FIELD OF THE INVENTION

This invention relates generally to coded modulation techniques for use in digital communication systems such as orthogonal frequency division multiplexed multiple access (OFDMA) systems. More particularly, the invention relates to methods and apparatus for multilevel coding and decoding. Different receivers coupled to different parallel downlink channels with different channel qualities decode different received signal constellations at different levels of resolution. This allows the overall OFDMA downlink to operate with a significantly higher data rate, thus significantly increasing system level bandwidth efficiency.

BACKGROUND OF THE INVENTION

The OFDMA downlink of LTE (long term evolution) mobile wireless data systems operated by carriers in the licensed bands has always been hindered by key system level bottlenecks and congestion. Also, WiFi has been solely deployed in unlicensed bands where huge spectrum is available in 2.4 Ghz (UHF) and 5 GHz band. Up until now, WiFi has been the only major player in these bands. But as the licensed bands are hitting congestion, operators are looking to deploy LTE in unlicensed spectrum as well. There are two main types of LTE in unlicensed band, 1) LTE-U (Qualcomm) and LAA (3GPP). With these new technologies, incumbent WiFi has no choice but to share spectrum resources in TDD (time division duplex) with LTE-U and LAA. Hence, spectrally efficient utilization of the unlicensed bands is becoming more critical for WiFi down links. Similarly, LTE on the unlicensed bands has to cooperate with incumbent WiFi so that new technologies offering enhanced downlink efficiency for any or all of LTE, LTE-U, LAA, and WiFi systems are needed. Other higher bandwidth OFDMA technologies such as “GigaFi” are being introduced to provide higher data rates than can currently be achieved with WiFi. New technologies offering enhanced downlink efficiency for GigaFi or other types of next-generation enhanced short range or long range systems are also needed.

LTE and WiFi systems typically involve a multi-user downlink that employs orthogonal frequency division multiplexed multiple access (OFDMA). For example, in LTE systems, different mobile units will be assigned respective resource blocks that each identify a set of 12 OFDM tones (tone=line frequency). Each resource block includes 12 15 KHz wide sub-carriers (on the frequency axis) and 14 (or 12) OFDM symbols on the time axis. Hence each resource block will carry 168 (or 144) data or control symbols at a specified bit loading that can consist of QPSK (4-QAM), 16-QAM, 64-QAM, or 256-QAM. In LTE, each resource block will be transmitted in 1 ms and occupies 180 KHz bandwidth. Other technologies such as cable modems use a different downlink channel other than OFDMA, but have similar issues with different attached cable modems having different quality downlink channels.

Consider a simplified exemplary system where half of the mobile units have strong channels and are assigned resource blocks that use 256-QAM (eight bits per data tone) while the second half of the mobile units have weak channels and are assigned resource blocks that use 4-QAM (two bits per data tone). In such an example, due to the lower bit loading used in the downlink resource blocks assigned to the second half of the mobile units, the total downlink throughput is much less than the maximum that could be achieved if all of the mobile units had strong channels that could support resource blocks that used 256-QAM. It would be desirable to be able to increase the net throughput of the LTE and WiFi downlinks by allowing the data symbols of resource blocks associated with the weak channels to carry additional bits that could be received and decoded by mobile units that can see those same data tones through the lens of the strong channels.

Tiled-building-block trellis codes are a family of codes that typically use trellis codes such as convolutional codes and turbo codes (e.g., parallel concatenated convolutional codes) as distinct codes in a multilevel coded system. Such coded systems are described in detail in U.S. Pat. No. 8,077,290 and M. A. Naim, J. P. Fonseka, And E. M. Dowling, “A Building-Block Approach for Designing Multilevel Coded Systems,” IEEE Com Letters, Vol. 19, NO. 1, January 2015, “the Naim reference.” U.S. Pat. No. 8,077,290 is incorporated herein by reference and the reader is referred to this patent to better understand the background of tiled-building-block trellis codes. A building-block-trellis code is used to construct a small compact signal constellation building block which is called a “coded-constellation-building block,” or, a “building block” for short. A tiling code is also employed to allow the small powerful building blocks to be tiled to form larger constellations with a tile spacing that is selected to preserve the building block's MSED. Associated with the tiling code is a signal constellation called the “tiling constellation.” At each constellation point of the tiling constellation is placed a copy of the building block. Each constellation point of the tiling code is referred to as a “tiling point.” The “intra-block MSED” is defined as the MSED between constellation points within a building block, and the “tiling MSED” is defined as the MSED between the centers of the tiled building blocks, i.e., the MSED between tiling points in the tiling constellation. In terms of coded sequences, the “sequence-level intra-block MSED” is the distance between coded sequences of constellation points within a building block and the “sequence-level tiling MSED” is the distance between coded sequences of tiling points (tile center locations).

It would be desirable to have methods, apparatus and systems for improving a systems-level data rate on a communications link such as those used in the downlink of OFDMA systems. It would be desirable to have a technology that uses tiled-building-block encoding/decoding in a configuration that allows different receivers coupled with different parallel downlink channels with different channel qualities to decode different received versions of the transmitted signal constellation at different levels of resolution. It would be desirable to have a technology that could reduce or eliminate existing inefficiencies in the downlink of OFDMA systems to thereby allow the downlink to operate with a significantly higher data rate, thus significantly increasing system level throughput and spectral efficiency.

SUMMARY OF THE INVENTION

A first aspect of the invention centers on methods, apparatus, and communication systems that involve a headend such as a base station that is coupled to a first mobile unit via a first communication channel that is able to support a 2m1-ary signal constellation and a second mobile unit via a second communication channel that is able to support a 2m2-ary signal constellation. The first communication channel and the second communication channel can optionally correspond to respective first and second wireless communication paths traversed by an OFDMA (orthogonal frequency division multiple access) downlink signal that carries a shared/multi-resolution signal constellation. For example, such embodiments can include LTE, WiFi, GigaFi, or any of the types of systems mentioned in the background section above or in the detailed description below. The first mobile unit is directed to receive a 2m1′-ary signal constellation and the second mobile unit is directed to receive a 2m2′-ary signal constellation, where m1, m2, m1′ and m2′ are positive rational numbers (typically integers), and m1′≤m1and m2′≤m2, m1<m2. The headend (e.g., base station) typically includes an apparatus that includes a first encoder that is coupled to receive a first stream of bits associated with the first mobile unit. The first encoder is operative to encode the first stream of bits to form a first encoded stream of bits. The first encoder is typically implemented as a tile encoder, and encodes, for example, in accordance with a convolutional code, a block code, an LDPC code, a turbo code, or a composite/concatenation of any of the above mentioned codes. The apparatus also includes a second encoder that is coupled to receive a second stream of bits associated with the second mobile unit. The second encoder is operative to encode the second stream of bits to form a second encoded stream of bits. The second encoder is typically implemented as a building-block encoder that encodes using any of the types of codes that the tile encoder can use as stated above. The apparatus also includes a signal mapper that is coupled to receive the first encoded stream of bits and the second encoded stream of bits. The signal mapper is operative to form a symbol stream of symbols by mapping, during each of a plurality of symbol intervals, m1′ number of coded bits from the first encoded stream of bits and m2′ number of coded bits from the second encoded stream of bits onto a shared/multi-resolution signal constellation whose constellation points are drawn from a 2m-ary underlying constellation associated with a shared/multi-resolution resource block, where m≥m1′+m2′. The communication system is operative to transmit control information to direct the first mobile unit to extract first receiver demodulation information from a first sub-constellation portion of the 2m-ary underlying constellation and to use the first receiver demodulation information in a first decoder to form an estimate of the first stream of bits. The communication system directs the second mobile unit to extract second receiver demodulation information from a second sub-constellation portion of the 2m-ary underlying constellation. The second mobile unit then uses the second receiver demodulation information in a second decoder to form an estimate of the second stream of bits. The first and second receiver demodulation information typically correspond to respective first and second sequences of bit metrics calculated over a frame duration period. The first sub-constellation portion can sometimes be associated with a 2m1′-ary tile constellation and the second sub-constellation portion can sometimes be a 2m2′-ary building block constellation, in which case the 2m1′-ary constellation can optionally be selected to have an average energy E1that is selected to be received alone by a low resolution receiver or in combination with additional signal constellation points in a higher resolution receiver. In such embodiments, the 2m2′-ary constellation is selected to have has an average energy E2and to be received by a high resolution receiver that is able to decode a 2m-ary constellation. In other types of embodiments the first encoder is an upper level encoder in a multi-level code and the second encoder is a lower level encoder in the multi-level code. In some embodiments, there are additional encoders numbered three through L that are coupled to receive third through Lthstreams of bits associated with additional third through Lthmobile units. The third through Lthencoders are then operative to encode the third through Lthstreams of bits to form third through Lthencoded streams of bits, where L≥3. In such embodiments, the signal mapper is further coupled to receive the third through Lthencoded streams of bits and is operative to form the symbol stream by mapping, during each of the plurality of symbol intervals, m3′ through mL′ sets of coded bits from the third through Lthencoded streams of bits onto an 2(m1′+m2′+ . . . +mL′)-ary shared/multi-resolution signal constellation that is a sub-constellation of the 2m-ary underlying signal constellation, where m1′+m2′+ . . . +mL′≤m.

A second aspect of the invention relates to a second mobile unit apparatus and methods for communicating with a communication system base station that is coupled to both a first mobile unit apparatus via a first channel and to the second mobile unit apparatus via a second channel. The second mobile unit apparatus includes an OFDMA downlink receiver that is configured, in response to control information associated with a shared/multi-resolution resource block, to receive data symbols from selected data tones using a selected one of a plurality of different signal constellations which include a 2m1-ary signal constellation, 2m1′-ary signal constellation, 2m2-ary signal constellation, a 2m2′-ary signal constellation and a 2m-ary signal constellation, where m, m1, m1′, m2, and m2′ are positive rational numbers, m1′≤m1, m2′≤m2, m1<m2≤m, and m≥m1′+m2′. The data symbols are drawn from a shared/multi-resolution signal constellation that is a sub constellation of the 2m-ary signal constellation. The second mobile unit apparatus includes at least one processor coupled to a memory which holds instructions to be executed by the at least one processor, wherein the instructions cause the at least one processor to perform various action under stored program control. A first such action includes receiving the control information from the communication system base station. Based upon the control information, a second action includes configuring the OFDMA downlink receiver to extract second receiver demodulation information (e.g., a sequence of m2′ number of bit metrics per symbol over a frame of symbols) from a sequence of data symbols modulated onto respective OFDM tones associated with the shared/multi-resolution resource block, and to couple the second receiver demodulation information to a decoder that is configured to decode a coded 2m2′-ary signal constellation to form a sequence of decoded bits. In such a system, the m1′ number bits are associated with a first coded bit stream whose destination is the first mobile unit apparatus. The 2m2′-ary signal constellation typically corresponds to a building block constellation, and the first encoded bit stream is typically used to identify a sequence of tiles. Similar to the first aspect of the invention described above, additional third through Lthstreams of coded bits can also be transmitted to respective additional third through Lthmobile units via the shared/multi-resolution signal constellation. In such embodiments, the shared/multi-resolution signal constellation is typically a 2(m1′+m2′+ . . . +mL′)-ary constellation, where m1′+m2′+ . . . +mL′≤m.

A third aspect of the invention involves an methods, apparatus, and systems for use in a communication system that is coupled to a receiver via a communication channel. The receiver is directed to receive a 2m1′-ary signal constellation and also a 2m2′-ary signal constellation, where m1′ and m2′ are positive rational numbers (most often they are integers). Such an apparatus includes a first encoder that is coupled to receive a first stream of bits associated with a first application layer program that can withstand a first bit error rate. The first encoder is operative to encode the first stream of bits to form a first encoded stream of bits. A second encoder is also coupled to receive a second stream of bits associated with a second application layer program (or, in some embodiments, a lower layer stream that carries control information) that can withstand a second bit error rate. The second bit error rate is greater than the first bit error rate. The second encoder is operative to encode the second stream of bits to form a second encoded stream of bits. The apparatus also includes a signal mapper that is coupled to receive the first encoded stream of bits and the second encoded stream of bits. The signal mapper is operative to form a symbol stream of symbols by mapping, during each of a plurality of symbol intervals, m1′ number of coded bits from the first encoded stream of bits and m2′ number of coded bits from the second encoded stream of bits onto a shared/multi-resolution signal constellation whose constellation points are drawn from a 2m-ary underlying constellation associated with a shared/multi-resolution resource block, where m≥m1+m2′. The communication system is operative to transmit control information to direct the receiver to extract first receiver demodulation information from a first sub-constellation portion of a 2m-ary underlying constellation and to use the first receiver demodulation information in a first decoder to form an estimate of the first stream of bits. The communication system is also operative to direct the receiver to extract second receiver demodulation information from a second sub-constellation portion of the underlying constellation and to use the second receiver demodulation information in a second decoder to form an estimate of the second stream of bits. In many embodiments, the first encoder is a tile encoder, the second encoder is a building-block encoder, and the first and second receiver demodulation information corresponds to respective first and second sequences of bit metrics. The first sub-constellation portion is often associated with a 2m1′-ary tile constellation and the second sub-constellation portion is often a 2m2′-ary building block constellation. In other embodiments, different types of multilevel codes can be used instead of tiled-building-block coded constellations. Any of the code types listed in this section above can be used in different embodiments in the first and second encoders. In embodiments where variable selective mapping is used, the first sub-constellation portion is a 4-ary corner constellation, the second sub-constellation portion is a non-corner constellation.

