Multiplex communications – Generalized orthogonal or special mathematical techniques – Quadrature carriers
Reexamination Certificate
1999-10-20
2004-09-28
Duong, Frank (Department: 2666)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Quadrature carriers
C370S252000, C370S468000
Reexamination Certificate
active
06798735
ABSTRACT:
TECHNICAL FIELD
This application relates to the field of electronic communication and more particularly to the field of multiband digital signal communication.
BACKGROUND OF THE INVENTION
Conventional multicarrier digital communication is a technique for transmitting and receiving digital signals using a plurality of carriers (subchannels) having different frequencies. Each of the subchannels is used to communicate a different portion of the signal. The transmitter divides the signal into a number of components, assigns each component to a specific one of the carriers, encodes each of the carriers according to the component assigned thereto, and transmits each of the carriers. The receiver decodes each received carrier and reconstructs the signal.
The maximum amount of information that can be encoded onto a particular subcarrier is a function of the signal to noise ratio of the communication channel with respect to that subcarrier. The signal to noise ratio of a communication channel can vary according to frequency so that the maximum amount of information that can be encoded onto one carrier may be different than the maximum amount of information that can be encoded onto another carrier.
Bit loading is a technique for assigning bits to subchannels according to each subchannel's signal to noise ratio. A bit loading algorithm provides a bit allocation table that indicates the amount of information (in bits) that is to be encoded on each of the carriers. That is, for a multicarrier communication system with J carriers, a bit allocation table B[j] indicates, for each j=1 to J, the amount of information that is to be encoded onto each of the J carriers.
Shaping the transmission to match the channel characteristics is known. For example, a technique known as “water pouring” was introduced by Gallager in 1968 (“Information Theory and Reliable Communication”, page 389) and by Wozencraft in 1965 (“Principles of Communication Engineering”, pp. 285-357). Water pouring involves distributing the energy of the transmission signal according to the channel frequency response curve (a plot of the signal to noise ratio as a function of frequency). The frequency response curve is inverted and the available signal energy (the “water”) is “poured” into the inverted curve so that more of the energy is distributed into those portions of the channel having the highest signal to noise ratio. In a multicarrier system in which the transmission band is divided into numerous subchannels, throughput can be maximized by putting as many bits in each subcarrier as can be supported given the “water pouring” energy and a desired error rate.
Other techniques for allocating bits among carriers of a multicarrier signal are known. U.S. Pat. No. 4,731,816 to Hughes-Hartogs discloses a bit loading scheme where one bit at a time is incrementally added to each subcarrier until a maximum rate is achieved. Subcarriers that require the least amount of additional power to support an additional bit are selected first.
U.S. Pat. No. 5,479,477 to Chow et al. discloses a bit loading scheme that is capable of either maximizing the throughput or maximizing the margin for a particular target data rate. Unlike Hughes-Hartogs, Chow et al. determines the bit loading table one carrier at a time (rather than one bit at a time). In Chow et al., all the carriers are sorted in descending order according to the measured signal to noise ratio. The initial subchannels that are selected are the ones capable of carrying the most bits. Using the Chow et al. scheme to maximize the data rate provides a bit loading table similar to that provided by the Hughes-Hartogs algorithm.
In order for the receiver to correctly interpret the received data, both the transmitter and the receiver must use the same bit loading table. When the bit loading algorithm is performed during the initialization phase of communication, the resulting bit allocation table is communicated between the transmitter and receiver to ensure that both the transmitter and the receiver are using the same bit loading table. However, in the event that the communication channel signal to noise ratio characteristics change during communication, it may be necessary to update/change the bit allocation table to more appropriately match the transmission with the channel characteristics. However, when the bit allocation table is changed, it is necessary to synchronize use of the new table with both the transmitter and the receiver. If the transmitter and the receiver use different bit allocation tables at any time, the communications link will suffer significant errors in those subchannels in which the bit allocation tables do not agree.
In addition, determining a new bit allocation table can be time consuming, especially if the bit loading algorithm is computationally intensive, such as that disclosed by Hughes-Hartogs where the bit allocation table is constructed one bit at a time. If the bit allocation table is to be calculated many times during communication between the transmitter and receiver, then spending a relatively long amount of time recalculating the bit allocation table (and hence not communicating data) is undesirable.
One solution is to simply not change the bit loading table after initialization. However, this may be unacceptable in cases where the communication channel signal to noise ratio changes during data transmission. Accordingly, it is desirable to be able to determine a bit loading tableirelatively quickly and to be able to synchronize use of the new table by the transmitter and the receiver.
SUMMARY OF THE INVENTION
In accordance with the present invention, a pair of bit allocation tables are maintained at both the transmitter and the receiver. These tables are updated as needed, using measurements of the signal to noise ratio performed on known data transmitted to the receiver in a control frame separate from the data frame. The transmitter signals the receiver as to which of the two tables is to be used for subsequent communication. Preferably, this is done by transmitting a flag from the transmitter to the receiver at some point during the data transmission; this causes the receiver to thereafter switch the bit loading table it is using for communication to synchronize with the corresponding table at the transmitter.
In the preferred embodiment of the invention, although the invention is not restricted thereto, 69 “frames” of 245.5 microseconds duration each are used to form a “superframe” of 16.94 milliseconds. The first frame of each superframe comprises a control frame that is used to transmit a standard (known) data set from the transmitter to the receiver; the remaining frames contain data. The receiver measures the signal to noise ratios of the received data in this frame for each of the channels and uses this to calculate channel bit allocations for subsequent data transmissions. In practice, it has not been found necessary to calculate the signal to noise ratios for each and every superframe, although this can, of course, be done. Rather, we have found it sufficient for most data transmissions to measure the signal to noise ratios of the channels over several frames, average them, update the bit allocation tables based on the resultant values, and use the bit allocations tables so determined over hundreds or thousands of subsequent frames.
The bit allocation table updating is performed by comparing the measured signal to noise ratio (SNR) in each channel with a constellation signal to noise ratio SNR[c
j
], that has been augmented by a trial noise margin M, SNRa[c
j
]=SNR[c
j
]+M. The constellation signal to noise ratio, SNR[c
j
], specifies the number of bits c
j
(“constellation size”) that can be transmitted over a channel j given a specific signal to noise ratio SNP
j
, where c
j
may vary, for example, from 1 to 15. The value of the margin M is dependent on the difference between the amount of data (i.e., number of bits) that can be transmitted across the channels in accordan
Kechriotis George
Tzannes Marcos
Wu Pui
Aware Inc.
Duong Frank
Miles & Stockbridge PC
Vick Jason H.
LandOfFree
Adaptive allocation for variable bandwidth multicarrier... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Adaptive allocation for variable bandwidth multicarrier..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Adaptive allocation for variable bandwidth multicarrier... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3215636