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to k if cd = c, [I]. This value is used to select the binary digit
r d , - > required to determine which output has to be used for
routing the packet [I]. The packet ‘ad’ of the header comprises the first four bits representing the value cd and the last
four bits representing rd. The intermediate eight bits are Os
and are present in order to avoid interactions in the optical
routing operation. T o obtain all-optical routing, cd is conveniently represented in unary coding (c,,,). A signal in unary
form can be represented by a string of ‘0’s of length cd followed by a ‘1’.
The quantity ( k + c d - c c , ) mod k can be computed by
means of sum and mod k operations with the device shown in
Fig. IC.
The sum can be optically performed feeding the pulse representing cdl into a delay line DL,. The length of D,, is equal
to the value (k - c,)T/8 and depends only on the r NIU position.
To perform the operation mod k, the output of the delay
line is forward to a loop L, producing a delay kT/8, which
allows us to add the first k bits of the results to the second k
bits, obtaining the correct operation mod k expressed by the
second four bits. In this manner a pulse is produced in the
time position equal to xT/8. By means of DL, the pulse is
delayed by 2k - 1 in order to check, by means of the A N D
gate, the value of the (x - 1)th bit of the rd part of the address.
Based on the schematic diagrams of the devices in Fig. I we
propose in Fig. 2 the schematic diagram of an NIU that is
designed to operate with all-optical processing. Each NIU is
subdivided in two equal parts, part a and part b.
This switch allows the packet to be retransmitted by one of
the two outputs (0,.
0,).
Collisions among the packets coming from part a and part
b are avoided, delaying the packets by means of the buffer
loops L , and L , whose electro-optic switches S, and S , are
driven by the central unit that chooses the priority.
The gain of the optical amplifiers depends on their output
saturation power which has to be lower than 100-200mW,
and on the energy level at the optical gates. In particular for
pulses of 1 ps duration, the XOR gate requires pulses with an
energy of the order of 1 pJ [4], whereas AND gates require
that the two input pulses have to present a product of their
energy greater than 1 pJ2 [SI.
Starting from all these energy requirements the gain of the
amplifiers has to be chosen as follows: A, = 12 dB, A, = 6 dB,
A, = 9dB, A, = 27dB, A, = 21 dB, A, = lOdB, while all the
couplers but C, present a coupling ratio of 3dB. C , has to
‘spill’ a very low fraction of energy (lo/), which is detected
from the photodiode to determine the arrival of a packet.
According to all these considerations the total throughput
of the network is equal to 0.048 x 0.43 1 1Tbit/s, where 0.43
is the throughput eficiency per NIU, in a multihop network
with k = 4, p = 2 [3].
22ndJune 1992
F. Matera and M. Settembre (Fondazione Ugo Bordoni, Via B. Castigfione 59,00142, Roma, Italy)
E. Ripani (CNR, Via E . Castigfione 59,00142, Roma, Italy)
clock
References
1 ACAMPORA, A. s., and KAROL,
M. J.: ‘An overview of lightwave
packet network’, IEEE Network, January 1989, pp. 29-41
2 SOCCOLICH,
c. E., ISLAM,
M.N., HONG,
B. I., CHBAT,
M.,and SAUW,J.
R.: ‘Application to ultrafast gates to a soliton ring network‘. Proc.
of Nonlinear Guide-Wave Phenomena, Cambridge, 2nd-4th September 1991, pp. 366-369
3 MATFRA,
I.,ROMAGNOLI,
M.,SETTEMERE,
M.,TAMBURRINI,
M., and DE
MARCHIS,
G.: ‘Proposal of an all optical TDMA network. Proc.
LAN, EFOCiLAN 91, London, UK, pp. 132-137
4 WOOD, D.: ‘Constraints on the bit error rates in direct detection
optical communication systems using linear or soliton pulses’, J.
Lightwaue Technol., 1990, LT-4, (7),pp. 1097-1106
kj
part
b
M E M O R Y ARQ SCHEME W I T H LOW
RELIABILITY ZONE DETECTOR
C. Lau
Indexing terms: Digital communication systems, Automatic
repeat request
The performance improvement of a memory ARQ scheme
using majority voting decision and reliability zone detectors
is analysed.
Introduction: In a conventional ARQ system [l, 21, the receiver checks each received packet for possible errors. Any
packet detected in error is discarded and a request for
retransmission of the erroneous packet is made. In a memory
ARQ system [3-51, the receiver stores the erroneous packet.
It is then combined with subsequent retransmissions for possible error correction.
In this Letter, we consider a memory ARQ system as shown
in Fig. 1 in which a message to be transmitted is split into
groups of maximum length K bits. Each group is then
assembled into a packet of length n = K r bits where r
represents the overhead required for addressing, error detection, etc. It is assumed that the error detection code used can
detect all errors. Each packet denoted by (bi, b,, .. ., b,) where
b, E (0, l}, i = 1, 2 , . .., n, is then sent over an additive white
Gaussian noise channel using binary antipodal signalling. The
+
ELECTRONICS LETTERS
r
13th August 1992
Vol 28 No. 17
.
-
1571
input to the vector channel is x
if bi = 0; else xi = -A.
= (xl,
x2,. . ., xn) where xi = A
If Y, is a codeword, transmission is completed; otherwise a
request is made for retransmission.
T = 0 corresponds to the one-threshold memory ARQ
scheme in Reference 4.
Eoaluation ofrepeat request probability: Assume that an all '0'
codeword is transmitted. The bit error probability a t the hard
decision detector output is P, = f erfc [A/oJ(2)], where erfc
(a) is the complementary error function of a. After J transmissions, the probability that the decision accumulator has value
s is given by
PBJS) =
T
decision
ldetector
x
I
- PJ-1
I
s=j-2/
/ = 0 , 1 , 2,..., j
reliobility
packet
detector
Fig. 1 Block diagram of memory ARQ system
where Pb&) is the probability that the value s falls in the low
reliability zone
At the receiver, the received vector corresponding to the j t h
transmission of x is y j = (yj, yj, 2 r . .., yj,J where yj, = xi
nj, and {n,,;} represent outcomes of independent Gaussian
random variables, each with variance aZ.The hard decision
detector then carries out a hard estimate on the received
vector y j . A weight wj. = 1 is assigned if the amplitude of
the received signal yj,i is positive; otherwise, w j , i = -1 is
assigned.
A decision accumulator B j , i is maintained for each bit in
the packet. After the jth reception of the packet, the accumulators are updated using Bo, = 0 and B,, = B j - 1.
w j ,;. The
outputs of the accumulators are examined by the reliability
zone detector. The range of the weight stored in the accumulator is divided into a low reliability zone, - z j 5 B,, I zj, and
high reliability zone B j , , < - z j and B j ,z z j , where z j is a
predetermined zone threshold. This is illustrated in Fig. 2.
+
+
+
I
i
Zl-1
i
-21..
high reliability
zone
(2)
Because bit errors are independent, the repeat request probability is
decision
accumulator
updating
F
k
(;>.:(I
I
I
-2
-1
i
i
0
1
I
2 ...
i
ZI
low reliability zone
,.
PJj) is the probability that s is in the positive high reliability
zone
(5)
and Pb&) is the probability that s is in the positive low
reliability zone
PbrcCi)
= tPBi(0)
c
+ s = 1P B J S )
(6)
The repeat request probability, P,G),is plotted against the
signal-to-noise ratio (SNR) 20 log,,(A/o) for n = 512,
T = 5, j = 1, 2 , . . ., 5 and different values of z, in Fig. 3. The
I
i
21.1
(4)
J
high reliability
zone
Fig. 2 Weights associated with reliability zones
Let t be the number of accumulators of which the weights
fall within the low reliability zone, i.e. - z j IB j , i , ,
B j , i z ,..., B j , i ,5 zj. If f I T , where T is an integer number
which is small compared to the maximum number of errors
that the code can detect, the decoder input v, is determined
from the following:
(1) for those Bj, i , i # i,, i 2 . . .. , i,, which fall within the high
reliability zone, the corresponding uj, I is set to '0'if B j , I > z j
or to '1' if B , < -zi.
(2),for those B j . i , i = i,, i,, ..., i,, which fall within the low
reliability zone, the corresponding decoder inputs {uj.
uj,i2, .. .,t i j , i , } form the set of 2' binary combinations in which
one combination is equal t o {b;,, biz,..., bz,}.
In this case (t IT), there are 2' received words. If those B, ,
falling within the high reliability zone are correctly received,
then one of the 2' received words is a codeword and transmission of the packet is completed; otherwise a request is made
for packet retransmission.
For t z T, the decoder input is given by
1"
o j , I= 1
1 or 0 with equal probability
1572
for B , z 0
for B j , < 0
for B j , = 0
(1)
3
4
5
6
7
8
9
1011
1213
SNR
Fig. 3 Repeat request probability against SNR
curve for j = 1 corresponds to the repeat request probability
1 - (1 - PJ'. After j = 2 transmissions, the repeat request
probability drops substantially for SNR > 8 dB. We note that
in memory ARQ systems without the reliability zone detector
[4], there is no reduction in the repeat request probability
when j increases from 21 1 to 21 + 2, 1 = 0, 1, 2, ... . For
systems in which the number of packet transmissions, J, is to
be chosen to achieve a certain repeat request probability, significant savings in SNR can be obtained by increasing j. For
example, increasing; from 1 to 2 results in a saving of 3 dB for
a target repeat request probability of
ELECTRONICS LETTERS
+
13th August 1992 Vol. 28 No. 17
Fig. 4 shows how the repeat request probability decreases
with various values of j and T for S N R = 9dB. For a given j ,
the value of zj that gives the lowest P,(j) was used. From the
Figure, we note that odd values of j should be used for T = 0
[4]. It can be seen that there is a substantial reduction in P,(j)
for j 2 4 when T is increased to 1. Further increases in T
result in smaller reductions. However, for j = 2 and 3, significant reductions in P,G) can only be obtained when T > 3.
of coherent detection optical receivers, the direct detection
optical receiver has the advantage of simplicity. FSK is an
attractive modulation scheme because the transmitter laser
frequency can be directly modulated by the current. FSK
signals can be optically demodulated by converting them to
baseband ASK signals. This can be carried out using a solid
Fabry-Perot interferometer etalon as the FSK discriminator.
Recently, the attractive capabilities of the DFB and DBR
laser as a multifunction device have been investigated. The
laser can be used for light emission or optical filtering and
photodetection. Recent work has demonstrated FSK transmission using multielectrode DFB and DBR lasers for both
transmitter and receiver [2, 31. The laser receiver acts as a
filter, an FSK discriminator and a photodetector. More
recently, using a two-electrode DFB laser for both transmitter
and receiver, a 1.5 Gbit/s transmission with -20dBm sensitivities for
bit error rate has been achieved [4].
In this Letter, we describe the ability to use a Fabry-Perot
laser, with one antireflection coated facet, as an FSK
discriminator/photodetector. We report FSK transmission up
to 1.5Gbit/s with -30dBm sensitivities for
bit error
rate.
Experiment: The experimental setup for the different measurements is depicted in Fig. 1. At the transmitting end, a
number of transmissions J
!f P
Fig. 4 Repeat request probability against j
V
e
Conclusions: The repeat request probability of a memory
ARQ system can be significantly reduced by including a reliability zone detector in the receiver. This technique can also be
applied to a memory ARQ system with soft decision detectors.
I
Dattern
1
23rd June 1992
C . Lau (School of Applied Science, Nanyang Technological University,
Nanyang Avenue, Singapore 2263, Singapore)
m
References
Fig. 1 Experimental configuration of transmission system
s. R., and LIN, s.: ‘Selective-repeat-ARQ schemes for
broadcast links’, IEEE Trans., 1992, COM-40, pp. 12-19
BENICE,
R. I., and m y , I. A. H.: ‘An analysis of retransmission
systems’, IEEE Trans., 1964, CS-11, pp. 135-145
3 BENELLI,
G.: ‘An ARQ scheme with memory and soft error detectors’, IEEE Trans., 1985, COM-33, pp. 285-288
4 LAU,c., and LEUNG,
c.: ‘Performance analysis of a memory ARQ
scheme with soft decision detectors’, IEEE Trans., 1986, COM-34,
pp. 827-832
R. : ‘Performance evaluation of efficient continuous
5 FANTACCI,
ARQ protocols’, IEEE Trans., 1990, COM-38, pp. 773-781
1.524pm, two electrode GaInAsP/InP DFB buried ridge
structure (BRS) laser was used. This DFB laser was fabricated
on material grown by MOCVD, with a second-order corrugation. One of the two facets of the DFB laser is coated with a
I44 layer of silicon monoxide, using electron beam evaporation, giving a residual reflectivity of 2%. This laser has two
equal electrodes of 300pm each and presents a continuous
tuning range of over 90GHz with a minimum spectral linewidth of 5 MHZ [SI.
At the receiving end, a single-electrode 350pm long BH F P
laser was used. One of the two facets of the FP laser is coated
with an AR coating of 2.5 x
reflectivity at 1.52pm. The
electrodes of the FP laser were matched with a 47 R resistance
and a bias current connection. The two lasers were connected
bv a variable outical attenuator (VA) and a polarisation controller (PC).
An optical spectrum analyser with O.lnm resolution was
used to observe the spectral output of the two lasers.
1
CHANDRAN,
2
l.5Gbit/s TRANSMISSION SYSTEM USING
FP LASER AS FSK
DISCR I M INATORlPHOTO D ETECTOR
~
Results: Fig. 2 shows the spectral output of the F P laser
biased at three times the lasing threshold current (Ifp=
53 mA). From Fig. 2, it can be seen that the F P laser presents
M. J. Chawki, L. Le Guiner, D. Dumay and
J. C. Keromnes
-25
Indexing terms Optical receivers, Frequency shrft keying,
Semiconductor lasers
An optical FM receiver based on a monoelectrode FabryPerot laser, with one antireflection coated facet is reported
The device, biased far above threshold, acts as an FSK
discnminator/photodetector An FSK transmission system,
up to 15Gbit/s with -30dBm sensitivities for
BER,
has been demonstrated using this laser. The speed is lmited
by the transmitter FM response and not by the receiver
response
2
U
- 45
-39 9 4
-65
Introduction Performance analysis of direct detection optical
receivers has been the subject of interest for many researchers
over the past several years [l]. Compared to the complexity
ELECTRONICS LETTERS
13th August 1992
15224
15274
w
15324
138612
Fig. 2 Spectral output of FP laser
Vol. 28 No. 17
1573
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