5_1 Introduction
Now that the CDMA system and simulation model has been detailed, it is beneficial to examine certain aspects of the system in more detail. In the wireless communication industry, it is well known that the RF front-end transceiver is one of the key elements that detennines the overall performance of the communication system. The noise generated within these systems contributes significantly to the total noise. The usually high operating frequencies that are part of mobi le communication systems coupled with the fact that signal digitization and processing hardware are not commercially available for use at these frequencies, has inevitably led to the use of the mixer for frequency downconversion in the receiver. However, the mixer is the "noisiest"
component in the RF front·end. It is thus common to find receivers with low noise amplifiers (LNAs) in the RF front·end stages. These optimize the most critical perfomlance parameter, the noise figure. This quest for minimizing receiver noise (and hence the bit error rate) is usually achieved at the expense of a number of LNA stages. This chapter evaluates the impact of internally generated RF component noise in a simple DS·CDMA system with a view to identifying a method of quantifying the RF front-end component specifications for a given DS-CDMA system performance requirement.
5_2 Recei ve r noise and its characterization
Noise is a phenomenon inherent to all communication systems. It can be devastating in any communication system since it places a lower limit on the signals that can be detected and an upper limit on the amount of gain that can be used before distortion
5-1
EFFECT OF RF COMPONENT NOISE ON DS-CDMA SYSTEM PERFORMANCE CHAPTER 5
occurs. The noise generated by electronic systems can he categorized as thermal noise, shot, flicker (lIt) noise and burst noise ([18],[61]). Most of the noise present at the input of a typical receiver consists of thermal and shot noise. Shot noise is proportional to the bias current in devices such as diodes, transistors, etc. For the sake of the noise characterization of devices and systems, only the combined effect of all internally generated noise will be considered. This combined noise effect is often referred to as the thcmlal noise (N,h)of the system. It is proportional to temperature and bandwidth only as can he seen from Nyquist's equation [62]:
N" =kTB where Nth = thermal noise power in watts
T = absolute temperature in degrees Kelvin
=X °c + 273.15°
k = 1.38 X 10-23 watt.?K.Hz, Boltzmann's constant B = bandwidth in Hz in which the measurement is made
(5.1 )
For a noise source at room temperature (290K or 17°C), the noise power is - I 74dBm per Hertz of the system bandwidth. This thermal noise is characterized by a waveform that never repeats itself exactly i.e. it is purely random and, as predicted by the kinetic theory of heat, the power spectrum is flat with frequency_ Since all frequencies are present in this thermal random noise, it is referred to as 'Johnson noise' or white noise due to an analogy with white light which has a uniform power distribution over the band of optical frequencies.
In many conununication systems, the received signals are of low power and accompanied by noise. In these systems, the noise is generated within and outside the system. However, the noise generated within the system itself contributes a significant portion of the total noise, and in most cases, the internally generated noise is virtually the total noise [61]. Futher amplification is therefore necessary and this increases both signal and noise power levels. As the noise cannot be eliminated, it has been usual to use a criterion of performance, the ratio of signal power to noise power at various points in the system. This measure of system perfoffilance is not adequate in the case of certain active networks such as amplifiers or receivers. Consequently, it has been
found necessary to use other criteria for measuring system performance.
EFFECT OF RF COMPONENT NOISE ON DS-CDMA SYSTEM PERFORMANCE CHAPTER 5
The well-known concept of noise figure or noise factor F has thus been created to provide a standardized means of characterizing the internally generated noise of a system. In general, it measures the "noisiness" of a network which may be an amplifier or receiver by considering two signal-ta-noise ratios, onc at the input end and the other at the output of the network. This is illustrated in figure (5.1).
S,
Ni NETWORK
Fig. 5.1: Two port network
S.
N.
In terms of these quantities the noise factor F is widely defined as F- Si/Ni
I
- S / N T._290K
where Si = input signal power Ni = input noise power So = output signal power No = output noise power
• •
(5.2)
From equation (5.2), it can be seen that the noise figure of the system or network is the degradation in the signal-to-noise ratio as a signal passes through the system. In defining the noise figure, a standard temperature of To=290K was adopted, originally by the IRE (forerunner of the IEEE), and this standard is widely accepted. Since the definition of F in equation (5.2) uses the signal-to-noise ratio at two different points in the system, it is sometimes more convenient to use an alternative expression obtained as follows:
F = S,I Ni = N.
So/No So/S;xN, (5.3)
If G is the power gain of the network, then G = SJSi and also Ni = Nth= kToB. Hence F= N.
GkT.B (5.4)
In the above expression, No is the noise power output of the network, while GkT
oB
is the noise power output of an ideal network i.e. a network which adds no further noise.5-3
EFFECT OF RF COMPONENT NOISE ON DS-CDMA SYSTEM PERFORMANCE CHAPTER 5
Thus the noise figure indicates the amount of noise generated in the device above the kT,j3 thermal noise (referenced to a temperature of To = 290K).
The noise figure is also frequently expressed in terms of the internally generated noise Ne (equivalent noise power). Ne is the output noise power of the network when there is no noise power present at the input i.e. noise added by the system itself. Rearranging equation (5.2),
F= S,I N, So I No
Si I Nj
=
GS,IN.
= -==---:S,-,-,:-I N,-,,-::. =-:- GS,/(N, + GN,) Ne +GN,
ON; N, + GkT.B
GkT.B
(5.5)
It should be noted that the system noise figure is independent of the signal level as long as the system is levelled (constant gain).
The two-port network shown in figure (5.1) usually consists of a string of amplifiers andlor mixers as in the RF front-end of a typical communication system. In this instance, it would appear that every amplifier/mixer would degrade the signal-to-noise ratio by its noise figure. However, this is not the case. Each amplifier in a string of amplifiers produce a certain amount of noise over and above the thennal noise. As the input signal (and noise) is amplified, the signal becomes strong enough that the small amount of noise generated by succeeding stages adds relatively very little to the overall amount of noise. This concept leads to the familiar Friis' fomlUla ([18J, [61J, [63]) for calculating the overall noise figure ofa n-stage system:
F, -I F-I F'()/4' =
F.
+ - ' - + ... +-=---="-=--
G] G] G2 ••. G 11_] (5.6)
From equation (5.6) it can be seen that for a system processing very weak signals, F,o,al
must be small in order to allow a high enough signal-to-noise ratio at the system output for the output signal to be detected. It is also observed that the noise figure of the first stage of the system FI . contributes significantly to the overall noise figure, Flola[,
EFFECT OF RF COMPONENT NQISE ON OS-COMA SYSTEM PERFORMANCE CHAPTER 5
provided the gain G I in this stage is high. Hence it is necessary to have a low-noise and high-gain device as the first stage of a system processing weak signals.
5.3 System architecture and simulation work
In order to assess the effect that these RF components will have on system performance, it is necessary to simulate a complete CDMA transmit-receive system.
5.3.1 Transmitter architecture
Figure (5.2) shows the transmitter architecture for the proposed adaptive COMA system.