satellite communication -iv

MODULE 4 : 

MODULE 4

Multiple Access TechniquesTechnique used to access a satellite so that spectrum and power are shared efficiently between usersA multiple access scheme must optimize the following parameters: satellite radiated power RF spectrum connectivity adaptability to traffic and network growth handling of different types of traffic economics ground station complexity secrecy : 

Multiple Access TechniquesTechnique used to access a satellite so that spectrum and power are shared efficiently between usersA multiple access scheme must optimize the following parameters: satellite radiated power RF spectrum connectivity adaptability to traffic and network growth handling of different types of traffic economics ground station complexity secrecy

Base technologies : 

Base technologies Isolates data from different sources Three basic choices Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

Multiple Access Methods : 

Multiple Access Methods Time Frequency Codes TDMA: Time Division Multiple Access FDMA: Frequency Division Multiple Access CMDA: Code Division Multiple Access

It is a communication technique that divides a communications channel into a number of equally spaced frequency bands : 

It is a communication technique that divides a communications channel into a number of equally spaced frequency bands FDMA

FDMA : 

FDMA Simplest Best suited for analog links Each station has its own frequency band, separated by guard bands Receivers tune to the right frequency Number of frequencies is limited reuse frequencies in non-adjacent cells

Frequency division multiple access The assigned bandwidth is divided into n segments Multiplexed data are modulated and transmitted with allocated bandwidth Guard bands are allotted between segments Guard bands reduce the bandwidth utilization efficiency - directly related to the number of accessing earth station networks. Earth station (Rr) filters out (baseband filter) the frequency segment and demodulates this band a demux retrieves individual channels and feeds them to the terrestrial n/w for onward transmission. : 

Frequency division multiple access The assigned bandwidth is divided into n segments Multiplexed data are modulated and transmitted with allocated bandwidth Guard bands are allotted between segments Guard bands reduce the bandwidth utilization efficiency - directly related to the number of accessing earth station networks. Earth station (Rr) filters out (baseband filter) the frequency segment and demodulates this band a demux retrieves individual channels and feeds them to the terrestrial n/w for onward transmission.

based on the traffic demands- multiple channel per carrier (MCPC) single channel per carrier (SCPC). Multiple channel per carrier (MCPC) Several sub carriers are combined into a single bitstream before being modulated onto a carrier The major disadvantage of MCPC is that all of the signals must be sent to a single place first, then combined for retransmission each baseband filter in a receiver corresponds to a specific transmitting station. Any change in channel capacity requires a retuning of this filter. Thus changes in traffic are difficult to implement. : 

based on the traffic demands- multiple channel per carrier (MCPC) single channel per carrier (SCPC). Multiple channel per carrier (MCPC) Several sub carriers are combined into a single bitstream before being modulated onto a carrier The major disadvantage of MCPC is that all of the signals must be sent to a single place first, then combined for retransmission each baseband filter in a receiver corresponds to a specific transmitting station. Any change in channel capacity requires a retuning of this filter. Thus changes in traffic are difficult to implement.

Slide 9: 

A MCPC is further classified – FDM / FM / FDMA - when the incoming baseband signals are frequency division multiplexed. The multiplexed signal frequency modulates a carrier which is then transmitted using a FDMA scheme. TDM / PSK / FDMA - transmits a time division multiplexed digital signal using a PSK modulation into a FDMA mode

Single channel per carrier : 

Single channel per carrier using a single signal at a given frequency and bandwidth. used on broadcast satellites to indicate stations are not multiplexed as sub carriers onto a single video carrier, but instead independently share a transponder Advantages simple and reliable technology low-cost equipment easy to add additional receive sites Disadvantages inefficient use of satellite bandwidth for burst transmissions usually requires on-site control When used in remote locations, the transmitting dish must be protected.

Slide 11: 

SCPC systems may be – pre-assigned or demand-assigned. In pre-assigned SCPC - a few channels (5-10) are permanently assigned to an earth station. In demand-assigned SCPC – a pool of frequency is shared by earth stations. When necessary,station requests a channel from pool manager permits the sharing of circuits both in the terrestrial link and the satellite link. reduction in cost. the call handling capacity of each circuit is increased.

frequency pool can be - distributed and centralizedcentrally controlled frequency management scheme channels are assigned centrally from a network control station (NCS) Requests for channel assignment are directed to NCS on a signaling channel NCS then assigns a frequency from its pool using certain assignment rules. automatic frequency control (AFC) pilot is transmitted by designated reference earth stations all the earth stations use this reference to correct their transmission frequency An orderwire line is provided for comm. between personnel for system management, fault diagnosis etc. : 

frequency pool can be - distributed and centralizedcentrally controlled frequency management scheme channels are assigned centrally from a network control station (NCS) Requests for channel assignment are directed to NCS on a signaling channel NCS then assigns a frequency from its pool using certain assignment rules. automatic frequency control (AFC) pilot is transmitted by designated reference earth stations all the earth stations use this reference to correct their transmission frequency An orderwire line is provided for comm. between personnel for system management, fault diagnosis etc.

Slide 13: 

Advantages: simple to manage higher usage of channels - availability of all information at a single point offer a lower connection time They are more cost-effective. Disadvantage: lower reliability prone to single point failure. Examples : Inmarsat Standard A, B and M.

Distributed frequency management scheme each earth station can obtain a channel from the common pool on its ownadvantage: high reliability - the failure of a single earth station doesn't affect the availability of the system to other earth stations in the network. Disadvantage: The increased complexity in earth stations high cost.Example: INTELSAT’s SPADE system. Hybrid frequency management scheme in which the network provides a combination of distributed and centrally controlled frequency management functions, bringing together the advantages of each. : 

Distributed frequency management scheme each earth station can obtain a channel from the common pool on its ownadvantage: high reliability - the failure of a single earth station doesn't affect the availability of the system to other earth stations in the network. Disadvantage: The increased complexity in earth stations high cost.Example: INTELSAT’s SPADE system. Hybrid frequency management scheme in which the network provides a combination of distributed and centrally controlled frequency management functions, bringing together the advantages of each.

Design considerations : 

Design considerations Major sources of impairments: 1.Caused by satellite HPA – a) HPA exhibits non linearity as the output reaches saturation. this causes intermodulation noise as number of carrier pass through it. to minimize the effect, the drive level of the final stage of transmitter is reduced to operate it in linear region. hence full power of the amplifier is not utilized and it reduce its capacity. Slope in the Frequency response characteristics of TWT causes variation in the frequency of incoming signals appear as amplitude variations. this introduces phase modulation to other carriers sharing the TWT. Hence causes cross talk between carriers. reduced by using TWTs which minimize AM-PM conversion.

Slide 16: 

2.Other impairments – To maximize frequency utilization, adjacent carriers are brought closer. causes adjacent channel interference. adjacent carrier with smaller amplitude, causes convolution noise adjacent carrier with larger amplitude, causes impulse noise Intermodulation noise from adjacent transponder minimized by filtering the out of band intermodulation in each transponder Phase nonlinearity by carriers located at the band edges this is due to inadequate isolation at the band edges minimized by optimizing the transponder guard band and increasing out of band attenuation increases complexity of filter Group delay associated with each filter in transmission path less pronounced for SCPC because of narrow channels

Slide 17: 

Advantages of FDMA : Uses existing hardware ,hence technology is mature and cost is low.There is no need for network timing.no restriction regarding the type of baseband (voice or date) / modulation. Disadvantages : Intermodulation noise in transponder ,reduces the satellite capacity.Lack of flexibility -change in allocation,the receiver filters need to be retuned. Requires uplink power control to maintain the link quality.traffic containing strong & weak carriers, the weak ones are suppressed. Applications: AMPS ( Advanced Mobile Phone system ) DECT ( Digital European Cordless Telephone )

It is a digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. : 

It is a digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. TDMA

TDMA : 

TDMA stations transmit data on same frequency, but at different times Needs time synchronization digital mode of transmission is best suited. Applications: GSM ( Global System for Mobile ) US DC ( U. S Digital Cellular ) JDC ( Japanese Digital Cellular ) Pros - The capacity of the system is increased as only a single carrier is present at any given time users can be given different amounts of bandwidth mobiles can use idle times to determine best base station can switch off power when not transmitting

Slide 20: 

Cons - adds both hardware and network complexity - data need to be stored, compressed and transmitted during one (or more) specific time slots - synchronization overhead - greater problems with multipath interference on wireless links

Slide 21: 

Working principle of TDMA The transmissions are organized into frames earth station transmitting in its designated time slot in every frame stations receives all the bursts & removes data addressed to it Every frame contains at least one reference burst transmitted by a station,designated for synchronization purposes. Each time slot is separated from the adjacent slot by a guard time. The sizes of the time slots depend on the traffic requirement

Slide 23: 

The preamble consists of three parts. The carrier and bit time recovery (CBR) slot is used by receivers for recovering the carrier and bit time essential for coherent demodulation. The unique word (UW) is used for burst synchronization. This is achieved by correlating a stored replica of the unique word with the received bits. A correlation peak indicates the start of a new frame. Control bits (C) contain information such as station identification and engineering service messages for network management. Trail bits – allow synchronization of transmissions from mobile units Guard bits – used to avoid overlapping with other bursts CBR UW C

Two methods of TDMA synchronization- open-loop and closed-loop.Open-loop synchronization each earth station maintains an accurate clock, independently of other stations in the network relatively simple to implement hence useful for simple ground stations. It is possible to maintain guard bands of 100 µs by using this technique. : 

Two methods of TDMA synchronization- open-loop and closed-loop.Open-loop synchronization each earth station maintains an accurate clock, independently of other stations in the network relatively simple to implement hence useful for simple ground stations. It is possible to maintain guard bands of 100 µs by using this technique.

Slide 25: 

frame Reference burst Burst from first traffic station i Ta Tm error time Closed loop synchronization 1 Ta - desired time slot position Tm - measured position

Closed-loop synchronization An earth station, i, beginning transmission, must first acquire its assigned time slot. The initial burst is transmitted on an estimated position of burst position, Tm. The received burst is compared with the desired position Ta of the slot and a correction of (Ta – Tm ) applied to the initial estimate in the next transmission, to acquire the correct time slot. The burst is transmitted at a low power to minimize interference to adjacent slots in case of a gross error in the estimate. A carrier modulated with a pseudo-random signal may be transmitted. : 

Closed-loop synchronization An earth station, i, beginning transmission, must first acquire its assigned time slot. The initial burst is transmitted on an estimated position of burst position, Tm. The received burst is compared with the desired position Ta of the slot and a correction of (Ta – Tm ) applied to the initial estimate in the next transmission, to acquire the correct time slot. The burst is transmitted at a low power to minimize interference to adjacent slots in case of a gross error in the estimate. A carrier modulated with a pseudo-random signal may be transmitted.

Slide 27: 

The receiver consists of a correlation receiver to obtain the burst position. A correction equal to the difference between the received and the desired bursts positions is then applied to the original burst position. After entering the network, the process of measurement/ correction is repeated at frequent intervals (  1s ) so that the burst position errors remain within tolerable limits. In another synchronization scheme a reference station observes burst positions of each earth station in the network and provides feedback to each station to apply he necessary correction. useful in a multiple beam system when the transmitting earth station is unable to receive its own transmission.

Frame efficiency The frame efficiency ηf in a TDMA system is defined as the ratio of time devoted for useful transmission to the total frame length ηf = [ 1 - ( ∑ti / TF ) ] Where ti - the sum of all guard times and preamble including the reference burst TF - the frame time. The frame efficiency increases when the frame length is increased. Frame efficiencies of the order of 0.9 are possible at present. The voice channel capacity of a TDMA system can be obtained as Nc = [ Rs – ( kn / Tf ) ] / Rc where Rs = TDMA system bit rate k = number of bits in each preamble n = number of bursts/frame Tf = frame time Rc = voice channel bit rate. : 

Frame efficiency The frame efficiency ηf in a TDMA system is defined as the ratio of time devoted for useful transmission to the total frame length ηf = [ 1 - ( ∑ti / TF ) ] Where ti - the sum of all guard times and preamble including the reference burst TF - the frame time. The frame efficiency increases when the frame length is increased. Frame efficiencies of the order of 0.9 are possible at present. The voice channel capacity of a TDMA system can be obtained as Nc = [ Rs – ( kn / Tf ) ] / Rc where Rs = TDMA system bit rate k = number of bits in each preamble n = number of bursts/frame Tf = frame time Rc = voice channel bit rate.

Slide 29: 

The advantages of TDMA are: Maximum use from the available satellite power since intermodualtion noise is minimal. Uplink power control is not required. Transmission plans are easier to construct and modify. Capacity management is simple and flexible. utilization of all the advantages of digital techniques. For eg. techniques such as digital speech interpolation, source and channel coding, etc. can be incorporated. The disadvantages of TDMA are : It requires network-wide timing synchronization, hence relatively complex. Analog signals must be converted to digital form.Interface with analog terrestrial plant is expensive.

Speech interpolationA channel in use by user A is allocated to another user whenever a speech pause is detected during a conversation of user A.A new channel is subsequently allocated to the user A when the his next speech spurt begins. The resulting improvement in channel utilization, known as the DSI gain = Nt / Ns where Nt -number of incoming terrestrial channels at an earth station Ns-number of transmission channels.DSI implementation can improve the channel usage by a factor of 2. DSI system the percentage of speech lost while waiting for reassignment, called freeze-out fraction, is less than 0.5%. : 

Speech interpolationA channel in use by user A is allocated to another user whenever a speech pause is detected during a conversation of user A.A new channel is subsequently allocated to the user A when the his next speech spurt begins. The resulting improvement in channel utilization, known as the DSI gain = Nt / Ns where Nt -number of incoming terrestrial channels at an earth station Ns-number of transmission channels.DSI implementation can improve the channel usage by a factor of 2. DSI system the percentage of speech lost while waiting for reassignment, called freeze-out fraction, is less than 0.5%.

CDMA : 

CDMA based on the use of modulation technique known as spread spectrum Users separated both by time and frequency Send at a different frequency at each time slot (frequency hopping) Or, convert a single bit to a code (direct sequence) receiver can decipher bit by inverse process

Slide 35: 

Pros hard to spy immune from narrowband noise no need for all stations to synchronize no hard limit on capacity of a cell all cells can use all frequencies Cons implementation complexity need for power control to avoid capture need for a large contiguous frequency band (for direct sequence) problems installing in the field

spread spectrum The message signal is spread over a wide band by multiplying it with a noise-like or pseudo-random spreading signal. To demodulate the signal the receiver cross-correlates the received signal with an exact replica of the spreading function. The cross-correlation produces a maximum only when the codes are matched. : 

spread spectrum The message signal is spread over a wide band by multiplying it with a noise-like or pseudo-random spreading signal. To demodulate the signal the receiver cross-correlates the received signal with an exact replica of the spreading function. The cross-correlation produces a maximum only when the codes are matched.

Spread Spectrum - illustrated : 

f Power Density PDi PDSS Conventional Transmission Spread Spectrum Transmission Spread Spectrum - illustrated

Slide 38: 

Pseudo-random sequences a random sequence of signals within a specified time period Tr , at the end of which the sequence repeats itself. Shift registers with feedback are often used for its digital implementation. When an m-bit shift register is used, the maximum possible length of code is p = (2m - 1). Each sequence has randomly placed (2m-1) ones and (2m-1 -1) zeros, which results in a sequence appearing to be random. The maximum number, Nmax, of maximum-length linear codes using a shift register of size m is given by Nmax = [Φ (2m - 1 ) ] / m Where Φ (2m - 1) is a Euler number Nmax for m of 14,15 and 16 are

There are several ways of implementing a CDMA system - direct sequence spread spectrum frequency hopped spread spectrum : 

There are several ways of implementing a CDMA system - direct sequence spread spectrum frequency hopped spread spectrum frequency hopped spread spectrum It is a digital comm. technique in which the carrier frequency is varied in a pseudo random fashion within a wideband channel Multiple access is supported as carrier frequencies of multiple users do not collide (can be done by design) Eg: Bluetooth direct sequence spread spectrum Signal is generated by multiplying the data with a pseudo random sequence so that the resultant rate (chip rate) is high, resulting in a wideband signal Multiple access is supported as random sequences of multiple users have low correlation so that interference due to other signals are reduced Eg: IS- 95,cdma 2000

Direct sequence spread spectrum : 

Direct sequence spread spectrum

Slide 41: 

Bandwidth Effects of the Spreading Operation SS modulation is applied on top of a conventional modulation such as BPSK or direct conversion. All other signals not receiving the SS code will stay as they are, unspread.

Bandwidth Effects of the Despreading Operation : 

Bandwidth Effects of the Despreading Operation An SS demodulation has been made on top of the normal demodulation operations above. Signals added during the transmission (such as an interferer or jammer) will be spread during the despreading operation!

Direct sequence spread spectrum The information stream m1(t) at a bit rate of b1 bits/s is modulated and the modulated signal s1(t) multiplied by a spreading function g1(t). The spreading function g1(t) is a pseudo-random code with a bit rate Bs significantly larger than the information bit rate. Other users in the network transmit on the same channel but each user has a unique code.Thus the received signal may be given as Rx (t) = C1 (t) + C2 (t) + ….C n (t) + n (t) Where Cn (t) is the received signal from the nth transmitter n (t) is the system noise. The processing gain of the spread spectrum signal can be approximated by Gp = (Co / No ) / (Ci / Ni) Where Ci,Co are the i/p and o/p signals of the correlator Ni = I (f) Bc I (f) = noise power spectral density Bc = occupied channel bandwidth No = I(f)Bm Bm = occupied message bandwidth. : 

Direct sequence spread spectrum The information stream m1(t) at a bit rate of b1 bits/s is modulated and the modulated signal s1(t) multiplied by a spreading function g1(t). The spreading function g1(t) is a pseudo-random code with a bit rate Bs significantly larger than the information bit rate. Other users in the network transmit on the same channel but each user has a unique code.Thus the received signal may be given as Rx (t) = C1 (t) + C2 (t) + ….C n (t) + n (t) Where Cn (t) is the received signal from the nth transmitter n (t) is the system noise. The processing gain of the spread spectrum signal can be approximated by Gp = (Co / No ) / (Ci / Ni) Where Ci,Co are the i/p and o/p signals of the correlator Ni = I (f) Bc I (f) = noise power spectral density Bc = occupied channel bandwidth No = I(f)Bm Bm = occupied message bandwidth.

Slide 44: 

Substituting for Ni and No and assuming Ci = Co, the processing gain is given by Gp = Bc / Bm Note that Bc = 2Rc (channel bit rate), assuming Bm =2Rm (message bit rate). Then, Gp = Rc / Rm The processing gain is reduced because of losses in the correlation process caused by factors such as imperfect cross-correlation. The tolerance of a spread spectrum signal to interference is given in terms of an interference margin (Mi). Mi = Gp – L – ( Co / No) Where Gp = processing gain L = implementation loss Co/ No = desired carrier-to-noise ratio at the correlator o/p

Slide 45: 

Resistance to Interception Resistance to Fading (Multipath Effects) Without the right key, the SS signal appears as noise or as an interferer . signal levels can be below the noise floor as the spreading operation reduces the spectral density. The message is thus made invisible Other receivers cannot "see" the transmission; they only register a slight increase in the overall noise level! The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading. Because the despreading process synchronizes to signal D, signal R is rejected even though it contains the same key. Methods are available to use the reflected-path signals by despreading them and adding the extracted results to the main one.

Slide 46: 

Frequency hopping is less vulnerable to interference , because the frequency is always changed . Frequency Hopped Spread Spectrum

It is very difficult to intercept a frequency hopping communication ,one must jam the hole band to attack . : 

It is very difficult to intercept a frequency hopping communication ,one must jam the hole band to attack .

FHSS - illustrated : 

FHSS - illustrated Frequency Time Tune Time Frequency Hop Dwell Time

Slide 49: 

High security . Protection against fading . Low probability of interception (LPI). Anti jamming (AJ). Allows multiple access . Advantages of FH Challenges because of FH Devices within communication range of each need to synchronize their frequency hopping pattern in order to communicate . A high processing gain is hard ,there is a need for fast-hopping over the carrier frequencies.

Slide 50: 

Bluetooth channel is represented by a pseudo random hopping sequence through the entire 79 RF frequencies Nominal hop rate of 1600 hops per second Channel Spacing is 1 MHz Applications

Bluetooth FHSS : 

Bluetooth FHSS Employs frequency hopping spread spectrum Reduce interference with other devices Pseudorandom hopping 1600 hops/sec- time slot is defined as 625 microseconds Packet 1-5 time slots long

Slide 52: 

Frequency hopped spread spectrum Spreading function is used to alter the transmission frequency in discrete steps (Δf) using a frequency synthesizer. modulator X Frequency synthesizer Frequency hopped signature To transmitter Spreading function g1(t) Message m(t) transmitter

Slide 53: 

At the receiver, an identical code synchronized to the transmission is used to alter the frequency of the total oscillator. As a result of the mixing process, the frequency hopping element is removed. The mixer output is bandpass filtered to remove the undesirable frequency product generated by the non-linearity of the mixer and other system noise. The resulting signal is at a fixed IF which is then fed into the demodulator for retrieving the message signal. X Frequency synthesizer Spreading function g1(t) From receiver Despread signal To Filter and demodulator receiver

The spectrum Brf of the transmitted signal is given by Brf = ( 2n - 1) Δf Where n = number of stages in code generator shift register Δf = unit frequency increment Brf >> Bi (information bandwidth).The processing gain of a frequency hopped system is given by Gp = Brf / Bi = (2n – 1) Δf / Δf (assuming that Bi = Δf) The code rate of a frequency hopped system can be much lower than the code rate of a direct sequence type of spread spectrum system. Interference in the frequency hopped spread spectrum is caused at instants when an unwanted signal appears within the passband of the desired signal. : 

The spectrum Brf of the transmitted signal is given by Brf = ( 2n - 1) Δf Where n = number of stages in code generator shift register Δf = unit frequency increment Brf >> Bi (information bandwidth).The processing gain of a frequency hopped system is given by Gp = Brf / Bi = (2n – 1) Δf / Δf (assuming that Bi = Δf) The code rate of a frequency hopped system can be much lower than the code rate of a direct sequence type of spread spectrum system. Interference in the frequency hopped spread spectrum is caused at instants when an unwanted signal appears within the passband of the desired signal.

Capacity of spread spectrum systems determined by estimating the max. number of users that can access the channel simultaneously without exceeding the specified carrier-to-interference noise ratio at the receiver demodulator. Resistance to interference can be increased by coding the information with a suitable FEC coding technique, giving improved capacity. Interference margin of a spread spectrum signal is given by M = Gp - L – Co / IoSubstituting for Gp and rearranging, the maximal interference, Iim , at the receiver input as Iim = (Ci + M + L ) dB : 

Capacity of spread spectrum systems determined by estimating the max. number of users that can access the channel simultaneously without exceeding the specified carrier-to-interference noise ratio at the receiver demodulator. Resistance to interference can be increased by coding the information with a suitable FEC coding technique, giving improved capacity. Interference margin of a spread spectrum signal is given by M = Gp - L – Co / IoSubstituting for Gp and rearranging, the maximal interference, Iim , at the receiver input as Iim = (Ci + M + L ) dB

Slide 56: 

maximum number of users that can share the channel 10 log Σ in = Iim dB Where in = interfering power (in watts) from each active user.Assuming for simplicity equal power from each source 10 log (nin) = M + L + Ci 10 log(n) = M + L (note : 10 log in = Ci) = Gp – (Co / Io) Spread spectrum systems have an inherent capability to accommodate traffic growth without any change to the network configuration, provided that the system design has been dimensioned for this growth at the outset. A user can access the channel without any delay the grade of service in a spread spectrum system is always good.

Comparison SDMA/TDMA/FDMA/CDMA : 

Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation Advantages Dis - advantages Comment

Access protocols for data trafficAll the Fixed and demand assigned schemes are optimized mainly for speech or continuous stream of dataSpeech which requires several minutes of continuous connection, data messages are generally characterized by bursts of high activity. Another important difference is that in data communication user is more tolerant to delays. Examples of data traffic are: the transfer of large volumes of data a request for channel allocation in a demand-assigned system a query-response system, etc. They are fully random access - a terminal is allowed transmission at any time without coordination with other users. Drawback is an increased probability of message loss through collision.In Fully scheduled access - a channel is reserved by a user for the duration of a message.It is highly reliable but increases complexity of terminal and network control : 

Access protocols for data trafficAll the Fixed and demand assigned schemes are optimized mainly for speech or continuous stream of dataSpeech which requires several minutes of continuous connection, data messages are generally characterized by bursts of high activity. Another important difference is that in data communication user is more tolerant to delays. Examples of data traffic are: the transfer of large volumes of data a request for channel allocation in a demand-assigned system a query-response system, etc. They are fully random access - a terminal is allowed transmission at any time without coordination with other users. Drawback is an increased probability of message loss through collision.In Fully scheduled access - a channel is reserved by a user for the duration of a message.It is highly reliable but increases complexity of terminal and network control

Slide 59: 

Categories of accessing schemes for data traffic: channel reservation schemes contention protocols packet reservation protocols. Most accessing schemes attempt to minimize the conflict between users who require access to a channel at the same time by the use of certain rules or ‘protocols’. Hence multiple access schemes for data traffic are often called multiple access protocols.

The performance of accessing schemes for data traffic is measured as channel throughput. Throughput is defined as the number of messages successfully transmitted per unit at time. If P is the probabililty of successful transmission of a packet and G is the offered lad, then throughput S is given by S = GPEach message is segmented into blocks called packets.The burstiness of data traffic can be measured as the peak-to-average data rate. β = δ / T Where δ = permissible average message delay T = average inter-arrival time between messages.As β reduces, traffic gets more bursty : 

The performance of accessing schemes for data traffic is measured as channel throughput. Throughput is defined as the number of messages successfully transmitted per unit at time. If P is the probabililty of successful transmission of a packet and G is the offered lad, then throughput S is given by S = GPEach message is segmented into blocks called packets.The burstiness of data traffic can be measured as the peak-to-average data rate. β = δ / T Where δ = permissible average message delay T = average inter-arrival time between messages.As β reduces, traffic gets more bursty

Channel reservation By channel reservation scheme we mean schemes where physical channels are reserved for the duration of each message transmission. The channels may be pre-assigned or demand-assigned for a session. e. The upper bound on channel throughput S for a pre-assigned channel is given by S ≤ β Where β is the bursty factor. The upper bound on channel throughputs S for a demand-assigned channel is given by S ≤ (δ – ta ) / (δ + td ) Where ta = allocation time td = de-allocation time δ = averages message delay constraint. : 

Channel reservation By channel reservation scheme we mean schemes where physical channels are reserved for the duration of each message transmission. The channels may be pre-assigned or demand-assigned for a session. e. The upper bound on channel throughput S for a pre-assigned channel is given by S ≤ β Where β is the bursty factor. The upper bound on channel throughputs S for a demand-assigned channel is given by S ≤ (δ – ta ) / (δ + td ) Where ta = allocation time td = de-allocation time δ = averages message delay constraint.

ALOHA : 

ALOHA ALOHA is the best known random access protocol It refers to a simple communications scheme in which each source in a network sends data whenever there is a frame to send.  If the frame successfully reaches the destination, the next frame is sent. Collision can be sensed by each earth station by listening to its own burst or awaiting the acknowledgement of receipt. If the frame fails to be received at the destination, it is sent again.  This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific. as networks become more complex, eg. in an Ethernet system trouble occurs because data frames collide

Frames in a pure ALOHA network : 

Frames in a pure ALOHA network

Throughput (S) – number of Packets successfully (without collision) transmitted per unit time S = Ge-2G Where G -offered traffic on the channel including new transmissions and retransmissions, and measured in packets/packet time. Maximum throughput is 0.5e or 18.4%, corresponding to an offered load of 0.5 packet/packet time. : 

Throughput (S) – number of Packets successfully (without collision) transmitted per unit time S = Ge-2G Where G -offered traffic on the channel including new transmissions and retransmissions, and measured in packets/packet time. Maximum throughput is 0.5e or 18.4%, corresponding to an offered load of 0.5 packet/packet time.

ALOHA network : 

ALOHA network

Slotted ALOHA : 

11/14/96 S. Keshav 68 Slotted ALOHA A simple way to double ALOHA’s capacity Make sure transmissions start on a slot boundary Halves window of vulnerability Used in cellular phone uplink

Slotted ALOHA : 

Slotted ALOHA A packet is vulnerable to collision for a total period of 2T seconds, T being the packet duration. When the packet transmissions area confined to specific time slots, the vulnerable period of a packet collision is reduced Packet arrives before the start of a slot, its transmission is delayed until the beginning of the next slot. Throughput S = Ge-G The maximum throughput occurs at a traffic load of 1 packet/packet time and equals 1/e or 36.8%.

Slide 70: 

Used when long messages are to send Combines ALOHA with slotted ALOHA divide time into frames = fixed length set of slots station that wins access to a reservation minislot using S-ALOHA can keep slot as long as it wants station that loses keeps track of idle slots and contends for them in next frame Pros supports both circuit and packet mode transfer simple Cons arriving packet has to wait for entire frame before it has a chance to send variants of R-ALOHA avoid these problems Application Used for cable-modem uplinks Reservation ALOHA

Throughput SRA = SSA / (SSA + 1/L ) Where SSA = slotted ALOHA throughput L = average number of packets transmitted before a user releases captured slot. The throughput for an infinite population model is given by C = 1 / [ 1 + (e / L) ]Thus, 1 / ( 1 + e) ≤ C ≤ 1, for L ranging from 1 to infinity. When the end of message is announced 1/e ≤ C ≤ 1, for L ranging from 1 to infinity.Although simple to implement, ALOHA schemes tend to become unstable as traffic increases. : 

Throughput SRA = SSA / (SSA + 1/L ) Where SSA = slotted ALOHA throughput L = average number of packets transmitted before a user releases captured slot. The throughput for an infinite population model is given by C = 1 / [ 1 + (e / L) ]Thus, 1 / ( 1 + e) ≤ C ≤ 1, for L ranging from 1 to infinity. When the end of message is announced 1/e ≤ C ≤ 1, for L ranging from 1 to infinity.Although simple to implement, ALOHA schemes tend to become unstable as traffic increases.

Packet reservation slots are reserved on demand for transmission of packets.Reservation requests are sent on a separate channel ( fixed assigned / contention access scheme such as ALOHA. )these schemes provides a much higher throughput than contention protocol since losses due to contention are eliminated. The reservation scheme be simple. disadvantage - expense of an additional message delivery delay incurred during the reservation process.Delay incurred in reservation process is twice the propagation delay, ~ 0.54 S. : 

Packet reservation slots are reserved on demand for transmission of packets.Reservation requests are sent on a separate channel ( fixed assigned / contention access scheme such as ALOHA. )these schemes provides a much higher throughput than contention protocol since losses due to contention are eliminated. The reservation scheme be simple. disadvantage - expense of an additional message delivery delay incurred during the reservation process.Delay incurred in reservation process is twice the propagation delay, ~ 0.54 S.

Slide 73: 

The mechanisms involved in packet reservation schemes are : reservation, queue management recovery in the case of a packet loss caused by erroneous access. The reservation channels can be fixed assigned time slots or a contention scheme such as slotted ALOHA. The reservation packets are typically a fraction of the size of message packets, and hence very bursty. Queue management can be done either centrally or distributed control.

Slide 74: 

Reservation TDMA Users first reserve a slot by means of a reservation slot. The main traffic is then transmitted into the allotted traffic slot. The assignment may be done either by a central controller or by a suitable distributed algorithm.

Choice of data access protocol The selection of a protocol for data access depends on type of application. Some of the considerations are: message delay and throughput of the channel cost of earth stations network complexity. Mobile satellite serviceThe choice of multiple access scheme are - Traffic for mobile service is characterized by a large number of widely dispersed users each with low traffic demand. There is a need to accommodate growth in the user base. The need to operate in a multipath propagation environment. The need to keep terminals simple and low-cost, hence well established technology is preferred. The need to serve mobiles in aeronautical, land maritime environment. The need to conserve bandwidth. : 

Choice of data access protocol The selection of a protocol for data access depends on type of application. Some of the considerations are: message delay and throughput of the channel cost of earth stations network complexity. Mobile satellite serviceThe choice of multiple access scheme are - Traffic for mobile service is characterized by a large number of widely dispersed users each with low traffic demand. There is a need to accommodate growth in the user base. The need to operate in a multipath propagation environment. The need to keep terminals simple and low-cost, hence well established technology is preferred. The need to serve mobiles in aeronautical, land maritime environment. The need to conserve bandwidth.

Slide 76: 

Fixed assignment for this type of environment is – wasteful of bandwidth too restrictive for growth. Demand-assigned schemes- both in the time and frequency domains- provides higher channel usage through sharing inherently flexible to accommodate traffic growth well suited for the application. The use of the spread spectrum system which provides a natural resistance to noise and multipath together with a capability to accommodate growth and a high grade of service at all times.

Inmarsat network provides worldwide voice and data communication for various types of terminals mounted on ships, aircraft and land mobiles. Currently, data transmissions are more commonly used by aircraft. voice communication is the main type of traffic from ships.Inmarsat-A system is a first-generation system which provides analog voice transmission and telex to ships.Inmarsat-B system is an improved implementation of the Inmarsat-A using digital techniques for telephony, telex, fax and data.Inmarsat-M system operate with smaller terminals in the maritime and land, which support medium-bit rate telephony, data and fax Inmarsat-C is a digital store-and-forward low-bit rate system to provide comm. to low-costs maritime and land mobile terminals. Inmarsat-Aero system provides telephony, data and facsimile services to the aeronautical community. : 

Inmarsat network provides worldwide voice and data communication for various types of terminals mounted on ships, aircraft and land mobiles. Currently, data transmissions are more commonly used by aircraft. voice communication is the main type of traffic from ships.Inmarsat-A system is a first-generation system which provides analog voice transmission and telex to ships.Inmarsat-B system is an improved implementation of the Inmarsat-A using digital techniques for telephony, telex, fax and data.Inmarsat-M system operate with smaller terminals in the maritime and land, which support medium-bit rate telephony, data and fax Inmarsat-C is a digital store-and-forward low-bit rate system to provide comm. to low-costs maritime and land mobile terminals. Inmarsat-Aero system provides telephony, data and facsimile services to the aeronautical community.

Fixed satellite service The accessing scheme for FSS depends on: the type of service the traffic density between communicating points.A random access scheme may be suitable in a VSAT network whereas a high-bit rate TDMA for connecting two large earth stations.INTELSAT networks developed to meet the growing and changing demand over the years. Initially, fixed-assigned fdma was a natural choice because of its simplicity and the state of the technology. addition of earth stations having low traffic, the SCPC system- both fixed and demand-assigned schemes in the form of SPADE. increase in demand,availability of digital hardware and a growing need for b/w and network flexibility, tdma was introduced in the late 1970s. DSI was introduced to increase the capacity further. Multiple spot beams were introduced to enhance system capacity. To economize space segment, satellite switched TDMA was introduced. At present, the INTELSAT network consists of a mix of all these schemes. : 

Fixed satellite service The accessing scheme for FSS depends on: the type of service the traffic density between communicating points.A random access scheme may be suitable in a VSAT network whereas a high-bit rate TDMA for connecting two large earth stations.INTELSAT networks developed to meet the growing and changing demand over the years. Initially, fixed-assigned fdma was a natural choice because of its simplicity and the state of the technology. addition of earth stations having low traffic, the SCPC system- both fixed and demand-assigned schemes in the form of SPADE. increase in demand,availability of digital hardware and a growing need for b/w and network flexibility, tdma was introduced in the late 1970s. DSI was introduced to increase the capacity further. Multiple spot beams were introduced to enhance system capacity. To economize space segment, satellite switched TDMA was introduced. At present, the INTELSAT network consists of a mix of all these schemes.

Future trends : 

Future trends The basic multiple access schemes may not be able to meet the increasing demands of satellite resources sharing with the desired flexibility and cost. Maximum number of interconnections in a full mesh network of n earth stations is given as n (n – 1) / 2. hence the number of accesses is n( n -1). Therefore if a base of 100 customers (e.g. MSS users) needs simultaneous individual interconnections, the number of accesses totals 9900. Several hybrid schemes can be envisaged combine the time and frequency domains to form a two dimensional grid. In this scheme the available bandwidth is segmented into sub-blocks and each sub-block is then shared in a TDMA mode. A three-dimensional plane can be visualized by introducing CDMA in each sub-band, increasing the number of accesses further.

Two well established transport mechanisms – circuit mode and packet mode. Circuit mode provides a dedicated connection for the duration of a call allows information to be transferred at the maximum capacity wasteful if the data rate is variable. packet mode data are transferred in packets which are individually routed provides flexible resource sharing slow because of the need for software switching. To make the most efficient use of resources, while transferring variable bit rate services, there is a need of flexible transport mode which could serve fixed & variable rate, real & non real-time applications. : 

Two well established transport mechanisms – circuit mode and packet mode. Circuit mode provides a dedicated connection for the duration of a call allows information to be transferred at the maximum capacity wasteful if the data rate is variable. packet mode data are transferred in packets which are individually routed provides flexible resource sharing slow because of the need for software switching. To make the most efficient use of resources, while transferring variable bit rate services, there is a need of flexible transport mode which could serve fixed & variable rate, real & non real-time applications.

Asynchronous Transfer Mode (ATM) : 

Asynchronous Transfer Mode (ATM) encodes data traffic into small fixed-sized cells. each cell consists of 48 bytes of user data and 5 bytes of header. Information is transferred through virtual connection which does not reserve any space can be altered depending on the network load. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email. a connection-oriented technology, establishes a virtual circuit between the two endpoints before the actual data exchange begins

Slide 82: 

ATM will become the main transport mechanism for fixed terrestrial networks. developed mainly for terrestrial systems using optical fibre recently there is a great deal of activity in extending it to wireless access. Each service supported by ATM has different quality of service requirements which are defined in terms of information lost (BER), cell delay and cell delay variation. For example, cell delay variations in a video service may cause jitter in a picture. The ATM specifications for cell loss ratio are ranging from 10-8 to 10-12. Limitations: Bit error rate, cell dropping and propagation delay in satellite links must therefore become important considerations. Long propagation delay results in inefficient use of satellite resources because of the need for retransmissions under bad link conditions. ATM does not include mobility management in its definition and in general higher bit rates can be supported as the speed of mobility is reduced.

Slide 83: 

THANK YOU

Slide 84: 

Back up

Spread Spectrum : 

Spread Spectrum

Slide 86: 

The Inmarsat-A network consists of a network control station (NCS) which provides central control of the network, several fixed land earth stations (LESs) distributed all over the Earth, which serve as gateways to adjacent geographical regions, and the ship earth stations. The NCS broadcasts network-related information on a time division multiplexed broadcast channel. Call assignment (or rejection) messages are carried on this channel. All ship earth stations (SESs) and LESs stay tuned to the NCS broadcasts channel to receive the necessary network information. All the channel assignment requests are made to the LES. Ship earth stations use a pure ALOHA channel for sending requests. The NCS to LES communication takes place via the broadcast channel and pre-assigned time division multiplexed and time division multiplexed (TDM) broadcast channels. Voice communication between ship and shore takes place using demand-assigned single channel per carrier. Telex messages in the ship-to-shore direction take place via demand-assigned time division multiple access.

Transponder utilizationin the case of TDMA transponder utilization depends on the satellite EIRP the G / T values of the receiving earth stations the efficiency of the modulation scheme.Carrier to noise power spectral density C / N0 = EIRP – Lpd – Lmd + G/T – k dB-Hz …………….(1) we know that, C = EbR watts Where Eb = energy / bit R = bit rate (bits/s).Expressing in dB C = 10 log Eb + 10 log RSubstituting in (1), the bit rate Rp for a power-limited link is Rp = EIRP – Lpd – Lmd + G/T – k – (Eb / N0) dB (or) R = 10 RP/10 bits/s : 

Transponder utilizationin the case of TDMA transponder utilization depends on the satellite EIRP the G / T values of the receiving earth stations the efficiency of the modulation scheme.Carrier to noise power spectral density C / N0 = EIRP – Lpd – Lmd + G/T – k dB-Hz …………….(1) we know that, C = EbR watts Where Eb = energy / bit R = bit rate (bits/s).Expressing in dB C = 10 log Eb + 10 log RSubstituting in (1), the bit rate Rp for a power-limited link is Rp = EIRP – Lpd – Lmd + G/T – k – (Eb / N0) dB (or) R = 10 RP/10 bits/s

Slide 88: 

A more spectrally efficient modulation scheme provides a higher bit rate.In such a case Rb= Bη bits/s Where Rb = maximum bit rate in a bandwidth-limited case B = transponder bandwidth (Hz) η = modulation efficiency (bits/Hz).

FDMA/TDMA operation in a multiple beam environment if an earth station in beam 1 wishes to transmit to earth stations in all the n spot beams, it chooses transponder 1 to transmit to earth stations in spot beam 1 and transponder n to transmit in spot beam n. Earth stations switch between desired beams, transmitting in their designated frequency or time slot in each corresponding transponder. This is often known as transponder hopping because earth stations have to hop between transponders to route traffic to the desired spot beam.satellite switched TDMA provides a good solution by incorporating a certain degree of ob-board processing. - a programmable switch located on the satellite routes bursts to spot beams according to set plan. - earth station can direct its transmission to any spot by transmitting in the appropriate time slot. - Transmissions directed to a given beam are arranged in non-overlapping time slots. : 

FDMA/TDMA operation in a multiple beam environment if an earth station in beam 1 wishes to transmit to earth stations in all the n spot beams, it chooses transponder 1 to transmit to earth stations in spot beam 1 and transponder n to transmit in spot beam n. Earth stations switch between desired beams, transmitting in their designated frequency or time slot in each corresponding transponder. This is often known as transponder hopping because earth stations have to hop between transponders to route traffic to the desired spot beam.satellite switched TDMA provides a good solution by incorporating a certain degree of ob-board processing. - a programmable switch located on the satellite routes bursts to spot beams according to set plan. - earth station can direct its transmission to any spot by transmitting in the appropriate time slot. - Transmissions directed to a given beam are arranged in non-overlapping time slots.

Select and poll functions in polling access method : 

Select and poll functions in polling access method

CONTROLLED ACCESS : 

CONTROLLED ACCESS In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. We discuss three popular controlled-access methods.

Reservation access method : 

Reservation access method