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Issue 7: Current Developments - ArcLight Technology

Interactive VSAT Communications

Download the full paper in PDF (296 KB)


This paper presents some additional information about ViaSat's ArcLight Technology, a true breakthrough in satellite communications.

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This new system is the first interactive VSAT to use CDMA technology. Along with the ViaSat bandwidth saving technique called Paired Carrier Multiple Access (PCMA), this combination makes ArcLight the most bandwidth efficient and responsive VSAT available.

Introduction

ArcLight was engineered to answer two primary needs:

  • To support the business world's move to Internet Protocol (IP), packet-based networking
  • To reduce the cost of satellite networking.

Those needs break down further into the following requirements:

  • Handle IP packets efficiently
  • Match "bursty" nature of packet-based networks
  • Give users faster, more responsive access to networks and data centers
  • Cut use of satellite bandwidth
  • Increase the capacity of satellite transponders
  • Build on industry-standard (i.e., DVB) technology where practical.

To meet those requirements, ArcLight brings two technologies to interactive, point-to-multipoint (star) Very Small Aperature Terminal (VSAT) networking for the first time. One is CDMA, a proven technology in cellular phone markets that we bring to the satellite world with our Code Reuse Multiple Access (CRMA). The other is our patented bandwidth sharing technique called Paired Carrier Multiple Access (PCMA).

The ArcLight system, like many two-way VSAT networks, operates with a high rate outbound (hub-to-user) channel, and a large number of lower rate inbound (user-to-hub) channels. However, ArcLight achieves significantly greater capacity and throughput performance from these channels, compared to analogous competing systems, through the application of these two key ViaSat-developed technologies.

The rest of this paper explains the function of these technologies, how they are implemented in ArcLight, and gives examples of some customers already benefiting from the bandwidth savings of PCMA.

PCMA Basic Concept

The foundation for PCMA is captured in Figure 1 - PCMA Concept. Two satellite terminals are engaged in 2-way communication. For PCMA, we require the following assumptions for the satellite links (which are valid for most existing and planned fixed site systems):

  • The satellite operates in a "loopback" mode. That is, signals transmitted by each terminal can also be received by each terminal (with some, as yet undefined, signal to noise ratio).
  • The satellite uses a "bent-pipe" transponder. That is, the satellite does not digitally demodulate the uplink signals and remodulate them on the downlink. Functionally, the operations performed by the satellite transponder are bandpass filtering, frequency translation and amplification.

Each terminal transmits an uplink signal and receives a downlink signal from the other terminal. To date, all satellite links have separated the two uplink signals in one or more dimensions using one or more of the common multiple access techniques (e.g., TDMA, FDMA, and/or CDMA).

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For instance, with CDMA, the signals may use the same bandwidth at the same time, but they are discriminated via distinct CDMA codes. With PCMA we intentionally allocate the same multiple access dimensions to a pair of satellite terminals.

Overlapping Outbound Broadcast and Remote Returns Can Cut Transponder Use by Half

In the case of the ArcLight application, PCMA enables multiple, lower rate inbound channels to occupy the same physical bandwidth as the high-data-rate DVB, outbound transmission. As the following diagram illustrates, the outbound channel in other two-way broadband VSAT implementations occupies most (or all) of one transponder, and a second separate transponder is required to carry the return channels. With PCMA processing, only a single transponder is required, resulting in a reduction by half of recurring, space-segment costs.

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"Free" Return Channels

vEven more remarkably, this benefit is achieved essentially "free", from a remote terminal cost perspective. ArcLight remote terminals do not require any PCMA-related hardware or processor loading because of the asymmetric nature of the network transmissions. The Hub terminal uses PCMA to cancel its own transmission in order to detect the lower power transmissions from the small remote terminals. However, the remotes receive the outlink DVB transmission at a much higher signal-to-noise ratio than the transmissions from other remotes, and therefore do not require any PCMA processing. This one-sided nature of the processing is what gives the technique the title "Asymmetric PCMA."

CRMA - CDMA for Satellite

ViaSat's new CDMA technology gives users faster access to the network. because CDMA bursts do not require any particular time slots or synchronization. ArcLight Subscriber terminals access the satellite return channel using a multiple access technique called CRMA - or Code Reuse Multiple Access. CRMA is a direct sequence spread spectrum technique, similar to conventional Code Division Multiple Access (CDMA) used in cellular telephony.

Faster Access to the Network

Spread spectrum techniques accept a number of channels at once and give each a unique code. This method is opposed to others that have to fit data into a linear sequence of time slots each time they need to transmit. Typically, users are accessing data many times each "session," so the delay adds up with each access of the network. A system using CRMA-based return channels enjoys a throughput advantage by enabling any terminal - or all terminals at once - to burst information to the hub at any time. In addition, CRMA does not require frequency overhead for guard bands, so users gain access to a higher percentage of their full burst rate - up to 512 kbps in the case of ArcLight - and gain more bandwidth efficiency.

ArcLight CRMA subscriber terminals can achieve additional cost efficiency in regards to required RF sizing. The constant envelope of the GMSK waveform enables operation of the Transmit Block in saturation, so a CRMA terminal can provide a higher data rate for a given SSPA size than VSATs with traditional BPSK/QPSK modulation schemes. In addition, after an ArcLight CRMA terminal data burst, any unused capacity is instantaneously returned to the shared capacity pool for reuse by other terminals in the network.. This process optimizes capacity use, particularly under very bursty data traffic conditions.

Application Examples

ArcLight is just beginning to be deployed, but the technologies behind it are well proven. The benefits of CDMA are obvious from its success in the cellular phone industry. As for PCMA, several ViaSat customers are enjoying its benefits through the use of our StarWire product line.

One such case is the Beijing Application Institute of Information Technology (BAIIT). BAIIT, with branches in most major cities in China, is a leading IT service provider. It operates and manages The Private Satellite Communication Network of BAIIT, a set of integrated satellite network products that focus on multimedia applications including video, data, and voice. BAIIT uses PCMA to double the capacity of its network, so StarWire can serve as the primary infrastructure in this complex and dynamic network. StarWire with PCMA is also being used to replace SCPC modems for permanently assigned ("nailed up") clear-channel circuits. The China National Offshore Oil Corporation is in the process of replacing its existing VSAT and SCPC modems with StarWire PCMA VSAT terminals. CNOOC has a hub in Beijing, connected to 6 regional "sub-centers" which in turn connect to more than 50 remote oil drilling rigs. StarWire's PCMA advantage saves this customer 10 MHz of transponder rental.

Immeon, ViaSat's North American satellite services brand, is serving many customers with PCMA-enhanced networking as well. Customers include Global Communication Systems, Havertys, New York State Insurance Fund, Florida Power and Light, and National Distributing Company.

Multiple Needs Among Your Network Customers - ViaSat VSATs Match Them All

If you're already a ViaSat LinkStar customer, you can enhance your current system by adding ArcLight. Both networks build from the same DVB-S uplink chain at the hub, so you can start with one, change to the other, or keep both, all managed from a single network management system computer. You'll be assured that ViaSat broadband VSATs offer you a future technology path and multiple access methods tailored to your application.

Summary

Using new technologies from ViaSat, ArcLight will reduce the cost of IP networking over satellite, while giving network customers a better user experience. The combination of CDMA and PCMA overlaps hub and remote transmissions to cut bandwidth use by as much as half. CDMA technology in ArcLight remote terminals gives users high performance, low latency, and bandwidth efficiency. The latest in VSAT technology is here, and it's ArcLight. Please call the ViaSat representative in your region to discuss your networking needs.


ViaSat Inc.: http://www.viasat.com/

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