Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-07-10
2004-04-27
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C714S790000
Reexamination Certificate
active
06728323
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to signal communications and, in particular, to reception of encoded signals over a communications channel.
The use of wireless communications is expanding rapidly, particularly as more radio spectrum becomes available for commercial use and as wireless phones become more commonplace. In addition, analog wireless communications are gradually being supplemented and even replaced by digital communications. In digital voice communications, speech is typically represented by a series of bits which may be modulated and transmitted from a base station of a wireless communications network to a mobile terminal device such as a wireless phone. The phone may demodulate the received waveform to recover the bits, which are then converted back into speech. In addition to voice communications, there is also a growing demand for data services, such as e-mail and Internet access, which typically utilize digital communications.
There are many types of digital communications systems. Traditionally, frequency-division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. These carriers may be further divided into time slots, generally referred to as time-division-multiple-access (TDMA), as is done, for example, in the Digital-Advanced Mobile Phone Service (D-AMPS) and Global System for Mobile communication (GSM) standard digital cellular systems. Alternatively, if the radio channel is wide enough, multiple users can use the same frequencies using spread spectrum techniques and code-division-multiple-access (CDMA).
A typical digital communications system
19
is shown in FIG.
1
. Digital symbols are provided to the transmitter
20
, which maps the symbols into a representation appropriate for the transmission medium or channel (e.g. radio channel) and couples the signal to the transmission medium via antenna
22
. The transmitted signal passes through the channel
24
and is received at the antenna
26
. The received signal is passed to the receiver
28
. The receiver
28
includes a radio processor
30
, a baseband signal processor
32
, and a post processing unit
34
.
The radio processor typically tunes to the desired band and desired carrier frequency, then amplifies, mixes, and filters the signal to a baseband. At some point, the signal may be sampled and quantized, ultimately providing a sequence of baseband received samples. As the original radio signal generally has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex, baseband samples.
The baseband processor
32
may be used to detect the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values. The post processing unit
34
typically performs functions that depend on the particular communications application. For example, it may convert digital symbols into speech using a speech decoder.
A typical transmitter is shown in FIG.
2
. Information bits, which may represent speech, images, video, text, or other content, are provided to forward-error-correction (FEC) encoder
40
, which encodes some or all of the information bits using, for example, a convolutional encoder. The FEC encoder
40
produces coded bits, which are provided to an interleaver
42
, which reorders the bits to provide interleaved bits. These interleaved bits are provided to a modulator
44
, which applies an appropriate modulation for transmission. The interleaver
42
may perform a variety of types of interleaving.
The modulator
44
may apply any of a variety of modulations. Higher-order modulations are frequently utilized. One example is 8-PSK (8-ary phase shift keyed), in which 3 bits are sent using one of 8 constellation points in the in-phase (I)/ quadrature (Q) (or complex) plane. Another example is 16-QAM (16-ary quadrature amplitude modulated), in which 4 bits are sent at the same time. Higher-order modulation may be used with conventional, narrowband transmission as well as with spread-spectrum transmission.
In a conventional baseband processor, a baseband received signal is typically provided to a demodulator which produces soft bit values. These soft bit values are generally provided to a de-interleaver which reorders the soft bit values to provide de-interleaved soft bits. These de-interleaved soft bits may then be provided to a forward error correction (FEC) decoder which performs, for example, convolutional decoding, to produce detected information bits.
A second example of a conventional baseband processor employs multipass equalization, sometimes referred to as Turbo equalization, in which results, after decoding has completed, are passed back to the equalization circuit to re-equalize, and possibly re-decode, the received signal. Such a system is described, for example, in U.S. Pat. No. 5,673,291 to Dent et al. entitled “Simultaneous demodulation and decoding of a digitally modulated radio signal using known symbols” which is hereby incorporated herein by reference. In multi-pass equalization, the processor typically initially performs conventional equalization and decoding. After decoding, the detected information bits are encoded and then re-interleaved to provide information to the multipass equalizer and soft information generator which re-equalizes the received baseband signal using the detected bit values. Typically, because of diagonal interleaving or the fact that some bits are not convolutionally encoded, the second pass effectively uses error corrected bits, as determined and corrected in the first pass, to help detection of other bits, such as bits which were not error correction encoded.
Both single pass and multipass baseband processors as described above typically use conventional FEC decoders. Conventional FEC decoders typically treat each soft bit value as if it were independent of all other values. For example, in a Viterbi decoder for convolutional codes, soft bit values are generally correlated to hypothetical code bit values and added. As the soft bit values typically correspond to loglikelihood values, adding soft values corresponds to adding loglikelihoods or multiplying probabilities.
By way of background, error correction codes, such as convolutional codes, in essence, provide error correction capability by generating one or more “parity” bits for transmission from each data bit to be transmitted. The ratio of data symbols to parity symbols is referred to as a coding rate. For example, if two parity bits are generated for each data bit, the code is known as a rate 1/2 code, with twice as many parity bits as original data bits being transmitted. If the rate of transmission is fixed, the time required to transmit such parity bits is twice as long as the time required to transmit the original data bits. More generally, if r parity bits are generated for each data bit, the code is known as a rate 1/r code. Typically, the parity bit transmission rate is adopted to be r times the message information bit rate.
Tables of parity equations for various code rates and constraint lengths that result in optimum codes are published in the technical literature. See, e.g., G. Clarke, Jr., and J. Cain, Error-Correction Coding for Digital Communications, Appendix B, Plenum Press, New York (1981).
The known methods for decoding convolutional codes include threshold decoding, Sequential Maximum Likelihood Sequence Estimation (SMLSE), and the stack algorithm. The SMLSE technique is commonly known as the Viterbi algorithm, which is described in the literature including D. Forney, “The Viterbi Algorithm”, Proc. IEEE, Vol. 61, pp. 268-278 (March, 1973).
As noted above, r times more parity symbols than input data symbols are produced for a rate 1/r code, and, if all parity symbols are transmitted, an r-times redundancy has been provided to combat errors. It will, however, be appreciated that it is not necessary to transmit all of the parity s
Chen Dayong
Feli Evin
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Tran Khai
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