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ADVANCED COMMUNICATION SYSTEMS Module 2 Types of satellite communication system-FSS, DSS-Direct broadcasting and community broadcast - Multiple Access Techniques– Introduction- FDM-FM-FDMA, PSK-TDMA, SSMA, CDMA - Switching techniques – circuit – message - packet switching- Packet satellite network- Fixed Service Satellite (or FSS), is the official classification (used chiefly in North America) for geostationary communications satellites used for broadcast feeds for television and radio stations and networks, as well as for telephony and data communications. FSS satellites have also been used for Direct-To-Home (DTH) satellite TV channels in North America since the late 1970s. This role has been mostly supplanted by direct broadcast satellite (DBS) television systems starting in 1994 when DirecTV launched the first DBS television system. However, FSS satellites in North America are also used to relay channels of cable tv networks from their originating studios to local cable headends and to the operations centers of DBS services (such as DirecTV and Dish Network) to be re-broadcast over their DBS systems. FSS satellites were the first geosynchronous communications satellites launched in space (such as Intelsat 1 (Early Bird), Syncom 3, Anik 1, Westar 1, Satcom 1 and Ekran) and new ones are still being launched and utilized to this day. FSS satellites operate in either the C band (from 3.7 to 4.2 GHz) or the FSS Ku bands (from 11.45 to 11.7 and 12.5 to 12.75 GHz in Europe, and 11.7 to 12.2 GHz in the United States). FSS satellites operate at a lower power than DBS satellites, requiring a much larger dish than a DBS system, usually 3 to 8 feet (0.91 to 2.4 m) for Ku band, and 12 feet (3.7 m) or larger for C band (compared to 18 to 24 inches (460 to 610 mm) for DBS dishes). Also, unlike DBS satellites which use circular polarization on their transponders, FSS satellite transponders use linear polarization. Systems used to receive television channels and other feeds from FSS satellites are usually referred to as TVRO (Television Receive Only) systems, as well as being referred to as big-dish systems (due to the much larger dish size compared to systems for DBS satellite reception), or, more pejoratively, BUD, or big ugly dish systems. The Canadian Shaw Direct satellite TV service relies on FSS satellite technology in the Ku band. Primestar in the USA used Ku transponders on an FSS satellite as well for its delivery to subscribing households, until Primestar was acquired by DirecTV in 1999. FSS and the rest of the world The term of Fixed Service Satellite is chiefly a North American one, and is seldom used outside of the North American continent. This is because most satellites used for direct-to-home television in Europe, Asia, and elsewhere have the same high power output as DBS-class satellites in North America, but use the same linear polarization as FSS-class satellites. Dish Network and FSS The Dish Network satellite TV service also relies on FSS satellite technology in the Ku band to provide the necessary additional capacity to handle local channels required by FCC must-carry rules and make room for HDTV resolution. The old SuperDish system that Dish ceased manufacture of years ago, receives circularly-polarized DBS 12.7 GHz from both 110-degree (the Echostar 8 & 10 satellites) and 119-degree (the Echostar 7 satellite) orbital locations as well as linearly-polarized FSS 11.7 GHz from either the 121-degree (Echostar 9) or 105-degree (AMC 15) orbital locations depending on consumer choice. Those FSS satellites are no longer used for Dish Network home subscribers, and are now used exclusively for commercial or corporate services. Dish now uses the 118.7-degree (Anik-F3 -FSS) on their Dish 500+ and Dish 1000+ dishes. It has an oval LNB called a DP DBS/FSS Dual Band. This LNB will receive both the 119-degree and 118.7-degree satellites. While the original Dish Network satellites use circular polarity at 12.7 GHz, the newer Intelsat 13/Echostar 9 satellite at 121-degrees uses the older FSS technology to broadcast commercial and corporate services. As a result, newer DiSH Network receivers are designed to receive both circular and linearly-polarized signals at two different intermediate frequencies from up to five different orbital locations. Direct-broadcast satellite . Direct broadcast satellite (DBS) is a term used to refer to satellite television broadcasts intended for home reception, also referred to more broadly as direct-to-home signals. The expression direct-to- home or DTH was, initially, meant to distinguish the transmissions directly intended for home viewers from cable television distribution services that sometimes carried on the same satellite. The term predates DBS satellites and is often used in reference to services carried by lower power satellites which required larger dishes (1.7m diameter or greater) for reception. Types of broadcasters Public broadcasters Many countries have public broadcasters, which get funding from the government to broadcast television shows and radio programs. Examples of public broadcasters include the BBC in Britain, NHK in Japan, and the CBC in Canada. In the US, the public broadcaster is called PBS. It is different than the other public broadcasters such as BBC, NHK and CBC, because the PBS gets a lot of its funding (money) from donations by viewers and listeners. Public broadcasters make programs that the private companies are not interested in making, such as educational children's shows, documentaries, and public affairs shows about current issues. Private broadcasters As well, there are private broadcasting companies. These are companies that broadcast television and radio programs. To make money, private broadcasting companies sell advertisements called commercials. Community broadcasters A third type of broadcaster is community broadcasters. There are community television stations and community radio stations. Community television stations are often provided on cable networks. Community television stations usually have shows about local issues and community events. Some community television stations film and broadcast community cultural activities, such as musical performances or town hall meetings. Community radio stations play music and have public affairs shows about community issues. Community radio stations are usually small organizations that are run by volunteers. Community radio stations often get their funding (money) from local governments, local universities, and from donations by listeners. Some community radio stations also have poetry readings by local poets, or performances by local musicians or singers. Other meanings Broadcasting can also mean sending a message to many users on a computer network at the exact same time, or sending a message from one computer to many other computers, giving information about itself, such as its name and location. Sending information to a small selected group is called narrowcasting. multiplexing In telecommunications and computer networks, multiplexing (also known as muxing) is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications. The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX). Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream. Categories The two most basic forms of multiplexing are time-division multiplexing (TDM) and frequency- division multiplexing (FDM), both either in analog or digital form. FDM requires modulation of each signal. In optical communications, FDM is referred to as wavelength-division multiplexing (WDM). Variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statistical multiplexing, for example packet mode communication. Packet mode communication is an asynchronous mode time-domain multiplexing, which resembles, but should not be considered as, time-division multiplexing. Digital bit streams can be transferred over an analog channel by means of code-division multiplexing (CDM) techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). In wireless communications, multiplexing can also be accomplished through alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phased multi-antenna array combined with a Multiple-input multiple-output communications (MIMO) scheme. Time division multiplexing(TDM) Theory: Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample, byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc. In its primary form, TDM is used for circuit mode communication with a fixed number of channels and constant bandwidth per channel. What distinguishes time-division multiplexing from statistical multiplexing such as packet mode communication (also known as statistical time-domain multiplexing, see below) is that the timeslots are recurrent in a fixed order and pre-allocated to the channels, rather than scheduled on a packet-by-packet basis. Statistical time-domain multiplexing resembles, but should not be considered as, time division multiplexing. In dynamic TDMA, a scheduling algorithm dynamically reserves a variable number of timeslots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream. Frequency division multiplexing (FDM) Theory: Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central leftright sum channel, prior to the frequency modulation of the composite signal. A Television channel is divided into subcarrier frequencies for video, color, and audio. DSL also uses different frequencies for voice and for upstream and downstream data transmission on the same conductors. Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA). FDMA is the traditional way of separating radio signals from different transmitters. In the 1860 and 70s, several inventors attempted FDM under the names of Acoustic telegraphy and Harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile their efforts led to an elementary understanding of electro acoustic technology, resulting in the invention of the telephone Code Division Multiplexing (Synchronous CDMA) Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other (note: if u=(a,b) and v=(c,d), the dot product u·v = a*c + b*d). Some properties of the dot product aid understanding of how W-CDMA works. If vectors a and b are orthogonal, then Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An example of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95 64 bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes are orthogonal to one another, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each users signal can be encoded and decoded. Example Start with a set of vectors that are mutually orthogonal. (Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.) An example of orthogonal functions is shown in the picture on the left. These vectors will be assigned to individual users and are called the "code", "chipping code" or "chip code". In the interest of brevity, the rest of this example uses codes (v) with only 2 digits. An example of four mutually orthogonal digital signals. Each user is associated with a different code, say v. If the data to be transmitted is a digital zero, then the actual bits transmitted will be –v, and if the data to be transmitted is a digital one, then the actual bits transmitted will be v. For example, if v=(1,–1), and the data that the user wishes to transmit is (1, 0, 1, 1) this would correspond to (v, –v, v, v) which is then constructed in binary as ((1,– 1),(–1,1),(1,–1),(1,–1)). For the purposes of this article, we call this constructed vector the transmitted vector. Each sender has a different, unique vector v chosen from that set, but the construction method of the transmitted vector is identical. Now, due to physical properties of interference, if two signals at a point are in phase, they add to give twice the amplitude of each signal, but if they are out of phase, they "subtract" and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled by the addition of the transmission vectors, component by component. If sender0 has code (1,–1) and data (1,0,1,1), and sender1 has code (1,1) and data (0,0,1,1), and both senders transmit simultaneously, then this table describes the coding steps: Advantages of Asynchronous CDMA over other techniques 1. Efficient Practical utilization of Fixed Frequency Spectrum Asynchronous CDMA's main advantage over CDM (Synchronous CDMA), TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. (In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges – power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.) TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each timeslot must have a guard-time, which reduces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the unpredictable doppler shift of the signal spectrum which occurs due to the user's mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum. 2. Flexible Allocation of Resources Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of a PN codes to active users. In the case of CDM, TDMA and FDMA the number of simultaneous orthogonal codes, time slots and frequency slots respectively is fixed hence the capacity in terms of number of simultaneous users is limited. There are a fixed number of orthogonal codes, timeslots or frequency bands that can be allocated for CDM, TDMA and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in an Asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by Asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of the time, then 2N users can be accommodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users talking half of the time. In other words, Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (Synchronous CDMA), TDMA and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing to use more than N timeslots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time-slot or frequency channel resources. By comparison, Asynchronous CDMA transmitters simply send when they have something to say, and go off the air when they don't, keeping the same PN signature sequence as long as they are connected to the system. 3. Privacy protection in Spread Spectrum CDMA due to anti-jamming capabilities of PN sequences Spread Spectrum Characteristics of CDMA Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudorandom code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudorandom sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal. CDMA can also effectively reject narrowband interference. Since narrowband interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any given time. Like the narrowband interference this will result in only a small loss of data and can be overcome.
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