Satellite Radio - Seminar Report

Satellite Radio- a technological overview

Sagar Medikeri2BV06EC060

Department of Electronics and Communications,BVBCET, Hubli, India.

Contents

1 Introduction 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Review of Radio Technology 22.1 Analog Radio vs. Digital Radio . . . . . . . . . . . . . . . . . 2

3 Satellite Radio 33.1 The Market Players . . . . . . . . . . . . . . . . . . . . . . . 33.2 Service Features . . . . . . . . . . . . . . . . . . . . . . . . . 33.3 How Satellite Radio works . . . . . . . . . . . . . . . . . . . . 43.4 Satellite Technology . . . . . . . . . . . . . . . . . . . . . . . 4

3.4.1 Geostationary satellites . . . . . . . . . . . . . . . . . 53.4.2 Geosynchronous satellites . . . . . . . . . . . . . . . . 53.4.3 Automatic Transmission Identification Service (ATIS) 73.4.4 RF Power . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.5 Receiver Technology . . . . . . . . . . . . . . . . . . . . . . . 73.5.1 Signal Diversity . . . . . . . . . . . . . . . . . . . . . . 83.5.2 Maximal Ratio Combining . . . . . . . . . . . . . . . . 8

3.6 Perceptual Audio Coding (Baseband Modulation) . . . . . . . 93.7 Link Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . 103.8 Conditional Access . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Future of Satellite Radio 12

5 References 13

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List of Figures

3.1 Working of Satellite Radio . . . . . . . . . . . . . . . . . . . . 53.2 Satellite footprints of 1worldspace satellites - Afristar and

Asiastar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Orbital footprint of Radiosat-1,2,3 . . . . . . . . . . . . . . . 63.4 Block Diagram of Satellite Receiver . . . . . . . . . . . . . . . 73.5 Downlink Spectrum of Sirius . . . . . . . . . . . . . . . . . . 83.6 Maximal Ratio Combining for Signal Diversity . . . . . . . . 93.7 Perceptual Audio Encoder . . . . . . . . . . . . . . . . . . . . 93.8 Audio Masking Effect . . . . . . . . . . . . . . . . . . . . . . 103.9 Hierarchical Modulation . . . . . . . . . . . . . . . . . . . . . 11

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Satellite Radio 1

Chapter 1

Introduction

1.1 Introduction

A satellite radio or subscription radio (SR) is a digital radio signal that isbroadcast by a communications satellite, which covers a much wider geo-graphical range than terrestrial radio signals.

Satellite radio offers a meaningful alternative to ground-based radio ser-vices as they allow listeners to roam across an entire continent, listeningto the same audio programming anywhere they go. In any case, the an-tenna must have a clear view to the satellites. In areas where tall buildings,bridges, or even parking garages obscure the signal, repeaters can be placedto make the signal available to listeners.

1.2 Motivation

The last decade has seen the Radio change like never before. People todaycan listen to excellent Radio programmes in CD quality audio even in themost remote parts of a continent. There is no signal fading as in AM/FMRadio nor are there any commercials to annoy the listeners. People are spoiltfor choice tuning between the stations - all thanks to Satellite Radio. Com-petition in the Satellite Radio arena has led scientists and engineers to makegreat strides in communication capabilities of the channel. From modula-tion techniques used for transmission to diversity reception, the technologyincorporated by Satellite Radio is cutting-edge.

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Chapter 2

Review of Radio Technology

Radio can be classified as Analog Radio and Digital Radio. In Analog for-mat, the audio signal to be transmitted is piggybacked on a carrier signaland transmitted after applying some analog modulation techniques like Am-plitude Modulation or Frequency Modulation. In Digital format, the audioor modulating signal is first digitized, then modulated using a digital mod-ulation technique like Quadrature Phase Shift Keying (QPSK), QuadratureAmplitude Modulation (QAM), etc and then beamed into air. Examplesof Analog Radio are AM Radio, FM Radio. Examples of Digital Radio areSatellite Radio and High-Definition (HD) Radio.

2.1 Analog Radio vs. Digital Radio

Satellite Radio being Digital Radio has several advantages over its Analogcounterparts. The major ones are,

Analog Radio Digital RadioCompression Not possible PossibleBandwidth Usage Large; FM - 200KHz/channel EfficientLink Adaptation Not possible PossibleMetadata Not available AvailableEncryption Not possible PossibleInterference More susceptible More susceptible

Table 2.1: Advantages of Digital over Analog Radio

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Chapter 3

Satellite Radio

3.1 The Market Players

There aren’t much companies offering Satellite Radio services. This is be-cause the spectrum allotted to Satellite Radio services is too less. Cur-rently, there are three major Satellite Radio service providers namely, SiriusSatellite Radio and XM Satellite Radio Holdings, Inc. in Continental US,Canada and 1worldspace Inc. serving Africa and Asia. It was in 1998 thatWorldspace (now 1worldspace) began Satellite Radio services. In the US, theFederal Communications Commission (FCC) invited bids for Satellite Radiooperations from potential companies for spectrum allocation. Finally, Satel-lite CD Radio, Inc. (now Sirius) and American Mobile Satellite Corporation(now XM) were chosen for spectrum allocation. XM started operations onSeptember 25, 2001 and Sirius, on February 14, 2002. Since their inception,Sirius and XM together have more than 18 million subscribers as of April2010.

3.2 Service Features

1. Large area coverage - The area covered can be as large as an entirecontinent.

2. CD quality audio - The audio quality is exceptional mainly becauseDigital Radio is immune to interference and noise sources.

3. Subscription based - Satellite Radio is a subscription-based service.Subscription rates vary from USD 5-17 depending on geographical re-gion.

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4. Receivers - Each service provider uses different modulation techniquesfor transmission, different codecs and so each one has a different re-ceiver.

5. Commercial-free content - Since the service is subscription-based, com-mercials are not played on radio channels.

6. Lots of variety - 100s of channels - Each service providers airs upto 130-160 channels which gives listeners more choice than any other Radioservice.

7. Programming includes music, news, talk radio, sports, humor

8. Audio associated metadata transmitted - Metadata of audio beingplayed, like Name of the Song, Singer and Channel name are alsoavailable.

9. Some channels broadcast in 5.1 surround sound format

10. FCC does not regulate content - This means that the Radio serviceproviders can play anything they want. The FCC does not censor theprogramming content.

3.3 How Satellite Radio works

Each broadcaster has one or more Satellite Uplink facilities from where theprogramming content created in the studio is beamed to the satellite overthe Uplink frequency. The Satellite then transmits the signals over theDownlink frequency to the coverage region. Terrestrial Repeater Networkis concentrated in and around Urban areas where buildings can block line-of-sight link between the satellites and the receivers. One of the Uplinkfacilities also acts as the Control Room for monitoring the position andEIRP (Effective Isotropic Radiated Power) footprints of the satellites andalso the Terrestrial Repeater Network. The signals are then received by thereceiver sets which decode and play the original audio.

3.4 Satellite Technology

Satellite broadcasters use either geostationary or geosynchronous satellites.The Uplink frequency is allotted in the X-band (8-12GHz) and the Down-link is allotted in the S-band (2-4GHz) or L-band (1-2GHz). FCC has al-lotted 12.5MHz of bandwidth to both Sirius and XM. Sirius operates from

Dept of EC, BVBCET, Hubli, KA.

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Figure 3.1: Working of Satellite Radio

2320MHz to 2332.5MHz and XM operates from 2332.4MHz to 2345MHz.1worldspace operates from 1452MHz to 1492MHz.

3.4.1 Geostationary satellites

XM operates two geostationary satellites, namely ”‘Rhythm”’ at 85WL and”‘Roll”’ at 115WL. 1worldspace also operates two geostationary satellites,”‘Afristar”’ 21EL and ”‘Asiastar”’ at 105EL. A geostationary satellite staysfixed in position with respect to a point on Earth and broadcasts over thesame region all the time. The spotbeams of Afristar and Asiastar are shownbelow.

3.4.2 Geosynchronous satellites

Sirius operates three geosynchronous satellites, Radiosat-1, Radiosat-2, Radiosat-3 in a highly elliptical Tundra orbit. This ensures that each of the threesatellites spends atleast 16 hours over the Continental US. Geosynchronoussatellites have an orbital period of 24 hours and their orbital plane is inclinedat an angle to the equotorial plane of the Earth. The ground trace of thesatellites is shown below.


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Figure 3.2: Satellite footprints of 1worldspace satellites - Afristar and Asi-astar

Figure 3.3: Orbital footprint of Radiosat-1,2,3


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3.4.3 Automatic Transmission Identification Service (ATIS)

Satellites are equipped with ATIS. This enables broadcasters to uplink frommultiple locations. Each uplink facility is given a station ID. Once thesignals are received by the satellite, it verifies the source of the signals byreferring to the station ID for authentication.

3.4.4 RF Power

The satellites employ dual high-power Travelling Wave Tube Amplifiers(TWTAs) operating in parallel for RF amplification. The satellites areequipped withtwo or three parabolic reflectors or Unfurlable antennas de-pending on the spot beams required. The total power output is in ther rangeof 12-15KW.

3.5 Receiver Technology

Figure 3.4: Block Diagram of Satellite Receiver


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The basic receiver block diagram is as shown below. The signals receivedby the receiver are in the S-band (2.3GHz). The Antenna Tuner downcon-verts them to an Intermediate Frequency of about 75MHz. The MaximalRatio Combining unit is used to achieve Signal Diversity. The Basebandprocessor performs digitization, demodulation, error correction and decryp-tion.

3.5.1 Signal Diversity

The radio programming content is broadcast over several carriers. This isto prevent signal fading due to noise sources operating close to the carrierfrequency of the broadcast. With multiple carriers, the probability of sig-nal fading is greatly reduced. For example, Sirius’ programming contentis broadcast over 2322MHz, 2326.25MHz and 2330.5MHz; each of 4MHzbandwidth. The actually programming content is only 4MHz in bandwidth.The selection of the best signal from all the signals received is called SignalDiversity.

Figure 3.5: Downlink Spectrum of Sirius

3.5.2 Maximal Ratio Combining

The Maximal Ratio Combining unit determines the best estimate of the allthe signals received and passes it to the next stage. First, all the base-band signals are retrieved from the carriers. Then, each baseband signalis weighted by a gain depending on the signal strength. If Baseband-1 sig-nal is strong, then it is further amplified. If Baseband-2 signal is weak ornoisy, then it is attenuated. The same is done for all signals. Finally, all theweighted signals are summed up and averaged to obtain the best estimateof the actual signal.


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Figure 3.6: Maximal Ratio Combining for Signal Diversity

3.6 Perceptual Audio Coding (Baseband Modula-tion)

Perceptual Audio Coder (PAC) is an algorithm, like MPEG’s MP3 stan-dard, used to compress digital audio by removing extraneous informationnot perceived by most people. The input audio is first split up into its com-ponent spectral quantities. The number of bits to be used for quantizationand encoding is decided by a psychoacoustic model. Below is the plot of theminimum sound pressure required for a human to hear a sound in completesilence. It is clear that at around 3.8KHz, the least sound pressure is re-quired to be heard. This means that the human ear is most sensitive to thisfrequency. Even small changes in this frequency sound is easily detectableby the ear. So, more number of bits have to be allocated for representationof spectral content around this frequency. Similarly, less number of bits canbe allotted for representation of lower and higher frequency spectral contentas the ear is less sensitive to those frequencies.

Figure 3.7: Perceptual Audio Encoder

Also, Masking effect can is used to eliminate unperceptible sounds. If


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Figure 3.8: Audio Masking Effect

a loud sound is present in the audio stream, then all spectral content inthe masking threshold of this masker sound is unperceptible. The mask-ing threshold is an envelope of sounds of frequencies close to the masker’sfrequency with amplitude decreasing with difference in frequency as shownin the figure. All such unperceptible sounds are eliminated which leads tobetter compression. PAC is therefore lossy compression, but there is no lossof quality because of the psychoacoustic model used.

3.7 Link Adaptation

Link adaptation, or adaptive coding and modulation (ACM) denotes thematching of the modulation, coding and other signal and protocol parame-ters to the conditions on the radio link (e.g. the pathloss, the interferencedue to signals coming from other transmitters, the sensitivity of the receiver,etc.). Irrespective of whether a receiver is in clear view of the satellite or not,the audio quality has to be acceptable. Link Adaptation is achieved by Hi-erarchical Modulation which employs a hybrid approach to modulation. Forexample, the link can employ QPSK (for noisy channels) and 16QAM (forclearer channels). The former is more robust and can tolerate higher levels


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of interference but has lower transmission bit rate. The latter has twicehigher bit rate but is more prone to errors due to interference and noisehence it requires stronger FEC (forward error correction) coding which inturn means more redundant bits and lower information bit rate. The QPSKcarries the two MSBs and the remaining LSBs are modulated by 16-QAMor 64-QAM, etc. In poor link conditions, atleast the QPSK is decoded anddecent signal quality is ensured.

Figure 3.9: Hierarchical Modulation

3.8 Conditional Access

Conditional Access (CA) is the protection of content by requiring certaincriteria to be met before granting access to this content. In Satellite Radio,CA is used to limit reception to only legitimate subscribers. This preventspiracy and the resulting loss in revenues. Elliptical Curve Cryptography(ECC) is nowadays used for encryption. ECC is capable of providing thesame security as that of the RSA algorithm but with a smaller key. Thisreduces the transmission overhead without compromising on security.


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Chapter 4

Future of Satellite Radio

Satellite Radio has several challenges to overcome to become a success story.

1. The receivers and subscriptions are expensive.

2. Stiff competition from iPod, Internet Radio, local FM

3. Steady Attrition Rates

4. Sirius XM still in debt

So, it looks like the future of Satellite Radio is not big. But, the cutting-edge technology developed for Satellite Radio will likely pave the way fornext digital revolution, Satellite Digital Media Broadcasting (S-DMB).

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Chapter 5

References

1. Painter, T., and A. Spanias. ”‘Perceptual coding of digital audio.”’Proceedings of the IEEE 88 (2000)

2. Gledhill J., MacAvock P., Miles R. Whitepaper on ”‘Hierarchical Mod-ulation”’

3. Wikipedia entries on,

(a) Sirius Radio

(b) XM Radio

(c) Psychoacoustics

Satellite Radio - Seminar Report

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