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CDMA - Essay Example

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Code Division Multiple Access is a form of Direct Sequence Spread Spectrum communications where the channel resources are shared by all the service users. Qualcomm Inc. developed a CDMA based cellular system which was later standardized by the telecommunications industry association (TIA) as an Interim Standard (IS-95)…
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Download file to see previous pages Soft handover capabilities are offered to improve the voice quality whereas a RAKE receiver is used to reduce the effect of multi-path fading. J-STD-008 is the PCS version of IS-95; both were combined to IS-95B.
Modulation is a process by which the characteristics of a sinusoidal carrier waveform are modified using values of coded symbols that represent the original message. Once the carrier waveform is modulated, it carries the message along to the receiver, where demodulation will reverse the process and extract the original message from the carrier.
In the CDMA (IS-95) cellular system, Binary phase shift keying (BPSK) modulation is used on the forward link. Using algebraic notations, we can mathematically express a carrier wave with certain phase shift applied as the sum of a cosine or I(t) and a sine or Q(t) wave component. I(t) and Q(t) , or simply I and Q, are called the real (in phase) and the imaginary (quadrature-phase)data components respectively. This representation is not simply an algebraic identity; it also provides the required base for the necessary modulation and demodulation operations. With this representation we have two superimposed binary PSK waveforms. It is easier to modulate and demodulate such waveforms. The waves are generated by the transmitter at the same frequency, and then each of the components is modulated by 0 or 180 degrees phase shifting (detecting 180 degrees phase shifts is easier than detecting 90 degrees phase shifts). After that the components are summed together and transmitted. Once the signal arrives at the receiver, the receiver will generate two reference waveforms for the demodulation process to be carried out.
A 64-ary orthogonal modulation scheme using Walsh words are employed on the reverse link. This conveys 6 coded symbols per each Walsh word. Note that "Walsh words are groups of 2N vectors or words which contain 2N binary elements which with them selves and their logical inverses form a mutually orthogonal set."(Ernest) phase modulation remains the form of change the carrier will undergo. This means that a Walsh word w(t) with binary chips 1 will be multiplied by the carrier to mathematically produce either a positive or a negative version. The effect of the Walsh word will be to shift the phase of the negative version by 180 degrees, while the positive version will be shifted by 0 degrees (which means that it will be left intact).
The mobile determines the data bit on the forward link by simply examining all the received Walsh chips and checking if they are all either inverted or not relative to the chip values assigned to it. The base station will also recognize the 1 pattern of the transmitted Walsh word using the fast Walsh transform (FWT) on the reverse link. In this process, the 1 pattern received will be compared with all possible 64 Walsh words.
CDMA spreading codes are different than other conventional modulation schemes. They are designed to offer extremely low correlation between successive chips. As a result, the effects of the channels propagation delay spread are merely multiple versions of the original signal. If such multipath components experience a time large time delay compared to the chip duration. They will be discarded as uncorrelated noise by the CDMA receiver. This happens because the uncorrelated noise will be made negligible by the spread spectrum processing gain after ...Download file to see next pagesRead More
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