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Input: Tools Of The Trade

Paradise Datacom’s PCMA-70 BANDWIDTH EXTENDER is designed to provide satellite-based system operators with a way to greatly increase their utilization efficiency of transponder spectrum to reduce operating costs.

The degree of savings depends on a number of factors including satellite performance and link design as it relates to the transponder power and bandwidth required to accommodate the traffic. System design considerations are expressed later in this article.

PCMA-70 uses Paired Carrier Multiple Access (PCMA) a patented technology developed by ViaSat. PCMA uses an adaptive, self-interference cancellation technique to allow transmit and receive carriers of a duplex link to be superimposed on to one another to achieve space segment savings of up to 50 percent (See Figure 1.)

With the PCMA-70 installed in the modem’s IF loop, a sample of the transmit signal is routed to the cancellation circuits where it is modified to emulate propagation effects and then used to eliminate the transmit carrier signature from the composite local + distant signals being received from the satellite.

PCMA-70 is housed in a 1-RU (1.75”) chassis and is available in a number of band-pass configurations including 2, 5, 10, 20 and 36 MHz. Redundancy switching is also available.

System Design Considerations for PCMA
The extent of savings that can be realized by implementing PCMA is dependant upon a number of factors that should be confirmed by performing a thorough satellite link analysis. For optimum performance, the following conditions should be met:
  • Links must be bandwidth-limited as opposed to power limited. For maximum benefit, required XPDR power should be half of required bandwidth so that superimposed carriers are balanced
  • Each site in the network must be able to receive its own carrier (i.e., no cross-strapped transponders
  • Only signals with an occupied bandwidth of 150 kHz to 36 MHz will work with PCMA-70.
  • Automatic Uplink Power Control (AUPC) is recommended to maintain power levels during rain-fade conditions since maintaining the carrier levels to +/- 5 dB limits Eb/No degradation to ~0.2 dB.
  • Power asymmetry between carriers should be no more than 10 dB.
  • For existing links, signal power must be increased 0.15 - 0.5 dB, depending on relative carrier power ratio.
For cases in which the system architecture has a power limitation component preventing the ability to achieve maximum benefit from PCMA, changes in ground station components and settings can provide the additional power necessary. These changes include:
  • Increasing antenna size at one or both ends of the link
  • Improving coding gain i.e. changing from Viterbi to TPC/LDPC or modulation/FEC rate i.e. 8PSK 7/8 to QPSK ¾ therefore requiring less power to achieve a target bit error rate (BER).
  • Using a higher quality LNA/LNB
  • Switching to a higher power transponder
SCPC Applications
For single point-to-point or co-located multi-carrier point-to-point applications, a PCMA-70 can be placed on both ends of the link as depicted in Figure 2. This can be provided in the form of outboard PCMA-70s or Paradise modems with embedded PCMA, or a combination of both. The diagram on the next page shows a simplified diagram of a PCMA-70 installed in the TX and RX IF links of a Quantum satellite modem.

The principal benefit of using a PCMA-70 over the embedded PCMA is that multiple modems can be trunked into a single PCMA-70 using a passive combiner/divider network as long as the aggregate traffic does not exceed 36 MHz of bandwidth.

For point-to-point links, maintaining similar carrier power levels minimizes Eb/No degradation with the effect being less significant for lower order modulation schemes. For example, two TPC/QPSK carriers with a 10 dB power ratio will experience a maximum degradation of ~0.4 dB whereas changing the modulation to TPC/8 PSK increases degradation by ~0.5 dB.

Point-to-Multipoint Applications
The spectrum savings benefit from the PCMA-70 is equally applicable to many VSAT or point-to-multipoint system architectures. In the example depicted in Figure 3, multiple return signals are being transmitted within the larger outbound carrier’s spectral foot print. A single modulator generates an outbound carrier sending information to four remote sites. The remotes generate lower data-rate return carriers that are placed within the same transponder spectrum being occupied by the larger outbound carrier. The composite TX + RX carriers received at the hub are routed through the PCMA-70 where the outbound carrier originated at the hub is removed leaving only the four carriers received from the remotes.

In point-to-multipoint applications, maximum savings is achieved when the return carriers will fit within the spectral footprint of the hub’s outbound carrier. The hub carrier level should be a minimum of 10 dB above the level of the return carriers to insure that the remote sites receive the outbound carrier with sufficient carrier-to-interference (C/I). No cancellation is required at the remotes as the return carriers will be treated as interference or system noise in relationship to the larger outbound carrier.

The satellite link analysis typically includes a small amount of additional system loss to compensate for this effect. Since in many STAR and VSAT network architectures, the hub-to-remote ratio can be high, the symbol-rate asymmetries between the outbound and inbound signals must be accommodated by the signal canceller. Asymmetries of up to 20:1 have been recorded and higher asymmetries are certainly possible.

It is important to note that the system architecture described in Figure 3 does not require carrier cancellation at the remote sites. Consequently, a single PCMA-70 at the hub station is all that is required. This can result in significant savings over embedded technology for point-to-multipoint topologies.

Embedded PCMA
For sites that are equipped with a single modem, an embedded version of PCMA can be purchased as a field upgrade for any Paradise Datacom Evolution, Vision or Quantum modem shipped after January 2009. Modems shipped prior to January 2009 can be retrofitted at one of Paradise Datacom’s factories in the U.S. or the U.K. Currently, modems can be ordered with the internal PCMA hardware installed at the factory prior to shipment allowing the feature to be installed on site at a future date.

QUANTUM and VISION modems with PCMA provide the added benefit of DVB-S2 technology for an even greater degree of spectral efficiency. DVB-S2 alone can increase efficiency over Turbo Product Code by 15 percent and DVB-S by 30 percent.

Paradise Datacom system engineers can perform an in-depth satellite link analysis to identify the potential benefits of implementing this cutting-edge technology.