Results and Conclusions

For a sample network with typical values of various parameter as given in table I, results were computed. Main results are as follows. Without OAs only two branches can be supported. Each branch is assumed to have 32 users. In the case with unsaturated OAs, when first OA is placed, the optimal position is before first OADM. It increases the number of branches to 19. Second OA leads to maximisation of branches when placed before fifth OADM. After adding sixth OA only one branch adds up. With seventh OA, there is no increase in number of branches (It remains 38). Table II, III, IV, V and VI shows the results in tabular form.

  

Table I: Typical parameter values

Maximum allowed transmitter power	PT	0 dBm
Desired Bit error rate		10-9
Insertion loss of single 2$\times$2 coupler	Li	0.5dB
Fiber attenuation coefficient	$\alpha$	0.2 dB/km
Insertion loss of splice	Lsp	0.5dB
Distance between branches	L	5 km
Insertion loss for dropping signal in OADM	L12	0.5dB
Insertion loss for passing signal in OADM	Lm	0.7 dB
Receiver load resistance	RL	100 $\Omega$
Operating wavelength	$\lambda$	1.55 $\mu$m
Optical Bandwidth	Bo	25 GHz
Electrical Bandwidth	Be	2.5 GHz
Receiver temperature	T	300o K
Electron charge	e	1.602$\times$10-19 C
Quantum efficiency of photodiode	$\eta$	1
Responsivity of photodiode	Ro	1.28
Number of users per branch	n	32

  

Table: P_e for without OA

$\epsilon \rightarrow$	Pe
$b \downarrow$	0.00	0.05	0.10	0.15
1	$1.0\times 10^{-39}$	$8.1\times 10^{-33}$	$3.9\times 10^{-27}$	$1.9\times 10^{-22}$
2	$3.9\times 10^{-19}$	$6.1\times 10^{-16}$	$2.6\times 10^{-13}$	$3.9\times 10^{-11}$

b is number of branches, $\epsilon$extinction ratio

  

Table III: Number of supported branches after placing first ideal amplifier.

$\epsilon \rightarrow$	Number of supported branches, Pe
$P \downarrow$	0.00	0.05	0.10	0.15
I	19, 8.7 $\times 10^{-11}$	18, 4.7 $\times 10^{-18}$	18, 4.8 $\times 10^{-15}$	18, 1.5 $\times 10^{-12}$
II	18, 8.9 $\times 10^{-21}$	17, 4.7 $\times 10^{-18}$	17, 4.8 $\times 10^{-15}$	17, 1.8 $\times 10^{-12}$
III	17, 8.9 $\times 10^{-11}$	16, 4.8 $\times 10^{-18}$	16, 4.9 $\times 10^{-15}$	16, 1.5 $\times 10^{-12}$

P is position of amplifier placement, $\epsilon$extinction ratio

  

Table: P_e with one optical amplifier

$b \downarrow$	Pe
$\epsilon \rightarrow$	0.00	0.05	0.10	0.15
for b= 1 to 16, Pe is very small
17	2.8 $\times 10^{-45}$	2.4 $\times 10^{-37}$	7.4 $\times 10^{-31}$	1.7 $\times 10^{-25}$
18	1.0 $\times 10^{-21}$	4.7 $\times 10^{-18}$	4.8 $\times 10^{-15}$	1.5 $\times 10^{-12}$
19	8.9 $\times 10^{-11}$	$\ast$	$\ast$	$\ast$
20	$\ast$	$\ast$	$\ast$	$\ast$

b number of branches, $\ast$ means BER is greater than 10^-9.

  

Table V: Supported b and P_e after second ideal amplifier

  

Table VI: Number of supported branches vs. number of amplifiers

No. of amplifiers	Supported branches after placing the amplifier at optimum position
5th	37
6th	38
7th	38

With a) average gain saturation as well as b) average gain saturation with gain fluctuation, the trend is similar. For the later case there is no improvement after placing more that three OAs.

In this work a novel subscriber access network architecture has been proposed and OA placement problem has been investigated. Cosidering the average gain fluctuation with gain fluctuation case, only three amplifier can be used. This means network can support 384 users per access fiber. The system will need 12 WDM channels while providing very good bandwidth (100Mbps) to each of the subscribers.