12.3 Perspective ways of further increasing efficiency

Using the various methods of error-control coding considered in these module the designer of transmission system owning art of optimisation can flexibly change of the efficiency indicators approaching them to the limiting, potentially possible values which are established in the previous section. Efficiency of the digital transmission systems of transmission can be essentially increased by the application of M-ary signals and error-control codes, and also their combinations. The choice of signals and codes in these cases is defining for construction highly effective codems (the codecs matching among themselves and modems). Comparison of efficiency of systems with M-ary signals and error-control codes is convenient for making with using of diagram  =f (), presented on figure 12.1. Thus degree of perfection of a modulation/coding methods and can be estimated, comparing efficiency with limiting values. Results of the efficiency analysis are presented on figure 12.2. At the same time, comparison various modulation/coding methods is convenient for making comparison taking for "reference point" the efficiency of transmission system with modulation QPSK (without error control coding). From among simple methods it is the most effective and widely used method of modulation/coding with indicators  = 2,  = 9,6 dB,   0,47. Conveniently as well that the point representing on figure 12.2 values of efficiency QPSK is arranged in a central part of the diagram. If an origin of coordinates to transfer to a point corresponding QPSK, in a new frame (Δβ, Δγ) on a vertical axis the energy gain in comparison with QPSK, and on a horizontal axis a gain on a specific rate  will be counted. Let's notice, that all possible modulation/coding methods can be divided into four groups corresponding to four quadrants on the diagram  =f ():

Quadrant III in which the low efficiency methods are arranged having rather QPSK loss on  and ;

quadrant II including methods with high energy efficiency, ensuring a gain on  in exchange for loss on  (systems with error-control codes);

quadrant IV including modulation methods ensuring a gain on  in exchange for loss on  (systems with M-ary М-PSK and М-APSK signals);

quadrant I including perspective modulation/coding methods ensuring a simultaneous gain both energy and frequency efficiency.

Outcomes of calculations show (figure 12.2, quadrant I), that application such coded modulations allows to receive simultaneously a gain both in energy, and frequency efficiency and, anyway, to get a gain on one indicator, not worsening another. So, system 8PSK-CC by the using of a convolution code with R_code = 2/3 ensures a energy gain  = 2,8 dB without a decreasing of a specific rate , and system 16APSK-CC by R_code = 1/2 and  = 3 a gain on a specific rate  = 2 dB without a drop of energy efficiency . Information efficiency of these systems is   (0,6…0,7). The detailed analysis such coded modulations is reduced in manual [3, section 9.2]. Microelectronic progress last decade initiated attempts to realise the potentially possible efficiency, despite of growth of decoding complexity. In 1993 turbo-codes have been offered. Turbo-codes has been in details in manual [3, section 11.1] described. Intensive development of mobile transmission systems has led to the invention of a time/space coding, in details described in the manual [3, section 11.2].