Lecture 8 SATELLITE SYSTEM PARAMETERS

Satellite system link modelsA satellite system consists of three basic sections: an

uplink, a satellite transponder, and a downlink. The primary component within the uplink section is

the station transmitter. A typical earth station transmitter consists of an IF modulator, an IF-to-RF microwave up-converter, a high-power amplifier (HPA), and some means of band limiting the final output spectrum. Figure 8.1 shows the block diagram of a satellite earth station transmitter.

Figure 8.1 Satellite uplink model

Transponder

It consists of an input band limiting device (BPF), an input low-noise amplifier (LNA), a frequency translator, a low-level power amplifier, and an output band pass filter.

Figure 8.2 shows the simplified block diagram of a satellite transponder. This transponder is an RF-to-RF repeater. Other transponder configurations are IF and baseband repeaters.

Figure 8.2 Satellite transponder

Downlink model

An earth station receiver includes an input BPF, an LNA, and an RF-to-IF down converter. Figure 8.3 shows a block diagram of a typical receiver. The BPF limits the input noise power to the LNA. The LNA is a highly sensitive, low noise device.

Figure 8.3 Satellite downlink model

Satellite system parameters

• Back-off lossHigh power amplifiers (HPA) used in earth station

transmitters and the travelling wave tubes typically used in satellite transponders are non-linear devices. They are gain is dependent on input signal level. A typical input/output power characteristic curve is shown in figure 8.4. It can be seen that as the input power is reduced by 4 dB, the output power is reduced by only 1 dB. There is an obvious power compression.

Cont…

To reduce the amount of inter modulation distortion caused by the non-linear amplification of the HPA, the input power must be reduced (backed off) by several dB. This allows the HPA to operate in a more linear region. The amount the output level is backed off from rated levels is equivalent to a loss is called back-up loss (Lbo).

Figure 8.4 HPA input/output

• Transmit power and Bit energy

To operate efficiently a power amplifier should be operated closer to saturation. The saturated output power is Po(sat) or simply Pt. The output power of a typical satellite earth station transmitter is much higher than the output power from a terrestrial microwave power amplifier. Consequently, when dealing with satellite systems Pt is generally expressed in d BW (decibels in respect to 1W).

Cont…

Most satellite systems use Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). With PSK and QAM, the input baseband is generally a PCM encoded, time division multiplexed signal that is digital in nature. Also several bits may be encoded in a single transmit signalling element. Consequently, a parameter more meaningful than carrier power is energy per bit (Eb).

Cont…

Eb= Pt Tb [8.1]where Eb = energy of single bit (joules per bit) Pt= total saturated output power (watts or

joules per sec) Tb= time of a single bit (sec)Since Tb= 1/fb, where fb is the bit rate in bits per sec,

Eb= Pt / fb= [J/s]/ [b/s]= joules/bit [8.2]

• Effective Isotropic Radiated Power (EIRP)

It is defined as an equivalent transmit power and is expressed mathematically as

EIRP = Pin At [8.3]Where EIRP= effective isotropic radiated power (W) Pin= antenna input power (W) At = transmit antenna gain (unitless ratio)Expressed as a log,

EIRP(dBW) = Pin(dBW) + At(dB) [8.4]In respect to the transmitter outputPin= Pt – Lbo- Lbf

Cont…

Thus,EIRP = Pt-Lbo-Lbf +At [8.5]Where Pin = antenna input power (d BW per W) Lbo = back-off losses of HPA (dB) Lbf = total branching and feeder loss (dB) At = transmit antenna gain (dB) Pt = saturated amplifier output power (dBW

per W)

• Equivalent Noise Temperature

In satellite communication systems, it is often necessary to differentiate or measure noise in increments as small as a tenth or a hundredth of a dB. Noise figure is inadequate for such calculations. Consequently, it is common to use environmental temperature (T) and equivalent noise temperature (Te).

Cont…The total noise power is N=KTB [8.6]Rearranging and solving T givesT= N/KB [8.7]where N= total noise power (W) K=Boltzmann’s constant (Joules/Kelvin) B=bandwidth(Hz) T=temperature of the environment (K)

Cont…Noise factor F isF = 1+ Te/T [8.8]

where Te=equivalent noise temperature

Rearranging equation 8.6, we getTe= T(F-1) [8.9]

Typically Te of the receivers used in satellite transponders are 1000 K. For earth station receivers it between 20 K and 1000 K.

Cont…

Te is generally more useful when expressed logarithmically referenced to 1 K with the unit of dBK, as follows:

Te(dBK) = 10log Te [8.10]Equivalent noise temperature is a hypothetical

value that can be calculated but cannot be measured. It is often used rather than noise figure because it is a more accurate method of expressing the noise contributed by a device. Table 8.1 shows noise unit comparisons.

Table 8.1 Noise Unit ComparisonNoise factor (F) (unitless)

Noise figure (NF)(dB)

Equivalent temperature (T )(K)

dBK

1,2 0,79 60 17,78

1.3 1,14 90 19,54

1,4 1,46 120 20,79

2,5 4 450 26,53

10 10 2700 34,31

• Noise density

It is the noise power normalized to a 1-Hz bandwidth, or the noise power present in a 1-Hz bandwidth.

N0=N/B=KTeB/B=KTe [W/Hz]Where N0= noise density N= total noise power B= bandwidth K= Boltzmann’s constant Te= equivalent noise temperatureExpressed as a logN0(dBW/Hz) = 10logN -10logB [8.11] =10logK+10logTe [8.12]

• Carrier-to-noise density ratio

C/N0 is the average wideband carrier power-to-noise density ratio. The wideband carrier power is the combined power of the carrier and its associated sidebands. The noise density is the thermal noise present in a normalized 1-Hz bandwidth. The carrier-to-noise density ratio may also be written as a function of noise temperature

C/N0 = C/KTe [8.13]

Expressed as a logC/N0 (dB)=C(dBW)- N0(dBW) [8.14]

• Energy of bit-to-noise density ratio

Eb/N0 is one of the most important and most often used parameters. The ratio is a convenient way to compare digital systems that use different transmission rates, modulation methods, or encoding methods.

Eb/N0= [C/fb]/[N/B] = CB/Nfb [8.15] Rearranging the equation givesEb/N0= C/N x B/fb [8.16]The ratio is the product of the carrier-to-noise ratio

(C/N) and the noise bandwidth-to-bit rate ratio (B/fb). Expressed as a log

Eb/N0(dB) =C/N(dB)+B/fb(dB) [8.17]

• Gain-to-equivalent noise temperature ratio

The (G/Te) ratio is a figure of merit used to represent the quality of a satellite or earth station receiver

G/Te= G-10log(Ts) [8.18]

where G = receive antenna gain Ts = operating or system temperature

And Ts= Ta+Tr

where Tr= receiver effective input noise temperature

Ta= antenna temperature

Lecture 9SATELLITE LINK BUDGET

The error performance of a digital satellite system is quite predictable. Figure 9.1 shows a simplified block diagram of a digital satellite system and identifies the various gains and losses that may affect the system performance. When evaluating the performance of a digital satellite system, the uplink and downlink parameters are first considered separately then overall performance is determined by combining them in the appropriate manner.

Figure 9.1 Overall satellite system

Link equations

Link equations consider only the ideal gains and losses and effects of thermal noise associated with the earth station transmitter, earth station receiver, and the satellite transponder.

Uplink equation

Cont…Where Ld and Lu are the additional uplink and

downlink atmospheric losses respectively. The uplink and downlink signals must pass through Earth’s atmosphere, where they are partially absorbed by the moisture, oxygen, and particulates in the air. Depending on the elevation angle, the distance the RF signals travels through the atmosphere varies from one earth station to another. Because Lp, Lu, and Ld represent losses, they are decimal values less than 1. G/Te is the receive antenna gain plus the gain of the LNA divided by the equivalent input noise temperature.

Expressed as log

𝐶𝑁0 = 10log𝐴𝑡𝑃𝑖𝑛 - 20 log (4𝜋𝐷𝜆 ) + 10log(𝐺𝑇𝑒) − 10log𝐿𝑢-10 log K

[9.2]

= EIRP –Lp + 𝐺𝑇𝑒 - Lu- K [9.3]

Downlink equation

𝐶𝑁0 = 𝐴𝑡 𝑃𝑖𝑛ቀ 𝐿𝑝𝐿𝑑ቁ 𝐴𝑟𝐾𝑇𝑒 =𝐴𝑡𝑃𝑖𝑛൫𝐿𝑝𝐿𝑑൯𝐾 × 𝐺𝑇𝑒 [9. 4]

Expressed as log

𝐶𝑁0 = 10log𝐴𝑡𝑃𝑖𝑛 - 20 log (4𝜋𝐷𝜆 ) + 10log(𝐺𝑇𝑒) − 10log𝐿𝑑-10 log K

[9.5]

= EIRP –Lp + 𝐺𝑇𝑒 – Ld- K [9.6]

Link budget Table 9.1 lists the system parameters for 3

typical satellite communications systems. The systems and their parameters are not necessarily for an existing or future system. They are hypothetical examples only. The system parameters are used to construct a link budget. A link budget identifies the system parameters and used to determine the projected C/N and Eb/N0 ratios at both the satellite and earth station receivers for a given modulation scheme and desired P(e).

Table 9.1 System parameters

Lecture 10 SATELLITE MULTIPLE ACCESSING

Multiplexing and multiple access procedures combine signals that might have different characteristics or might originate from different sources, so that they can share a portion of the communications resource.

Multiplexing and multiple access mean very similar things. Both involve the idea of resource sharing. The main difference between the two is that multiplexing takes place locally and the multiple access takes place remotely.

Cont…• Multiplexing involves an algorithm that is

known a priori, usually, it is hard-wired into the system.

• Multiple access is generally adaptive and may require overhead to enable the algorithm to operate.

When multiple carriers are utilized in satellite communications, it is necessary that a multiple-accessing format be established over the system.

Cont…

This format allows for a distinct separation between the uplink and downlink transmissions to and from a multitude of different earth stations. Each format has its own specific characteristics, advantages, and disadvantages.

Cont…

Figure 10.1a shows a single-link (2 earth stations) fixed-frequency FDM/FM system using a single satellite transponder. With earth coverage antennas and for full-duplex operation, each link requires 2 RF satellite channels (i.e.4 RF carrier frequencies, 2 uplink and 2 downlink).

In figure 10.1a earth station 1 transmits on a high-band carrier (f11, f12, f13, …) and receives on a low-band carrier (f1, f2, f3, …).

Cont…To avoid interfering with earth station 1, earth station

2 must transmit and receive on different RF carrier frequencies. The RF carrier frequencies are fixed and the satellite transponder simply an RF-to-RF repeater that provides the uplink/downlink frequency translation. This arrangement is economically impractical and inefficient. Additional earth stations can communicate through different transponders within the same satellite structure (Figure 10.1b), but each additional link requires 4 more carrier frequencies.

Figure 10.1 Fixed frequency earth station: a) single link, b)multiple link

Cont…

It is unlikely that any two-point link would require the capacity available in an entire RF satellite channel. Consequently, most of the available bandwidth is wasted. Also, with this arrangement, each earth station can communicate with only one other earth station. The RF satellite channels are fixed between any two earth stations. Thus, the voice-band channels from each earth station are committed to a single destination.

Cont..

In a system where three or more stations wish to communicate with each other, fixed-frequency or dedicated channel systems are inadequate. There is a requirement of multiple accessing. That is, each earth station using the satellite system has a means of communicating with each of the other earth stations in the system through a common satellite transponder.

Cont…

Multiple accessing is sometimes called multiple destination because the transmissions from each earth station are received by all the other earth stations in the system. The voice-band channels between any two earth stations may be pre-assigned (dedicated) or demand-assigned (switched).

Cont…

When pre-assignment used, a given number of available voice-band channels from each earth station are assigned a dedicated destination. With demand assignment, voice-band channels are assigned on an as-needed basis.

Cont…In an FDM/FM satellite system, each RF channel

requires a separate transponder. It is impossible to differentiate (separate) multiple transmissions that occupy the same bandwidth. Fixed-frequency systems may be used in a multiple access configuration by switching the RF carriers at the satellite, reconfiguring the baseband signals with multiplexing/demultiplexing equipment on board, or using multiple spot beam antennas (reuse).

Cont…

Communications satellites operating in the C-band are allocated a total bandwidth of 500MHz symmetrical around the satellite’s center frequency. This often referred to as one satellite channel, which is further divided into radio channels. Most communication satellites carry 12 transponders, each with 36 MHz of bandwidth.

Cont…

The carriers of the 12 transponders are frequency division multiplexed with a 4 MHz guard band between each of them and a 10 MHz guard band on both ends of the 500 MHz assigned frequency spectrum.

If adjacent transponders in the 500 MHz spectrum are fed from a quadrature polarized antenna, the number of transponders (radio channels) available in one satellite channel can be doubled to 24.

Cont…The 12 odd numbered transponders transmit and

receive with vertically polarized antenna, and 12 even numbered transponders transmit and receive on horizontally polarized antenna. The carrier frequencies of the even channels are offset 20 MHz from the carrier frequencies of the odd numbered transponders to reduce crosstalk between adjacent transponders. This method of assigning adjacent channels different electromagnetic polarizations is called frequency reuse.

Multiple accessing

Satellite multiple accessing (multiple destination) implies that more than one user has access to one or more radio channels (transponders) within a satellite communication channel. Figure 10.2 illustrates the three most commonly used multiple accessing arrangements: frequency division multiple accessing (FDMA), time division multiple accessing (FDMA), and code division multiple accessing (CDMA).

Figure 10.2 Multiple accessing: a) FDMA, b) TDMA, c) CDMA

Cont…

With FDMA, each earth station’s transmissions are assigned specific uplink and downlink frequency bands within an allotted satellite bandwidth. They may pre-assigned or demand-assigned.

With TDMA, each earth station transmits a short burst of information during a specific time slot (epoch) within a TDMA frame. The bursts must be synchronized so that each station’s burst arrives at a satellite at a different time.

Cont…

With CDMA, all earth stations transmit within the same frequency band. The entire satellite transponder bandwidth is used by all stations on a continuous basis. Signal separation is accomplished with envelope encryption/decryption methods.

FDMA FDMA is a method of multiple accessing where a

given RF bandwidth is divided into smaller frequency bands (subdivisions). Each subdivision has its own IF carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same subdivision at the same time. If each subdivision carries only one 4-kHz voice-band channel, this is known as a single-channel per carrier (SCPC) system. When several voice-band channels are frequency-division multiplexed together it is called multiple-channel per carrier (MCPC).

Cont…

When several voice-band channels are frequency-division multiplexed together it is called multiple-channel per carrier (MCPC).

Carrier frequency and bandwidths for FDM/FM satellite systems using multiple-channel-per carrier formats are generally assigned and remain fixed for a long period of time. This is referred to as fixed assignment, multiple access (FDM/FM/FAMA).

Cont…An alternate channel allocation scheme is

demand assignment, multiple access (DAMA).Demand assignment allows all users continuous

and equal access of the entire transponder bandwidth by assigning carrier frequencies on a temporary basis using a statistical assignment process. Figure 10.3 shows the block diagram for single-channel-per-carrier PCM multiple-access demand-assignment equipment (SPADE DAMA) satellite system.

Figure 10.3 FDMA, SPADE earth station transmitter.

Cont…

With SPADE, 800 PCM-encoded voice-band channels separately QPSK modulate an IF carrier signal. Each 4 kHz voice-band channel is sampled at an 8-kHz rate and converted to an 8-bit PCM code. This produces a 64-kbps PCM code for each voice-band channel. The PCM code from each voice band channel QPSK modulates a different IF carrier frequency.

Cont…With QPSK, the minimum required bandwidth is

equal to one-half the input bit rate. Consequently, the output of each QPSK modulator requires a minimum bandwidths of 32 kHz. Each channel is allocated a 45 kHz bandwidth, allowing for a 13 kHz guard band between pairs of frequency-division multiplexed channels. The IF carrier frequencies begin at 52,0225 MHz (low-channel 1) and increase in 45 kHz steps to 87,9775MHz (high band channel 400).

Cont…

The entire 36 MHz band (52 MHz to 88 MHz) is divided in half, producing two 400 channel bands. For full duplex operation, 400 45 kHz channels are used for one direction transmission, and 400 are used for the opposite direction.

Channels 1,2, and 400 from each band are left permanently vacant. This reduces the number of usable full-duplex voice channels to 397.

Cont…

The 6-GHz C-band extends from 5,725 GHz to 6,425 GHz (700 MHz). This allows for approximately 19 36-MHz RF channels per system. Each RF channel has a capacity of 397 full-duplex voice band channels.

Figure 10.4 shows the IF frequency assignments for SPADE. Each RF channel has a 160 kHz common signalling channel (CSC).

Figure 10.4 Carrier frequency assignments

Cont…

The CSC is a time division multiplexed transmission that is frequency division multiplexed into the IF spectrum below the QPSK encoded voice band channels. Figure 10.5 shows the TDM frame structure for the CSC. The total frame time is 50 ms, which is subdivided into 50 1-ms epochs.

Figure 10.5 FDMA, SPADE common signaling channel

Time Division Multiple Access

TDMA is the predominant multiple access method used. It provides the most efficient method of transmitting digitally modulated carriers (PSK). Only one earth station’s carrier is present in the transponder at any given time, thus avoiding a collision with another station’s carrier. Each earth station receives the bursts from all other earth stations and must select from them the traffic destined only for itself.

Cont…

Figure 10.6 shows a basic TDMA frame. Transmissions from all earth stations are synchronized to a reference burst. The reference burst as a separate transmission, but it may be the preamble that precedes a reference station’s transmission of data. Also, there may be more than one synchronizing reference burst.

Figure 10.6 Basic TDMA frame

Cont…

The reference burst contains a carrier recovery sequence (CRS), from which all receiving stations recover a frequency and phase coherent carrier for PSK demodulation. Also included in the reference burst is a binary sequence for a bit timing recovery (BTR, i.e., clock recovery). The UW sequence is used to establish a precise time reference that each of the earth stations uses to synchronize the transmission of its burst.

Cont…

The UW is typically a string of 20 successive binary 1s terminated with binary 0. Each earth station receiver demodulates and integrates the UW sequence. Figure 10.7 shows the result of the integration process. The integrator and threshold detector are designed so that the threshold voltage is reached precisely when the last bit of the UW sequence is integrated.

Cont…

This generates a correlation spike at the output of the threshold detector at the exact time the UW sequence ends. Each earth station synchronizes the transmission of its carrier to the occurrence of the UW correlation spike. Each station waits a different length of time before it begins transmitting. Consequently, no 2 stations will transmit the carrier at the same time. There is a guard time between transmissions.

Figure 10.7 Unique word correlator

Cont…

Figure 10.8 shows the block diagram for CEPT (Conference of European Postal and Telecommunications Administrations) primary multiplex frame. This is a commonly used TDMA frame format for digital satellite systems.

Figure 10.8 TDMA, CEPT primary multiplex frame transmitter

Cont…

The CEPT frame is shown in figure 10.9 is made up of 8-bit PCM encoded samples from 16 independent voice-band channels. Each channel has a separate codec that samples the incoming voice signals at a 16-kHz rate and converts those samples to 8-bit binary codes. This results in 128 kbps transmitted at a 2.048 MHz rate from each voice channel codec.

Cont…

The 16 128-kbps transmissions are time-division multiplexed into a subframe that contains 1 8-bit sample from each of the 16 channels (128 bits). It requires only 62,5 µs to accumulate the 128 bits( 2.048-Mbps transmission rate).

Figure 10.9 TDMA, CEPT primary multiplex frame

Cont…

The CEPT multiplex format specifies a 2-ms frame time. Consequently, each earth station can transmit only once every 2 ms and, therefore, must store the PCM-encoded samples. The 128 bits accumulated during the 1-st sample of each voice-band channel are stored in a holding register, while the 2-nd sample is taken from each channel and converted into another 128-bit subframe.

Cont…

This 128 bit sequence is stored in the holding register behind the 1-st 128 bits. The process continues for 32 subframes (32x62,5 µs = 2 ms). After 2 ms, 32 8-bit samples have been taken from each of 16 voice-band channels for a total of 4096 bits (32x8x16=4096). At this time, the 4096 bits are transferred to an output shift register for transmission.

Code Division Multiple Access (CDMA)

With CDMA there are no restrictions on time or bandwidth. Since there is no limitation on the bandwidth, CDMA is sometimes referred to as spread-spectrum multiple access. Each earth stations’s transmissions are encoded with the unique binary code called a chip code. To receive a particular earth station’s transmission, a receive station must know the chip code for that station.

Cont…

Figure 10.10 shows the block diagram of a CDMA encoder and decoder. In the encoder figure 10.10a the input data is multiplied by a unique chip code. The product code PSK modulates an IF carrier, which is up-converted to RF for transmission. At the receiver in figure 10.10b the RF is down converted to IF. From the IF, a coherent PSK carrier is recovered.

Figure 10.10 CDMA: a) encoder, b) decoder

Cont…

Also, the chip code acquired and used to synchronize the receive station’s code generator. The recovered synchronous chip code multiplies the recovered PSK carrier and generates a PSK modulated signal that contains the PSK carrier plus the chip code. The received IF signal that contains the chip code, the PSK carrier, and the data information is compared with the received IF signal in the correlator.

Cont…

The function of the correlator is to compare the two signals and recover the original data.

The correlation is accomplished on the analog signals. Figure 10.11 shows how the encoding and decoding is accomplished. Fig 10.11a shows the correlation of the correctly received chip code.

Figure 10.11 CDMA code/data alignment:a) correct code, b) orthogonal code

Cont….

A +1 indicated an in-phase carrier, and -1 indicates an out-of-phase carrier. The chip code is multiplied by the data (+1 or -1). The product is either an in-phase code or one that is 180 out of phase with the chip code. In the receiver, the recovered synchronous chip code is compared in the correlator with the received signalling elements.

Cont…

If the phases are the same, a +1 is produced; if they are 180 out of phase, a -1 is produced. It can be seen that if all the recovered chips correlate favourably with the incoming chip code, the output of the correlator will be a +6 (which is the case when logic 1 is received). If all the code chips correlate 180 out of phase, a -6 is generated (which is the case when a logic 0 is received) the bit decision circuit is simply a threshold detector.

Cont…

Depending on whether a +6 or -6 is generated , the threshold detector will output a logic 1 or a logic 0. The correlator looks for a correlation (similarity) between the incoming coded signal and the recovered chip code. When a correlation occurs, the bit decision circuit generates the corresponding logic condition.

Cont…

The chip code from one station must not correlate with the chip codes from any of the other earth stations. Fig 10.11b shows how such a coding is achieved. If half the bits within a code were made the same and half were made exactly the opposite, the resultant would be zero cross correlation between chip codes. Such a code is called an orthogonal code.

Cont…

It can be seen that when the orthogonal code is compared with the original chip code, there is no correlation. The orthogonal code, although received simultaneously with the desired chip code, had absolutely no effect on the correlation process. For this example, the orthogonal code is received in exact time synchronization with the desired chip code; this is not always the case.

Cont…

The primary difference between spread spectrum PSK transmitters and other types of PSK transmitters is the additional modulator where the code word is multiplied by the incoming data. Because of the pseudorandom nature of the code word, it is often referred to as pseudorandom noise (PRN).

Cont…

A spread spectrum signal cannot be demodulated accurately if the receiver does not possess a dispreading circuit that matches the code word generator in the transmitter. Three of the most popular techniques used to produce the spreading function are direct sequence, frequency hopping, and a combination of direct sequence and frequency hopping called hybrid direct sequence frequency hooping (hybrid-DS/FH).

Direct sequence spread spectrum (DS-SS)

DS-SS is produced when a bipolar data-modulated signal is linearly multiplied by the spreading signal in a special balanced modulator called a spreading correlator. The spreading code rate Rcw= 1/Tc, where Tc is the duration of a single bipolar pulse (i.e., the chip). Chip rates are 100 to 1000 times faster than the data message; therefore, chip times are 100 to 1000 times shorter in duration than the time of a single data bit.

Cont…

As a result, the transmitted output frequency spectrum using spread spectrum is 100 to 1000 times wider than the bandwidth of the initial PSK data-modulated signal. The block diagram for a direct-sequence spread-spectrum system is shown in Figure 10.12. The data source directly modulates the carrier signal, which is then further modulated in the spreading correlator by the spreading code word.

Cont…The spreading (chip) codes used in spread spectrum

systems are either maximal-length sequence codes, sometimes called m-sequence codes or Gold codes. The advantages of CDMA are:

1. The entire bandwidth of a satellite channel or system may be used for each transmission from every earth station.

2. CDMA is immune to interference (jamming). Disadvantage is that it requires time synchronization.

Figure 10.12 Simplified block diagram for direct-sequence spread spectrum transmitter

Frequency hopping spread spectrum (FH-SS)

FH is a form of CDMA where a digital code is used to continually change the frequency of the carrier. The carrier is first modulated by the data message and then up converted using a frequency synthesized local oscillator whose output frequency is determined by an n-bit pseudorandom noise code produced in a spreading code generator. The simplified block diagram of a FH-SS transmitter is in figure 10.13.

Figure 10.13 Simplified block diagram of a frequency-hopping spread spectrum transmitter

Cont…With frequency hopping, the total available

bandwidth is partitioned into smaller frequency bands, and the total transmission time is subdivided into smaller time slots. The idea is to transmit within a limited frequency band for only a short time, then switch to another frequency band and so on. This process continues indefinitely. The frequency hopping pattern is determined by a binary spreading code. Each station uses a different code sequence.

Cont…

A typical hopping pattern (frequency-time matrix) is shown in figure 10.14.

With FH, each earth station within a CDMA network is assigned a different frequency-hopping pattern each transmitter switches (hops) from one frequency band to the next according to their assigned pattern. Each station uses the entire RF spectrum but never occupies more than a small portion of that spectrum at any one time.

Figure 10.14 Frequency time-hopping matrix

Cont…

• FSK is the modulation most commonly used with frequency hopping. When it is a given station’s turn to transmit, it sends one of the two frequencies for the particular band in which it is transmitting. The number of stations in a given frequency hopping system is limited by the number of unique hopping patterns that can be generated.

Lecture 11 SATELLITE NETWORKING

In this lecture we discuss the issues associated with routing of information from origin to destination earth stations.

Advantages and disadvantages of multi-beam satellites

Single beam satellites have one of these disadvantages:

• The satellite may provide coverage of the whole region of the earth which is visible from the satellite and thus permit long-distance links to be established. In this case the gain of the satellite antenna is limited by its bandwidth as imposed by the coverage.

Cont…

• The satellite may provide coverage of only part of the earth by means of a narrow beam. One thus benefits from a higher antenna gain due to a reduction of the aperture angle of the antenna beam, but the system can be connected to stations situated outside its coverage only by terrestrial or inter satellite links.

Cont…

• With a single beam satellite it is therefore necessary to choose between interconnection of a large number of stations and provision of a favourable link budget by means of a high satellite antenna gain.

• With multi-beam satellite the coverage is extended from the several beam coverage and each beam provides an antenna gain which increases as the antenna beam width decreases.

Advantages

The performance improves as the number of beams increases.

Frequency reuse Frequency re-use consists of using the same

frequency band several times in such a way as to increase the total capacity of the network without increasing the allocated bandwidth.

Cont…

The frequency re-use factor is defined as the number of times that the bandwidth B is used. In practice the frequency re-use factor depends on the configuration of the service area which determines the coverage before it is provided by the satellite. If the service area consists of several widely separated regions, it is possible to re-use the same band in all beams. The frequency re-use factor can then attain the theoretical value of M.

Disadvantages

Interference between beams The figure 11.1 illustrates interference

generation in a multi-beam satellite system. The allocated bandwidth B is divided into 2 sub-bands B1 and B2. The figure shows 3 beams. Beam 1 and 2 use the same band B1. Beam 3 uses band B2.

Figure 11.1 Interference on: a) uplink, b) downlink

Cont…

On the uplink Fig 11.1a the carrier at frequency fu1 of bandwidth B1 transmitted by the beam 2 earth station is received by the antenna defining beam 1 in its side lobe with a low but non-zero gain. The spectrum of this carrier superimposes itself on that of the carrier of the same frequency emitted by the beam 1 earth station which is received in the main lobe with the maximum antenna gain.

Cont…

The carrier of beam 2, therefore appear as interference noise in the spectrum of the carrier of beam 1. This noise is called co-channel interference (CCI). Part of the power of the carrier at frequency fu2 emitted by the earth station of beam 3 is introduced as a result of imperfect filtering. In this case it consists of adjacent channel interference (ACI).

Cont…

On the downlink Fig.11.1b the beam 1 earth station receives the carrier at frequency fd1 emitted with maximum gain in the antenna lobe defining beam 1. Downlink interference originates from the following contributions of power spectral density superimposed on the spectrum of this carrier:

Cont…

• The spectra of the uplink adjacent channel and co-channel interference noise retransmitted by the satellite,

• The spectrum of the carrier at the same frequency fd1 emitted with maximum gain in beam 2 and with a small but non-zero gain in the direction of the beam 1 station. This represents additional co-channel interference.

Cont…

A multi-beam satellite system must be in a position to interconnect all network earth stations and consequently provide interconnection of coverage areas. Using transponders, 3 techniques can be used for interconnection of coverage areas:

• Interconnection by transponder hopping• Interconnection by on-board switching (SS/TDMA)• Interconnection by beam scanning

Cont…Multi-beam satellite systems reduce the size of

earth stations and the cost of the earth segment.

Frequency re-use permits an increase in capacity without increasing the band width allocated to the system. However, interference between adjacent channels, which occurs between beams using the same frequencies, limits the potential capacity increase. The interference is greater with earth stations with small antennas.

Interconnection by transponder hopping

The band allocated to the system is divided into as many sub-bands as there are beams. A set of filters on board the satellite separates the carriers in accordance with the sub-band occupied. The output of each filter is connected by a transponder to the antenna of the destination beam. It is necessary to use a number of filters and transponders at least equal to the square of the number of beams.

Cont…

Figure 11.2 shows this concept for an example with two beams. According to the type of coverage, the earth stations must be able to transmit and/or receive on several frequencies and polarizations in order to hop from one transponder to another (transponder hopping).

Figure 11.2 Example of beam interconnection by transponder hopping

Table 11.1 Frequency agility required to ensure beam interconnection in accordance with the type of coverage

TYPE OF COVERAGE FREQUENCY AND POLARISATION AGILITY

Uplink Downlink

Global Global On transmission or on reception

Spot Global On reception

Global Spot On transmission

Spot Spot On transmission or on reception

Interconnection by on-board switching (SS/TDMA)

The principle is illustrated in figure 11.3. The payload includes a programmable switch matrix having a number of inputs and outputs equal to the number of beams. This matrix connects each up-beam to each down-beam by way of a receiver and a transmitter. The number of repeaters is equal to the number of beams.

Cont…

The distribution control unit (DCU) associated with the switch matrix establishes the sequence of connection states between each input and output during a frame in such a way that the carriers arriving at the satellite in each beam are routed to the destination beams.

Cont…

Since interconnection between two beams is cyclic, stations must store traffic from users and transmit it in the form of bursts when the required interconnection between beams is realised. This method can be used in practice only with digital transmission and TDMA access. This is why it is called satellite switched time division multiple access (SS-TDMA).

Figure 11.3 The principle of on-board switching

(SS-TDMA)

Cont…

Figure 11.4 shows the organisation of a frame for a three-beam satellite. The frame contains a synchronising and a traffic field. Bursts from traffic stations are routed to their destinations in the traffic field. The traffic field contains a succession of switch states. During a given switch state the switching matrix retains the same connection state.

Cont…

The traffic field also contains a growth space in case traffic demand is less than capacity. The duration of a connection between an up beam and a down beam is called a window. A window can extend over the duration of several switch states. Figure 11.4b shows the switch state sequence implemented by the switch matrix in order to route traffic according to the frame organisation of figure 11.4a.

•

Figure 11.4 Three –beam SS-TDMA satellite:a) frame organisation b) switch state sequence during the active part of the traffic.

Cont…

Figure 11.5 shows the way in which bursts are arranged in the time interval of one window. It shows bursts that are transmitted by stations A, B and C in the window corresponding to a connection from beam 3 to beam 2. Each bust transmitted by a station during the window time considered consists of several sub-bursts which contain station-to-station information.

Figure 11.5 Window organisation

Cont…

The assignment of bursts in the frame must maximise the use of satellite transponders. Transponders are exploited best when the windows are occupied entirely by traffic bursts. This is possible only when traffic distribution between beams is balanced. In practice this is not always the case.

Cont…

• A traffic matrix can be established which describes the traffic demand from one beam to another. For example, for a three-beam satellite (1,2,and 3), this matrix is as shown where tXY represents the traffic demand from beam X to beam Y.

Matrix

To beam 1 2 3From beam

1 t11 t12 t13 S1

2 t21 t22 t23 S3

3 t31 t32 t33 S3

R1 R2 R3

Cont…

The sum of each row Si (i =1, 2, 3) represents the traffic uplinked by all stations in beam i. The sum of each column Rj (j = 1,2, 3) represents the beam traffic down linked in beam j. In the case of balanced traffic distribution between beams they are equal. Otherwise, one of these sums is grater than all the others. The corresponding line of the matrix (row or column) is called the critical line.

Cont…

There are 2 aspects of network synchronisation:• Synchronisation of earth stations • Synchronisation of the earth segment with the

satellite.

Interconnection by beam scanning

Each coverage area is illuminated cyclically by an antenna beam whose orientation is controlled by a beam-forming network which is part of the antenna sub-system on board the satellite. The area stations transmit or receive their bursts when the area is illuminated by a beam.

In the absence of on-board storage, at least two beams are necessary at a given instant- one to establish the uplink and the one to establish the downlink. The illumination duration is proportional to the volume traffic to be carried between the two areas. Figure 11.6 shows this concept.

Figure 11.6 Interconnection by scanning beams

On-board processing

Error correcting coding can be used on one of the up-and downlinks. For the downlink, the encoder is located on board the satellite and is activated by tele command. The link benefits from the decoding gain. The transmission rate increases by a factor equal to the inverse of the coding ratio.

This implies that the downlink is limited in power but not in bandwidth.

Cont…

If the link is limited in bandwidth, the transmission rate must be maintained, and consequently the information rate is reduced, on both the downlink and the uplinks which feed it.

The availability of bits on board the satellite at the output of the uplink carrier demodulators permits switching between receiving and transmitting antennas to be no longer at radio frequency but at baseband.

Cont…

Onboard processing offers several advantages. Regenerative satellite systems tolerate a higher level of interference compared to transparent ones and make earth stations simpler. The advantages gained have to be balanced with the additional onboard complexity and its impact on the payload reliability.

Cont…

Moreover, the implementation of fast computing circuitry onboard the satellite places strong demand on the power consumption of the satellite payload, hence the satellite mass. Finally, onboard processing payloads imply some form of a priori selection of given transmissions formats which make the payload specific to predefined types of services that may turn out to be popular once the satellite is in operation.

Cont…

This also poses the problem of coping with unexpected changes in traffic demand (both volume and nature), and new operational procedures. This issue is dissuasive to satellite operators, who tend to prefer minimising risk rather than acquiring substantial but uncertain advantages.

Intersatellite links (ISL)

ISL can be considered as particular beams of multi beam satellites. The beams in this case are directed not towards the earth but towards other satellites. For bidirectional communication between satellites, two beams are necessary-one for transmission and one for reception. Network connectivity implies the possibility of interconnecting beams dedicated to inter satellite links and other links at the payload level.

Cont…

Three classes of inter satellite link can be distinguished:

• Links (GEO-LEO) between geostationary earth orbit (GEO) and low earth orbit (LEO) satellites; also called inter-orbital links.

• Links between geostationary satellites (GEO-GEO).

• Links between low orbit satellites (LEO-LEO).

GEO- LEO link

GEO-LEO link serves to establish a permanent relay via a geostationary satellite between one or more earth stations and a group of satellites proceeding in a low earth orbit at an altitude of 500-1000 km. For economic and political reasons, one does not wish to install a network of stations which is so large that at every instant the passing LEO satellites are visible from at least one station.

Cont…

One or more geostationary satellites are therefore used which are permanently and simultaneously visible both from stations and low earth orbit satellites. They serve to relay communications. This method also permits to overcome possible limitations of the terrestrial network

GEO-GEO links

Consider a multi-beam satellite network. Figure 11.7 illustrates the case of a three beam satellite (Fig.11.7a). To ensure inter connectivity among all stations, it is necessary to equip all stations with two antennas each pointing towards a different satellite (Fig 11.7b). With satellites provided with inter satellite transponders one can either:

Cont…• Equip the stations of region 1, assumed to be

generating the excess traffic, with a second antenna and retain the same configuration for the stations of regions 2 and 3(Fig. 11.7c). The intersatellite link carries the excess traffic of region 1. Or

• Distribute the stations, each with a single antenna, into two groups, each associated with one satellite (Fig.11.7d). The inter-satellite link carries the traffic between the two groups.

Figure 11.7

Cont…

The choice is economic and depends on the case considered. An intersatellite link permits earth stations of two networks to be interconnected and hence the geographical coverage of the two satellites to be combined (Fig 11.8a). The alternative solutions are either:

Cont…

• To install an interconnecting earth station equipped with two antennas in the common part of the two coverages, if it exists (Fig. 11.8b). Or

• To make the connection, by means of the terrestrial network, from the stations of one network to a station of the other network situated on the common border of the two coverages (Fig. 11.8c).

Figure 11.8 Extension of system coverage.interconnection of the stations of each coverage by an intersatelite link.Interconnection without an intersatellite link by a station common to the two networks.Interconnection without an intersatellite link by a terrestrial network.

Cont…

Figure 11.9 shows the design of a global network based on nine geostationary STAR satellites, which establish a basis for wordwide communication, and a set of local satellites connected to these by regional intersatellite links (GOL-82).

Figure 11.9 A global network [GOL-82].

LEO-LEO links

The advantages of low orbit satellites and the increasing congestion of geostationary satellite orbits suggest the future development of orbiting satellites. In fact, the disadvantages of an orbiting satellite (limited duration of communication time and relatively small coverage) can be reduced in a network containing a large number of satellites which are interconnected by intersatellite links and equipped with a means of switching between beams.

Cont…

Intersatellite links permit the following:• The use of a geostationary satellite as a relay

for permanent links between LEO satellites and a network of a small number of earth stations.

• An increase in system capacity by combining the capacities of several geostationary satellites.

Cont…

• The planning of systems with a higher degree of flexibility.

• Consideration of systems providing a permanent link and worldwide coverage using low orbit satellites as an alternative to system using geostationary satellites.