ICT Projects: PERFORMANCE ANALYSIS ON MODULATION TECHNIQUES IN WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS-MODULATION TECHNIQUES IN WCDMA

Saturday, April 28, 2012

PERFORMANCE ANALYSIS ON MODULATION TECHNIQUES IN WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS-MODULATION TECHNIQUES IN WCDMA

PERFORMANCE ANALYSIS ON MODULATION TECHNIQUES IN

WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS

MODULATION TECHNIQUES IN WCDMA

3.1     Introduction

The evolution objective of wireless cellular technology from 1G to 3G is capable of delivering high data rate signal so that it can transmit high bit rate multimedia content in cellular mobile communication. Thus, it has driven many researches into the application of higher order modulations [19][20].

In cellular system, different users have different channel qualities in terms of signal to noise ratio (SNR) due to differences in distance to the base station, fading and interference. Link quality control adapts the data protection according to the channel quality so that an optimal bit rate is obtained for all the channel qualities [19][20]. Thus, the system adopts AMC to suit the link quality. W-CDMA systems can employ the high order modulation (8PSK or M-QAM) to increase the transmission data rate1 with the link quality control.

However, there is a trade off in employing bandwidth efficient M-QAM modulation scheme. The complexity of the receiver increases linearly with M (number of orthogonal sequences) and exponentially with the number of bits per symbol. The achievable bandwidth efficiency of the system is limited by the maximum possible number of orthogonal sequences and by acceptable complexity of the receiver [20].

To minimize Inter-symbol Interference (ISI), noise and channel fading, a wireless system needs to have a robust system to minimize, if not to eliminate, these unfavorable effects. A typical W-CDMA transmitter system consists of bit generator, TC (Tele command) encoder, rate matcher, interleaver, spreader, modulator, scrambler, and pulse shaper. On the other hand, a receiver consists of a matched filter, channel estimator, rake receiver, despreader, demodulator, deinterleaver, and TC decoder. Maximal ratio combining of rake results amplitude boost is very favorable for M-PSK demodulation due to its greater separation of the received symbol constellation. However, it is not the case for the MQAM.

For an amplitude-modulated signal (M-QAM), amplitude change could produce incorrect symbol detection [19].

3.2     Bit Rate and Symbol Rate

To understand and compare different modulation format efficiencies, it is important to understand the difference between bit rate and symbol rate. The signal bandwidth for the communications channel depends on the symbol rate or also known as band rate.. Bit rate is the sampling frequency multiplied by the number of bits per sample. For example, a radio with an 8-bit sampler is sampled at 10 kHz for voice. The bit rate, the basic bit stream rate in the radio, would be 8 bits multiplied by 10k samples per second giving 80 kbps. In this example, extra bits required for synchronization, error correction, etc are ignored for simplicity. In GMSK, only one bit can be transmitted for each symbol. Thus, the symbol rate for this modulation technique is 80 kbps. However, high data rate like 8-PSK, as it will be reviewed in the next section, can transmit 3 bits per symbol. Thus, the symbol rate, if this

                               (3.1)

Bit rate is the sampling frequency multiplied by the number of bits per sample. For example, a radio with an 8-bit sampler is sampled at 10 kHz for voice. The bit rate, the basic bit stream rate in the radio, would be 8 bits multiplied by 10k samples per second giving 80 kbps. In this example, extra bits required for synchronization, error correction, etc are ignored for simplicity. In GMSK, only one bit can be transmitted for each symbol. Thus, the symbol rate for this modulation technique is 80 kbps. However, high data rate like 8-PSK, as it will be reviewed in the next section, can transmit 3 bits per symbol. Thus, the symbol rate, if this modulation scheme is employed, is 26.7 kbps. The symbol rate for 8-PSK is three times smaller than that of GMSK. In other words, 8-PSK or any high order (M) modulation scheme can transmit same information over a narrower piece of RF spectrum.

3.3     Bit Error Rate (BER)

Bit-error rate (BER) is a performance measurement that specifies the number of bit corrupted or destroyed as they are transmitted from its source to its destination. A simple definition is given by:

                                                                                                        (3.2)

An error is defined as discrepancy between corresponding points in the two sets of data. Several factors that affect BER performance includes bandwidth, signal to noise ration (SNR), transmission speed and transmission medium. For example, one way to lower the spectral noise density is to reduce the bandwidth, but it is limited by the bandwidth needed to transmit the desired bit rate (Nyquist criteria). A lower bit rate will increases the energy per bit, but lose in capacity. Ultimately, optimizing E_b/N_ois a balancing act among these factors [21].

3.4     Signals-to-Noise Ratio (SNR)

SNR is defined as the ratio between signal power to noise power and it is normally expressed in decibel (dB). The SNR can degrade in two ways, through the decrease of the desired signal power and through the increase of noise power, or the increase of interfering signal power.

The mathematical expression of SNR is

                                                                                             (3.3)

where and are noise power and signal power respectively.

3.5     Quadrature Phase Shift Keying (QPSK)

Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal.

QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the BER — twice the rate of BPSK. Analysis shows that this may be used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed.

Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers. With this interpretation, the even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature-phase component of the carrier. BPSK is used on both carriers and they can be independently demodulated.

QPSK is one example of M-ary PSK modulation technique (M = 4) where it transmits 2 bits per symbol. The phase carrier takes on one of four equally spaced values, such as 0, /2, and 3/2, where each value of phase corresponds to a unique pair of message bits as it is shown in figure 3.1. The basis signal for QPSK can be expressed as

                        (3.3)

                                                                    Q                                      for i = 1, 2, 3, 4



                                   (-1,1)                                         (1,1)



                                                                                              I

                                                               (-1,-1)

                                 (-1,-1 )                                      (1,-1)

                          Figure 3.1:    Constellation diagram of a QPSK system

Special characteristics of QPSK are twice data can be sent in the same bandwidth compared to Binary PSK (BPSK) and QPSK has identical bit error probability to that of BPSK. When QPSK is compared to that of BPSK, QPSK provides twice the spectral efficiency with the efficiency with the same energy efficiency. Furthermore, similar to BPSK, QPSK can be differentially encoded to allow non-coherent detection.

The modulated signal is shown below for a short segment of a random binary data-stream. The two carrier waves are a cosine wave and a sine wave, as indicated by the signal-space analysis above. Here, the odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quardrature component (taking the first bit as number 1). The total signal — the sum of the two components — is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period

Figure 3.2:     Timing diagram for QPSK.

Due to these advantages of QPSK, it has been employed as the modulation technique in UMTS 3G wireless cellular networks where the following data rate can be achieved depending on the channel quality.

i. 144 kbps for high mobility

ii. 384 kbps for low mobility

iii. 2 Mbps for indoor or static environment.

3.6     M-ary Quadrature Amplitude Modulation (QAM)

QAM is a modulation technique where its amplitude is allowed to vary with phase. QAM signaling can be viewed as a combination of Amplitude Shift Keying (ASK) as well as Phase Shift Keying (PSK). Also, it can be viewed as ASK in two dimension. Figure 3.3 shows the constellation diagram of 16-ary QAM (16-QAM). The constellation consists of a square lattice of signal points. The general form of an M ary signal can be defined as

   i=1,2,…..M.

                                                                                                                                  (3.4)

where is the energy of the signal with the lowest amplitude and
and are a pair of independent integers chosen according to the location of the particular signal point.

Figure 3.3:      Constellation diagram of a 16-QAM system

Theoretically, higher order of M-ary QAM enables data to be transmitted in a much smaller spectrum. However, the symbols are easily subjected to errors due to noise and interference because the symbols are located very closed together in the constellation diagram. Thus such signal has to transmit extra power so that the symbol can be spread out more and this reduces power efficiency as compared to simpler modulation scheme. Also the radio equipment is more complex.

3.7     Noise and Interference

3.7.1 Additive White Gaussian Noise (AWGN)

For ideal communication system, the channel is anticipated only with additive white Gaussian noise where it is free from intersymbol interference (ISI). This is usually a good starting point for understanding basic performance relationships. The primary source of performance degradation is thermal noise generated in the receiver.

Often, external interference received by the antenna is more significant than thermal noise. The term additive means the noise is superimposed or added to the signal that tends to obscure or mask the signal where it will limit the receiver ability to make correct symbol decisions and limit the rate of information transmission. Thus, AWGN is the effect of thermal noise generated by thermal motion of electron in all dissipative electrical components i.e. resistors, wires and so on [22].

Mathematically, thermal noise is described by a zero-mean Gaussian random process where the random signal is a sum of Gaussian noise random variable and a dc signal that is

                                                                                                                  (3.5)

where pdf for Gaussian noise can be represented as follows where is the variance of n.

                                                                                   (3.6)

A simple model for thermal noise assumes that its power spectral density G_n (f) is a flat for all frequencies and is denoted as

                                                                                                               (3.7)

where the factor of 2 is included to indicate that G_n (f) is a two-sided power spectral density. When noise power has such a uniform spectral density, it is referred as white noise. The adjective "white" is used in the same sense as it is with white light, which contains equal amounts of all frequencies within the visible band of electromagnetic (EM) radiation. Since thermal noise is present in all communication systems and is a prominent noise source for most system, the thermal noise characteristics that are additive, white and Gaussian are most often used to model the noise in communication systems.

3.7.2 Rayleigh Fading

In a wireless mobile communication system, a signal can travel from transmitter to receiver over multiple reflective paths; this phenomenon is referred to as multipath propagation. The effect can cause fluctuations in the received signal’s amplitude, phase, and angle of arrival, giving rise to the terminology multipath fading. The end-to-end modeling and design of systems that mitigate the effects of fading are usually more challenging than those whose sole source of performance degradation is AWGN [23].

Generally, fading effects in mobile communication can be categorized as large scale fading and small scale fading. Large scale fading represents the average signal power attenuations or path loss due to motion over large areas. This phenomenon is affected by prominent terrain such as hills, forests, building and others. The receiver is often being shadowed by such prominences. On the other hand, small scale fading refers to dramatic changes in signal amplitude and phase that can be experienced as a result of small changes (as small as a half-wavelength) in the spatial separation between a receiver and transmitter [23]. Small scale fading is also called Rayleigh fading because the envelope of the received signal is described by a Rayleigh pdf. The received signal consists of large number of multiple reflective paths and there is no line-of-sight signal component. When there is a dominant non-fading signal component present, such as a line-of-sight propagation path, the small-scale fading envelope is described by a Rician pdf [24].

Figure 3.3:      Relationship among channel correlation function and power density function

3.7.2.1        Doppler Shift

Doppler shift is categorized as time variance in frequency domain of small scale fading. Referring to Figure 2.3(d) shows a Doppler power spectral density, S(v) plotted as a function of Doppler frequency shift, v. For the case of the dense-scatterer model, a vertical receive antenna with constant azimuthal gain, a uniform distribution of signals arriving at all angles through the range (0,2) and an unmodulated continuous wave (CW) signal, the signal spectrum at the antenna terminal is

                                                                                                   (3.8)

where f_d is Doppler spread. Figure 2.3(d) also shows that the sharpness and steepness of the boundaries of the Doppler spectrum are due to the sharp upper limit on the Doppler shift produced by a vehicular antenna traveling among the stationary scatterers of the dense scatterer model. The largest magnitude (infinite) of S(v) occurs when the scatterer is directly ahead of the moving antenna platform or directly behind it. Thus the magnitude of the frequency shift is given by

                                                                                                                   (3.9)

where is relative velocity, and is the signal wavelength. . f_d is positive when the transmitter and receiver move toward each other, and negative when moving away from each other. Knowledge of S(v) illustrates how much spectral broadening is imposed on the signal as a function of the rate of change in the channel state. The width of the Doppler power spectrum is referred to as the spectral broadening or Doppler spread, denoted by f_d, and sometimes called the fading bandwidth of the channel. Equation 6 describes the Doppler frequency shift. In a typical multipath environment, the received signal arrives from several reflected paths with different path distances and different angles of arrival, and the Doppler shift of each arriving path is generally different from that of another path. Therefore, the effect on the received signal is seen as a Doppler spreading or spectral broadening of the transmitted signal frequency rather than a shift [23].