This invention relates to the transmission and reception of digital data over radio links, and more particularly, to the transmission and reception of 4-level and 8-level signaling symbols according to prevailing conditions so as to achieve a greater data rate when conditions allow.
In a typical cellular telephone system, speech compression is used in order to accommodate as many mobile users as possible within the spectrum allocated to any particular operator. Speech compression involves a tradeoff between the total number of users that can be served by the system, and the speech quality delivered by the system. Where less speech compression is used, a better speech quality is achieved; however, less users can be served by the system. Where a greater amount of speech compression is used, the total number of users that can be served by a system is increased; however, the speech quality is degraded. Similarly, there is a tradeoff between the quality of a data communication service and the number of users that can be accommodated by the service. The quality of a data service may be measured in terms of the time required to transfer a data file. Thus, using a higher bit rate may succeed in completing data transfer faster, thereby leading to increased quality of data communications; however, other users are then prevented from using the spectrum at the same time. Additionally, the number of users that can be accommodated in the spectrum may be increased; however, the quality of data communications is degraded. Thus, an improved system would be one which is able to use less speech compression or use a higher bit rate to transmit data, without reducing the number of users that can be accommodated by the system.
Further, in a cellular telephone system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example, a direct path and one or more reflected paths. In digital cellular phone systems, an equalizer and an error correction decoder are used to process the received signal which may have traveled over multiple paths. In situations where the received signal contains two modulation types, for example, 4-level modulation and 8-level modulation, two equalizers and two error correction decoders are required to process the received signal, where the first equalizer and first error correction decoder are suitable for processing 4-level modulation and the second equalizer and second error correction decoder are suitable for processing 8-level modulation. This increases the hardware cost of a cellular station since multiple equalizers and error correction decoders are required. Furthermore, in a field where products are being manufactured increasingly smaller for convenience, the user of multiple equalizers and error correction decoders require greater printed circuit board (PCB) real estate, resulting in a physically larger product than cellular stations using only one equalizer and error correction decoder.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention, there is disclosed a method and system for the transmission of both 4-level and 8-level signalling symbols using an 8-level modulator. There is further disclosed a method and system for demodulating and error correction decoding a signal containing both 4-level and 8-level modulation.
It is an object of the present invention to provide a method for modulating 4-level signaling symbols using an 8-level modulator, where the method comprises receiving 4-level signaling symbols to be modulated, where each 4-level signaling symbol contains 2-bits of information. The method further includes expanding the 4-level signaling symbols into eight-level signaling symbols, where each 8-level signaling symbol contains 3-bits of information. The method further includes modulating the 8-level signaling symbols.
In a preferred embodiment, the step of expanding the 4-level signaling symbols into 8-level signaling symbols includes manufacturing a third bit for each 4-level signaling symbol and appending the third bit to the 2-bits of information. In this preferred embodiment, the third bit is the exclusive-OR of the 2-bits. In another preferred embodiment, the method includes performing a progressive 45xc2x0 phase shift between a modulation of consecutive symbols. In a further preferred embodiment, modulating and expanding 4-levels signaling symbols with the 8-level modulator causes the 8-level modulator to produce 4-level modulation. In a yet further embodiment of the invention, the 4-level signaling symbols are a subset of the 8-level signaling symbols. In another preferred embodiment of the invention, the method includes the step of sending an indication signal indicating which of expanded 4-level signaling symbols and 8-level signaling symbols are being transmitted.
It is another object of the present invention to provide a method for demodulation of a received signal containing at least one of 4-level modulation and 8-level modulation where the method includes demodulating the received signal under a first assumption that the received signal contains 4-level modulation and determining a value of a first quality factor by performing error correction decoding on symbols modulated under the first assumption. The method further includes demodulating the received signal under a second assumption that the received signal contains 8-level modulation and determining a value of a second quality factor by performing error correction decoding on symbols modulated under the second assumption. Additionally, the method determines a received modulation present in the received signal using the first and the second quality factors, and demodulates the received signal as the received modulation.
In a preferred embodiment, the step of determining from the first and second quality factors may be performed before the demodulation under the first and the second assumptions is completed. In a further preferred embodiment, the first and second quality factors are determined using metrics generated by an error correction decoder.
It is another object of the invention to provide a method for demodulating a received signal containing at least one of 4-level modulation and 8-level modulation, where the method determines whether 4-level modulation or 8-level modulation is present in the received signal. The method continues by operating a demodulator in a constrained 4-level demodulation mode by restricting possible levels for the received signal to four levels, determining which level out of the four levels is being received, and producing 4-level signaling symbol decisions having 2-bits each if it is determined that 4-level modulation is present. If however, it is determined that 8-level modulation is present in the received signal, the method continues by operating the demodulator in an unconstrained 8-level demodulation mode by determining which level out of 8 possible levels is being received, and producing 8-level signaling symbol decisions having 3-bits each.
In a preferred embodiment, the step of determining whether 4-level modulation or 8-level modulation is present includes the step of receiving an indication signal indicating whether 4-level modulation or 8-level modulation is present in the received signal. In a further preferred embodiment, the indication signal is transmitted using at least one of a SYNCWORD, a CDVCC, and a FACCH message. In a further preferred embodiment, the step of determining whether 4-level modulation or 8-level modulation is present includes the step of using a predetermined transmission format to determine whether a 4-level signaling symbol or an 8-level signaling symbol is present in the received signal.
It is another object of the invention to provide a method for decoding a received signal containing at least one of 4-level signaling symbols and 8-level signaling symbols, the method comprises receiving a signal, demodulating the received signal as 8-level modulation, and determining whether 4-level signaling symbols or 8-level signaling symbols are present in the received signal. If it is determined that 4-level signaling symbols are present in the received signal, the method operates an error correction decoder in a constrained 4-level decoding mode and produces 4-level signaling symbol decisions having 2-bits each. If it is determined that 8-level signaling symbols are present in the received signal, the method operates the error correction decoder in an unconstrained 8-level mode and produces 8-level signaling symbol decisions having 3-bits each.
In a preferred embodiment, the step of determining whether 4-level signaling symbols or 8-level signaling symbols are present further includes the step of receiving an indication signal indicating whether 4-level signaling symbols or 8-level signaling symbols are present in the received signal. In a further preferred embodiment, the step of determining whether 4-level signaling symbols or 8-level signaling symbols are present further includes the step of using a predetermined transmission format to determine whether 4-level signaling symbols or 8-level signaling symbols are present in the received signal. In another preferred embodiment, the step of decoding the received signal in the unconstrained 8-level mode includes removing a third bit from the 3-bits of the 8-level signaling symbol decision.
It is another object of the invention to provide a method for decoding a received signal containing at least one of 4-level signaling symbols and 8-level signaling symbols, where the method comprises receiving a signal, demodulating the received signal as 8-level modulation, and performing error correction decoding under a first assumption that 4-level signaling symbols are present in the received signal and determining a first quality factor. The method continues by performing error correction decoding under a second assumption that 8-level signaling symbols are present in the received signal and determining a second quality factor. The method then performs the step of determining a received modulation in the received signal using the first and the second quality factors, and decodes the received signal as the received modulation.
In a preferred embodiment, the step of determining from the first and second quality factors may be performed before the decoding under the first and the second assumptions is completed.
It is another object of the invention to provide a system for modulating 4-level signaling symbols using an 8-level signaling symbol modulator, where the system comprises a processor for processing information into a plurality of 4-level signaling symbols to be modulated. The system also includes an expander coupled to the processor for expanding the plurality of 4-level signaling symbols each containing 2-bits of information to expanded 8-level signaling symbols each containing 3-bits of information. The system further includes an 8-level modulator coupled to the expander for modulating the expanded 8-level signaling symbols.
In a preferred embodiment, the system applies a progressive 45xc2x0 phase shift between the modulation of consecutive symbols. In a further preferred embodiment of the system, the 4-level signaling symbols are a subset of the expanded 8-level signaling symbols. In another preferred embodiment of the invention, the 8-level modulator produces 4-level modulation when modulating the expanded 8-level signaling symbols.
It is yet another object of the invention to provide a system for demodulating a received signal containing at least one of 4-level modulation and 8-level modulation, where the system comprises a receiver for receiving the received signal, and a demodulator coupled to the receiver. The demodulator operates under at least one of a first assumption that the received signal contains 4-level modulation, and a second assumption that the received signal contains 8-level modulation. If the demodulator is operating under the first assumption that the signal contains 4-level modulation, the demodulator operates as a constrained demodulator by restricting the possible levels of the received signal to a set of four levels, determining which level out of the four levels is received, and producing 4-level signaling symbol decisions having 2-bits each. If the demodulator is operating under the second assumption that the received signal contains 8-level modulation the demodulator operates as an unconstrained demodulator by restricting possible levels of the received signal to a set of eight levels, determining which level out of the eight levels is received, and produces 8-level signaling symbol decisions having 3-bits each.
In a preferred embodiment, the system may further include a first error correction decoder coupled to the demodulator and a second error correction decoder coupled to the demodulator, where the first error correction decoder decodes the 4-level signaling symbol decisions when the demodulator is operating under the first assumption, and the second error correction decoder decodes the 8-level signaling symbol decisions when the demodulator is operating under the second assumption. In a further preferred embodiment, a controller is coupled to the first and the second error correction decoders and to the demodulator where the first error correction decoder determines a first quality factor, the second error correction decoder determines a second quality factor, and the controller uses the first and the second quality factors to determine whether 4-level modulation or 8-level modulation is present in the received signal. If the controller determines that 4-level modulation is present, the controller directs the demodulator to operate as the-constrained demodulator, and if the controller determines that 8-level modulation is present, the controller directs the demodulator to operate as the unconstrained demodulator. In-yet another further preferred embodiment, a controller is coupled to the demodulator where the controller determines whether 4-level modulation or 8-level modulation is present in the received signal by using at least one of an indication signal within the received signal and a predetermined transmission format.
It is another object of the invention to provide a system for decoding a signal containing at least one of 4-level modulation and 8-level modulation, where the system comprises a receiver for receiving a signal, and a demodulator coupled to the receiver for demodulating the signal as 8-level modulation. The system also includes an error correction decoder coupled to the demodulator for operating as at least one of a constrained 4-level mode for performing error correction under a first assumption that 4-level signaling symbols are present in the signal, and an unconstrained 8-level mode for performing error correction decoding under a second assumption that 8-level signaling symbols are present in the signal.
In a preferred embodiment, the system comprises a controller coupled to the error correction decoder wherein the error correction decoder determines a first quality factor when operating under the first assumption, and a second quality factor when operating under the second assumption, and the controller determines a received modulation from the first and the second quality factors. The controller then sends a control signal to the error correction decoder to decode the signal as the received modulation. In another preferred embodiment of the invention, the error correction decoder removes a third bit from the 3-bits created by the demodulator when performing error correction decoding under the second assumption. In a further preferred embodiment, the system includes a controller coupled to the error correction decoder wherein the controller determines whether 4-level or 8-level signaling symbols are present in the demodulated signal, and the controller directs the error correction decoder to operate in the constrained mode if it is determined that 4-level signaling symbols are present, and directs the error correction decoder to operate in the unconstrained mode if it is determined that 8-level signaling symbols are present. In a further preferred embodiment, the controller uses at least one of an indication signal within the received signal and a predetermined transmission format to determine whether 4-level or 8-level signaling symbols are present.