By Andrew Aroh

The next several years will see a rapid progression in deployment of broadband satellite services. Transitional satellite payload designs will be based on conventional transponders.

But eventually, on-board switching and processing payload designs will exploit broadband satellite and frequency resources.

The key to success? Deliver a cost-competitive service and do so in a manner that is flexible enough in design to keep up with a very dynamic decade of telecommunications changes. Satellites that will successfully complement terrestrial telecom networks must deliver services that are better provided from space.

Traffic modeling has shown that successful designs will be those that provide the largest amount of traffic capacity while also providing flexibility to allow use of the bandwidth in a manner that is not fixed but market responsive.

The progression of satellite capabilities has led to an entirely new emerging generation of satellites that process signals in space to achieve both capacity and flexibility. Future satellite systems will have market demand similar to terrestrial telecom – more bandwidth, improved Functionality, lower price.

The broadband systems with the greatest capacity and simplest terminals to use will have a market advantage. But there are important differences. This is where the flexibility of broadband satellite services becomes important.

Consumers will become accustomed to always-on, high-speed data connections as part of daily existence. Unfortunately, terrestrial broadband links, whether wired or wireless, typically have shorter range that the voice-based systems they will replace.

This can lead to pockets of broadband isolation even in relatively urban areas, let alone areas where low density population or economic deprivation render dedicated terrestrial systems uneconomical.

For these areas, satellites appear the natural broadband choice. Satellites have always been providers of true universal service – signals from space do not care whether they land on urban or rural antennas.

What differentiates the coming broadband services from past satellite systems is that the target price will be comparable to terrestrial delivery, not a premium. This means that satellite delivered broadband has the potential to reach beyond hard to reach areas to become a true mass market service, just as direct-to-home satellite TV has become.

The large geographic scope, however, means that each satellite will serve multiple countries with a wide range of bandwidth demands and service expectations over its lifetime. For a time some countries may have a high demand for internet access, while others may be heavy generators of video multicast or corporate extranets.

Not only will these change gradually with the economic climate but there will also be more rapid changes on an hourly basis, as the day sweeps across a continent or special events suddenly shift demand. Only by employing the latest bandwidth-on-demand techniques can satellite designers ensure that such systems will be cost effective over their entire life cycle.

SATCOM professionals are aware that Ka-band allows satellite to provide access oriented circuits to an entire continent at prices comparable to terrestrial delivery in cities. These will find broad residential applications where access services are more price sensitive.

Moreover, in order to provide the flexibility required by the long life and broad geographic scope, a satellite must be able to shift capacity among beams. This requires onboard switching, which gets more complex as the number of beams increases. But if the traffic from many users is combined in a satellite terminal to fill the data channel continuously this will be a very efficient system.

However, as satellite terminals get smaller and cheaper, they become more single-user-dedicated (User Oriented) and the circuit takes on a bursty traffic pattern. The power-limited satellite downlinks are not completely filled with data and therefore the capacity used less efficiently.

By moving the traffic routing function from a central ground facility (HUB/NOC) up to the spacecraft, system designers can combine the traffic from many bursty users before it gets to the satellite downlink transmitters dramatically improving efficiency and with it the billable bits result in higher revenues for the system.

The efficiency can provide higher bandwidth-on-demand availability and quality of service (QoS) to more users because it dynamically and automatically allocates unused capacity as needed. Customers pay only for what services they need at the moment they need it, at whatever quality level they choose.

So, what are the relevant questions now?

The processed payload design is almost identical to the transponded design. However, the on-board circuit switch found on transponded designs is replaced by onboard demodulators, a switch and a set of downlink modulators. What are the main advantages?

ANS:

In addition to highly efficient use of the downlink capacity, the use of on-board demodulation leads to improved link performance, which reduces both user terminal size and transmit power requirements, since the signal is regenerated before downlink noise is added.

In addition to isolating the uplink and downlink interference, on-board demodulation also provides a high degree of access security, ensuring that rogue terminals do not rob power from the downlink.

The heart of processed broadband satellite is a fast packet switch. How?

ANS:

The irregular incoming data streams are pooled and distributed to the output channels, ensuring that the downlink remains maximally loaded under all traffic conditions.

This is called statistical multiplexing – the multiplexing of several partially utilized uplink channels (bandwidth allocations) into downlink channels which fully utilize the capacity for user data. This process is an extremely important capability since most services present statistically bursty data traffic to the satellite uplinks.

For network operators transitioning from a traditional transponded to the emerging generation of processing architectures raises the question as to which architecture maximizes throughput and revenues?

ANS: THE TRANSPONDED ARCHITECTURE:

The transponded architecture requires extensive ground infrastructure interconnections to deliver full-mesh connectivity. Even then, with only one beam per satellite, frequency reuse is not achievable. This limits the throughput and revenue-generating capacity of the transponded architecture.

The circuit switch architecture enables frequency reuse by employing multiple beams. Inter-beam connectivity is only achieved via ground gateway interconnections.

The advantage of the circuit switch over the transponded architecture comes from the frequency reuse gains, and its ability to route bandwidth to others smaller geographical regions. These architectures are ideally suited for traditional broadcast television services, but do not efficiently transport multimedia applications from a large numbers of geographically distributed users.

ON-Board Processing Architecture:

After quantifying and analyzing the throughput potentials of the two architectures, the result shows the improvement that on-board processing provide to the average throughput, a measure of deliverable user data, under mixed traffic conditions.

The raw capacity and average throughput (billable bits) of the processing architectures are significantly larger than the transponded architecture as a result of the frequency reuse possible without a terrestrial infrastructure.

The most flexible and efficient approach is the fast packet switch architecture. Why?

ANS:

1.  It allows a user in one beam to connect directly to any other terminal in the network without investing in significant ground infrastructure.
2. The fast packet switch also takes advantage of the bursty nature of variable rate multimedia sources to most efficiently utilize the raw downlink capacity to achieve billable user throughput.
3. The fast packet switch architecture delivers a substantial advantage over the other architectures in terms of available billable throughput and ground infrastructure requirements, enabling the network service provider to market and sell gigabits of capacity and very profitable support a variety of traffic scenarios.
4. In addition to the capacity advantages that processed payloads provide, onboard switching allows the capability to dynamically configure the satellite connectivity to service any number of specialized uses simultaneously. This allows multiple services to be offered with an ability to shift the satellite bandwidth allocation as market demands change.
5. A nimble fast packet switch can handle basic internet protocol (IP) services, point-to-point and multicast video distributions/contributions, local-into-local retransmission, local content insertion into national feeds, corporate mesh virtual private networks (VPN) and a host of other specialized broadband services.
6. The primary difference between the processed and transponded payloads is the switch.

The recurring cost associated with the complex digital implementation is a bit higher than a simple circuit switch, but is relatively minor cost increase when compared to the investment required for the remainder of the satellite and the ground network infrastructure.

The additional capacity or billable bits far outweigh the small incremental cost increase for the processed system compared to the transponded systems.

SOFTWARE ASPECT:

• Recall that software is a crucial part of the correct functioning of the OBP. It must:
  • Cope with diagnostics from the hardware
  • Be fault tolerant
  • Enable reconfiguration
  • Be self-checking
• For on-board processing Satellite, it is important to realize that there are varying degrees of processing that can be applied:
  • Regenerative transponder: Demodulation to baseband and re-modulation takes place, subsequent to baseband regeneration of the digital bit stream-modulation and coding.
  • On-board switching/Networking control
  • Access format conversion: (e.g. FDMA-TDM)
  • Flexible baseband routing
  • Onboard ARQ
• On-board processing aims to put the complexity in the Satellite and to reduce the cost of the use of the space segment and cost of the earth terminals.
• Both Modulation and Coding which takes place subsequent to baseband regeneration of the digital bit stream can be matched to the channel characteristics (essential in mobile systems) and adaptive techniques can also be incorporated with control onboard the satellite to further improve the performance.

The regenerative transponders can withstand much higher levels of interference for the same overall C/N_T, even though the Eb/No are cumulative and not the C/No as in a transparent transponder. Hence in an interference-limited environment, which will progressively be the case in future, on-board processing has an important role in easing co-ordination problems.

• In the area of multirate (LDR, MDR and HDR) digital processing, on-board processing satellite contains Multi carriers demodulators which consist of a digital demultiplexer (Transmultiplexer-TMUX) plus digital demodulator.
• In the digital TMUX architecture, filtering is the fundamental process used. The three major types of transmultiplexer are:
  • Single-state single-path
  • Multistage single-path
  • Multipath
• It can be seen that new onboard hardware and software is required for OBP. In hardware we need on-board switches, multi carrier demodulators and distributed process control. Technology and architectures are the major areas of concern. Technology imposes the following constraints:
  • Power dissipation
  • Mass/Volume
  • Radiation hardness (note difference between orbits)
  • Reliability
  • Thermal dissipation
  • Device and unit package
• Andrew Aroh is President SSPI Nigeria