Note: Descriptions are shown in the official language in which they were submitted.
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SrSTEM AND METHOD FOR WIRELESS TRANSMISSION OF DIGITAL
DATA USING DIFFERENTIALLY ENCODED PILOT WORDS.
FIELD OF THE INVENTION
This invention relates to wireless dighal In particular, this invention relates to
wireless ! ~ ' systems that use pilot symbols to estimate the state of a channel.
e.A OF THE INVENTION
Various modulation techniques exist for varying the phase andlor amplitude of a carrier to convey
digital data from a transmhter to a receiver. Examples of such modulation techniques include phase shift
keying IPSK) and quadrature amplhude modulation IDAMI. PSK involves the swhching of the phase of the
carrier between a plurality of discrete phase offsets l-elative to a reference carrier phase), whh each discrete
phase offset representing one or more information bits. QAM involves the swhching of both the phase and
amplitude of the carrier, whh each discrete combination of carrier amplitude and phase representing one or
more information bhs.
Every signaling interval, the transmhter transmits one of a plurality of possible "symbols," with each
symbol being a signal (at the carrier frequencyl that has a certain phase and amplhude. Each symbol (i.e.,
each unique I . ~ j ~ combination~ represents one or more bhs of information in accordance whh a
given modulation technique. With 8-PSK, for example, one of eight possible symbols is transmitted every
siynaling interval, whh each symbol having a unique phase (and fixed amplitude~ that corresponds to a three
bit value (since 23 - 81. Illustratively, phase offsets of 0, 45, 90, 135, 180, 225, 270 and 315 degrees
may represent binary values 0' ~~~2~ ~~12, ~1~2, ~112, 1002, 1012, 1102, and 1112 respectively. Similarly,
with 16 OAM, each of 16 symbols has a unique, , . ~ combination that represents a unique four
bit value.
The symbols for a given modulation scheme can be conveniently represented as a set of complex
values, whh the imaginary portion of each complex value representing the phase offset and the magnhude
of the complex value representing the amplhude. The plot of the complex symbol values in a complex plane
for a modulation scheme is commonly referred to as the symbol constellation" for the modulation scheme.
(See, for example, Fig. 1, which illustrates a conventional 16 ~AM symbol constellation).
In wireless systems, the information signal is transmitted to the receiver over a
channel thal comprises multiple propagation paths or multipaths" between the transmitter and the receiver.
~ 30 These multipaths are caused by the reflection of the transmitted signal off hills, buildings, airplanes,
!' ' in the atmosphere, and the like. As the result of multipaths, the signal received by the
- receiver consists of multiple components that vary in both phase and amplhude.
The complex addition of these muhiple components at the receiver results in a phenomenon known
as fading, wherein the phase and amplitude of the received signal varies with time. Thus, at any given time,
the state of the channel between the transmitter and the receiver can be described generally by the amplaude
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attenuation and phase shift caused bV the channel. These channel bi ~ can significantly affect
the abilhy of a wireless receiver to detenmine the phase and amplitude of the transmhted signal, and can
thus impair the abilhy of the recenver to decode the transmhted symbols. This impairment is particularly
significant when the receiver encounters "deep fades," which are periods of significant signal attenuation
caused by the destructive addhion of muhipath components.
Various techniques have been developed to combat the effects of fading. One technique involves
the periodic insertion by the transmhter of predetenmined symbols known as "pilot symbols" into the stream
of data symbols to allow the receiver to estimate the state of the channel. The receiver knows when the
pilot symbols will be transmhted, and further knows the value ~i.e., the phase and amplhude) of pilot symbols
upon transmission. Thus, upon receipt of a pilot symbol the receiver can determine the extent to which the
channel is cunently impairing the phase and amplhude of the transmitted signal by comparing the value of
the received pilot symbol with the expected (i.e., transmitted) value.
Once the receiver estimates the current phage and amplitude effects of the channel, the receNer
compensates for these effects by appropriately adjusting the phase and amplhude of tha received si~nal.
Channel estmates are updated each time a pilot symbol is received by the receiver. Since the channel state
is generally quasi-static over small numbers of consecutive symbols, the method works well provided that
pilot symbols are inserted at a rate that is whh the rate at which the channel state varies.
The rate at which the channel state varies depends on a variety of factors, including the relative speed
between the transmhter and receiver (h any).
Before a receiver can extract pilot symbols from the symbol stream and estimate the channel state,
the receiver must become synchronized whh the transmhter so that h knows when the pilot symbols will
be transmhted. Since pilot symbols are affected and often corrupted by the channel, the process of
synchronizing on the periodic pilot symbols is typically prohibitively slow. Pilot symbol :, ' is
further complicated by the fact that the pilot symbols are symbols that may also appear in the stream of
data symbols.
To solve this problem, conventional transmhters periodically transmh a separate ,. ' .
sequence of known symbols (e.g., 20 consecutive symbols) to permit the receiver to synchronize with the
transmhter. The use of a, ' sequence, however, occupies bandwidth that could otherwisa be
used for the transmission of data symbols. Further, a receiver of such a system must wait for the
transmission of a, ' seqoence before it can extract pilot symbols and perform channel
estimation.
SUMMARY
The present invention solves these problems using a data stream format that includes periodically-
inserted, differentially encoded pilot words. The differentially encoded pilot words permit receivers to
synchronize whh the transmhter, obviating the need for a ,. ' sequence. In a preferred
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embodiment of a wireless ~ system, each differentially encoded pilot word consists of two
~; pilot symbols that are a fixed difference (or "pilot differencen) apart, with the pilot
difference being the same for each differentially encoded pilot word. The pilot difference may alternatively
be varied, provided that the se~uence of pilot words is known to the receiver so that the receiver can use
pilot words for performing channel estimation. In other embodiments, each pilot word comprises three or
more differentially encoded pilot symbols, whh each pilot symbol of a pilot word being a fixed difference from
another pilot symbol of the pilot word. Pilot differences used to encode differentially encoded pilot words
preferably include differences in phase, but may include differences in phase andlor differences in amplhude.
ReceNers of the system monhor differences between 1~: (and channel impaired)
symbolsinordertodetecttheperiodic,dhferentiallyencodedpilotwords. Differences(or differencevaluesnl
between ! ' '~; ~ ' symbols tend to be robust when transmhted over a wireless channel in
comparison to the symbols themselves, since channe~ impairments to such difference values are typically small
in comparison to the impairments to the symbol values. Dhferential encoding of pilot words thus enables
receivers of the system to locate the periodically transmitted differentially encoded pilot words. Detection
of the differentially encoded pilot words permits the recenvers to become synchronized with the transmhter,
as is necessary to receive meaningful data.and thereby become synchronized whh the transmitter.
The pilot symbols of the dhferentially encoded pilot words are predetermined symbols that are
known to receivers of the system. Once a receiver determines the periodic location of the differentially
encoded pilot words within the symbol stream, the recenver extracts the: - , ~ pilot symbols from
the symbol stream and compares the -, ~ pilot symbols to their known or expected" values.
The receiver thereby generates estimates of the state of current state of the channel. These channel
estimates then are used by the recenver to compensate the , data symbols it receives (in
phase and amplitude~ on a ,. b~ ~ basis.
In accordance whh one aspect of the present invention, there is thus provided a transmitter for
transmhting data over a wireless channel, wherein the transmhter comprises a constellation mapper, a pilot
word generator, a pilot symbol inserter, a filter, and a radio frequency (RF~ up converter. The constellation
mapper receives a data stream and generates a data symbol stream by transforming binary values of the data
stream into corresponding symbols in accordance with a particular, coherent modulation technioue. The pilot
word generator generates differentially encoded pilot words, whh each differentially encoded pilot word
~ 30 comprising at least two pilot symbols. The pilot symbol insener periodically inserts the pilot symbols of the
differentially encoded pilot words into the data symbol stream to produce a composhe symbol stream. The
filter filten the composhe symbol stream prior to transmission, and the RF up-converter generates an RF
of the composite symbol stream for transmission over the wireless channel.
According to a further aspect, one embodiment of the invention is a method of estimating the state
of a wireless channel when a signal is transmitted from a transmhter to a receiver. The method comprises
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the steps of: (a) periodically nnserting a pilot word into a stream of data symbols, the pilot word comprising
at least two pOot symbols that are separated by a fixed, predetermined pilot dlfference, at least one pilot
symbol having a value that is l nown to the receiver; (b~ transmhting a stream of symbols resuhing from step
(a~ from the transmhter on the wireless channel; (c~ receiving a stream of ~ , symbols at the
receiver, the stream of -, ~ symbols being the stream of symbols transmitted in step (b~ as
modified by the wireless channel; (d~ detecting the periodic position of the pilot word inserted in step (a)
whhin the stream of ~ symbols received in step (c~ by monitoring a difference between
recenved -~, ~ symbols; (e~ extracting a pilot word inserted in step (a) from the stream of channel-
impaired symbok received in step (c) using the periodic position detected in step (d); and If) comparin~ at
least one pilot symbol of the pilot word extracted in step ~e) with the value of the pilot symbol upon insenion
in step (a) to thereby estimate the state of the channel.
In a particularhy preferred embodiment, step (d) comprises the step of monitoring the dlfference
between received -~ symbols over a pluralhy of pilot periods to detect a periodic occurrence
of the predetermined pilot difference. The periodic occurrence of the predetermined pilot difference indicates
the periodic poshion of the pilot word.
In another particularly preferred embodiment, the pilot word periodically inserted in step (a) consists
of two pilot symbols that are separated by a predetermined phase difference.
In still another particularly preferred embodiment, the pilot word consists of two pilot symbols that
are separated by a predetermined phase difference and a predetermined amplnude difference.
In yet another particularly preferred embodiment, the pilot word consists of first, second and third
pilot symbols, and wherein the first and second pilot symbols are separated by a first predetermined
difference and the second and third pilot symbok are separated by a second predetermined difference.
In another preferred embodiment, the method further comprises the step of: (g) using an estimate
generated in step (f) to compensate the amplitude and phase of '-~, data symbol.Under another aspect of the invention, an advantageous embodiment of the invention is a
system including a transmhter that transmits a sequential stream of data. The sequential
stream of data comprises a stream of data symbols and a series of pilot sequences spaced at substantially
equal time intervals within the stream of data symbols. At least a portion of each of the pilot sequences
is identical. A plurality of the pilot sequences include two pilot symbols that differ from one another.
In a particularly preferred embodiment, each pilot sequence is identical. Ahernatively, every other
pilot sequence is identical.
In another particularly preferred embodiment, each pilot sequence comprises two pilot symbols that
are separated by a fixed difference. Alternatively, every other pilot sequence comprises two pilot symbols
that are separated by a fixed difference.
In stOI another particularly preferred embodiment, the two pilot symbols dUfer from one another by
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a fixed phase.
In yet another preferred embodiment, the system is in combination with a recejver
that is designed to receive the sequential stream of data. The transmitter transmits the sequential stream
of data as a radio frequency signal on a wireless channel.
BRIEF DESCRIPTION OF THE ORAWINGS
Fig. 1 illustrates a conventional 16 ~AM symbol constellation.
Fig. 2 illustrates an example sequence of transmhted symbols in accordance with the prior art.
Fig. 3 illustrates a sequence of transmhted symbols in accordance with the present invention.
Fig. 4 is a functional block diagram of a transmhter in accordance with the present invention.
Fig. 5 is a functional block diagram of a recenver in accordance with the present invention.
Fig. 6 is a functional block diagram of a preferred embodiment of the differentially encoded pilot
word detector of Fig. 5.
Fig. 7A is a graphical illustration of a result generated by the differentially encoded pilot word
detector
after a single heration.
Fig. 7B is a qraphical illustration of a result ~qenerated by the differentially encoded pilot word
detector
after two iterations.
Fig. 7C is a ~raphical illustration of a result generated by the differentially encoded pilot word
detector after N herations.
OETAILEO DESCRIPTION OF THE PREFERREO EMBOOIMENTS
The prior art is further described with reference to Figs. 1 and 2. Fig. 1 illustrates a conventional
16 ~AM symbol constellation. Each symbol is shown as a complex point in a cartesian coordinate system
that has a real ~R) axis and an imaginary ~j~ axis. Each of the 16 symbols corresponds to the unique 4 bit
binary value shown in parenthesis below the symbol. For example, the symbol 102 in Fig. 1 corresponds to
the value 001 Oz. To convey digital data to a receiver, a transmhter groups output data bits into nibbles ~four
bits), and maps each nibble to the corresponding symbol. One or more of the 16 symbols may additionally
be used as a pilot symbol, andlor may be used as a, ' symbol.
As used herein, the term 'symbol~ refers generally to the RF signals of discrete phases andlor
amplitudes that are used to convey data in accordance whh a given modulation technique. However, since
such amplhude and phase information can be represented or conveyed in a variety of forms ~e.g., as comple
- numbers, points on a complex plane, analog or digital pulse signals that represent complex values, etc.), the
term will also be used herein to include the various forms by which the 9, ~' 'e'~ ' information may be
represented or conveyed. Where applicable, the term ~complex value~ or "complex symbol value- will be used
to emphasize that numerical data is being operated on or conveyed.
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The 16 ~AM tonstellation shown in Fig. 1 will be used herein to describe the prior art and anillustrative embodiment of the present invention. However, as will be reco~qnized by those
skilled in the art,
the present invention is fully applicable to other coherent modulation techniques that use drfferent symbol
. .. ..
Fig. 2 shows an illustrative symbol stream for a conventional wireless digital system
that transfers one symbol every signaling interval T. The symbol sequence is , of packet radio
or wireless modem system for which multiple receivers monitor an information signal that is transmitted
continuously from a single transmhter. Referring to Fig. 2, a :~ ' sequence 202 comprises
~,. ' symbols 204A-204L. The ,. ' sequence is transmhted on a periodic basis to
permh the recenver or receivers of the system to synchronize with the transmitter using techniques that are
well known in the art. Conventional ~, ' sequences typically comprise at least 11 symbols, which
may be provided consecutively in the symbol stream (as shown in Fi~q. 2), or may be interleaved with symbols
that carry data, control information, pilot information and other types of information.
The , ' sequence 202 is followed by a stream of data symbols with, " 'I~ h~
pilot symbols 206A, 206B, 2û6C. Each pilot symbol is the same symbol (i.e., each pilot symbol 206A, 206B,
206C has the same amplitude and phase), and the symbol used as the pilot symbol is known to the receiver.
Illustratively, the symbol 3 + j3 (symbol 104 in Fig. 1) may be used as the pilot symbol, in which case the
receiver will ~expect" to recenve a symbol of 3 + j3 during each pilot symbol signalling interval.
A pilot symbol is insehed into the stream of data symbols every pilot period 210. The pilot period
210 for the sequence shown in Fig. 2 is equal to 12T (twelve signaling intenals), resulting in the
transmission of one pilot symbol for every eleven data symbols. The pilot period 210 generally must be
selected to be with the expected rate of change of the channel for the particular application.
For mobile applications, factors that are considered in sebcting an appropriate pilot period 210 include the
malimum expected velochy between transmhters and receivers of the system, the transmission frequency,
and the baud rate for the system. By way of example, for a cellular phone system having a frequency of
operation of 910 MHz (or wavelength of 1.06 feet) and a maximum expected vehicular velocity of 60 miles
per hour (88 feet per second), the maximum Doppler shift will be 2 X (velocity)lA - (2)(88)1(1.08) - 163
Hz. From Nyquist's sampling theorem, the bandlimited channel variation should be sampled at a rate of at
least twice the maximum Doppler shrft, or 326 H2, requiring a pilot period of 11326 - 3.1 milliseconds. In
practice, to ensure accurate channel pilot symbols may be inserted at twice this rate, or every
1.55 milliseconds. The number of symbols falling between the inserted pilot symbols will then depend upon
the baud rate of the system and the modulation technique (8 PSK, 16 DAM, 64 ~AM, etc.) employed. A
detailed description of the hL ' ' " of mobile RF channels can be found in William C. Jakes, Jr.,
Microwave Mobile ~ (New York: John Wiley and Sons, 1974).
In operation, when a receiver of a typical wireless system is first turned on, the receiver monhors
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the stream of symbois recenved and attempts to lotate a ,. ' ~ sequence. The receiver cannot
extract pilot symbols or receive meaningful data during this time period. Thus, even rf the receiver could
become synchronized whh the recerver on the first, ' ' sequence h receives, it would still have
to wah for the transmission of this, ' ~ sequence before h could extract pilot symbols or recenve
meaningful data.
Once the receiver locates a, ' sequence and becomes synchronized with the
transmhter, the recenver begins to extract the transmhted pilot symbols and estimate the state of the
channel. Depending upon the nature of the particular system (for example, packet radio, wireless modemlfax,
cellular phone, beeper, or PDA~, the receiver may begin to passively receive data, or may transmit a signal
to inform the transmhter that is has become synchronized.
Fig. 3 illustrates a symbol stream in accordance whh the present invention. The periodic pilot
symbols 206A-206C of Fig. 2 are replaced with differentially encoded pilot words 306A-3D6D (also referred
to herein as "pilot words 306A-306Dn). ~ ' ~ Iy, each differentially encoded pilot word comprises
two or more pilot symbols, wherein the pilot symbols are separated by (or "encoded withn) a fixed difference
(i.e., a fixed difference in amplhude andlor phase). Illustratively in Fiq. 3, each pilot word 3D6A-3D6D
comprises two consecutnve pilot symbols that are a fixed difference apart. Referring to Fig. 1, for example,
each pilot word 3D6A-3D6D may consist of the symbol lD4 (3 I j3) followed by the symbol 106 (-3 - j3),
thus having a phase difference of 180 degrees and an amplitude drfference of zero. Each pilot word 3D6A
306D could alternatively comprise three pilot symbols, whh the first and second pilot symbols separated by
a first fixed difference and the second and third pilot symbols separated by a second fixed difference.
Regardless of the number of pilot symbols per differentially encoded pilot word, each pilot word is encoded
whh the same drfference (or differences). Preferably, each pilot word consists of the same sequence of pilot
symbols.
For radio channels that are quasi-static on a, ' '-b~ ~ ' ' basis, the effect of the channel on
the encoded difference is negligible. For example, rf the differentially encoded pilot word 31 j3, ~3j3 is
transmhted at a time when the channel is approximated by an amplhude attenuation of 50% and a phase
offset of 30 degrees, the phase and amplhude differences between the consecutive pilot symbols will still
be approximately 180 degrees and zero respectnvely. Since the encoded differences are not significantly
affected by the channel, the differentially encoded pilot words in the symbol stream can be detected by
monhoring the differences between symbols (using a correlator or COM8 filter, as described below). This
eliminates the need for a separate pilot, ' ~ sequence.
As illustrated by Fig. 3, the present invention requires the insertion of an additional pilot symbol
every pilot period 210. However, the bandwidth occupied by the added pilot symbols is generally less than
the bandwidth occupied by the prior art pilot ,. ' ~ sequence 210. Thus, an overall bandwidth
reduction is achieved. Further, since the use of differentially encoded pilot words enables receivers to
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become synchronr2ed whh the transmhter whhout waiting for the transmission of a pilot,
sequence, a reduction in the average, ' ' tame is typically achieved.
In general, phase differences between consecutive symbols tend to be less susceptible to channel
impairments than the amplhude differences between the symbols. Thus, pilot words may be differentially
encoded with fixed phase differences only, in which case the receiver may i~qnore (or give less weight to)
differences in amplhude when attempting to synchroni2e with the transmhter. However, for variable
amplhude systems (for example, QAM) h may be desirable to additionally or ahernatively encode pilot words
whh differences in amplhude. Thus, as used herein, the term "difference" refers to the complex difference
between two symbols, and thus encompasses differences in phase and differences in amplitude. The term
~pilot difference" refers to the difference that is used to encode two pilot symbols of a differentially encoded
pilot word.
The pilot symbols of the differentially encoded pilots words 306A-306D are shown in Fig. 3 as being
transmhted in consecutive symbol poshions. It will be recognized, however, that the two pilot symbols of
each pilot word 306A-3060 could alternatnvely be transmhted in : , symbol poshions provided
that the wirebss channel remains quasi-static during the time period between the respective pilot symbol
transmissions (whh the pilot symbols transmitted in the same symbol positions each pilot period so that the
receiver can extract the pilot symbols). Illustratively, the two pilot symbols of each pilot word 306A-306D
could be separated by a single data symbol, provided that the channel is expected to remain quasi-static for
2T (two signaling intervals).
Fig. 4 is a functional block diagram of a preferred embodiment of a transmitter 400 in accordance
with the present invention. The transmhter 400 comprises a forward error correction encoder 404, an
interleaver406, a constellation mapper408, a differentially encoded pilot word generator412, a pilot symbol
inserter (denoted by reference numbers 416 and 416' to indicate two alternative locations), a filter 418, a
radio frequency (RF) up-converter 422, and an antenna 426. As will be recognr2ed by those skilled in the
art, the functional blocks shown within the box 430 can be impbmented in a straight forward manner using
one or more dighal signal processing (DSP) chips, such as the C50 DSP chip available from Texas
Instruments, Inc., under the control of software. Preferably, however, the functional blocks are implemented
using dedicated digilal circuitry integrated into an ,, " ~, " integrated circuit (ASIC). It will also
be recogni2ed that the forward error correction encoder 404 and the interleaver 406 are not necessary
components of a transmhter in accordance whh the present invention, and can thus be omitted if desired.
Referring to Fig. 4, the forward error correction encoder 404 receives a digital data stream and
adds redundancy bhs in accordance with techniques that are well known in the art. The digital data stream
may represent voice, video or data (or a combination thereof), and may come from any of a variety of
possible sources. Illustratnvely, the dighal data stream may be the output of a data link layer of a node of
an open systems interface (OSI) computer network. The forward error correction encoder 404 encodes the
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dighal data stream using convolutional coding, trellis coding, Reed-Solomon coding, andlor any other well
known type of encoding technique suhable for the wireless transmission of dighal data.
The output of the forward error correction encoder 404 is fed to a conventional interleaver 406.
The interleaver performs convolutional mterleaving or block interleaving in accordance with techniques that
are well known in the art. Regardless of the type of interleavinû employed, the primary function of the
interleaver 406 is to reanange the data stream in order to mhigate the effects of burst errors. Illustratnvely,
the rnterleaver 406 interlea~es each block of data (for example, 64 bits) with preceding and following blocks
of data in the digital data stream so that each block of data is transmitted over a greater period of time,
with this period of time exceeding the expected average duration of deep fades. Burst errors caused by deep
fades are then spread out over multiple blocks of data in a manner that enables the receiver to correct the
errors lusing forward error correction, CRC, etc.) once the data stream has been d~ . ' '
The output of the interleaver 406 is fed to a conventional constellation mapper 408. The
constellation mapper 408 maps groups of data bits (for example, four consecutive bits) into symbols in
accordance whh any linear, coherent modulation technique. Illustratively, the constellation mapper may map
data bhs into symbols according to the 16 QAM symbol constellation shown in Fig. 1. The mapping function
can be perfonmed, for example, using a lookup table stored in read-only memory IROM).
The constellation mapper 408 outputs dighal signals that represent the data symbols to be
transmhted. These dighal signals are preferably in the form of ~ ~ ' ' . , " ' pulses that represent the
real and imaginary portions of the symbols. For each generated symbol the constellation mapper 408
generates one pulse that represents the real component of the symbol Ireferred to as the in-phase pulse or
"I pulsen), and a second pulse that represents the imaginary portion of the symbol Ireferred to as the
quadrature pulse or "~ pulsen), with the real and rmaginary symbol components specified by the amplitudes
of the respective pulses. Illustratively for the symbol 102 in Fig. 1, the constellation mapper 408 would
generate an I pulse of amplitude +1 to represent the real component of the symbol 102, and a ~ pulse of
amplhude +3 to represent the imaginary component of the symbol. Each I-O pulse pair represents a complex
value that corresponds to the carrier amplhude and phase to be transmhted. The I and ~ pulses may be
provided on separate I and Q channels, and are ultimately used to control the amplitude and phase of the
carrier. The I and O channels are represented in Fig. 4 by the arrow that connects the constellation mapper
408 to the pilot symbol inserter 416.
~ 30 The output of the constellation mapper 408 is fed to a pilot symbol inserter 416 that periodically
inserts pilot symbols of differentially encoded pilot words in accordance with the present invention. A
- drfferentially encoded pilot word generator 412 generates differentially encoded pilot words, and provides the
pilot symbols of the pilot words to the pilot symbol inserter 416 for insertion into the symbol stream. The
pilot symbols of a pilot word are preferably inserted in consecutive symbol positions, but may alternatnvely
be separated by one or more data symbols. The inserted pilot symbols are preferably in the form of dighal
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I and O pulses that represent the real and imaginary components of the pilot symbols respectbely.
In an ahternative embodiment, illustrated by dashed lines in Fig. 4, the differentially encoded pilot
word generator412 and pilot symbol inserter416' are ahernatively provided upstream from the constellation
mapper 408 (after the interleaver 406), and the pilot symbol inserter 416' periodically inserts digital values
Igenerated by the differentially encoded pilot word generator 412~ that correspond to the pilot symbols to
be generated. The constellation mapper 408 then generates the pilot symbols of the differentially encoded
pilot words. Illustratively for a pilot word consisting of the consecutive symbols 104 and 106 in Fig. 1, the
pilot symbol inserter 416' would insert the binary values ~~112 and 1100z into the data stream.
The output of the pilot symbol inserter 416 is provided to a conventional filter 418. This output
is preferably in the form of separate I and ~ signals, with each signal comprising sequences of concatenated
pulses of varying amplitude. The fiher 418 bandlimits these signals, and thus smooths the instantaneous
transrtions between consecutive pulses. The filter 418 thereby smooths the instantaneous transitions in
carrier amplitude and phase that result from transitions between consecutive non-like symbols. Such filtering
is generally necessary for effective wireless transmissions of dighal data, as is weil understood in the art.
The filter 418 is preferably a conventional, ~ ' cosine filter, and is preferably
implemented as a digital fiher. The output of the digital filter is fed to a ~is ' to . '1 ~ converter Inot
shown) that converts each dighal signal into its analog equivalent. In an alternative embodiment, the digital-
to-analog fiher is provided upstream from the filter 418, and the filter 418 is an analog filter that processes
analog I and Q signals.
The output of the filter 418 is provided to the RF up-converter 422 that converts the baseband,
filtered pulse signals into an RF signal at a carrier frequency, with the amplitudes of the filtered I and O
pulse signals controlling the amplitude and phase of the RF signal. In the preferred embodiment, the
baseband signal is up-converted to a 900 MHz Rf signal. The RF signal is then radiated from the antenna
426.
Fig. 5 is a functional block diagram of a receiver 500 in accordance with the present invention.
The receiver 500 corresponds to the transmitter 400 of Fig. 4. The receiver 500 comprises an antenna 504,
an RF ' : ;~.. 508, a filter 512, a sampler 514, a differentially encoded pilot word detector 516,
a pilot symbol eltractor 518, a channel estimator 520, a channel compensator 522, a demodulator 524,
deinterleavers 528 and 530, and a forward error correction decoder 534. The deinterleavers 5ZB and 530
and the forward error correction decoder 534 are optional components that may be omitted d no interleaving
or forward error correction encoding is performed by the transmitter 400. The functional blocks shown in
the box 540 can be implemented using one or more general purpose DSP chips Isuch as the Texas
Instruments C50), but are preferably implemented using dedicated digital hardware integrated into an ASIC.
The fiher 512 can alternatively be implemented as an analog filter using analog components.
Referring to Fig. 5, the RF signal received by the antenna 504 is converted to a baseband signal
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by a conventional RF ' : t,,, 508. The RF ' : t,,. 508 preferably outputs separate I and
Q signals that represent the in-phase Ireal~ and quadrature (imaginary) components of the received signal
respectnvely. These I and Q signals represent the transmhted symbols as affected or impaired by the wireless
channel.
The output of the RF ' : t~. is fed to the filter 512. The filter 512 bandlimits the
baseband signal so that , ~ ' to ! ~ value conversion is limited to the bandwidth of the transmhted
signal. The filter 512 is thus preferably identical in frequency response to the filter 418 used with the
transmitter 400. As in conventional in the art, , : '- ' cosine fihers can be used for the
transmnter and receiver filters 418 and 512, resulting in an overall root-raised cosine filtering technique that
produces a low degree of intersymbol interference. The filter 512 outputs I and Q signals that are in the
form of ~ ' ' ., ' pulses, whh transhions between pulses smoothed by the filtering process.
The output of the filter 512 is fed to the sampler 514. Each signaling interval T the sampler 514
samples the filtered I and Q signals to produce a single complex value that ~epresents a ' ';,
symbol. As is well known in the art, the ~' .: ' root-raised cosine filtering technique
~ produces output I and Q pulses that, at a specific, known instant in time, have the same
amplhudes as the corresponding unfiltered I and Q pulses (ignoring channel effects). The sampler 514
samples the I and Q signals at this point in trme (during each signaling interval T) so that the amplhude
effects of the filtering process on the I and Q signals are effectively negated.The output of the sampler 514 is fed to the differentially encoded pilot word detector 516
Ihereinafter ~pilot word detector 516n), and is also fed to the pilot symbol extractor 518. The pilot word
detector 516 detects the periodic poshion of differentially encoded pilot words within the symbol stream by
repetitively calculating the difference between received symbols (i.e., the difference between the complex
values that represent ' '-;, ' symbols), and by effectively searching for the periodic occurrence of
the pilot difference within the calculated stream of difference values. If pilot symbols of pilot words are
inserted by the transmhter 400 in consecutive symbol positions las in Fig. 3), the pilot word detector 516
monhors the difference between consecutive symbols. The process of searching for differentially encoded
pilot words is normally performed when the recenver 500 is initially placed in a receive mode, or when the
receiver 500 otherwise attempts to become synchronized with the transmitter 400.To distinguish the differentially encoded pilot words in the symbol stream from groups of data
symbols that are coincidentally separated by the same difference, the pilot word detector 516 monitors the
symbol stream over multiple pilot periods 210 (Fig. 3) before determining the periodic position of the pilot
- words. In the preferred embodiment, the pilot word detector 516 analyzes the symbol stream over 32 pilot
periods before determining the periodic position of the pilot words.
Once the pilot words have been detected, the pilot word detector 516 informs the pilot symbol
extractor 518 of the periodic location of the pilot words. The pilot symbol extractor 518 then periodically
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samples the symbol stream at the proper times to extract the pilot symbols of the differentially encoded pilot
words. The e1~tracted pilot symbols are provided to the channel estimator 520. The remaining symbols in
the symbol stNam are fed to the channel compensator 522~ These remaining symbols are in the form of
comple~ values that represent ~ , ' data symbols.
The channel estimator 520 compares the amplhudes and phases of the extracted pilot symbols whh
the e~pected amplhudes and phases, to thereby estimate the effects of the channel on the transmitted signal.
The channel estimator 520 preferably performs the comparison with each pilot symbol of a differentially
encoded pilot word, and then uses the resuhs of each comparison to calculate the average attenuation and
average phase impairment as the estimate. The estimate is recalculated every time a pilot word is received
li.e., every pilot period Z10). Estimates obtained over multiple pilot periods 210 are then interpolated or
otherwise fihered to generate, ' ', ' amplitude and phase adjustments to apply to the channel-
impaired data symbols. The channel estimator 520 outputs these ,. ' ' !, ~, ' ' phase and amplitude
estimates to the channel compensator 522 and the deinterleaver 530.
The channel compensator 522 uses the output of the channel estimator 520 to adjust or correct
the amplhudes and phases of ' '-~, ' data symbols. Each ' '-~, ~ ' data symbol is
compensated by adjusting the real and imaginary portions of the comple~ value that represents the channel-
impaired data symbol, using a channel estimate that was calculated for that data symbol. Compensated data
symbols are fed to the demodulator 524, which converts the compensated data symbols to their
corresponding binary values If or e~ample, the four bit values shown in Fig. 1 ) in accordance whh the specific
modulation technique used by the transmhter400. The demodulator 524 performs this function by matching
or assigning the compensated data symbols received from the channel compensator 522 with the constellation
symbols to which the compensated data symbols most closely correspond in value. Assuming that the data
stream was interleaved by the transmhter 400, the output of the demodulator 524 is fed to the deinterleaver
528 to return the binary values of the data stream to the original '~ sequential order.
If forward error correction encoding was performed by the transmhter 400, the deinterleaved data
is fed to the forward error correction decoder 534 to correct for errors. As is conventional in the art, the
forward error correction decoder 534 makes use of soft decision informatiDn generated by the channel
estimator 520 to improve the reliabilhy of the error correction process. The soft decision information is in
the form of the , ' ' b~ ~ ' ' channel estimates generated by the channel estimator 520, with the
estimates deinterleaved by the deinterleaver 530 so that the estimates are in a sequential order that
corresponds whh the data stream. These estimates are an indication of the reliability of the binary values
generated by the demodulator 524.
To further improve the effectiveness of the forward error correction process, the forward error
correction decoder 534 may also consider the values of the compensated data symbols that were used by
the demodulator 524 to generate the data stream. The demodulator 524 may retain these symbol values
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as h converts the compensated data symbols to binary values.
In a ' ~ embodiment of the transmitter 400 and recenver 500, the receiver 500 can
begin to receive and process data as soon as the differentially encoded pilot word detector 516 locates the
pilot words ~for example, after 32 pilot periods 210~. In ' ' embodiments, the receiver 500 must
buffer data for a certain amount of time before the deinterleaver 534 can deinterleave the stream of dighal
data.
As will be recognized by those skilled in the art, the transmitter 400 and receiver 500 can
'~ be designed to use the periodic pilot word positions as reference points for sending units of
data, thereby allowing the transmitter 400 and receiver 500 to communicate once the pilot words have been
located ~disregarding any delay caused by interleavingl. For example, the transmitter 40û can be designed
to place receiver address fields immediately after differentially encoded pilot words, so that receivers 500
can rmmediately begin to receive and decode addresses once the differentially encoded pilot words have been
located.
Fig. 6 is a functional block diagram of a preferred embodiment of the pilot word detector 516 of
Fig. 5. The pilot word detector 516 shown is in the general form of a conventional correlator or COMB
fiher, modified to operate on differences between consecutnve symbol values ~rather than operating on the
symbol values themsebesl. The pilot word detector 516 shown in Fig. 6 is designed for use with a symbol
stream that has a pilot period 21û of 12 symbols and which uses differentially encoded pilot words of two
1~; ' pilot symbols. A symbol stream of this format is illustrated in Fig. 3.
The pilot word detector 516 comprises a unit delay ~Dl element 602, a complex conjugate generator
604, a complex multiplier 606, a complex adder 618, twelve sequentially-connected unit delay elements 622A-
622L ~ . . ' to a pilot period of twelve symbolsl, and a twelve-input comparator 632. The unit delay
element 602, complex conjugate generator 604, and complex multiplier 606 form a differential detector 610.
The unh delay element 602 and the complex conjugate generator 604 are provided in series along
a muhi bh path 607, providing a first data path from the input 600 of the pilot word detector 516 to the
complex muhtiplier 606. A second muhi-bit path 608 is provided directly between the input 600 and the
complex multiplier 606. The output of the complex muhiplier 606 is provided as a first input to the complex
adder 618. The output of the complex adder 618 is provided as an input to the first unh delay element
622A of the string of twelve ~ ' unit delay elements 622A-622L. A multi-bit feedback
path 626 provides the output of the last unit delay element 622L of the string as the second input to the
complex adder 618. Multi-bh paths 628A-628L provide the outputs of the respective unit delay elements
622A-622L as inputs to the twelve-input comparator 632. The output of the comparator 632 is provided
on a muhti bn path 636 as the output of the pilot word detector 516.
The pilot word detector 516 shown in Fig. 6 is suhable for ~ ,' using standard
combinational and sequential logic components. However, as noted above, the pilot word detector 516 can
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ahernatrvely be implemented using a DSP chip under the tontrol of software.
The operation of the pilot word detector 516 will now be described. The differential detector 610
receives complex symbol values from the sampler 514 (Fig. S) at a rate of one symbol value per siynaling
interval T. Each complex symbol value corresponds to a , ~ symbol that may be either a data
S symbol or a pilot symbol.
Every signaling intenal T the dlfferential detector 610 outputs a complex value that represents the
complex drfference between two consecutive symbols li.e., the difference between two consecutive complex
symbol values). Each complex d'lfference is generated by mu~tiplying the symbol for the current signaling
interval T, with the complex conjugate of the symbol for the immediately preceding signaling interval T,,l.
The symbol for the current signaling interval TD jS provided to the complex muhiplier 606 along the direct
path 60û. The complex conjugate of the symbol from the preceding signaling interval To~ is provided along
the path 607, which includes the unit delay element 602 (which delays each symbol by one signaling interval
T) and the complex conjugate generator 604 ~which inverts the sign ot the imaginary portion of each symbol~.
This method of calculating the drfference between two complex values is known in the art, and produces a
complex numerical value that represents both the phase difference and the amplitude difference between two
symbols. However, alternative methods for generating a numerical,, of the difference could be
used, including methods that yield only the phase difference between two symbols.
The outputs of the unit delay elements 622A-622L are reset to zero whenever the pilot word
detector 516 inhiates a pilot word detection operation. Thus, for the first twelve signaling intervals T after
the pilot word detector 516 inhiates pilot word detection, the feedback path 626 provides values of zero to
the complex adder 618, and the complex ddference values generated by the differential detector 610 are
shlfted sequentially through the unh delay elements 622A-622L.
vrnh each successive heration of twelve signaling intervals, the complex adder 618 adds the
complex drfference values for the current heration with the corresponding complex difference values from the
previous herations. For example, on the second iteration, a complex difference calculated during a signaling
interval Tn will be added to the complex difference calculated during the signaling interval Tnl2, and a complex
difference calculated during a signaling interval T,~l will be added to the complex difference calculated during
the signaliny interval Tnll. Thus, the complex values stored by the unit delay elements 622A-622L represents
cumulative summatiùns of complex symbol differences for each of the twelve possible periodic positions
where the pilot word may be found.
As described above, the complex difference between the two pilot symbols of a differentially
encoded pilot word will be approxrmately the same each heration, since channel effects on differences
between consecutive symbols are typically negligible. Thus, the cumulative sum corresponding to the pilot
word position will grow in magnhude with each successive iteration. The other eleven cumulative sums will
fluctuate in magnhude whh successive iterations, assuming that data symbols are not transmitted in a
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repethive pattern from pilot period to pilot period. After a sufficient number N of iterations, the pilot word
poshion tan therefore be determined by comparing the magnhudes of the cumulative summation values. The
comparator 632 performs this function by comparing the magnhudes of the cumulative summation values
after N herations. In the preferred embodiment, the comparator 632 performs this comparison after N -
32 nerations, which has been found to produce a high probabibty of accurate, ' in a mobile,
cellular environment. A lower value for N will decrease the minimum ,. ' . ~ time for a receiver, but
will increase the probability that the comparison process will produce an erroneous ,. ' ~ result.
Once the symbol constellation for a system has been selected, the pilot symbols of pilot words are
preferably selected so as to maximize the pilot difference for the particular symbol conttellation. Maximizing
the pilot difference tends to increase the rate at which the cumulative summation value that represents the
pilot difference increases in magnhude whh successive herations, as is desirable for rapidly detecting the
location of the paot word. Illustratively for the symbol constellation shown in Fig. 1, the svmbols 104 and
106areseparatedbyadifferenceof~3+ j3)1-3 l j3)-I-9 9) ' l9 9b--18lusingthe ' .' "
method for calculating the difference). It can further be verified that no other pair of symbols in the
constellation of Fig. 1 produces a diflerence that is greater than 18 in magnitude. Thus, the symbols 104
and 106 are an optimum pair of pilot symbols for ~,. ' purposes.
The comparator 632 outputs a number that indicates the position (1-12) of the cumulative
summation value whh the greatest magnhude. This number corresponds to the periodic position of the
differentially encoded pilot word, and is used by the pilot symbol extractor 518 ~Fig. 5) to extract pilot
symbols of pilot words from the symbol stream. The extracted pilot symbols are then compared whh their
expected values ~i.e., their values prior to transmission over the wireless channel) to perform channel
estimation and compensation using techniques that are known in the art.
The operation of the pilot word detector 516 is further illustrated by Figs. 7A-7C, which are
graphical ., of example cumulative summation values after 1, 2, and N iterations respectively.
The twelve magnhude values in each of Figs. 7A-7C represent the magnhudes of complex values stored by
the unh delay elements 622A-622L of Fig. 6.
Referring to Fig. 7A, after one neration ~i.e., twelve signaling intervals T), the twelve difference
values vary in magnitude, and the poshion of the pilot word (position 1n) cannot be reliably determined since
the magnitudes of one or more other difference values are greater than or approximately equal to the
magnitude of the pilot difference. Referring to Fig. 7B, the magnitude of the cumulative summation value
(or cumulative dlfference) corresponding to position 10 begins to stand out after the second heration as the
result of the constructive addnion of like pilot difference values. However, additional iterations are desirable
to minimize the effects of the channel, and to ddferentiate between the periodic pilot differences and like
dlfferences that may coincidentally occur in the symbol stream. Referring to Fig. 7C, after 11 iterations
(wherein N preferabiy equa(s 32) the poshion of the pilot word can be readi(y ascertained by inspection of
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the relatrve magnhudes of the twelve cumulative summation values. It is noted that the pilot word detettor
516 determines the periodic pilot word pos'rtion by this method whhout knowing the pilot difference used
the transmhter 400 to encode pilot words.
To further increase reliability, the comparator 632 may include logic to detect cumulative summation
values that are close in value after N iterations, to thereby detect resuhs that have a high probabilhy of
error. Addhional herations can then be performed to ensure reliable pilot word detection.
As will be apparent to those skilled in the art, various modifications to the symbol stream format
used by the system can be made whhout departing from the spir'rt of the invention. By way of example,
the transmhter could be designed to transmit a differentially encoded pilot word every nth pilot period (for
example, every third pilot period), and to transmit only a single pilot symbol during pilot periods for which
no drfferentially encoded pilot word is transmitted. Such a modification to the symbol stream would desirably
reduce the bandwidth occupied by pilot symbok, but would increase the average, ' ' time.
While various embodiments of the system and method of the ,oresent invention have been described,
h should be understood that these embodiments have been presented by way of example only, and are not
intended to Irmh the scope of the present invention. Thus, the breadth and scope of the present invention
should be defined only in accordance with the following claims and their equnvalents.
