| Beyond Line Of Sight ("BLOS") communications | - Comms BLOS |
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GSM 900 has the advantages of the higher permissable transmit power and the ability of the lower frequency signals to reach places, leading to a slightly better reception for GSM 900 signals.
- from http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations/tables/gsm.html Phase and frequency synchronization must allow for Doppler shift for vehicle speeds up to 250 km/h as well as for frequency standard drift, and timing advance to compensate for propagation delay due to round trips for paths, in cells up to 35km radius. - from http://www.cellular.co.za/gsm-frequencies.htm In the UK, O2 (the old BT Cellnet) and Vodafone use GSM 900. Question posted on http://www.privateline.com/PCS/GSM07supplement.html "In GSM technology, I just need an explanation on how we are able to fit a gross bit rate of 270 Kbps in the 200 KHz channel on the air interface in the GSM system. GSM uses Gaussian-filtered Minimum Shift Keying or GMSK. That technology has a spectral efficiency of 1 bit/symbol/Hz. Does that mean we use 1 bit per symbol and not more?" Professor Levine responds: I am writing this quickly and may not remember all the numbers exactly, so if you find other numbers in other source documents, I may have the numbers wrong. GMSK modulation has a "spectral efficiency" of APPROXIMATELY 1 bit per symbol or 1 bit per hertz of bandwidth. The word "approximately" is used because there are several different ways to measure the bandwidth of a signal. ot; A normal or Gaussian distribution is also called a "bell-shaped curve" in some statistics books. e 99% bandwidth (called B), and the total bit or symbol duration (called T). This product is called BT. In GSM, a transition time giving a BT product of about 0.3 was chosen by the designers. It puts 99% of the signal power into a 200 kHz bandwidth centered at the carrier frequency. It also allows an almost error-free (about 1% bit error rate -- BER) demodulation of the binary data in the presence of noise at the ratio of 8 to 1 (signal power to noise power). Power ratio of 8/1 corresponds to 9 dB signal to noise ratio in logarithmic decibel units. This 1% BER can be handled adequately by the forward error correcting codes and other error protection methods used in the GSM system design.In the GSM system, with BT=0.3, the spectral efficiency is therefore about 1.35 bits/second/Hz (270/200). GMSK designs with different BT values have a different spectral efficiency value as well. Regards, Richard Levine Levine's page is here
The GSM-1800 system is as for the GSM 900 system, except for the following differences:
In the UK, Orange and One2One use GSM 1800.
- from http://www.rfsolutions.co.uk/acatalog/DS-GSMT35I-2.pdf
uplink path loss = L P
isotropic power gain of the antenna = G T
A Very Small Aperture Terminal (VSAT) is a device, known as an earth station, that is used to receive satellite transmissions. The "very small" component of the VSAT acronym refers to the size of the VSAT dish antenna, typically about 0.55 - 1.2 m in diameter, that is mounted on a roof or wall, or is placed on the ground. That size is appropriate for Ku band communications which is most used for current systems. In 1985, Schlumberger Oilfield Research co-developed the world's first Ku band (12-14 GHz ) VSATs with Hughes Aerospace to provide portable network connectivity for oil field drilling and exploration units. Ku Band VSATs make up the vast majority of sites in use today for data or telephony applications. Nearly all VSAT systems are now based on IP , with a very broad spectrum of applications. As of December 2004 , the total number of VSATs ordered stood at over 1 million, with nearly 650,000 in service. - from http://en.wikipedia.org/wiki/Very_small_aperture_terminal
Satellite orbits - from http://www.gilat.com/Content.aspx?Page=introduction_sat There are three areas for satellite orbits:
GEO satellites orbit the earth directly over the equator, approximately 35 400 km (22 000 miles) up. At that altitude, one complete trip (orbit) around the earth takes 24 hours. Thus, the satellite remains over the same spot on the surface of the earth (geo) at all times, and stays fixed in the sky (stationary) from any point on the surface from which it can be "seen." MEO is defined simply as the area between LEO and GEO. The primary satellite systems there are the GPS (Global Positioning System) satellite constellations. LEO is between 200 and 1400 km above the earth. Satellites in LEO rapidly circle the earth and are typically in range of one location for only 90 minutes. Their main advantage is how close they are, providing shorter delays for faster communications. However, for consistent communications they require a constellation of satellites so that communications can be maintained as one satellite moves out of range and another moves within range of the ground station. LEO satellites are less expensive to build, typically less powerful, and have a shorter average life span.
Above: satellites operating in the Ku microwave band COMMUNICATIONS SATELLITES Most communications satellites are in GEO. A single geostationary satellite can cover as much as 40 percent of the earth's surface; so, in theory, three such satellites can provide global coverage. To ensure accurate and strong coverage of a specific region, continent or country, the transponders are often “shaped” to focus transmission and increase signal strength for a service area. A satellite’s job in the communications network is to serve as a repeater. That is, it receives a signal from one location and rebroadcasts it so another station can receive the signal. Reception and retransmission are accomplished by a transponder. A single transponder on a geostationary satellite is capable of handling approximately 5,000 simultaneous voice or data channels. A typical satellite has 32 transponders.
Transponders each work on a specific radio frequency wavelength, or “band.” Satellite communications work on three primary bands: C, Ku and Ka. C was the first band used and, as a longer wavelength, requires a larger antenna. Ku is the band used by most current VSAT systems. Ka is a new band allocation that isn’t yet in wide use. Of the three, it has the smallest wavelength and can use the smallest antenna.
Because of attenuation and business competition, there are far more than three GEO satellites. Satellites of similar frequency can be as close as 3 degrees apart without causing interference. Since there are 360 degrees in a circle, that means 120 satellites of a specific frequency can be placed in GEO orbits. The combination of individual transponder volumes and the number of transponders in orbit means today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content, from simple data to the most complex and bandwidth-intensive video, audio and data content.
The Iridium satellite system
The Iridium Satellite Phone System is the only provider of truly global, truly mobile satellite voice and data solutions with complete coverage of the Earth (Iridium satellite telephone coverage includes oceans, airways and Polar regions). Through the Iridium constellation of 66 low-earth orbiting (LEO) satellites operated by Boeing, Iridium satellite phones delivers essential communications services to and from remote areas where terrestrial phone services are not available. Iridium satellite phone service is ideally suited for industrial applications such as heavy construction, defense/military, emergency services, maritime, mining, forestry, oil and gas and aviation. Iridium satellite currently provides satellite phone services to the United States Department of Defense and launched commercial satellite phone service in March 2001. - from http://www.satphone.co.uk/networks/iridium/...
The 375 gram Iridium / Motorola 9505A satellite phone, which can also be used as a data modem to support a data rate of 9.6 kbps. The Thuraya SO-2510 Satellite Phone
- from http://store.sattransusa.com/thu-bas-2510.html?gclid=CJmpseWV9pACFQqwQwodT2ik0w
9522 Iridium Data Modem (L-Band Transceiver, LBT)
- from http://www.voicemall.com/iridium_9522.htm
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