Evolution and Applications

SCM systems have evolved with optical communication, which was first employed as a commercial transmission medium in early 1980s. In its initial stages they replaced point- to-point transmission links [22], employed in long distance communication. The superior performance of optical links were preferred to the existing coaxial and microwave links, whose transmission capacities were reaching their limits. Pioneering the move towards embracing SCM technology were telephony companies, and SCM was used in the trunk lines of the public telephony network [23].

As the source power and linearity improved the point-to-point systems evolved into point-to-multi-point systems. In multiple access networks employing time division multiple access (TDMA) technology, the bit rate that each user must handle grows linearly as the number of users increase. Hence the cost and complexity of each receiver is a problem in large networks [22]. Since SCM enables the bandwidth per node to remain constant as the number of nodes increase, it has found applications in local area network (LAN)s [24-25], multi-channel video distribution networks and bi-directional broadband networks [26-27].

Two techniques have been widely deployed in LANs using SCM. In the first technique, subcarriers are not part of the access method, but are simply carriers for conventional multiple access procedure. Standard LAN traffic is used as a data source and modulated on to a carrier to be transmitted along other services in a multi service environment [11].

The second technique is to use subcarrier frequencies for addressing in order to implement a multiple access procedure [25]. In these type of networks each node transmits on a given frequency and the receivers wishing to tune in select the appropriate subcarrier frequency. Figure 2.2 illustrates a NxN star coupler interconnecting N users. The user “J” transmits on a given frequency and the receiver can tune into any of the 1-N users. The speed at which the receiver can tune to any given subcarrier is very important, since it directly effect the access delay time of the implemented access procedure.

mod BPF demod Receiver user in NxN star coupler out

Alternatively the receivers can be allocated a unique carrier frequency with transmitters capable of transmitting at any of the receiver frequencies. In such a system the access delay depends on the speed of tuning of the subcarriers of the transmitter [11].

One of the most important applications of SCM is the distribution of community antenna television (CATV) channels. In the mid 1980s the feasibility of SCM for multi-channel video distribution was evaluated [28-29] and in early 1990s SCM was adopted as the industry standard to deliver analogue video channels [10, 30, 31]. Amplitude modulation-vestigial sideband (AM-VSB) is used as the modulation format as it is compatible with the current television receivers, but frequency modulation (FM) is also employed, especially in super trunking applications.

SCM-CATV networks offer increased reliability, repeater distance and power budgets, compared to the former coaxial networks. Additionally both CATV and telephone industries seek to extend fibre closer to the home and SCM optical networks are a platform to deliver future broadband services [22].

One of the drawbacks of the present network is the high signal to noise ratio (SNR)s required by AM-VSB channels, which can limit the number of channels and subscribers.

NTT Transmission Systems Laboratories, Kanagawa, Japan, has developed a fully engineered mutli-channel FM-SCM video distribution system (see Fig. 2.3) using FM [31]. The system performance is enhanced, due to FM modulation and upto 50 video channels can be distributed to more than 10,000 customers through three cascaded erbium doped fibre amplifiers. Several tests have also been carried out on the feasibility of SCM to deliver compressed and uncompressed video channels and future networks

may use more complex modulation methods such as multilevel quadrature amplitude modulation [32]. Customer Premises O -I csu -ONU SM Fiber Cable ^ ^ ^ 1 6 Branch ' V Multichannel/ Video Signal

ONU :Optlcal Network Termination Unit CSU :Channel Sellection Unit EDFA:Erblum Doped Fiber Amplifier OA&M C enter: Operation, Administration

and Maintenance Center

Headend Office C AfaP/BRCjTQ ■ . S D ,.0.0 r _,r n .O_X. <u 0 . o_r < u C L O 5 “ < n . C) ■pDC < CU rSYS-1 ’ __ i-SYS-0; i o - j : e d f a j - t optsw i<* [ J..JDFB-LD HfMMOCK* Ch1 - jFMMOcf** Ch2 <~)I2 • HfMMOdI ^ Ch50 ©T;i5o Status Mon.

&System Cont. ■to OA&M Canter

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Figure 2.3 System configuration of FM multi-channel video distribution system [31].

As the demand for broadband services grew in the late 1980s, SCM was one of the first techniques to be investigated to evaluate the possibility of broadband services delivery to home and business users. The attraction of SCM in a broadband environment was due to its low cost, high bandwidth, flexibility and upgradability.

The first field test experiment using optical fibres for the distribution of broadband

services was implemented by GTE laboratories in 1989 [33], employing binary phase

shift keying (BPSK) subcarriers. The system consists of 20 digitised video channels (16 broadcast, 4 switched) and one 4 Mbit/s data channel for downstream operation and a video and data channel for upstream operation. Services are distributed to 5 homes and the system is run for a trial period of 5 years to determine the public response to broadband services. A simplified block diagram of the central office downstream transmitter is illustrated in Fig. 2.4.

Distribution 107Mb/s _Network N b’cast channels Fibre Laser M switched channels Data _{2 Mb/s} A/D A/D A/D A/D Mod. Mod. Mod. Mod. Mod.

Figure 2.4 Simplified block diagram of the central office transmitter in BPSK subcarrier system [33].

Olshansky has also demonstrated a bi-directional hybrid fibre-coax network employing SCM [34]. Downstream traffic consists of 64 FM video channels and 32 data channels, each at a data rate of 1.5 Mbit/s. Thirty two customers are served from four optical network unit (ONU)s, each serving 8 subscribers via coaxial drops. Each subscriber is allocated a 1.5 Mbit/s upstream baseband digital channel which is modulated at the ONU and transmitted upstream to the central office. The topology of the distribution network is shown in Fig. 2.5.

:ONU :ONU

64 FM video channels

Central