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Spread Spectrum Communications: Fundamentals, Applications, and Products
4. Classification of SS Modulation Systems
Bandwidth spreading by direct modulation of signals by a wideband spread signal (also called code) is called direct sequence spread spectrum (DS SS). The DSSS signal is then modulated by a carrier before final transmission. In DSSS, the base band signals are usually called bits, and the code bits are called chips. Typically, the baseband signal bandwidth is multiplies several times by the spreading signals. In other words, the chip rate is much higher than the bit rate. The spreading signal sequence is unique for a transmitter, and the same chip sequence is used at the receiver to re-construct the signals (data bits). A mechanism, by name correlation is used to synchronize the received spread signals (that contain data) with the locally generated code. At maximum received signal strength, correlation said to have occurred. The receiver then enters the tracking mode, such that the spread signal modulated signals are received without interruption.
A simple DSSS system is described below:
1. DSSS Transmitter:
d(t) is the input data bits
c(t) is the code bits
x(t) is the frequency converted signal, ready for transmission.
A note about why frequency up conversion is required for radio transmission: Base band, and very low frequencies are susceptible to heavy attenuation during transmission. In addition, imagine every transmitter transmitting in the base band frequencies. It is practically impossible for everyone to transmit in base band frequencies (A base band frequency is the frequency spectrum that is occupied by the unmodulated signals). Hence up-conversion of frequency is normally done to comply with the transmission requirements.
In the DSSS transmitter, a code generator is a pseudo random generator that generates a known pseudo noise code sequence. Normally, the code has finite length (say 1024 chips), and repeats periodically. The requirements for a good PN code is discussed in a later section.
An XOR gate can be used for spreading the data bits. The input, code, and the resulting output are displayed in the figure below:
In the figure shown above, each data bit is coded with 8 chips. In practice, this would be much higher, of the order of 1024 or even more. Higher the number of chips per bit, higher will be the processing gain. Processing gain is defined below:
Processing Gain: One important parameter of DS SS receiver is the processing gain. Consider a data rate of 10KBPS, and Chip rate of 1MBPS. The processing gain is given by 10 log [rc/rb], where rc is the chip rate, and rb is the data rate. For a chip rate of 1MBPS, and a data rate of 1KBPS, the processing gain is 10log or 30dB. The processing gain is a measure of immunity to noise, and jamming signals. Higher the processing gain, more the band spread of the signals.
Higher processing gain results in greater immunity to noise, and interfering signals.
After spreading, the signals are unconverted and transmitted.
2. DSSS receiver:
A simplified DS SS receiver block diagram is shown below.
It consists of a PN generator that feeds the matching chip sequence to an XOR gate to reproduce the original bit sequence. The PN generator is driven by an error signal from the output of the LPF, so that chip timing is adjusted to produce maximum signal threshold. Normally, the acquisition of the data is done through a two step process. The first is acquisition, and the second is tracking. Acquisition refers to acquiring the chip timing of the received signals. This may further be sub-divided into course acquisition, and fine acquisition. The two are differentiated by the amount of chip timing adjustment. Once the acquisition is achieved, then the received signals must be tracked properly. Otherwise, you may loose the lock, resulting in loss of data bits. As with conventional receiver operation, an error voltage at the output of the LPF (or an Integrator) provides necessary correction to the PN Generator.
3. Pseudo Noise Codes (PN Codes): The PN codes used for DSSS require certain mathematical properties.
1. Maximum Length Sequences: These are PN sequences that repeat every 2n -1, where n is an integer. These sequences can be implemented using shift registers. The PN sequences must exhibit good correlation properties. Two such sequences are Barker Codes, and Willard Codes.
2. Maximum Auto-Correlation: When the received signal is mixed with locally generated PN sequence, it must result in maximum signal strength at the point of synchronization.
3. Minimum Cross-Correlation: When the received signal with a different PN sequence than that of the receiver, is mixed with the locally generated PN sequence, it must result in minimum signal strength. This would enable a DSSS receiver to receive only the signal matching the PN code. This property is known as Orthogonality of PN Sequences.
Here the transmitted signal appears as a data modulated carrier which is hopping from one frequency to next, and therefore, it is called frequency hopping spread spectrum (FH-SS). FH systems work by driving a frequency synthesizer with pseudorandom sequence of numbers that result in the synthesizer hopping different frequencies at different points, and thus achieving signal spread. At the receiver end, the same principle works. A synthesizer is driven by a matching code to achieve maximum threshold detection of received signals.
A simplified block schematic of a FH SS system is shown below:
The transmitter consists of a baseband modulator followed by frequency synthesizer. The frequency synthesizer is driven by a PN generator. A PN generator may be built internal to the synthesizer.
2. FH-SS Receiver:
A FH-SS receiver consists of a down converter followed by a demodulator. A synthesizer, driven by a matching PN generator is used to down convert the received signals. A miximum received signal threshold signifies locking.
Time hopping is not used as frequency as DSSS and FHSS. Time Hopping to spread the carrier is achieved by randomly spacing narrow transmitted pulses.
The multiplicity factor is given by:
Multiplicity factor = (average pulse spacing) / (pulse width).
In all cases of SS receivers, faithful recovery of the transmitted signals require the following:
a. Correlation Interval Synchronization: Receiving bits is achieved by proper correlator (or integrator) timing. Proper start/stop times for correlator are required for minimizing the received bit errors.
b. SS Generator Synchronization: Timing signals are required to control the SS wave form generator signals. Direct Sequence systems employ a clock ticking at the chip rate 1/tc, and FH systems have a clock operating at the hopping rate 1/th.
c. Carrier Synchronization: Faithful reproduction of the transmitted signals to baseband requires down-conversion, and demodulation. This can be achieved only if the locally generated frequency and phase are in sync with the received carrier frequency.
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