DBS and MPEG compression

DIRECT BROADCAST SATELLITE

Direct Broadcast Satellite (DBS) is a satellite-delivered program service meant for home reception. DBS programming is, in most respects, the same as that available to cable television subscribers. DBS subscribers, however, do not access their programs from terrestrial cable systems but rather directly from high powered telecommunications satellites stationed in geosynchronous orbit some 22,000 miles above the earth.

Direct Broadcast Satellite (DBS) TV service is a high-powered broadcast service to homes using satellites as the primary form of signal transmission. Its high transmission power makes possible use of relatively small dish antennas for efficient signal reception and utilization. Commercial satellite TV, as we know it, is a DBS service.

Advantages of Direct Broadcast Satellite (DBS) service

More channels per dollar
The receiver components which interface the satellite signal to your television also serve as decoders. Therefore, the DBS providers are able to use digital compression techniques to carry up to ten times as many channels on the satellite transponders as would otherwise be possible. As a general rule, you will get many more channels per dollar spent with a satellite service than with cable TV.

Rural availability
For some people in remote rural areas, the cable vs. DBS argument is moot because cable is simply not available. Satellite service, on the other hand, is available anywhere in the contiguous 48 states (with more limited availability in Alaska and Hawaii) as long as there is a clear line of sight to the position of the satellite in the sky.

Reliable service
The cable infrastructure is always at the mercy of accidents resulting in downed lines or severed cables. The only anamolies that typically affect satellite broadcasts (aside from accidents involving your own dish antenna) are extremely severe weather, or solar interferences during the equinoxes, and these are rare.

Digital picture/sound
The analog signals sent over standard cable lines are subject to degradation, interference, and other factors that can result in a less than stellar picture. Satellite signals are digital, and like a compact disc, are not subject to depreciation in picture or sound quality. Satellite TV generally looks and sounds far superior to cable transmissions.

Interactive channel guides
Some cable systems include a channel which shows program listings. These usually scroll along at an unbearably slow pace and cannot be advanced to show more than a couple of hours of upcoming shows. DirecTV and Echostar include highly interactive program guides that can be manually advanced, or can display additional program information, and sometimes can provide one-touch timed VCR recording. Certain programming can also be locked out based upon content ratings.

Moving Picture Experts Group (MPEG)

It is a working group of experts that was formed by ISO and IEC to set standards for audio and video compression and transmission. It was established in 1988 by the initiative of Hiroshi Yasuda (Nippon Telegraph and Telephone) and Leonardo Chiariglione, group Chair since its inception.

MPEG algorithms compress data to form small bits that can be easily transmitted and then decompressed. MPEG achieves its high compression rate by storing only the changes from one frame to another, instead of each entire frame. The video information is then encoded using a technique called Discrete Cosine Transform (DCT). MPEG uses a type of lossy compression, since some data is removed. But the diminishment of data is generally imperceptible to the human eye.

The major MPEG standards include the following

MPEG-1: The most common implementations of the MPEG-1 standard provide a video resolution of 352-by-240 at 30 frames per second (fps). This produces video quality slightly below the quality of conventional VCR videos.

MPEG-2: Offers resolutions of 720×480 and 1280×720 at 60 fps, with full CD-quality audio. This is sufficient for all the major TV standards, including NTSC, and even HDTV. MPEG-2 is used by DVD-ROMs. MPEG-2 can compress a 2 hour video into a few gigabytes. While decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power.

MPEG-3: Was designed for HDTV but was abandoned in place of using MPEG-2 for HDTV.

MPEG-4: A graphics and video compression algorithm standard that is based on MPEG-1 and MPEG-2 and Apple QuickTime technology. Wavelet-based MPEG-4 files are smaller than JPEG or QuickTime files, so they are designed to transmit video and images over a narrower bandwidth and can mix video with text, graphics and 2-D and 3-D animation layers. MPEG-4 was standardized in October 1998 in the ISO/IEC document 14496. See MPEG-4.

MPEG-7: Formally called the Multimedia Content Description Interface, MPEG-7 provides a tool set for completely describing multimedia content. MPEG-7 is designed to be generic and not targeted to a specific application.

MPEG-21: Includes a Rights Expression Language (REL) and a Rights Data Dictionary. Unlike other MPEG standards that describe compression coding methods, MPEG-21 describes a standard that defines the description of content and also processes for accessing, searching, storing and protecting

Encoding

Prior to MPEG 1 Audio, audio compression usually consisted of removing statistical redundancies from an electronic analog of the acoustic waveforms. MPEG 1 Audio achieved further compression by also eliminating audio irrele-vancies by using psychoacoustic phenomena such as spectral and temporal masking. One way to explain this is that if the signal at a particular frequency is sufficiently strong, weaker signals that are close to this frequency cannot be heard by the human auditory system and, therefore, can be neglected entirely.

In general, the MPEG 1 Audio encoder operates as follows. Input audio samples are fed into the encoder. For DBS, these samples are at a sample rate of 48 Ksamples per second (for each of the stereo pairs), so this rate will be used exclusively in the rest of this chapter. Each sample has 16 bits of dynamic range. A mapping creates a filtered and subsampled representation of the input audio stream. In Layer II, these 32 mapped samples for each channel are called subband samples.

Decoding

The basic structure of the audio decoder reverses the encoding process. Bit-stream data is fed into the decoder. First, the bitstream is unpacked with the main data stream separated from the ancillary data. A decoding function does error detection if an error_check has been applied in the encoder. The bitstream data is then unpacked to recover the various pieces of information. A reconstruction function reconstructs the quantized version of the set of mapped samples. The inverse mapping transforms these mapped samples back into a Pulse Code Modulation (PCM) sequence. The output presentation is at 48 Ksamples per second for each of the two stereo outputs. The specific parameters utilized by DBS are shown in table below.

References

http://www.museum.tv/eotvsection.php?entrycode=directbroadc

http://dish-cable.com/DBS.htm

http://www.maplenet.net/~trowbridge/DBSintro.htm

http://en.wikipedia.org/wiki/Direct-broadcast_satellite

http://www.webopedia.com/TERM/M/MPEG.html

Satellite Multiple Access

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon

In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. So, all the signal transferring is happening in space. Thus this type of communication is known as space communication

Multiple Access Defined

Multiple access in satellite terms involves running communication streams between multiple satellite conduits or terminals at the same time. Normally, in simple traffic a terminal only handles one stream at a time. This approach doesn’t work when a satellite’s owner needs it to function managing thousands of points simultaneously. As a result, satellite technology today works with three different systems that offer multiple access ability.

Types of Multiple Access Methods

There are three types of Multiple Access Methods:

1. Frequency Division Multiple Access (FDMA) – flexible and simple
2. Time Division Multiple Access (TDMA) – popular
3. Code Division Multiple Access (CDMA Spread Spectrum) – highly secure

Frequency Division Multiple Access (FDMA)

• Satellite frequency is already broken into bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA)

• Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization)

• The number of sub-channels is limited by three factors:
  • Thermal noise (too weak a signal will be effected by background noise)
  • Inter modulation noise (too strong a signal will cause noise)
  • Crosstalk (cause by excessive frequency reusing)

• FDMA can be performed in two ways:
  • Fixed-assignment multiple access (FAMA):      The sub-channel assignments are of a fixed allotment.  Ideal for broadcast satellite communication
  • Demand-assignment multiple access (DAMA):  The sub-channel allotment changes based on demand.  Ideal for point to point communication

 

Time Division Multiple Access (TDMA)

• TDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter
• TDMA is increasingly becoming more widespread in satellite communication
• TDMA uses the same techniques (FAMA and DAMA) as FDMA does

Advantages of TDMA over FDMA.

• Digital equipment used in time division multiplexing is increasingly becoming cheaper
• There are advantages in digital transmission techniques.  Ex: error correction
• Lack of inter-modulation noise means increased efficiency

Code Division Multiple Access (CDMA Spread Spectrum)

• CDMA, also called spread spectrum communication, differs from FDMA and TDMA because it allows users to literally transmit on top of each other
• This feature has allowed CDMA to gain attention in commercial satellite communication
• It was originally developed for use in military satellite communication where its inherent anti-jam and security features are highly desirable
• CDMA was adopted in cellular mobile telephone as an interference-tolerant communication technology that increases capacity above analog systems
• Two forms of CDMA are applied in practice:

1.   Direct sequence spread spectrum (DSSS)
2.   Frequency hopping spread spectrum (FHSS)

FHSS has been used by the OmniTracs and Eutel-Tracs mobile messaging systems for more than 10 years now, and only recently has it been applied in the consumer’s commercial world in the form of the Bluetooth wireless LAN standard. However, most CDMA applications over commercial satellites employ DSSS (as do the cellular networks developed by Qualcomm)

• A typical CDMA receiver must carry out the following functions in order to acquire the signal, maintain synchronization, and reliably recover the data:
  • Synchronization with the incoming code through the technique of correlation detection
  • De-spreading of the carrier
  • Tracking the spreading signal to maintain synchronization
  • Demodulation of the basic data stream
  • Timing and bit detection
  • Forward error correction to reduce the effective error rate

Summary

• The bottom line in multiple access is that there is no single system that provides a universal answer
• FDMA, TDMA, and CDMA will each continue to have a place in building the applications of the future
• They can all be applied to digital communications and satellite links
• When a specific application is considered, it is recommended to perform the comparison to make the most intelligent selection

References

http://www.en.wikipedia.org/wiki/Channel_access_method

http://www.jsati.com

http://www.ehow.com/about_6814542_multiple-access-techniques-satellite-communication.html

courses.missouristate.edu/huiliu/csc690/slides/satellite.ppt

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