QUADRATURE AMPLITUDE MODULATION (QAM) TECHNIQUE

Critical study and analysis of the Quadrature Amplitude Modulation (QAM) technique

SECTION I. INTRODUCTION

AM, Quadrature amplitude modulation is widely used in many digital data radio communications and data communications applications. A variety of forms of QAM are available and some of the more common forms include 16 QAM, 32 QAM, 64 QAM, 128 QAM, and 256 QAM. Here the figures refer to the number of points on the constellation, i.e. the number of distinct states that can exist.

The various flavours of QAM may be used when data-rates beyond those offered by 8-PSK are required by a radio communications system. This is because QAM achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. And in this way the points on the constellation are more distinct and data errors are reduced. While it is possible to transmit more bits per symbol, if the energy of the constellation is to remain the same, the points on the constellation must be closer together and the transmission becomes more susceptible to noise. This results in a higher bit error rate than for the lower order QAM variants. In this way there is a balance between obtaining the higher data rates and maintaining an acceptable bit error rate for any radio communications system.

A method for imprinting digital signals onto a carrier for transportation from one place, such as a cable headend, to another, such as a home. The business take-away from QAM is the message of throughput, or how many bits per second can be crammed into a single, 6 MHz channel, modulated with QAM(Campopiano & Glazer, 1962: 90).  Early implementations used 64-QAM, which afforded about 27 Mbps per channel. Contemporary networks generally use 256-QAM modulation now, which yields 38 Mbps per 6 MHz channel. Even though the technique is called quadrature amplitude modulation, it also involves the use of "phase modulation. What happens, loosely, is this: Digital bits are grouped into symbols, which are then imprinted onto a carrier by manipulating two transmission characteristics: Amplitude (power) and phase (frequency shape). The advantage of this dual modulation is the ability to stuff twice as much information into the transmission channel, at the same time(Campopiano & Glazer, 1962: 90).

Although 64-QAM and 256-QAM are the workhorses of digital modulation, a slimmer variation -- 16-QAM -- is available for upstream, home to headend transmissions. The advantage of 16-QAM in the upstream is more carrying capacity inside the skinny, 5-42 MHz upstream (headend-to-home) signal path. However, using 16-QAM in the upstream requires a carrier-to-noise ratio that is difficult to support in that inherently noisy spectral zone.

From a services perspective, QAM is used in the downstream (headend to home) signal path for all digital offers: Video, broadband Internet, and voice (circuit switched or IP). (In some cases, digital signals from traditional broadcasters are accepted, and particularly if the broadcasters send signals using vestigial sideband modulation.)

There's no easy way to describe this complicated form of ...