DCHPPA Noise Analysis

The advantage of the PPA approach is the power gain associated with it at the front

stage, and the main attraction compared to other optical wireless approaches is the

optimum overall noise figure, as the total noise figure of the receiver is a consequence of

the Friss Formula, with the assumption that all the stages have the same modulation

bandwidth. . ... G G 1 F G 1 F F F 2 1 3 1 2 1 r      

The above formula shows the overall noise factor of the receiver. It is clear thatG1should

be set to as high a value as possible, to minimise the effect of F2at the second stage (i.e.

pre-amplifier), and this makes the first stage crucial; this is also the case with respect toG2

andG3. The whole DCHPPA system can slightly increase the NF of the front-end system

over the up-converter PPA first stage, as mentioned in the previous chapter. Practically

speaking, it would be inadequate to compare the PPA carrier to the noise ratio with the

whole DCHPPA system, due to different bandwidth at each sub-circuit; the DCHPPA has

low loss IF devices with very high gain and low NF at as early a stage as possible, so as to

minimise the effect of the later stage. It is clear that the up-converter PPA first stage is the

main core for improving SNR, and makes the first stage crucial. In contrast, the PPA can

provide a better quality of reception, and generally higher communication accuracy and

reliability than low SNR ratios, but with the expense of bandwidth.

For example, in case1: for DD/IM technique, thepinPD have F1=1.87 (NF=2.7dB)

and unity gain as no amplification inside the junction itself. Case2: for PPA technique, the

PPA as stage one has F1=1.96 (NF=2.92dB) with G1=20dB gain. Both the techniques were

followed by LNA with F2=3.16 (NF=5dB) and G2=20dB gain. By using the Friss formula,

the total receiver noise figure in case one isNFr1=5.54 with a 20dB gain, whereas the total

receiver noise figure in case two is NFr2=2.97 with a 40dB gain. The photoparametric

technique exhibited a smaller noise figure compared to PD, followed by the pre-amplifier,

which clearly shows that the high gain at stage one will provide better total noise figure for

the whole receiver. It may be concluded that PPA can provide better noise performance

compared to the photodetector, followed by the preamplifier, hence, the better the noise

figure is, the less the degradation for the receiver SNR.

The same analysis may be applied to the DCHPPA implemented receiver. It is clear

that high gain with low insertion loss at early stages is more favourable with respect to

receiver noise performance, and as mentioned in previous chapter, the choice of receiver

components such as IF filters and IF Amplifiers were highly selected with respect to low

loss and noise figures, even with cost and power efficiency (i.e. passive devices or low

power devices). The research has computed the total noise figure for both receivers

configurations, DCHPPA and Gain Chain DCHPPA, as reported in section 5.4 and 5.5

respectively; the first configuration has a 3.04dB noise figure with almost 24.7dB gain,

almost 62dB gain NF. This result verified that high gain and low insertion loss at an early

stags will outperform, and the last stage will have a very low effect, and results in a low

increase in the total noise figure, and hence a very small degradation on the SNR. Any high

loss device at a later stage may have an insignificant effect on the overall performance of

the receiver with respect to SNR. Implementing the whole receiver in a single board

(MMIC chip) can help to reduce the parasitic effect (series resistances), and hence provide

better SNR.

6.3 Summary

Performance analysis was conducted on theoretical, simulation and practical

approaches; this presented a very good agreement, and a close result. This showed that the

photoparametric amplifier may be able to offer unexpected gains and bandwidth

improvement at low load impedance in comparison to a standard optical wireless receiver,

but sensitivity is limited by thermal noise. Noticeable improvements in up-conversion gain

are seen at zero bias modes. The three analyses follow the same trend, presenting a close

result, and the practical result verified the mathematical and simulation models presented

in preceding chapters.

Shot noise and thermal noise are predominate noise sources in both the

Photodetector (DD/IM) and the PPA. A reduction of shot noise is possible by using a low

power transmitted signal or a very narrow optical filter, or operating at very low ambient

background light. A reduction of thermal noise is possible by increasing the load resistance

which acts adversely with the bandwidth and the gain. Furthermore, a reduction of reverse

bias current to very low steady state dc level will reduce unwanted broadband noise (i.e.

to reach the compression point. A high nonlinear photodiode can perform much better with

respect to noise performance. Larger responsitivity (R) may reduce the quantum noise and

improve the optical detecting efficiency. A measurement of the signal and noise was

carried out with parametric amplification, and without amplification. From the

measurement of NF in both cases, NF was determined at NF=2.92dB, with a power gain of

20dB. Consequently, the PPA was shown to have added a very small noise over

photodetection, due to the parametric effect (i.e. ∆NF=0.22dB), but with the advantages of

offering power gain and low noise by mixing products in a single junction. Overall PPA

noise performance is shown to be potentially better than the photodetection receiver,

followed by the preamplifier, and provides better receiver sensitivity, but with a bandwidth

penalty. Furthermore, as mentioned in chapter 2, the PPA was shown to outperform APD,

Chapter 7