System resources and efficiency
System resources. Basic resources, requiring the system to provide given conditions (1.1, a, b), could be represented as frequency band and signal to noise ratio at the receiving point
В ≥ Вnom , Hz (1.3, a)
Eb/No ≥ (Eb/No)nom (1.3, b)
where Вnom - nominal bandwidth; Eb/No, (Eb/No)nom - is the necessary signal to noise ratio at the receiving point and its nominal value, consequently; Eb – an energy of one bit; No – Fourier density of noise power.
Therefore the system should require availability of the following technical resources:
a) b) c)
Fig. 1.2. Basic system resources: (a) – signal to noise ratio, Eb/No; (b) – frequency band, B; (c) - Eb/No and B.
In this case a similar situation occures, as with reqirements considering for the wireless communication system (1.1, a, b). On the one hand, it is advantageous to provide a communication system with best resources (increased values Eb/No and В). Consequently, system could operating more properly and robustly under influence of destabilizing factors. On the other hand mentioned values increasing is more expensive for system. Therefore, we considering conditions close to equalization of (1.3) (with reserve amount 10-20%).
It should be noted that at the receiving point signal to noise ratio exists in the form of energy resource Pr/No (Pr – the power of informative signal), and it is just following formula:
Eb/No= (Pr/No)/R (1.4)
But for digital communication systems analysis it is more convenient to use information about signal to noise ratio of the form Eb/No. An advantage of the frorm Eb/No for SNR definition is dimensionless ratio (in contrast to Pr/No ratio).
System efficiency. The objective of wireless communication systems design includes not only provision of set requirements, but also needs of efficiency increasing for such systems.
In the general case an efficiency of some system is determined by created product quantity and quality. The product quantity in communication systems is predefined by information transfer rate R, provided with necessary frequency band В. The system quality could be defined as bit error rate Pb, provided at the recieveing point with some SNR value. On the basis of abovementioned, the system efficiency is determined by the performance of utilization of the spectral resource and transmitter power, and therefore how close the transfer rate to its theoretical limits – throughtput capacity (1.2, b). Following indexes are often using for system efficiency, each of them is related with transfer rate R [54,55]:
frequency efficiency, as utilization index of wireless system frequency resource:
(1.5,
a)
energy efficiency, as utilization index of wireless system energy resources:
(1.
5,
b)
informative efficiency, as proximity index of transfer rate to the throughtput capacity С:
(1.
5,в)
Thus, basic system efficiency indexes were considered (1.5, a-c), in relation to the reqirements of the system - bit error rate Pb, and transfer rate R; also to effective system resources utilization – frequency band B and SNR value Eb/No. Analyzing wireless systems, it is more expedient to use normalized (desirable dimensionless) indexes. Following dimensionless normalized indexes are widely used for design of wireless communication systems: normalized transfer rate R/B; normalized signal to noise ratio Eb/No.