Ulti-rate Fiber-optic Time Division Multiple Access ... - IEEE Xplore

E E E Transactions on Consumer Electronics, Vol. 43, No. 2, MAY 1997

228

MULTI-RATE FIBER-OPTIC TIME DIVISION MULTIPLE ACCESS NETWORKS FOR DIGITAL TV/HDTV DISTRIBUTION APPLICATIONS Jian-Guo Zhang

ABSTRACT

deployment of FTTC systems marks the beginning of displacement of copper in the distribution portion of the netIn this paper, a new type of high-speed fiber-optic T z m e work serving smaller business and residential customers [l]. Dzvzszon Multzple Access (TDMA) networks is proposed to Using FTTC systems can achieve a cost-effective fashion if support multi-rate data communications for hybrid digital customer densities can support about 4 to 8 living units per TV/high-definition TV distributions. The basic principle optical-network node. FTTC systems can also be easily upof multi-data-rate time division multiplexing is presented, graded to offer broadband and multimedia communication and the structure of the TDMA frame is discussed. A sim- services by using, for example, optical Time Division Mulple algorithm is also proposed for the time-slot assignment tiple Access (TDMA), Wavelength Division Multiplexing, of users with different data rates. The design of the pro- or Wavelength Division Multaple Access. With the further posed network is described, and architectures for multi- reduction in costs of lightwave components and systems, rate optical TDMA receivers and fast optical clock-rate Fiber t o the Home will be very attractive for multimedia multipliers are presented. By using the proposed optical communications in future information age. TDMA technique, the network flexibility can be signifiConventional fiber-optic networks use electronic signal cantly improved with respect to conventional equal-rate processing and routing control which impose the throughoptical TDMA schemes. It is shown that a multi-rate put bottleneck in network nodes, so the processing speed fiber-optic TDMA network can be efficiently used to dis- is ultimately limited. To fully exploit the enormous fiber tribute multi-channel high-definition T V and standard dig- bandwidth, advanced optical processing techniques should ital TV signals simultaneously or to support future multi- be used to remove the bottlenecks at electrooptic and oprate broadband communication services. toelectronic interfaces. Therefore, future high-speed fiberoptic networks will be able to perform signal sampling, transmission, processing, detection, and regeneration in the 1, INTRODUCTION optical domain where the transmission bandwidth can supToday's single-mode optical fibers have a very low attenua- port bit rates in excess of 100 Gbit/s [2]-[4]. tion (e.g., 0.2 dB/km at 1.55 pm wavelength) and a vast Fiber-optic TDMA (FO-TDMA) is a synchronous actransmission bandwidth (e.g., 30 THz). Consequently, cess scheme which shares fiber bandwidth by scheduling optical fibers are used as an efficient transmission medium all the users to transmit data in distinct short time slots for telecommunication and consumer-electronics applica- per frame and using fast optical signal processing t o multitions. This in turn provides a capability to support emerg- plex/demultiplex data. Thus, FO-TDMA is very attractive ing multi-gigabit-per-second or multi-ten-gigabit-per-second for high-speed circuit-switching networks which can supnetworking for advanced multimedia communications. port fixed data-rate services and continuous-type traffic, Therefore, it is expected that such high-speed networks especially for digital T V or HDTV distribution and broadare vital to future broadband communication services. casting. Recently, T i m e Division Multiplexing (TDM) has In recent years, Fiber t o the Curb (FTTC) systems have been proposed for satellite transmissioii of digital combeen receiving considerable attention because of advance in pressed T V channels to cable headend earth stations [5], lightwave technology and huge potential market for both which in turn affords an opportunity to implement TDM service providers and network operators. In local exchange scheme in a fiber-optic transmission system having a much networks, optical fibers are used for offering access to many wider bandwidth than the satellite system. As a result, the large business customers [l]. While for FTTC applications, fiber-optic TDM system can support much more HDTV or optical fibers provide a very high bandwidth to remote elec- standard digital TV channels than a satellite TDM system, tronic curb sites where the final connection to customers and can be more attractive for consumer electronics. is achieved by using coaxial cables. This hybrid fiber/coax At present, FO-TDMA networks are normally designed distribution technique has the potential to deliver multiple to operate in an equal-data-rate environment [2]-[4][B] [7]. channel per household and there is sufficient bandwidth to This can only support either HDTV or standard digital TV support high-definition T V (HDTV) signals. Moreover, the distributions as shown in Figure 1, but it can not simultaneously accommodate both types of signals. The requireT h e author IS with the Telecommunications Program, School of Adment on keeping the same data rate for all the users will vance& Technologies, Asian Institute of Technology, P.O. Box 4, Klong N

-

Luang, Pathumthani 12120, Thailand. Email: [email protected].

Contributed Paper

Revised manuscript received March 31, 1997

0098 3063/97 $10.00

'1997 IEEE

Zhang: Multi-Rate Fiber-optic Time Division Multiple Access Networks for Digital TV/HDTV Distribution Applications

Figure 1: A self-synchronizing equal-rate FO-TDMA broadcast network using a multistar topology for MDTV distributions.

229

tional electronic signal processing should be used, which in turn causes a throughput bottleneck at network interfaces. Furthermore, it is difficult to achieve high-speed ATM switching for HDTV services without cell loss during a long holding time (e.g., 2 hours) [8]. In this paper, a new FO-TDMA network is proposed to effectively support real-time high speed communications in which data rates of all the users are positive integer multiples of a common clock rate, but without cell loss as encountered by using ATM. The proposed technique can cost-effectively transmit multi-channel HDTV and standard digital TV signals in a given fiber-optic network. The basic principle of multi-data-rate time-division multiplexing is presented in Section 2 . A simple algorithm is proposed for the time-slot assignment of users with different rates in Section 3. The design of the proposed networks is described in Section 4, and the architecture for multi-rate optical TDMA receivers is also discussed. In Section 5, the design of fast all-optical clock-rate multipliers is described, and an experimental demonstration of such a multiplier is reported. In Section 6, we explain some possible applications of the proposed multi-rate FO-TDMA to multichannel HDTV and standard digital TV distributions.

significantly limit applications of FO-TDMA networks. For example, Synchronous Transfer Mode (STM) multi-rate switching networks will be used to carry TV and HDTV channels at 52 Mbit/s and 156 Mbit/s respectively [8]. Moreover, Synchronous Digital Hierarchy (SDH) has become an international standard [9]. SDH systems can efficiently multiplex various digital signals and then build cost-effective networks, which allow payloads of approxi- 2. PRINCIPLE OF MULTI-DATA-RATE OPTImately 50 Mbit/s, 100 Mbit/s, 150 Mbit/s, and multiples CAL TIME DIVISION MULTIPLEXING of 150 Mbit/s [lo]. Furthermore, in regard to consumer electronics, the digital hierarchy concept has been recently Since TDMA is a synchronous scheme, a common time refproposed, which allows a single standard to accommodate erence is required by all the users t o guarantee the correct multiple delivery media and different data rates with com- time-domain multiplexing. This can be feasibly achieved in mensurate levels of picture quality [ll]. As known, higher a TDMA network by synchronizing all the transmitters to a data-rate levels of the digital hierarchy can have higher res- common clock source. There are two kinds of user’s slot asolution or fewer compression artifacts. For example, the signments [3][4]. One is called the fixed-transmitter assignuse of optical fibers can easily support a data rate of 120 ment TDMA, for which all the transmitters are scheduled Mbit/s for HDTV broadcast; the direct broadcast satellite to transmit the bit(s) of their information in distinct time can allow a data rate of 60 Mbit/s for HDTV applications; slots within a clock frame (called the basic frame), and rewhile a 20 Mbit/s data rate for HDTV is offered by ca- ceivers can select the information sent by any transmitter. ble and terrestrial broadcast systems. Moreover, VCR’s The other is referred to as the fixed-receiver assignment with extended play mode need only a 10 Mbit/s data rate TDMA. In this case, all the receivers are assigned distinct for HDTV applications [ll]. Therefore, the flexibility, of- time slots in a basic frame, and transmitters can be tuned fered by digital delivery and digital hierarchy concept, is to distinct time slots per basic frame to deliver their inforvery important for helping service providers and consumer mation to the desired receivers respectively. In an equal-rate FO-TDMA network, all the users have electronics manufacturers to meet the increased demands an identical data rate being equal to the clock rate, so the from the consumer. In order to support these mentioned optical time-division multiplexing (OTDM) can be impleapplications, FO-TDMA networks should have an ability mented by gating the ultrashort optical clock pulses (e.g., to offer high-speed data communications of which the data from a centralized mode-locked laser) in electrooptic modrates are integer multiples of a common rate. ulators located at the transmitters. The gated pulses are Although Asynchronous Transfer Mode (ATM) has been then directly delayed t o the assigned or desired time slot recently proposed t o efficiently support variable-rate serin each frame for fixed-transmitter or fixed-receiver assignvices, ATM switching networks require to use the complex hardware [8]. For example, each user should package its ment, respectively. In a multi-rate TDMA environment, data and transmit them in the form of the fixed-length the N users can operate at different data rates, however, packets (cells). At each receiver, the fast pulse packet they can be synchronized by a common clock source which should be deinterleaved to recover the relatively low-speed can operate at the baszc clock rate Fo. original data signal. At present, this can not be easily realFo=min{f, I i = 1 , 2 , 3 , . . . , N } (1) ized in the optical domain if the transmission and switching speeds are very high, because of the lack of fast and where f, is the i-th user’s data rate. Users which need low-loss all-optical logic devices. Otherwise the conven- higher data rates can use optical clock rate multipliers

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IEEE Transactions on Consumer Electronics, Vol. 43, No. 2, MAY 1997

= 1 s( - 4 at basic ramplc rate Fo s' L...-- = 1 e - - , designed in Section 5 to generate the desired rates, even s?. \ they are much higher than those available by using today's U6 u 4 U3 u2 u4 U1 U3 u 4 uz U6 u 4 U3 u4 U1 U1 uz U3 u4 mo de-lo cked 1asers. 1 2 3 4 5 6 7 8 9 10 il 12 13 14 15 16 17 18 19 20 21 22 23 24 From now on, f z is assigned and fixed for the i-th transa baric frame mitter unless otherwise specified. In a multi-rate data com5 channels a t Fa. :3 channel at 2Fo, I channel at 3F0, 1 channel at 4F0, inunication network, the data rates f , can be different from 1 channel at 6Fo one another. If all the fz are equal, the scheme is thus the (a) conventional equal-rate TDMA. At present, due to the lack of available all-optical logic at bartc sample rate Fo devices, complex data processing and buffering should be I 1 1 rI t I r t + i t 1 avoided for ultra-high-speed applications. In fact, electronic processing would cause a throughput bottleneck at the network interfaces, which would prevent ultrafast oper- I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 241 a basic frame 4 ation and waste the advantage of the enormous bandwidth - 1 10 channels at Fo, I channel at ZFo, 1 channel at 4F0, 1 channel at 8Fo offered by the fibers. In the following, we propose a feasible scheme for multi(b) rate OTDM which does not require any pulse permutation or interleaving technique at the transmitters, then we at baric sample rate FO present a simple architecture which is suitable for multirate FO-TDMA based on mature optical delay-line procesU1 uz U1 U1 U1 U1 u2 U1 U1 U1 U1 u2 U1 U1 U1 sors. Let K be the set of positive integer numbers and M be a buic frame _________i the number of total slots per basic frame (i.e., a time inter9 channels at Fo, 1 channel at 3Fo, 1 channel at 12Fo val of duration l/Fo), respectively. To correctly multiplex ic) N users with non-uniform data rates fi , the following conditions shall be satisfied.

+

Condition 1

u1 U2

U1

U2

- }

a

19 channels at Fa,

Condition 2

(d)

M

-EK

Li

,

Conditions 1 and 2 ensure that the data period (if

fi

=

If all t h e users satisfy Conditions 2 and 2, t h e correct time-division multiplexing i s achieved by straightforwardly delaying t h e gated Si-rate optical clock stream of user i t o t h e assigned t i m e slot(s) p e r basic f r a m e of M for either fixed-transmitter or $xed-receiver assignment, so that N users occupy distinct t i m e slots in each basic f r a m e . T h a t i s

all the users is exactly equal to the length of the basic frame 1/Fo as illustrated in Figure 2. In this case, the basic-rate (if fi = Fo)or integer-multiple-rate (if f; > Fo) optical clock stream with pulsewidth r Fo)for

'

-

(3)

0

Szr,j$-k

basic frame

1 channel at ~ F o ,

(4)

The newly proposed OTDM principle can be thus described as:

where all the s;,] form the set of the successfully occupied time slots by N users, U , ( N ) , i.e

Ue(N)

e

{si,]

I s:,] # si,,, V i # i'

E [l, NI

( r = 1 , 2 , . , L,, q = 1,2,. . , L , ) ) , and s;,] # sel,] , V r # T' E [l, L,]

(Lz > 1))

(7)

Zhang:

Multi-Rate Fiber-optic Time Division Multiple Access Networks for Digital TV/HDTV Distribution Applications

23 1

Step 1: Are the values of fi/Fo integers (i = 1 , 2 , 3 , 4 , 5 , . ’ . , N )? Then the set of the unused time slots b y N users, U e ( N ) , If so, performing the following steps, is defined as otherwise it stops the search U e ( N ) Ue ( N ) = { 1,213, . ’, M} (8) Step 2: Set M ( 0 ) = N , i = 0, and j = 0 Step 3: Set M ( j 1) = M ( j ) 1 In this way, no pulse permutation technique is used for Step 4: Are the values of M ( j l)/Li, integers for i’ = rearranging the gated optical pulses at each transmitter, so 1 , 2 , . . ., N ? the interval between two successive time slots of the i-th If so, doing Step 5, user per basic frame is: otherwise set j = j 1 and return to Step 3 to do If Li > 1 again ~ i + l , j - ~ i=,M j /Li, q=1,2;..,Li-l (9) Step 5: Set i = i 1 Step 6: If i > 1, set t = 1 and perform Step 7, r = 1 , 2 , . . . , Li (10) &+I - s:,j = M 1 otherwise set ~ f =, ( r~- 1) . M ( j 1)/L1 1 for r = 1 , 2 , . . . , L1 and return to Step 5 t o do again and if Li = 1 Step 7: Set s i , j = at(i - 1) and check whether all the s : , ~ +-~ =M (11) remaining slots = si,j ( q - 1) . M ( j 1)/Li ( q = 2,3, . . . , Li) superimpose on the existing occupied We call this scheme the direct-sample and forward-shift slots by the first i - 1 users (or only by the first user multiplexzng. The proposed network is expected to have the if i = a)? That is, is any same complexity as the equal-rate FO-TDMA network exE Ue(i- 1) ? cept that the former requires cptical clock-rate multipliers Answer 1: If any U e ( i- 1), the current i-th at some transmitters/receivers, and therefore, the former user’s assignment succeeds. Now if i = N , the search can operate as fast as the latter in practical cases where process is finished and the result of the current timethe maximum user’s data rate is normally limited to a few slot assignment is obtained, otherwise set j = j 1 Gbit/s (e.g., HDTV). Note that the equal-rate TDMA is and return to Step 3 to do again a special case of the proposed multi-rate TDMA, in which Answer 2: If any E U e ( i - 1), check whether Li = 1 for i = 1 , 2 , . . ., N and M = N . Thus, the time-slot the current at(i - 1) is the last element of the set arrangement for N equal-rate users is simplified to choose Ue(i- 1). If so, set i = 0 and j = j 1, then redistinct positive integer for each user’s time slot. turn to Step 3 to do again; otherwise set t = t 1 and begin Step 7 again. 3. ALGORITHM F O R TIME-SLOT ASSIGN- 0 MENT O F MULTI-RATE USERS Figure 2 illustrates four examples of possible time-slot In this section, we discuss the frame structure of multi- assignments for the multi-rate OTDM with a frame length rate TDMA based on the direct-sample and forward-shaft of M = 24. It is obvious that, for a given value of M , many multzplexcing scheme. In order to design the TDMA frame possible time-slot assignments can be feasibly obtained by structure for any given number of multi-rate users, a fast a computer search based on the above algorithm if the data computer algorithm will be developed. Here the fixed- rates orland the number of users are changed. When the transmitter assignment FO-TDMA is considered, because total number of high-data-rate channels is far lower than it is more suitable for digital TV or HDTV distribution and that of basic-rate channels and the difference between high broadcasting than the fixed-receiver assignment scheme. and low data rates is not very large, the proposed scheme Moreover, using the fixed-transmitter assignment FO- is very efficient. For example, if M = 24 and there are only TDMA can eliminate collisions, i.e., several transmitters 2 channels at rates 2Fo and 3F0, respectively, 19 channels simultaneously trying to communicate with the same re- at basic rate would be offered as shown in Figure 2d. The ceiver, as possibly encountered in the fixed-receiver assign- total number of channels is 21 which is very close to the 24 ment scheme [3]. channels offered in the equal-rate TDMA case. When the For convenience, let M ( j ) denote the j-th i t a v e value value of M becomes larger, this difference can be negligible. of M , and let ut(i) be the element of the set U e ( i ) However, if we define the aggregate throughput of a TDMA as the summation of all the individual user’s data rates, U e ( i ) = {ut(i) 11 5 at(i)5 M and (12) there is no throughput difference between equal-rate and t = 1,2,3,...,tmaz} multi-rate TDMA networks in this sense. The fact is that, Assume also that all the elements are listed in the increas- when the i-th user operates at data rate fi = IF0 for 1 > 1, it exactly occupies 1 time-slots per basic frame compared ing order. to only one slot occupied by a Fo-rate user, so the number ~ l ( i Fo.In this paper, we consider fixed-transmitter assignment FOTDMA which is more suitable for TV or HDTV distribution and broadcasting than the fixed-receiver assignment scheme. However, there is no fundamental difference in the network architectures of both assignments. Network synchronization is a very important issue for FO-TDMA networks, because the timing error becomes more critical with the increase of transmission speed on the channel and this error should be kept to be very small (e.g., a few tens or a few picoseconds) for ultra-high-speed TDMA applications. To ensure the correct OTDM, at the transmitting end all the transmitters are synchronized with a common clock source. This can be feasibly achieved by using a single mode-locked laser (MLL) which distributes the Fo-rate clock pulse stream to all the transmitters through fibers of appropriate lengths to maintain the correct phase synchronization at all the electrooptic modulators (EOM's) [ 2 ] [ 6 ] However, . to achieve the whole network synchronization, the basic-rate optical clock signal should be also distributed to each receiver with the known phase difference with respect to the transmitted FO-TDMA signal. There are two feasible approaches to the distribution of the optical clock. One is to use separate fibers to connect each receiver to the clock laser by carefully

lower bound on length of a basic frame is given by N

M > C L i=l

All the frame structures shown in Figure 2 can achieve this lower bound on M . If the M is a positive even number, the maximum data rate for the given multi-rate TDMA is limited to MFo/2. Using computer-aided slot assignment method based on the proposed algorithm can facilitate the implementation of multi-rate TDMA and to find the optimal or suboptimal M for the given number of users with the fixed fi (i = I,2, . . , N ) . In order to simplify the design of multi-rate TDMA frame structure, we suggest that higher data-rate users have higher slot-assignment priority over lower datarate users, because it is easier to specify the time slots for the latter than for the former ones.

ARCHITECTURE FOR MULTI-RATE FOTDMA NETWORKS

4.

Owing to the difference between the basic clock rate Fo and data rates of some users, optical clock-rate multipliers (CRM's) are required by such users to generate the

.

f"' Clock Source

IML,L

ODtical Clock a t A7 I

; Electrical NRZ I

I

i

-

I

21

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I

Data Input 1

DEMUX

Combiner

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I

I 1

I

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Detector

Selector

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.

I I I

FO-TDMA Transmitter 1

Electrical NRZ I ~~t~ ~~~~t 1 IA-

3 p tical Fixed Optical Delay Line 1

n

I I I

I

Electrical NRZ D a t a O u t p u t at Rate J ,

1

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I I I

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Data Regenerator

Basic-Rate F O - T D M A Receiver

+

Electrical NRZ D a t a O u t p u t a t R a t e Fo

Figure 3: Block diagram of the proposed multi-rate FO-TDMA network. AMP: electrical amplifier.


choosing the fiber lengths [2][6]. The other is to use a dualwavelength technique which uses two different wavelengths to carry the optical clock and the TDMA frame, respectively [7]. The dual-wavelength technique efficiently uses the fiber by sharing its bandwidth. This in turn allows t o build cost-effective large scale broadcasting networks where the number of receivers is much greater than that of transmitters as shown in Figure 1, which is very attractive for future broadband home distribution services (e.g. HDTV) [7]. On the contrary, the use of the former scheme results in the difficulty of phase synchronization between optical clock and FO-TDMA signals at each receiver, especially in a multistage distribution network. The block diagram of the proposed FO-TDMA network is shown in Figure 3 . It is similar to the equal-rate FOTDMA network except that optical clock-rate multipliers and the multi-rate receivers are used by some users in a multi-rate network. Mode-locked laser 1 at wavelength XI is used for generating ultrashort optical sample pulses of width T I , while laser 2 at wavelength Xz is employed to generate the clock pulses of width r2 5 7 1 . The maximum possible number of users in a FO-TDMA network is then determined by

233

electrooptic modulators of all the users are optically multiplexed into the assigned time slots by using the fixed optical delay lines. The outputs of the transmitters are then fed into a star coupler via fibers with lengths equal to integer multiples of the length whose propagation time just corresponds to one basic frame. The first slot in the frame is shared by the first sample pulse of user 1 and a clock pulse at the different wavelength as shown in Figure 4a. To reduce the effect of fiber nonlinearity, the transmitted power in the fiber can not be too high, but the power budget can be improved by using optical amplifiers whenever needed. The whole multiplexed signal is then broadcast to all the receivers. Two cases must be considered when designing the optical transmitters for a multi-rate FO-TDMA network, i.e. Case 1: f ; = Fo In this case, the implementation of the optical transmitters is the same as that of equal-rate transmitter, which can be essentially treated as a simplified transmitter in Case 2 (see Figure 3), i.e., no optical clockrate multiplier is required before the electrooptic modulator.

2 2) The design of optical transmitters is slightly different from Case 1. Now an optical clock-rate multipler is required to ensure that the sample rate is exactly equal to fi at the i-th transmitter as shown in Figure 3 . The construction of optical clock-rate multiplers will be described in Section 5 .

Case 2: fi = LiFo ( L i

where NmaX2 2.

1

2

3 4

5

6

7

8

10 11 12 13 14 15

9

a

16 17 18 19 20 2 1 22 23 24

baric frame

(a) TDMA signal plus clod, at the outputs

of a time multiplexer (star coupler)

I ///I 1 n nn n 1

2

3 4

5

6

7

(b) Signal at the input of

1

2

3 I

5

6

7

8

a

8

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

9

PD

I 1 10 11 12 1 3 14 15 16 17 18 19 20 2 1 22 23 24

9

(c) Electrical outpat s t p a l frow

I 1

2

3 4

5

6

7

8

Figure 5: An all-optical tunable delay line with the tuning range of (A4- AT.

9

il threshold

detector

I

10 11 12 13 14 15 I6 17 18 19 20 2 1 22 23 24

(d) Kecovered elertrlcai "117. data

Figure 4: Illustration of the multiplexed/demultiplexed and recovered signals.

N users occupy M time slots in the basic frame if they satisfy (2) and (3). The gated optical sample pulses from

At each receiver, the incoming dual-wavelength optical signals are demultiplexed by using a wavelength-divisionmultiplexing (WDM) demultiplexer. If a receiver only needs to select the channels at f; = Fo, the receiver design would be the same as the multi-rate case except that no any clockrate multiplier is used now (see Figure 3 ) . However, when the receiver is required to tune over all the channels, a tunable optical clock-rate multiplier should be used t o correctly multiply the clock rate before the tunable optical delay line (ODL), because the distributed clock signal has the basic rate Fo. For the fixed-transmitter assignment case, the reconfiguration time of the network is mainly dependent on the tunable optical delay line and clock-rate multiplier. As pointed out in [13], a tunable optical delay line using 2 x 2 LiNb03 electrooptic directional couplers and fiber delay lines (see Figure 5) can offer a subnanosec-

234


ond reconfiguration time. The same devices can be also used for the tunable clock-rate multipliers as discussed in Section 5. The rate-multiplied optical clock pulses are then delayed to the desired user’s slot(s) per basic frame and are incoherently added to the phase-adjusted FO-TDMA signal at the input of a photodetector (PD) as shown in Figures 3 and 4b. This results in three signal levels: a high level corresponds to a data bit 1 sent in the desired user’s slot, a middle level denotes a data bit 1 sent in other users) slots or a data bit 0 sent in the desired user’s slot, and a low level means a data bit 0 sent in other users’ slots [2]. The data regeneration is performed by setting the threshold level between the middle and high levels (see Figures 4b and 4c). Since the data rate of each transmitter is known, the NRZ waveforms with different data rates can be easily generated at the receiver by using a data regenerator array.

ri, in principle. Then

where T,,= l/fznand Tout = 1 / f O u t . The “Lz]” denotes the largest integer not greater than 2. A kfi,-rate multiplier inserts le - 1 additional pulses to exactly divide the period T,, of the input clock signal into k equal-length segments which form a new periodic pulse sequence with period Tout= T , , / k . The principle of a clock-rate multiplier can be thus stated as : For any input clock signal C ( t ) of rate f i n , the k f i n rate multiplier delays C ( t ) by k - 1 different times i ( i = 1 , 2 , 3 , . . ’ , k - 1)’ and generates the signals kfin (neglecting power loss and pulse broadening) Ci(t) = C(t - i / k f i n ) ,

Figure 6: A proposed very-wideband FO-TDMA receiver. DEMUX: demultiplexer. At present, the progress in AlGaAs/GaAs heterojunclion bipolar transistors (HBT), InAlAs/InGaAs and InP/ InGaAs HBT’s allows to implement 40-GHz amplifier IC [14], 32-Gbit/s hybrid optical receiver [15], and 20-GHz decision circuit [15]. Moreover, photodiodes of 45-GHz bandwidth are commercially available [16],which can be used to realize an ultrafast optical receiver at 50 Gbit/s as shown in Figure 6. An advantage of this configuration is that no electrical amplifier is used after the photodiode and the receiver bandwidth is thus determined by the photodiode. To avoid the pulse distortion from RF reflection, the HBT decision circuit can be inserted close to the photodiode [17].With advances in optoelectronic integrated circuits, the proposed FO-TDMA receivers in Figure 3 may operate at a speed of 50 to 60 Gbit/s. Further improvement on the network throughput can be achieved by using the so-called all-optical digital TDMA receiver based on a nonlinear optical loop mirror which can support 100 Gbit/s time-demultiplexing applications. 5. ARCHITECTURE FOR OPTICAL CLOCKRATE MULTIPLIERS

In a multi-rate FO-TDMA network, the optical clock signal is distributed over the whole network only at the basic rate Fo. To make higher clock rates available for the users with higher rates, optical clock-rate multipliers must be used at their locations. In this section, feasible architectures for both fixed and variable clock-rate multipliers are presented. The function of a clock-rate multiplier is to generate an output clock signal with data rate fout which is an integer multiple IC of the input clock-signal rate f i n . The output pulsewi3th rout should be equal to the input pulsewidth

i = 1 , 2 , ..

k

-1

(18)

then it combines the undelayed clock C ( t ) with the k - 1 delayed clock signals Ci(t) to form a new clock stream Gout ( t ) Cout(t)

=

C ( t )+

k-1

C(f - i / k f i n ) i=l

k-1

i‘=O

so Cout(t)has the period Tout=

and

‘out

2 2.

0

Optical Clock Input

Optical Clock Output

Combiner

Figure 7: Block diagram of a fixed optical clock-rate multiplier. Figure 7 shows the block diagram of the fixed optical clock-rate multiplier. It has an all-optical structure and only uses passive optical power splitter/combiner and k optical delay lines, so the proposed clock-rate multiplier can operate up to terabit-per-second. In order to provide an optical clock signal with many possible data rates, an optical variable-rate multiplier should be used. Assume that all the required clock rates L k f Z , , k = 1,2,3,...,6,, satisfy Conditions 1 and 2 as well as

Zhang: Multi-Rate Fiber-optic Time Division Multiple Access Networks for Digital TVMDTV Distribution Applications

L-rrl Clack-Rate Selector

r: Optical Optical Clock Input

Optical

r

Optical Clock Output

S&ch Combiner Fixed CRM, / I ,

‘

Figure 8: Block diagram of a variable optical clock-rate multiplier comprising 1, individual fixed clock-rate multipliers. eq. (17), where 1 < L1 < Lz < . . . < LI,. A variable-rate multiplier may be simply constructed by using 1, individual fixed-rate multipliers, each of them designed for a specific rate fk = Lkfin as shown in Figure 8. Then a clock-rate selector is used t o choose the desired rate. However, when l, is high, this multiplier becomes very large and complex. The alternative structure shown in Figure 9 is more efficient. It comprises one optical 1 x Ll_ splitter and one Ll, x 1 combiner, Ll, - 1 parallel tunable optical delay lines and electrooptic switches (EOS’s), and a clock-rate selector. Tunable optical delay lines have been designed in [3][13], and here all of them should have an identical reference time t o which is equal to the propagation delay when the delay line is set in the “zero-delay” state (also equal t o the propagation time of the reference path). A tunable optical delay line can introduce any delay At, which contains any desired positive integer multiple of the unit delay AT = Z n / M

This is achieved by appropriately setting each electrooptic switch of the corresponding delay line (saying j ) ,either in the cross state if ai = 0 or in the bar state if a: = 1, as shown in Figure 5. For example, t o generate the desired time delay of 2 A r for a delay line with 5-stage switches, we need to set U( = 0, U; = 1, u i = 1, a{ = 0, and a i = 1. Tunable optical delay lines can have a subnanosecond reconfiguration time [13].

Clock-Rate Selector ., . . . , Reference Path

. .

I

I

Optical Clock Input

I

. I I

Oprtcal Clock Output

.

When a variable-rate multiplier operates at the rate the rate selector sets the first LI, - 1 electrooptic switches to the “ON” state and the others to the “OFF” state. It also sets the delays of the first L k - 1 tunable delay lines to 2 L k f l n +to for i = 1 , 2 , 3 , . . ., LI, - 1. Therefore, the output clock rate is equal to Lkfin. Since electronic processing is only used to set the states of electrooptic switches, it does not limit the processing speed of the clock-rate multiplier, but only affects the network reconfiguration time. Therefore, the reconfiguration time of the proposed multi-rate network can be very fast, i.e., less than one nanosecond [ 131. Although the use of different optical clock-rate multipliers at the transmitting end results in different output powers for the transmitters, this problem can be solved by using amplified zero-loss splitters/combiners [18] in each clock-rate multiplier. To verify the operation principle and performance of proposed clock-rate multipliers, an experiment on the fixed alloptical rate multiplier is demonstrated only due to limited research facilities (i.e., no any electrooptic switch at hand). Instead of using high-performance mode-locked lasers, we only adopt a gain-switched Fabry-Perot (FP) laser diode of very low cost in this experiment. The output optical short pulses from the 1.55-pm gain-switched FP laser diode are first converted into the electronic pulses by a wideband photodetector. Then we use a high-speed oscilloscope to measure the waveform at the output of this photodetector. The obtained short pulse is shown in Figure 10, and has a pulse width of 60 picoseconds (ps) of which the bandwidth is much larger than the direct modulation bandwidth of 500 MHz for this low-cost F P laser diode. The optical short pulse stream (i.e., optical clock signal) from the gain-switched FP laser diode is fed into a fixed all-optical clock-rate multiplier of rate 5fin using the configuration shown in Figure 7. The measured new clock signal of rate 5 f z n is shown in Figure 11. The time intervals between the 1st and the 2nd pulses, the 2nd and the 3rd pulses, the 3rd and the 4th pulses, the 4th and the 5th pulses are equal to 923 ps, 926 ps, 912 ps, and 906 ps, respectively, while the theoretically calculated interval is 909 ps for the input clock-rate fin = 220.022 MHz and output clock-rate fout = 5fzn = 1100.110 MHz. It is clear that the timing error is very small (i.e. $17 ps and -3 ps) compared to 60-ps pulses with an interval of 909 ps, and therefore, the experimental results are in agreement with the theoretical ones. Note that no special effort is made to control the length precision of optical fiber delay lines in the experiment. Actually, such delay lines are only of the pigtail fibers of passive optical power splitter and combiner used in Figure 7, and they are thus obtained by carefully cutting off a certain length of each pigtail fiber through a manual fiber cleaver which, in turn, can not guarantee a high precision in length of the cut optical fibers. Even so, the obtained fiber delay lines can still provide a good time-delay performance which is sufficient enough t o support a FO-TDMA network with a throughput up t o 25 Gbit/s. This can accommoLkfzn,

Fixed CRM, /li

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235

,

Figure 9: An efficient architecture for variable optical clock-rate multipliers.

-

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236

6.1 FO-TDMA Networks for DigitalTV and HDTV

Distributions

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Figure 10: The measured waveform (generated by a gain-switched FP laser diode) at the output of a wideband photodetector.

____

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With the deployment of digital T V broadcasting and the present trend to use HDTV for consumer-electronics applications, future T V broadcast systems and distribution networks should cost-effectively support both types of TV signals with different data rates, e.g., 4 or 5 Mbit/s for standard digital T V while 20 Mbit/s for HDTV. Although standard digital T V has a lower resolution than HDTV, the former uses less expensive circuitry in all planned digital TV sets than the latter [19]. Recently, direct-to-the-home digital broadcast services have been provided by satellite systems in USA, Europe, and Japan, respectively. By considering a large number of newly installed digital-TV receivers, the broadcast of hybrid digital TV and HDTV signals will be a cost-effective solution for customers. For example, standard digital T V can be used for morning operas and kiddie shows, while HDTV will be used for sports and prime-time lineup [19]. Moreover, HDTV applications can be extended beyond the entertainment to other services. For example, distance learning or remote education also needs HDTV, especially for experiment-oriented teaching and laboratory demonstration. Figure 12 illustrates some possible applications of a multi-rate FO-TDMA network based on Faber t o t h e Home technique. 6.2 Multi-Rate Multi-Channel HDTV Distribution Networks

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Figure 11: The obtained new clock signal from a fixed all-optical clockrate multiplier of rate 5fin = 1100.110

MHz. date several hundred HDTV and digital TV channels for consumer-electronics applications. It is expected that, with the precise control of length for fiber delay lines, a higher network throughput can be achieved. Furthermore, the use of integrated optics for a clock-rate multiplier can feasibly keep the timing error within a few ps, and therefore, allows us to implement a very-high-speed optical TDMA network.

6. APPLICATIONS

In the field of consumer electronics, there are several reasons to require the use of multi-rate data communication techniques. In this section, we discuss possible applications of the proposed multi-rate FO-TDMA networks to hybrid digital TV/HDTV distributions and multi-rate multichannel HDTV distributions.

As we discussed in Section 1, the use of digital hierarchy allows a single standard to accommodate multiple delivery media and different data rates with commensurate levels of picture quality [ l l ] . To efficiently utilize the available bandwidth offered by transmission media, various video compression techniques have been employed to reduce the data rate of HDTV signals from 1 Gbit/s to several ten Mbit/s. For example, a 20 Mbit/s data rate for HDTV is offered by cable and terrestrial broadcast systems, while the direct broadcast satellite can allow a data rate of 60 Mbit/s for HDTV applications. However, the compression process is inherently lossy, and the compression typically introduces artifacts into the decoded signal [20]. It is also true that higher data-rate levels of the digital hierarchy can have higher resolution or fewer compression artifacts. Once a larger bandwidth is available, this in turn allows us to send additional information t o further reduce the noise level of the picture or to add the additional high-frequency detail to the picture [ll].Wideband optical fibers can be easily used to support a data rate of 120 Mbit/s for high-performance HDTV applications. For example, telemedicine or teleradiology may require HDTV technique to transfer x-ray and ultrasound images [all. Therefore, 120 Mbit/s H D T V optical fiber systems can provide high-quality images of lesions and operating procedures for diagnosis and surgical training, respectively. Figure 13 shows the use of a multi-rate FO-TDMA network to support multi-channel HDTV signals for such applications as entertainment, teleradiology, and distance learning.


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Figure 12: Applications of a multi-rate FO-TDMA network to digital TV/HDTV distributions based on Fiber t o the H o m e technique.

I n t e r n a t i o n a l a n d Regional H D T V

Direct B r o a d c a s t Satellite H D T V Set for Entertainment

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Figure 13: Using a multi-rate FO-TDMA network to support multi-channel HDTV signals for such applications as entertainment, teleradiology, and distance learning, which is based on Fiber t o the H o m e technique.

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7. CONCLUSIONS A novel multi-rate FO-TDMA network, which can support high-speed data communications with rates being positive integer multiples of a common clock rate, has been proposed in this paper. Compared to ATM switching, no complex hardware is required and no cell loss is encountered during a long holding time (e.g., several hours) when the proposed network is used. Thus, multi-rate FOTDMA is very suitable for hybrid digital TV/HDTV distributions, STM switching, and SDH applications. An alloptical time domain multiplexing scheme, called the directsample and forward-shift multiplexing, has been also proposed to achieve extremely high aggregate throughput in a multi-rate environment. To fastly design the structure for multi-rate TDMA frames, a simple algorithm which is suitable for the computer-aided design has been suggested. Moreover, we have outlined a possible architecture for the proposed network. The construction of both fixed and variable all-optical clock-rate multipliers, which can ensure ultrafast processing speed and fast reconfiguration time, has been described. An experiment on the fixed all-optical rate multiplier has been also demonstrated to verify its operation principle and performance. Finally, various applications of the proposed multi-rate FO-TDMA network have been explained. It is shown that the use of the proposed technique opens to new applications of FO-TDMA and allows us to build, for example, a high-performance hybrid digital TV/HDTV distribution network.

ACKNOWLEDGMENT The author would like to thank Prof. G. Picchi of the University of Parma (Italy) for many useful suggestions and helpful discussions. The author also gratefully acknowledges Dr. G. Migliorini and the Faculty of Engineering of the University of Parina for the “Ing. Migliorini” Scholarship during his Ph.D. study there,

References [I] D. L. Waring, D. S.Wilson, and T. R. Hsing, “Fiber upgrade strategies using high-bit-rate copper technologies for video delivery,” J. Lzghtwave Technol., vol. 10, pp. 1743-1750, Nov. 1992. [a] P. R. Prucnal, M. A. Santoro, and S.K. Sehgal, “Ultrafast all-optical synchronous multiple access fiber networks,” IEEE J . Sel. Areas Commun., vol. SAC-4, pp. 1484-1493, Dec. 1986. 131 P. R. Prucnal, S. T. Johns, M. F. Krol, and J . L. Stacy, “Time-division optical micro-area networks,” Proc. SPIE, vol. 1389, pp. 462-476, 1990. [4] H. S. Hinton, “Photonic switching fabrics,” IEEE Commun. Mag., vol. 28, pp. 71-89, April 1990. [5] M. Freeling, “The impact of compression technology on satellite transmissions to cable headend earth staTions,” 1992 N C T A Technical Papers, pp. 291-302, 1992.

[6] P. R. Prucnal, D. J . Blumenthal, and M. ,4.Santoro,“12.5 Gbit/s fibre-optic network using all-optical processing,” Electron. Lett., vol. 23, no. 12, pp. 629630, June 1987. [7] J.-G Zhang, “Dual-wavelength optical fiber HDTV distribution networks using self-synchronization technique and multistar topology,” IEEE Trans. Consumer Electronics, vol. 40, pp. 985-991, Nov. 1994. [8] S.Kikuchi, N . Yamanaka, and Y. Shimazu, “Optical wavelength-division multiplexing high-speed switching system for B-ISDN,” in Proc. IEEE GLOBECOM’91, Phoenix, 1991, pp. 1235-1239. [9] H. Shirakawa, K. Maki, and H. Miura, “Japan’s network evolution relies on SDH-based systems,” IEEE LTS, vol. 2, pp. 14-18, Nov. 1991. [lo] J . A. McEachern, “Gigabit networking on the public transmission network,” IEEE Commun. Mag., vol. 30, pp. 70-78, April 1992. [Ill G. Reitmeier, C. Carlson, E. Geiger, and D. Westerkamp, “The digital hierarchy - A blueprint for television in the 21st century,” S M P T E Journal, vol. 101, pp. 466-470, July 1992. [la] J.-G. Zhang, “Novel techniques and architectures for all-optical code-division and time-division multipleaccess networks,” Ph.D. Dissertation (in Italian), University of Parma, 1994. [13] P. R. Prucnal, M. F. Krol, and J . L. Stacy, “Demonstration of a rapidly tunable optical timedivision multiple-access coder,” IEEE Photonics Technol. Lett., vol. 3, pp. 170-172, Feb. 1991. [14] E. Sano, S. Yamahata, and Y. Matsuoka, “40GHz bandwidth amplifier IC using AlGaAs/GaAs ballistic collection transistors with carbon-doped bases,” Electron. Lett., pp. 635-636, April 1994. [15] K. C. Wang, “Advanced in IC technologies for highspeed fiber communications,” in Technical Digest, OFC’94, San Jose, Feb. 20-25, 1994, pp. 209-210. [16] Products catalog, NEW FOCUS, Inc., 1994. [17] T. Kataoka, Y. Miyamoto, K. Hagimoto, and K. Noguchi, “2OGbit/s long distance transmission using a 270 photon/bit optical preamplifier receiver,” Electron. Lett., pp. 715-716, April 1994. [I81 H. M. Presby and C. R. Giles, “Amplified integrated star couplers with zero loss,” IEEE Photonics Technol. Lett., vol. 3, pp. 724-726, Aug. 1991. [19] R. Braham, “Consumer electronics,” IEEE Spectrum, vol. 33, pp. 46-50, Jan. 1996. [20] R. Aravind, G. L. Cash, D. L. Duttweiler, H. M. Hang, B. G. Haskell, and A. Puri, “Image and video coding standards,” A T & T Technical Journal, vol. 72, pp. 6788, Jan./Feb. 1993. [21] D. F. Parsons, C. M. Fleischer, and R. A. Greenes, Eds., Extended Clinical Consulting by Hospital Computer Networks, New York: The New York Aaademy of Sciences. 1992.


J i a n - G u o Zhang was born in Chongqing, China, in January 1964. He received the M.Sc. degree in electronic engineering from the Beijing University of Aeronautics and Astronautics, Beijing, China, in 1988 and the Ph.D. degree in information technology from the University of Parma, Italy, in 1994, respectively. He spent two years a t the Chengdu Aircraft Company, P.R. China, where he conducted research on ophical fiber communication systems and MIL-STD-l773/1553B avionics optical/ electrical data buses. He was a Visiting Associate and then a Research Associate in the Department of Information Engineering of the Chinese University of Hong Kong for one year. He is presently an assistant professor in the Telecommunications Program, Asian Institute of Technology, Thailand. His research interests include new optical CDMA and TDMA techniques, high-speed optical fiber communication networks for digital TV and HDTV applications, digital communication systems, and coding theory. Up to now, he has published about 30 international journal papers in these fields. Dr. Zhang was a recipient of the “Ing. Migliorini” Scholarship from the Faculty of Engineering of the University of Parma, the 1991 Alcatel Face Research Center Scholarship, and the Special 1993 EMCSC Scholarship (granted by the World Federation of Scientists) for participating in the International School of Quantum Electronics in June 1993. He is a recipient of the Young Scientist Award from the 1995 International Symposium on Signals, Systems and Electronics (ISSSE’95) and is a recipient of the 1996 URSI Young Scientist Award from the International Union of Radio Science (URSI). He is also a member of the New York Academy of Sciences.

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