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  1. Abstract
  2. Introduction
  3. Upstream DWDM Network Architecture
  4. References
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Upstream DWDM Network Architecture

Dense wavelength division multiplexed (DWDM) technology utilizing a frequency-stacking scheme has emerged as a viable option for cable TV operators to solve their current return-path HFC network architecture design problems. Figure 1 shows a simplified block diagram of an upstream DWDM network architecture.1,2 It is assumed that each subscriber is connected to the closest fiber node via a coaxial cable plant. At the fiber node, a low-cost uncooled DFB or even Fabry-Perot laser transmitter operating at either the 1310 nm or 1550 nm wavelength band can be used to transmit the high-speed data in the 5–42 MHz band to the secondary hub. These low-cost upstream laser transmitters typically have an optical link loss of 5 dB at 1550 nm or 7 dB at 1310 nm. At the secondary hub, the upstream data can be aggregated to drive a directly modulated DWDM laser transmitter using time-division multiplexing (TDM) and frequency-stacking methods. Figure 2 shows, for example, the composite RF spectrum of four up-converted return-path frequencies blocks (5–42 MHz) using a commercial frequency-stacking system (FSS).2 A reference pilot tone is generated at a frequency of 370 MHz in the block up-converter in order to synthesize the four-band stack. The pilot tone is transmitted along with the up-converted signal and utilized in the block down-converter unit to synchronize the down-conversion, thus removing any frequency-offset errors. The composite RF signal is used to drive each of the DWDM upstream laser transmitters at the secondary hub. The DWDM upstream laser transmitters are operating in the 1550 nm wavelength band with an optical budget link of 8 dB. Using a 1x4 DWDM multiplexer, the optical signal is transmitted, for example, through 40 km of a standard SMF to the local head end or primary hub. At the primary hub, the optical signal is amplified, for example, with a 13 dBm inline Er-doped fiber amplifier, and optically demultiplexed to four optical receivers. The composite output RF signal from each optical receiver is transmitted to the block down-converter unit, which extracts the four separate 5–42 MHz bands. Then each of the high-speed data bands can be routed via the various return-path burst receivers or other network control and management equipment. It should be pointed out that the FSS is best utilized at the optical fiber nodes instead of the secondary hubs. The combined DWDM and FSS architecture provides, for example, a 32x return-path bandwidth expansion factor for an 8-wavelength DWDM network. The minimum required C/(N+I) at the optical receiver in the local head end is 37 dB with a spurious output level of -45 dBc. Typical output power level for each upstream DWDM transmitter is 7 dBm.

Figure 1 The return-path DWDM network architecture.

Figure 2 The composite RF spectrum of the four up-converted frequency bands.

Digital Return-Path Transport

There are two important factors that make the replacement of the analog return-path transport with a digital technology desirable. These factors are simplification of the reverse-path alignment, afforded by digital components, and lowering of the return-path system cost. These have been enabled by the continuous improvement in the performance of digital components while their cost declines, driven by high-volume applications.3

Figure 3 shows the basic elements of a digitized return-path in a DWDM network architecture. The composite analog return-path waveform is converted to a sequence of digital words by an analog-to-digital converter (ADC) operating at a typical sampling rate of 100 MHz with 8–12 bits of resolution. The parallel digital words are converted to a serial bit stream with the appropriate synchronization at the fiber node in order to recover the transmitted serial bit stream at the optical receiver output in the local head end. At the fiber node or at the secondary hub, several digital streams can be combined using a TDM scheme. For example, using two 12-bit ADCs (as shown in Figure 3), which are currently being deployed by cable TV operators, the laser transmitter is modulated by an approximately 2.5 Gbps TDM data stream (OC-48). The TDM data stream is optically multiplexed at the hub, and then demultiplexed at the local head end, where it is digitally demultiplexed and deserialized before it is presented to the digital-to-analog converter (DAC). The performance of the ADC is typically limited by its quantization noise, clipping noise, and distortions. Given the number of required resolution bits and the sampling frequency, the ADC noise density per unit bandwidth can be easily calculated from its SNR for a full-scale sine wave input. In analog fiber-optic links, the corresponding noise source is the laser relative-intensity noise. The performance of various components, particularly laser transmitters, in return-path cable TV networks is typically characterized in terms of the noise power ratio (NPR) curve. The NPR is simply the signal-to-noise plus the in-band clipping and distortion ratio. The NPR measurement loads the tested device with a Gaussian noise containing a notch at a test frequency. The depth of the notch is measured with respect to the noise power as it varies over the operating range of the tested device.4 The NPR curve is bounded by a Gaussian noise at low-input signal levels, and by a clipping noise and distortions at high-input signal levels relative to the optimum input level for a full-scale sine wave. The dynamic range (DR) of the component under test captures the range of input levels between the Gaussian noise limit and the clipping noise limit for a given NPR. Assuming a fully loaded return-path DWDM network, Table 1 shows the required NPRs and the corresponding DRs at the optical receiver output in the local head end using FSS and digital return-path technologies with 10-bits and 12-bits ADC and DAC. The cumulative NPR from the fiber node to the local head end includes a 1 dB RF contribution. Several points can be drawn from Table 1. First, a 12-bits digitized return-path technology at the fiber node provides the necessary NPR and DR at the local head end with about 2 dB margin over a 10-bit return-path technology. The return-path signals can also be digitized at the secondary hubs (12-bits), where typically an uncooled 1310 nm DFB laser transmitter with an output power of 3 dBm can be used at the fiber node. It should be pointed out that the required NPR and DR at the fiber node must be achieved over a wide temperature range. Second, the 10-bits digitized return-path technology does not meet the necessary NPR and DR at the local head end. The digitization of the return-path with TDM in a DWDM network has a significant advantage over analog fiber links. When the analog return-path signals are digitized, the DWDM network becomes transparent to them. In other words, the digital return-path signal can be efficiently transmitted throughout the network without a noticeable degradation. Furthermore, digital signal processing techniques can be used to minimize signal degradation before the ADC.

Figure 3 Block diagram of a digital return-path DWDM network architecture.

In conclusion, the use of FSS and digital return-path technologies in DWDM network architecture is very promising, since it offers cable TV operators the opportunity to significantly expand their return-path capacity while allowing network transparency and flexibility with cost savings. As the prices of the digital components continue to decline, driven down by high volumes, while their performance improves, the deployment of digital return-path technology becomes more and more attractive.

Table 1 Performance Requirements for Various Return-Path Technologies

Technology

NPR/DR (Node to SH)

NPR/DR (SH to PH)

Total NPR/DR (including RF)

DWDM (from fiber node)

40-dB/11-dB

42-dB/11-dB

37-dB/11-dB

DWDM+FSS (from fiber node)

38-dB/11-dB

37-dB/11-dB

DWDM+FSS (from SH)

40-dB/11-dB

42-dB/11-dB

37-dB/11-dB

Digitized at node (12-bits)

38-dB/11-dB

37-dB/11-dB

Digitized at SH (12-bits)

40-dB/11-dB

42-dB/11-dB

37-dB/11-dB

Digitized at node (10-bits)

36-dB/11-dB

Insufficient

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