Ford is making Sirius an OEM feature while GM vehicles and others have already had XM as a factory option. Both Sirius and XM use left-hand circular polarization (LHCP). The authorizations allow future flexibility to use RHCP to get polarization diversity multiplexing, which could someday increase throughput within the limits of cross-polarization performance of end user antennas.
Over the holidays, I noticed that a friend with XM Radio had reception inside a metal structure (walls and ceiling) where we were unlikely to be within range of a terrestrial repeater. The key factor to XM Radio reception indoors from the vehicle was that the metal garage door nearest the vehicle were open and south-facing. I got out a pen and paper for my parlor trick: SDARS link budget.
SDARS XM/Sirius Radio link budget
Let’s use general consulting engineer principles–take the figures you do know, especially if they’re worst case, so that you help even out some of the unknown factors where you might be optimistic in a first back of envelope. While XM uses a significantly more sophisticated modulation and interleaving scheme than Sirius, for simplicity let’s use Sirius’ simpler 4.5 MHz QPSK single stream as the model since it will perform worse than XM in general. That is, a single QPSK stream at 4.5 MHz RF bandwidth will get a Sirius radio to play music.
EIRP satellite ~ 90 dBm (1 MW) Thermal Noise power in 4.5 MHz BW: -107.4 dBm Elevation angle: 30 degrees (roughly based on latitude ~ 49 degrees ~ worst azimuth) Path loss to geosync at 2.345 GHz ~ 192 dB Rain loss ~ negligible
Sirius/XM Radio link margin [dB] = 90 - 192 -(-107.4) ~ 5.4 dB This is within reason for 10^-4 BER with QPSK and Reed-Solomon. While I would assume the highly simplified calculation above neglects a few dB (is too conservative) this shows it’s within the realm of reason, and that a big unfurling antenna on the satellite is important for more gain.
Spatiotemporal diversity is a very important factor, yielding ~ 10 dB of effective gain under many scenarios. Foliage drains back another few dB statistically. Basically, it’s give and take, and it seems likely a great amount of effort was spent modeling and measuring.
XM Radio FCC Authorization
XM Radio FCC Authorization cost $90 million in its winning bid, FCC DA 97-2210 authorization order coming in October 1997. The frequencies from 2332.5-2345 MHz were authorized for XM Radio broadcast of various services including voice and music subscriptions.
Late in 2001, XM Radio finally became publicly available using two geostationary satellites: “Roll” at 85 West and “Rock” at 115 West using LHCP S-band antennas. The terrestrial repeaters use vertical polarization, which is received with 3 dB loss by the LHCP end user antennas as implicit in linear vs. circularly polarization antennas in the same system.
XM Radio Technical characteristics
March 2001 saw FCC Order DA 01-699 increasing XM Radio EIRP from 62 dBW to 68.5 dBW (that is, from 1.6 MW to 7.1 MW). The frequency authorization consists of four 1.84 MHz space-to-earth channels and two terrestrial repeater channels each of 2.53 MHz. The center frequencies are:
Roll: 2333.465 MHz, 2344.045 MHz Rock: 2335.305 MHz, 2342.205 MHz Terrestrial: 2337.490 MHz, 2340.020 MHz
XM Satellites are bent-pipe transponders from X-band to S-band, like Sirius.
FCC Order DA 01-2172 on Sept. 17, 2001 gave an STA (Special Temporary Authority) for the long-anticipated terrestrial SDARS repeaters, ultimately necessary even for the elliptical Sirius orbit and much more so for the geostationary XM orbit (parked over the equator). This STA gave the OK for 180 days of 2 kW EIRP repeaters in general and specifically listed > 2 kW EIRP repeaters. The repeaters had to be the whole channel set completely matching the satellite broadcast–working to stave off fears of custom local terrestrial channels only on the repeaters for a particular city.
Sirius Radio FCC Authorization
May 18, 1990 was in a way the initial firm public step towards US mobile satellite radio with Sirius FCC application. October 1997 Sirius was initially authorized in FCC Order DA 97-2191 for the 2320-2332.5 MHz band using LHCP (left hand circular polarization), for which it paid $83M the same calendar year. 7025-7075 MHz was authorized for co-primary uplink for SDARS. Service was planned to launch in late 1999 for the “lower 48” USA. Initially Sirius planned to have two geostationary satellites – before launch, altered to be elliptical “tundra” orbits. The Sirius service was initially planned to have:
- thirty 128 kbps channels “CD quality”
- twenty 32 kbps channels “FM quality” for voice programming
- option to divide these channels as much as needed for auxiliary non-voice service
Of course what happened was that these channels were sliced even more finely to get the ~100 channels. The digital compression artifact are readily apparent on Sirius as well as XM. However, to most users they are less annoying than changing the radio station frequently or the pops and fizz of VHF WBFM multipath and fading combined with the high SNR requirement for FM stereo, and receivers that wait till SNR is too low to revert to monaural reception.
Terrestrial repeater authorization was explicitly deferred for a future authorization.
Early in 2002, Sirius finally became publicly available after years of wrangling with diverse and even humorous opposition reaching FCC filings over 12 years. Distinctive from XM Radio receivers’ six simultaneous RF broadcast streams (two from each satellite, and two terrestrial), Sirius receivers use three streams (one from each of two active satellites) and one terrestrial. Sirius receivers (end user) have three channel simultaneous receivers for time and spatial diversity (simultaneous tuning of two satellites and one terrestrial repeater). Sirius rotates which two satellites are active as the three (FM1,FM2,FM3) Sirius satellites pass over North America for best coverage of CONUS.
Each of the two Sirius satellite channels are 4.5 MHz using QPSK modulation. Each channel is a copy of the other, with a 4 second time delay for time diversity (e.g. overpass, tree cluster). The net Sirius satellite data throughout is 4.4 Mbps all-inclusive – voice, music, data, telemetry. Each Sirius satellite is a bent-pipe transponder from X-band to S-band, and the downlink channel is selectable in case of needing to swap out for a bad satellite. The single Sirius terrestrial repeater channel is also 4.5 MHz, but uses a different modulation scheme.
Terrestrial Repeater frequency assignment
July 2000 brought a joint FCC-State Dept announcement that agreement had been reached with Mexico for SDARS frequency bands, including terrestrial repeaters that had already been agreed to by Canada and the USA.
The specific terrestrial SDARS repeater frequencies were:
- XM Radio: 2336.225-2341.285 MHz
- Sirius: 2324.3-2328.3 MHz
Elbert, Bruce R. The Satellite Communication Applications Handbook. Artech House Space Applications Series. 2nd Ed. (2003).