RESOURCE ALLOCATION AND MODULATION TECHNIQUES

RESOURCE ALLOCATION

It is assigning the available resources in an economic way. It is a part of resource management. This allocation is done by strategic planning in which resorce allocation is a plan for using available resources e.g human resources to achieve gaols in future. In computing it is necessary for any application to be run on the system. When the user opens any program this will counted as a process and therefore requires the computer to allocate certain resources for it to be able to run. Such resources could be memory, files etc.

ORTHOGONALITY

In communications, multiple access schemes are orthogonal when an ideal receiver can completely reject strong unwanted signals from the desired signal using different basis functions. E.g TDMA where the orthogonal basis functions are time slots.

MODULATION

It is the process of varying one or more properties of a periodic waveform called the carrier signal with the modulating signal which contains the information to be transmitted.e.g a digital bit stream or an analog audio signal inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal.the device which perform modulation is called modulator and the device which performs demodulation is called demodulator.

Modulation is of two types:

1.   Analog modulation

2.   Digital modulation

Analog modulation : the main aim of this is to transfer an analog baseband which could be audio signal or TV signal over an analog bandpass channel at a different frequency.

Digital modulation : the main aim of this is to transfer a digital bit stream over an analog bandpass channel e.g PSTN

Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels (several different carrier frequencies).

The aim of digital baseband modulation methods, also known as line coding , is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial bus  or a wired local area network.

The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband  baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.

In music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform.

In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques are:

·       Amplitude modulation(AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

Double-sideband modulation (DSB)

Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band)

Double sideband suppressed carrier transmission (DSB-SC)

Double sideband reduced carrier transmission (DSB-RC)

      Single sideband modulation (SSB, or SSB-AM)

SSB with carrier (SSB-WC)

SSB suppressed carrier modulation (SSB-SC)

Vestigial sideband modulation (VSB, or VSB-AM)

Quadrature amplitude modulation (QAM)        

    Angle modulation, which is approximately constant envelope

Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols.

Schematic of 4 baud (8 bit/s) data link containing arbitrarily chosen values.

A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bitrate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems.

According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital to analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to datatransmission; very few would consider it as analog transmission.