Patent Application
20060285607
The present invention generally relates to radio frequency communication systems and, more particularly, to providing high link availability for a narrowband channel on a wideband bandwidth efficient modulation (BEM) radio frequency (RF) link such as a satellite link or terrestrial communications link.
Bandwidth efficient modulation (BEM) is a new approach in RF communications that achieves extremely high data rates over limited spectral allocation. BEM technology enables ultra-wideband data transfer over satellite, for example, often enabling up to five times the data throughput over a given amount of spectrum than other, more conventional techniques. Due to higher signal-to-noise ratio (SNR) requirements for BEM reception, however, a BEM channel's ability to power through severe rain attenuation events may be limited. Thus, link availability—e.g., the percentage of time that a link can provide a signal of acceptable quality for accurate reception of the transmitted signal—for a wideband BEM link may be lower than that for a lower data rate narrowband link at the same location. A small portion of the data that needs to be transmitted, however, requires a more reliable, i.e., higher availability, link than that which a wideband BEM link affords. Such data may include, for example, spacecraft telemetry and important user data, and may be transmitted over a narrowband channel.
Prior art satellite communication systems use a “bent-pipe” architecture (e.g., frequency translating repeater, or transponder) that does not incorporate sophisticated digital modulation. These bent-pipe satellites do not have sophisticated digital modulators to share spacecraft size, weight and power resources with, so with smaller payload demands they can incorporate a dedicated transmitter for narrowband channels that is designed to achieve link margins high enough to burn through all but the most extreme rain events for link availability of 99.9% and above. These narrowband transmissions use a separate part of the RF spectrum from the wideband transmissions. These systems with separate wideband and narrowband RF frequency bands require additional allocation of spectral resources and severe output filtering on the wideband channel to mitigate self-interference. Such filtering is undesireable for the much more distortion-sensitive BEM waveform. Therefore, an alternate spectral allocation would need to be found for the previous approach to be feasible.
The BEM approach with its need for higher SNR drives the satellite architecture away from previous bent-pipe systems to an architecture that demodulates and then remodulates the signal (demod-remod architecture) incorporating sophisticated digital modulation. The demod-remod architecture is favored to achieve better link performance (e.g., better SNR), since demod-remod architecture in a certain sense decouples the two links, uplink and downlink, from each other so that the downlink performance only depends on downlink SNR, and the uplink only on uplink SNR. Due to higher signal-to-noise ratio (SNR) requirements for BEM and susceptibility to rain fades, noted above, the increased RF link power requirements of BEM make the BEM type of link non-optimal for transmission of lower rate data with very high availability requirements, as high rain degradation margins (i.e., high ability to overcome rain fades) are extremely costly using BEM. The BEM architecture relies on demod-remod architecture with sophisticated digital modulation so it is not desirable to include separate downlink transmitters for the wideband BEM channel and the narrowband channel, as this would increase satellite cost. Furthermore, a separate transmission frequency for narrowband would present a spectrum allocation and self-interference challenge for a BEM satellite system.
As can be seen, there is a need for combining a narrowband channel with high availability requirements with a wideband data link—such as a BEM link—and delivering a significantly higher rain availability while minimally impacting satellite link communication hardware complexity, size, weight, power, and, therefore, cost. There is also a need for a solution to the problem of upgrading current satellite systems to BEM and other state of the art wideband communications.
In one embodiment of the present invention, a communication system includes a transmitter that transmits both wideband data and narrowband data on a link and a receiver that receives the wideband data and the narrowband data on the link. The receiver demultiplexes the wideband data and the narrowband data into separate data streams so that the link effects transmission of a narrowband channel and a wideband channel and so that the communication system achieves higher relative availability of the narrowband channel by utilizing different modulation and error control coding formats on the narrowband data and the wideband data.
In another embodiment of the present invention, a transmitter for a communication system includes: a wideband symbol mapper that maps wideband data to wideband frames using symbols for a higher order modulation format; a narrowband symbol mapper that maps narrowband data to narrowband frames using symbols for a lower order modulation format; and a data formatter that multiplexes the narrowband frames and the wideband frames together into a single stream of coded symbols using symbols for both the higher order modulation format and the lower order modulation format.
In still another embodiment of the present invention, a receiver for a communication system includes a data demulitplexer. The data demulitplexer detects a beginning of narrowband frames in a single data stream of narrowband frames and wideband frames, in which the wideband frames comprise symbols of a wideband modulation format. The data demulitplexer separates the narrowband frames from the wideband frames. The narrowband frames comprise symbols of a narrowband modulation format that provides more reliable resolution of symbols than that for the wideband modulation format. The narrowband frames may also be sent using a lower rate code that provides more reliable resolution of symbols than that for a higher rate code used for the wideband frames.
In yet another embodiment of the present invention, a communication system includes: a wideband symbol mapper that maps wideband data to wideband frames using symbols for a first modulation format; a narrowband symbol mapper that maps narrowband data to narrowband frames using symbols for a second modulation format; a data formatter that multiplexes the narrowband frames and the wideband frames together into a single stream of coded symbols using symbols for both the first modulation format and the second modulation format; and a data demulitplexer. The data demulitplexer detects a beginning of narrowband frames in a single data stream of narrowband data coded symbols and wideband data coded symbols. The data demulitplexer separates the narrowband data coded symbols from the wideband data coded symbols. The second modulation format provides more reliable resolution of symbols than that for the first modulation format.
In a further embodiment of the present invention, a satellite communication system includes a transmitter and receiver. The transmitter includes: a wideband symbol mapper that maps wideband data to wideband frames using symbols for a first modulation format; a narrowband symbol mapper that maps narrowband data to narrowband frames using symbols for a second modulation format; a data formatter that multiplexes the narrowband frames and the wideband frames together into a single stream of coded symbols using symbols for both the first modulation format and the second modulation format; and a wideband modulator that modulates a carrier using the first modulation format and the second modulation format. The receiver has a wideband demodulator and includes a data demulitplexer. The demodulator feeds the data demulitplexer the single data stream of narrowband frames and wideband frames. The data demulitplexer detects a beginning of narrowband frames in the single data stream of narrowband data coded symbols and wideband data coded symbols and separates the narrowband data coded symbols from the wideband data coded symbols. The second modulation format provides more reliable resolution of symbols than that for the first modulation format.
In a still further embodiment of the present invention, a method for achieving high link availability on a link includes formatting narrowband data and wideband data together into a single data stream using a different symbol mapping for the narrowband data than for the wideband data to enable higher availability for a narrowband channel carrying the narrowband data on the link.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Broadly, the present invention provides a communications downlink for a communication system, such as radio frequency (RF) communications over a satellite link or over a ground link, where the link is subject to interference from atmospheric disturbances such as rain, and where two different types of data are to be transmitted over two channels with different data rates and different quality of service (QoS) requirements. The two channels, for example, may be a narrowband channel with a lower data rate and wideband channel with a higher data rate, the narrowband channel having higher availability requirements than the wideband channel because, for example, of the type of data to be transmitted—such as spacecraft telemetry, or important user data. In one embodiment, the two channels of data are combined into one coded symbol stream and sent over a wideband satellite link to provide high link availability for the narrowband channel on the wideband link while providing generally lower link availability for the wideband channel on the same wideband link. In one particular embodiment, the wideband link may be an RF bandwidth efficient modulation (BEM) satellite link that provides high link availability for a narrowband channel and the same or lower link availability for a wideband channel on the wideband BEM satellite link.
In another embodiment, a satellite communications downlink combines a narrowband channel with high availability requirements with a wideband BEM data link (which may have lower availability requirements); delivers significantly higher rain availability while minimally impacting satellite link communication hardware complexity, size, weight, power, and, therefore, cost; and provides a simple, elegant solution to the problem of upgrading current satellite systems to BEM. For example, one embodiment makes novel use of existing spacecraft resources to apply coding and modulation to the narrowband channel and multiplex with the wideband coded data, eliminating need for additional hardware and spectral allocation for high availability narrowband data.
By combining channels on a single link using only one transmitter and receiver, one embodiment of the present invention differs from prior art communication systems that provide separate links, separate transmitters and receivers for each channel. As described above such prior art approaches are impractical for implementing high availability, wideband (e.g., BEM) communication links.
Referring now to
In one embodiment, for example, transmitter 200 may be a sophisticated BEM digital transmitter 200 that may provide a high availability narrowband channel 112 for BEM applications by muliplexing narrowband data 201 and wideband data 203. Narrowband channel 112 may comprise, for example, narrowband coded frames 302, as shown in
Higher availability may be achieved on the narrowband channel 112 than on the wideband channel 114 by (1) using lower order modulation format—such as BPSK (binary phase shift keyed) or QPSK (quaternary phase shift keyed) symbols—compared to higher order modulation formats—such as 12-4 APK (amplitude phase keyed) symbols, 64-QAM (quadrature amplitude modulation), 256-QAM, and so on; and (2) using lower rate forward error correction (FEC) coding—such as rate 1/2 or rate 1/3 FEC—compared to higher rate, more bandwidth efficient, FEC coding—such as rate 2/3 FEC and above. Lower order modulation formats may be described as placing symbols farther apart in an in-phase, quadrature (I/Q) phase space so that resolution of one symbol from the next closest symbol in the space may be more reliably achieved than for higher order modulation formats. Coding rate may be described as the ratio of input information bits to output information bits of coded data so that the lower the rate the more bits are used to convey the same amount of information. Thus, a lower rate code may also increase the reliability of information transmission.
Operation of one embodiment may be summarized as multiplexing short bursts of less bandwidth-efficiently modulated and encoded data with long bursts of wideband data. Wideband data is typically bandwidth-efficiently modulated and coded, although it may be desired to transmit backwards-compatible waveforms that may not be bandwidth-efficiently modulated. A summary of several of the factors involved may be given as follows:
One embodiment may be described as taking advantage of the ability of a wideband demodulator—such as a BEM demodulator 402 of a receiver 400 in one example embodiment—to maintain tracking through extremely deep fades—such as those caused by atmospheric disturbances, as described above. In the exemplary embodiment, advantage is taken of the fact that the coded BPSK data—e.g., narrowband coded frames 302- requires much lower signal-to-noise ratio (SNR) to achieve acceptable bit error rate than the coded BEM waveform—e.g., wideband coded frames 304. As shown in the example given below and illustrated by
Continuing with
Transmitter 200 at symbol mapper 206 may format the narrowband encoded data 207 for a low order of modulation, for example, as binary phase shift keyed (BPSK) symbols, one for in-phase (I) and one for quadrature (Q), corresponding to BPSK symbols, mapping the encoded data 207 to digital words, e.g., narrowband data coded symbols 209. Narrowband data coded symbols 209 may be comprised of any symbols in phase space, BPSK symbols being used as one illustrative example. Symbol mapper 206 may output narrowband data coded symbols 209 to data formatter 210. Likewise, transmitter 200 at symbol mapper 208 may format the wideband encoded data 211 for a higher order of modulation, for example, 12-4 APK or 64-QAM, mapping the encoded data 211 to digital words, e.g., wideband data coded symbols 213, which may be comprised of higher order modulation symbols corresponding to the modulation used, for example, 12-4 APK or 64-QAM. Symbol mapper 208 may output wideband data coded symbols 213 to data formatter 210.
The narrowband data coded symbols 209 may be inserted by data formatter 210 between wideband data coded symbols 213 into wideband data code blocks—such as data code block 300 seen in
The unique word 306 may be included in data code block 300 and may permit synchronization and location of wideband data code block boundaries—such as boundaries 308 and 310. These unique words—such as unique word 306, may enable synchronization of the decoder, e.g., decoder 404 of receiver 400 shown in
Receiver 400, shown in
Referring to table 600 shown in
For Miami (again see column 601), with its higher probability of more severe rain fade (according to the Crane Rain Model, see column 602), the same margins (D)=2.0 dB and (H)=20 dB (see column 603 of table 600, interpolate between lines 6 and 7 for D, approximate line 9 for H) imply that the availabilities are, respectively, less than 99.0% and 99.9% for the BEM wideband channel 114 and the narrowband channel 112 (see column 605). This means that the BEM wideband channel 114 will experience a rain loss greater than its margin and experience an outage greater than 1.0% of the time, while the narrowband channel 112 outage time is approximately 0.1% (see column 604).
In both cases, the FEC coding and lower order modulation format for the narrowband channel 112 enable it to perform with a significantly higher availability (significantly less outages) than the BEM wideband channel 114. For example, in the first example above, the narrowband channel performance may be described as being higher by a factor of about 50 (ratio of outage percentages of narrowband to wideband), and in the second example above, the narrowband channel performance may be described as being higher by a factor of about 10. Thus, between the two examples, narrowband channel performance may be described as being higher by a factor of at least about 10.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
