWM signal by applying it and the input pulses to an exclusive-or gate.