Friday, December 17, 2010

Wireless Signal Modulation

Signal modulation is a technique used to combine a signal being transmitted with a carrier signal for transmission. The receiver demodulates the transmitted signal and regenerates the original signal. Normally the carrier signal is a sine wave of a high frequency. The input signal could be digital (digital modulation) or analog (analog modulation). In either case, the three basic characteristics of a signal are utilized for modulation. The device that performs this modulation and demodulation is the modem. Modulation is often referred to as signal encoding. Analog signals can be modulated by the following methods.

Amplitude Modulation
For AM signals, the output signal is a multiplication of the input signal with a carrier wave. The amplitude of the carrier wave is determined by the input analog signal. The frequency of the resulting output signal is centered at the frequency of the carrier. As its name implies, AM radio that operates in the frequency band of 520 to 1605.5 KHz uses AM.

Frequency Modulation
Rather than vary the amplitude of the carrier wave, FM alters the transient frequency of the carrier according to the input signal. Again, as its name implies, FM radio that operates within the frequency band of 87.5 to 108 MHz uses FM.

Phase Modulation
In phase modulation (PM), the phase of the carrier signal is used to encode the input signal. Like AM, FM and PM shift the frequency of the input signal to a band centered at the carrier frequency. Both FM and PM require higher bandwidths. Analog modulation is necessary for transmitting a wireless analog signal such as voice over a long distance. Directly transmitting the signal itself to the receiver without applying modulation would require a large antenna to be effective, as the frequency of voice signals falls into the range of 30 to 3000 Hz. For digital data, if the medium only facilitates analog transmission (e.g., air), some digital modulation techniques will have to be employed. A carrier wave is also used to carry binary streams being transmitted, according to some keying schemes in digital modulation. Below is a list of digital modulation schemes:

» Amplitude-shift keying (ASK): ASK uses presence of a carrier wave to represent a binary one and its absence to indicate a binary zero. While ASK is simple to implement, it is highly susceptible to noise and multipath propagation effects. Because of that, ASK is primarily used in wired networks, especially in optical networks where the bit error rate (BER) is considerably lower than that of wireless environments.

» Frequency-shift keying (FSK): Similar to FM, FSK uses two or more frequencies of a carrier wave to represent digital data. Binary FSK (BFSK), which employs two carrier frequencies for 0 and 1, is the most commonly used FSK. The resulting signal can be mathematically defined as the sum of two amplitude-modulated signals of different carrier frequencies. If more than two carrier frequencies are used for modulation, each frequency may represent more than one bit, thereby providing a higher bandwidth than ASK.

» Phase-shift keying (PSK): PSK uses the phase of a carrier wave to encode digital data. Binary PSK simply reverses phase when the data bits change. Multilevel PSKs use more evenly distributed phases in the phase domain, with each phase representing two or more bits. One of most commonly used PSK schemes is quadrature PSK, in which the four phases of 0, π /2, π , and 3 π /2 are used to encode two digits. PSK can be implemented in two ways. The first is to produce a reference signal at the receiver side and then compare it with the received signal to decide the phase shift. This method somewhat complicates things at the receiver end, as the transmitter and the receiver must be synchronized periodically to ensure that the reference signal is being generated correctly. Another method is differential PSK (DPSK). In DPSK, the reference signal is not a separated signal but is the one preceding the current wave in question. One of the second generation cellular systems, Digital-Advanced Mobile Phone Service (DAMPS), uses DPSK.

ASK and PSK can be combined to offer more variations of phase shifts on the phase domain. Quadrature amplitude modulation (QAM) is such a scheme in which multiple levels of amplitudes coupled with several phases provide far more unique symbol shifts over the same bandwidth than used by PSK over the same bandwidth. QAM is widely used in today’s modems.

Apart from analog and digital modulation, another category of modulation that should be discussed for wireless communication is analog-to-digital data modulation, a procedure sometimes referred to as digitization. Two major digitization schemes are pulse-code modulation (PCM) and delta modulation. PCM samples an input analog signal in short intervals, and each sample is converted into a symbol representing a code. To reconstruct the original input signal from samples, the sampling rate must be higher than twice the highest frequency of the input signal. In other words, given a sample rate offs, a frequency higher than 2 fs in the input signal will not be recovered in the reconstruction. Delta modulation uses a staircase-like sample function to approximate the input signal. The resulting digital data comprise a series of 1’s and 0’s indicating the ups and downs, respectively, of the staircase function.

In the wireless world, signals transmitted through the air are primarily high-frequency analog signals. In wireless voice communication, the user’s voice is digitalized into digital data and then modulated to analog-based band signals (digital modulation), which are finally modulated with a carrier wave for transmission. For wireless data transmission, the first step of this procedure is not necessary. In either case, the receiver takes the reverse order of these steps to recover the transmitted data or voice.

Source of Information :  Elsevier Wireless Networking Complete 2010
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