A typical satellite digital audio radio service (SDARS) system, such as that provided by XM Satellite Radio Inc. of Washington, D.C. or Sirius Satellite Radio Inc. of New York City, N.Y., uses two or more satellites with orbits that provide a usable signal over most of North America at all times. However, the signals from the satellites tend not to be received well in cities or anywhere a satellite receiver does not have an unobstructed view of at least one of the satellites. Thus, in cities and other areas where direct reception from a satellite is impossible or unlikely, the SDARS provider may have installed terrestrial repeaters that provide a digital audio data signal carrying the same digital audio data that the satellites are broadcasting. The use of redundant digital audio data channels minimizes service outages as the satellites orbit the earth or as a user moves about. To minimize interference and provide redundancy, the terrestrial repeaters transmits its digital audio signal on a common channel and each satellite transmits its digital audio data signal on its own channel different from the terrestrial channel, each channel having a different carrier frequency. Moreover, the modulation method used for the terrestrial channel (e.g., a coded-orthogonal frequency division multiplexed (COFDM) modulation technique) is chosen for its resistance to fading caused by multipath interference and is more complicated than the conventional modulation method used for the satellite channels (e.g., a phase shift keying time division multiplexed (PSK-TDM, herein referred to as TDM) modulation technique).
Customers of SDARS service providers are demanding additional services, such as video, over existing SDARS systems. To do so, one SDARS provider has proposed sending a data stream, carrying the additional services, as an overlay (“overlaid” data) to the existing digital audio data (“legacy” data) transmission without significantly compromising the “legacy” data transmission. As will be described in more detail below, the “overlaid” data is sent using a conventional data modulation method (used to send the “legacy” data) modified in a way that is transparent to “legacy” receivers (receivers incapable of receiving the “overlaid” data) and allowing the “legacy” receivers to continue to receive the “legacy” data, i.e., the modified modulation method is “backward compatible” with the “legacy” receivers. It is generally desirable that a receiver capable of receiving signals modulated with the modified modulation method, whether or not the “overlaid” data is used by a user of the receiver, also be capable of receiving signals modulated using the unmodified modulation method.