The present invention relates to broadcast communications utilizing hierarchical modulation, and more particularly to systems and methods to jointly synchronize a legacy signal with one or more overlay modulation signals.
In certain broadcast communications systems, such as, for example, satellite radio, in order to optimize the utilization of a fixed bandwidth, hierarchical modulation (“HM”) can be used to overlay data for new services on top of a legacy transmission. Such a scheme can be used, for example, to offer additional channels or services. For example, in the Sirius Satellite Radio, Inc. (“Sirius”) Satellite Digital Audio Radio Service (“SDARS”), video channels can be sent over existing audio channels via such an overlay modulation scheme, where the video signal is sent in a “Layer 2” or overlay modulation layer, on top of an existing audio service, known as the “legacy” signal.
There are many approaches to hierarchical modulation, each utilizing a further modulation of a transmitted legacy bit or symbol as to amplitude, phase or a combination of the two. For example, hierarchical modulation can involve the perturbation of original legacy QPSK symbol constellation points, which can, for example, carry audio and data traffic, to convey additional information, such as, for example, video. Thus, for example, such an overlay modulation scheme can carry video data fully independently of the legacy data (original QPSK symbol) carrying a variety of audio channels. For example, Sirius' Backseat TV™ service uses an overlay modulation technique to send video on top of its existing legacy audio channels.
In such systems it is important that the legacy signal be synchronized with the HM signal. It is also advantageous to utilize synchronization techniques at an HM layer that can complement whatever synchronization methods are used at the legacy layer. What is thus needed in the art are techniques, apparatus and methods to jointly synchronize such legacy and HM signals.
Methods and apparatus are presented for the joint synchronization of legacy signals with overlay modulation signals in a communications system utilizing a hierarchical modulation scheme. In exemplary embodiments of the present invention, a synchronization signal can be sent in each of the legacy and overlay bit streams, each using a different approach to frame synchronization, and the two synchronization signals can be used in a complementary manner to synchronize both bit streams. In exemplary embodiments of the present invention a legacy physical frame and an overlay physical frame can be aligned in time. In exemplary embodiments of the present invention a key synchronization signal in a legacy bit stream can be time distributed throughout a legacy transmission frame and can be utilized to assist in both the synchronization of such legacy bit stream and of an overlay bit stream. Additionally, in exemplary embodiments of the present invention, a key synchronization signal in an overlay bit stream can be sent in one fixed physical frame of an overlay transmission frame, and can be utilized to assist in the synchronization of such overlay bit stream as well as legacy data, in a manner that complements the use of the synchronization signal provided in the legacy bit stream.
In general, the present invention can be applied to any communications system which employs the use of hierarchical modulation to transmit secondary information. For example, in order to support future services within an original system design (i.e., a “legacy system”), additional information bandwidth can be acquired by using hierarchical modulation to overlay data for new services on top of the legacy transmission. In particular, for example, in a satellite communications network, such as, for example, Sirius' SDARS, such overlay data can be transmitted by applying a programmable angular offset to legacy QPSK symbols, thus forming a new constellation similar to 8PSK. Alternatively, various other techniques for implementing hierarchical modulation can also be utilized, including modulation of the amplitude, phase or a combination of amplitude and phase, of a legacy bit stream that has already been modulated at a first layer.
In the case of Sirius' SDARS, for example, coded information for each of legacy audio data and overlay data can be conveyed by the use of extensive Forward Error Correction (FEC) and interleaving coding schemes. To successfully decode either data stream, a first task is, for example, to recover a frame synchronization signal, which indicates the boundary point for the FEC scheme and the interleaving scheme.
For example, as a frame synchronization signal, an SDARS legacy data stream can use, for example, a time distributed 255-bit PN (pseudo-random noise) sequence, each bit being repeated, for example, five (5) times within a transmission frame. It is often the case that legacy data de-interleaving processes, as well as FEC decoding processes, are highly dependent upon recovering such a key synchronization signal.
The initial bit is labeled “CS” for “Cluster Sync.” Thus, an exemplary synchronization signal for the exemplary legacy transmission frame can, for example, be sent 1 bit at a time following each physical frame boundary point, i.e., PF1 to PF1275 in
In exemplary embodiments of the present invention, an overlay data stream can, for example, also provide a sync signal, and can also, for example, use a different approach to frame synchronization. For such an overlay data stream, in exemplary embodiments of the present invention a 523 bit alternating PN sequence, known as an Overlay Identification Marker (“OIM”) synchronization signal, can, for example, be placed within the last physical frame of each overlay transmission frame, as shown in the exemplary overlay transmission frame 110 of
Thus, in exemplary embodiments of the present invention, unlike the legacy sync signal, the overlay sync signal is not time spread over the entire frame. It is noted that in the exemplary legacy and overlay transmission frames of
In exemplary embodiments of the present invention, a key system requirement for the generation of each of the legacy data stream and the overlay data stream can, for example, be that each transmission frame, i.e., legacy and overlay, must be of the same duration and exactly aligned in time. This feature can, for example, offer considerable advantages for improved reception of both the legacy and the overlay signals. The fact that in such a method two different synchronization markers are used to identify the same transmission boundary, said synchronization markers being in different time slots and of different duration, can be exploited to improve the reception for both data streams.
At a receiver, in exemplary embodiments of the present invention, the detection of each PN sequence can, for example, be performed by independent correlators.
With reference to
Thus, in exemplary embodiments of the present invention, QPSK soft decisions can be, for example, applied to a legacy correlator directly. Then, for example, OIM correlation first requires overlay data to be sliced from the received legacy soft symbols. These sliced decisions can then, for example, be applied to an overlay correlator, as shown for example, in
For the exemplary sync signals of the exemplary transmission frame format of
As noted, Sirius' SDARS utilizes two TDM signals and one COFDM signal, as is described in U.S. Pat. No. 6,618,367. In Sirius' COFDM transmission, for example, the legacy frame sync bit (i.e., the CS bit sent in each physical frame, or PF) can be sent in the same exact FFT bin. This leads to the possibility that a static multipath null can possibly preclude detection. That is, such a cluster sync pattern does not use any of the frequency diversity that is available in an COFDM system. Spectral nulling of the cluster sync FFT bin is known as the slow speed mute problem. Here again, in exemplary embodiments of the present invention, OIM detection can be used to completely avoid this problem. Since the OIM is spread over an entire physical frame, which is mapped to an entire FFT symbol, the OIM thus takes full advantage of the frequency diversity in the COFDM system. Static nulls generally do not cause loss of the OIM signal. Thus, detecting the OIM (and thus OIM sync) to reset the legacy flywheel circuit completely avoids the slow speed mute issue.
While the present invention has been described with reference to certain exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
This application claims the benefit of and hereby incorporates by reference U.S. Provisional Patent Application No. 60/962,780 entitled “METHOD AND APPARATUS TO JOINTLY SYNCHRONIZE A LEGACY SDARS SIGNAL WITH OVERLAY MODULATION”, filed on Jul. 31, 2007.
Number | Date | Country | |
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60962780 | Jul 2007 | US |