Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-06-06
2004-02-24
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S324000, C714S786000
Reexamination Certificate
active
06697441
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to signal communications and, in particular, to reception of encoded and modulated signals over a communications channel.
One type of communications channel which is expanding particularly rapidly is wireless communications, particularly as more radio spectrum becomes available for commercial use and as cellular 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 cellular communications network to a mobile terminal device such as a cellular 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 the global system for mobile communication(GSM) standard digital cellular systems. Alternatively, if the radio channel is wide enough, multiple users can use the same channel 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 material, 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 any type of interleaving. One example is block interleaving, which is illustrated in FIG.
3
. Conceptually, bits are written into rows of a table, then read out by column.
FIG. 3
shows an example of 100 bits, written into a 10×10 table.
Another example of interleaving is diagonal interleaving, in which data from different frames are interleaved together. Diagonal interleaving is illustrated in FIG.
4
. Each frame is block interleaved using block interleavers
50
a
,
50
b
, and
50
c
. Using switches
52
a
,
52
b
, and
52
c
, interleaved bits from each frame are split into two groups. The multiplexors
54
a
and
54
b
combine groups of bits from different frames to form transmit frames. In TDMA systems, different transmit frames generally would be sent in different time slots.
The modulator
44
may apply any of a variety of modulations. Higher-order modulations, such as those illustrated in
FIGS. 5A and 5B
, are frequently utilized. One example is 8-PSK (eight phase shift keying), in which 3 bits are sent using one of 8 constellation points in the in-phase (I)/quadrature (Q) (or complex) plane. In
FIG. 5A
, 8-PSK with Gray coding is shown in which adjacent symbols differ by only one bit. Another example is 16-QAM (sixteen quadrature amplitude modulation), in which 4 bits are sent at the same time as illustrated in FIG.
5
B. Higher-order modulation may be used with conventional, narrowband transmission as well as with spread-spectrum transmission.
A conventional baseband processor is shown in
FIG. 6. A
baseband received signal is provided to the demodulator and soft information generator
60
which produces soft bit values. These soft bit values are provided to the soft information de-interleaver
62
which reorders the soft bit values to provide de-interleaved soft bits. These de-interleaved soft bits are provided to the FEC decoder
64
which performs, for example, convolutional decoding, to produce detected information bits.
A second example of a conventional baseband processor is shown in FIG.
7
. This processor employs multipass 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. For the circuit illustrated in
FIG. 7
, the processor typically initially performs conventional equalization and decoding. After decoding, the detected information bits are re-encoded in the re-encoder
74
and then re-interleaved in the re-interleaver
72
to provide information to the multipass equalizer and soft information generator
70
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 forward error correction (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. As the Viterbi decoder corresponds to maximum likelihood sequence estimation (MLSE) decoding, multiplying probabilities generally assumes that the noise on each bit value is independent.
For lower-order modulation, with Nyquist pulse shaping and nondispersive channels, independent noise is often a reasonable assumption. For example, for quadrature phase shift keyed (QPSK) modulation, one bit is generally sent on the I component and a second bit is sent on the Q component. Because noise is typically uncorrelated between the I and Q components, the noise on these two bits would generally be independent. However, with higher-order mo
Arslan Huseyin
Bottomley Gregory Edward
Dent Paul Wilkinson
Khayrallah Ali S.
Ericsson Inc.
Le Amanda T.
Myers Bigel & Sibley & Sajovec
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