Code Division Multiple Access - Assignment Example

CDMA INTRODUCTION 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). The project aimed at dual-mode operation with already available analog cellular systems. The IS-95 system deploys a 1.25MHz wideband RF channel. Each of the RF channels is shared by network users, however, with different codes assigned to each one of them. 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 & DEMODULATION TECHNIQUE 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. RAKE RECEIVER 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 dispreading. And as a result, equalization is not necessary. However, there still exists some constructive information in the components of multipath. Such multipath components may be combined by the CDMA receiver in order to enhance the overall signal to noise ratio at the receiver. This is the function of the RAKE receiver which was first proposed by price and green. The receiver simply attempts to gather all the multipath versions of the transmitted signal by establishing a separate correlation receiver for each of the multipath signals. All the established correlation receivers may be controlled electronically in time delay so that the each receiver may search in a different time window for significant multipath signals. In case fading corrupts the output of one of the receivers, the other outputs will not be affected and the fading output can be discarded. In this way, multiple correlators are employed to individually sense the strongest multipath components. The elected components may then be combined to provide a better estimation of the transmitted signal. In IS-95, RAKE receivers are being used by both the base station and the mobile device. The RAKE receiver's correlators are known as the RAKE receiver fingers. In case of the mobile receiver, RAKE-receiver finger outputs are combined coherently (voltage addition of the outputs). On the other hand, the base station's rake receiver conducts a non-coherent combination of the RAKE-receiver finger outputs (this means that they are added in power). According to Ernest, "mobile receivers have 3 RAKE-receiver fingers while base station receivers have 4 or 5 depending on the equipment manufacturer."(Ernest) The rake receiver fingers outputs are generally combined using one of two methods. The first method is defined by weighting each output equally. That is why this method is called "equal gain method". In the second method, data is used to estimate weights for each output to improve the signal to noise ratio (SNR) of the total output. This is called the maximal ratio combining method. There are many methods for the generation of the weighting factors. It should be remembered that strong multipath amplitudes may not necessarily produce strong correlation outputs. Using data from actual outputs of the correlation to choose the weighing factors is proved to produce better receiver performance. Practically, both the first and second techniques are known to yield similar results. Sources: Ernest Simo, "CDMA online website", 6991 Scenic Pointe Place Manassas VA 20112 USA, 2004, 20 November 2007, Theodore S. Rapaport, "wireless communications-principles and practices", second edition, prentice hall,2002 Read More