Method and apparatus for modifying amplitude of at least one symbol

In a modulator (300), a symbol weight determiner (301) determines a scaling factor (310) for at least one symbol (308). An integrator (303) and a complex symbol generator (305) operate to produce at least one complex symbol (314) based on the at least one symbol. The scaling factor (310) is used to scale the at least one complex symbol. In one embodiment, a scaler (307) comprises a gain-modifiable amplifier (609) and, in a second embodiment, the scaler comprises a gain-modifiable filter (809). When such a modulator is incorporated into an RF communication device (600, 800), the energy used to transmit any given symbol can be varied according to the relative importance of that symbol. Additionally, the overall received sensitivity of a communication, at a given signal strength, can be improved.

FIELD OF THE INVENTION 
The present invention relates generally to wireless communications and, in 
particular, to a method and apparatus for modifying symbols. 
BACKGROUND OF THE INVENTION 
The use of symbols in wireless communication systems is generally known. 
Typically, a symbol is a representation of one or more binary bits and is 
used to modulate a parameter of a wireless communication resource, such as 
a radio frequency (RF) carrier. Thus, digitally represented information is 
used to create a stream of symbols which are then transmitted to a 
receiver. Upon recovery of the symbols at the receiver, the digital 
information can be reconstructed. This allows more efficient use of 
available communication resources in wireless communication systems. 
For example, in systems conforming to requirements recently promulgated by 
the Association of Public Safety Communication Officers (APCO) such 
symbols cause, and are therefore represented by, phase shifts in a carrier 
signal. More precisely stated, the APCO 25 Common Air Interface (CAI) 
specifies a version of .pi./4-quadrature phase shift keying (.pi./4-QPSK) 
as the required modulation type. A modulator 100, as specified by the APCO 
25 CAI, is shown in FIG. 1. 
At the front end of the modulator 100, symbols 106 are input to an 
integrator 101. The symbols 106 are real-valued representations of one or 
more binary bits. The integrator 101 produces phase representations 108 of 
the symbols 106. A complex symbol generator 103 modulates the phase 
representations 108 into complex symbols 110. The resulting complex 
symbols 110 are then processed by a splatter filter 105 to produce 
splatter-filtered complex symbols 112. (The operations of the integrator 
101, complex symbol generator 103, and splatter filter 105 are well known 
in the art and will not be discussed further.) In practice, the 
splatter-filtered complex symbols 112 are then transmitted via a suitable 
device, such as an RF transmitter. 
The modulator 100 of FIG. 1 results in a constellation pattern 200 as shown 
in FIG. 2 In particular, the constellation pattern 200 (shown within a 
complex I-Q plane) results when the splatter-filtered complex symbols 112 
are transmitted. Each splatter-filtered complex symbol 112 transmitted 
results in a phase shift between constellations points (represented by 
alternating x's and o's). Each of the constellation points lies along a 
single level of constant energy (shown as a circular dotted line) as 
determined by a radius 201. Using this method, the average transmitted 
power will approximately remain the same, regardless of the symbols being 
transmitted. 
In particular, symbols that may be relatively unimportant to a given 
communication (e.g., symbols related to encryption synchronization in an 
unencrypted communication) are still transmitted with the same amount of 
energy as other symbols more crucial to the overall success of the 
communication. Given that most portable communication units (e.g., a 
hand-held portable radio) have a limited battery life, the unnecessary use 
of excess power is to be avoided. 
In addition to the above, it is well known in the art that each symbol or 
block of symbols does not have the same importance as some other symbol or 
block or symbols. A method to equalize this disparity is to apply 
different error correction methods to improve the sensitivity of different 
symbols. Differently encoded symbols will likely have dissimilar 
probabilities of being successfully decoded by a receiver for a given bit 
error rate (BER). 
For example, in a land mobile radio environment, certain functions (e.g., 
audio unsquelch) depend upon a sequence of successful decodes (in the 
sense of no uncorrectable bit errors) of specific symbols. As a result, 
the overall sensitivity of a given communication may not be better than 
the sensitivity of any one portion of the communication. That is, while 
other symbols important to a communication (e.g., symbols related to 
vocoder parameters in a voice message) are successfully decoded, those 
symbols required to allow the communication to proceed at the receiver 
(i.e., audio unsquelch) may not be successfully decoded, thereby 
preventing the successful completion of the communication in the first 
place. 
In light of the above, it would be advantageous to provide a technique that 
allows the reliability of various symbols or blocks of symbols to be 
varied such that the overall sensitivity for a communication is maximized 
for a given signal strength. Additionally, such a technique should allow 
the energy used to transmit symbols to vary according to the relative 
importance of each symbol to a communication such that power conservation 
can be improved, if necessary.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention provides an apparatus and method for modifying at 
least one symbol based in part upon its relative importance to a given 
communication. A modulator comprising a symbol weight determiner, an 
integrator, a complex symbol generator, and a scaler is provided. Such a 
modulator can be incorporated into an RF communication device that 
utilizes phase-modulation techniques. In operation, the symbol weight 
determiner determines a scaling factor for at least one symbol based on a 
symbol weighting function. The integrator produces a phase representation 
of the at least one symbol, and the complex symbol generator produces at 
least one complex symbol corresponding to the phase representation of the 
at least one symbol. The resulting at least one complex symbol is scaled 
by the scaler according to the previously determined scaling factor. In 
one embodiment, the scaler comprises a gain-modifiable amplifier and, in a 
second embodiment, the scaler comprises a gain-modifiable filter. When 
such a modulator is incorporated into an RF communication device, the 
energy used to transmit any given symbol can be varied according to the 
relative importance of that symbol. Additionally, the overall received 
sensitivity of a communication, at a given signal strength, can be 
improved. 
The present invention can be more readily described with reference to FIGS. 
3-9. FIGS. 3 and 4 illustrate a general embodiment of the present 
invention. FIG. 3 is a block diagram of a modulator 300 for modifying at 
least one symbol, and FIG. 4 is a flow chart of a method for doing the 
same. In practice, the modulator 300 and its constituent components can be 
implemented using hardware devices, although a software-based 
implementation (i.e., algorithms stored as instructions in 
machine-readable memory for execution by a processing device, such as a 
microprocessor) is preferred. At step 401, a symbol weight determiner 301 
determines, based on a symbol weighting function, a scaling factor 310 for 
at least one symbol 308. The at least one symbol 308 is a real-valued 
representation of one or more binary bits. As discussed in greater detail 
below, the symbol weighting function can be any decision function that 
incorporates objective or subjective determinations of the relative 
importance of various symbols to be transmitted. 
At step 402, an integrator 303 produces a phase representation of the at 
least one symbol 312 using well known techniques. Generally, the phase 
representation of the at least one symbol 312 comprises a real-valued 
scalar in the range from 0-2.pi. radians. At step 403, a complex symbol 
generator 305 produces at least one complex symbol 314 based on the phase 
representation of the at least one symbol 312. The operation of the 
integrator 303 and the complex symbol generator 305 are well known in the 
art. 
At step 404, a scaler 307 is used to scale the at least one complex symbol 
314 according to the scaling factor 310. In a first embodiment of the 
present invention, the scaler 307 comprises a gain-modifiable amplifier. 
In a second embodiment of the present invention, the scaler 307 comprises 
a gain-modifiable filter. Regardless of the particular implementation 
used, the resulting at least one scaled complex symbol 316 reflects the 
relative importance of its corresponding at least one symbol 308. (As 
discussed below, the at least one scaled complex symbol 316 will also be 
splatter filtered if the second embodiment is used.) A constellation 
pattern 500 corresponding to the at least one scaled complex symbol 316 is 
depicted in FIG. 5. 
The constellation pattern 500 comprises two levels of constant energy 
(shown as circular dotted lines), as determined by a first radius 501 and 
a second radius 502. Although two levels of constant energy are shown, any 
constellation point (represented by alternating x's and o's) at a given 
phase angle can lie along only one of the two levels of energy at any 
given time. That is, although there appear to be 16 constellation points 
shown, FIG. 5 actually depicts the two possible energy levels that each of 
the 8 constellation points can occupy. 
For example, consider the constellation point corresponding to a positive Q 
component and zero I component. As shown, the constellation point can 
occupy either the first level of energy 504 or the second level of energy 
505, but not both. Whether a particular symbol results in a phase 
transition ending on either the first or second energy level is determined 
by the scaling factor for that symbol. The present invention anticipates 
that the first radius 501 and the second radius 502 can be chosen so as to 
maintain, decrease, or even increase the average transmitted power 
relative to another constellation pattern, e.g., constellation pattern 200 
of FIG. 2. 
Furthermore, it is anticipated that more than two levels of constant energy 
can be used. 
FIG. 6 is a block diagram of an RF communication device 600 that 
incorporates a first embodiment of a modulator. In particular, the RF 
communication device 600 comprises a symbol source 601, a symbol weight 
determiner 603, an integrator 605, a complex symbol generator 607, a 
gain-modifiable amplifier 609, a splatter filter 611, and an RF 
transmitter 613 operably coupled as shown. The first embodiment of the 
modulator encompasses the symbol weight determiner 603, the integrator 
605, the complex symbol generator 607, the gain-modifiable amplifier 609 
and, in practice, the splatter filter 611. Operation of the RF 
communication device 600 is further described with reference to FIG. 7. 
At step 701, the symbol source 601 provides at least one symbol 614. 
Typically, the symbol source 601 comprises any of a number of hardware 
devices or software processes that cause symbols to be generated. For 
example, a voice compression algorithm (commonly referred to as a vocoder) 
can create a stream of digitally represented parameters (later converted 
to symbols) corresponding to voice signal input. Alternatively, a high 
level software routine may create symbols representing required encryption 
synchronization information, which symbols are periodically interleaved 
with the other information to be transmitted. In practice, the symbol 
source 601 provides a constant stream of symbols so long as the RF 
communication device 600 is in the process of transmitting. 
Furthermore, such a constant stream of symbols will typically follow a 
repetitive pattern of symbols. That is, symbols representing various types 
of information will always be periodically sent, regardless of the 
operating mode of the RF communication device 600. For example, the APCO 
25 CAI standard specifies two different link data units (LDUs), each of 
which is a unique frame format. Each is sent alternately, in the form of a 
stream of symbols, during any given transmission. Between the two types of 
LDUs, all fields necessary to implement all possible system features are 
represented. However, it is rare that all fields in all transmitted LDUs 
contain information crucial to the successful completion of a particular 
communication. 
At step 702, the symbol weight determiner 603 determines a scaling factor, 
in the form of a scalar gain value 616, for the at least one symbol 614 
based on a symbol weighting function. As mentioned above, the symbol 
weighting function can be any decision function that incorporates 
objective or subjective determinations of the relative importance of 
various symbols to be transmitted. For example, an objective determination 
of relative importance would be the operating mode of the RF communication 
device 600. Thus, the relative importance of various symbols can be 
predicated on whether the RF communication device 600 is in a conventional 
or trunking mode, an encrypted or non-encrypted mode, or even if the RF 
communication device 600 is a mobile/portable radio or a fixed base 
station. An example of a subjective determination is perceptual weighting 
of various parameters (i.e., how important a given parameter is in the 
reconstruction of compressed voice signals) for a given vocoder, or even 
between different vocoders. 
Additionally, the symbol weighting function takes into account the type of 
symbol being transmitted. For example, in a given operating mode, an 
encryption-related symbol could be of different importance relative to a 
trunking-related symbol. In one embodiment, the symbol weighting function 
could be implemented as a lookup table, indexed by the current operating 
mode. The lookup table would be further sub-indexed by the type of the at 
least one symbol 614 being sent, wherein the lookup table lists the 
appropriate scalar gain values for any given symbol. 
Furthermore, the symbol weighting function can take into account a value of 
the at least one symbol or its value relative to a predetermined sequence 
of prior and subsequent symbols. Thus, the symbol weighting function 
provides a potentially different scalar gain value for each possible value 
for the at least one symbol as indexed above. Furthermore, by tracking the 
values of symbols both prior and subsequent to the at least one symbol, 
the symbol weighting function could detect a predetermined sequence of 
symbols and, when detected, supply various scalar gain values for the at 
least one symbol. Although several objective and subjective determinations 
and symbol types have been given as examples, the present invention is 
equally applicable to a large variety of criteria, depending upon the 
particular application. 
At step 703, the integrator 605 produces a phase representation of the at 
least one symbol 618. In essence, the phase representation of the at least 
one symbol 618 is representative of the difference between the at least 
one symbol 614 and a previous symbol, as known in the art. 
At step 704, the complex symbol generator 607 produces at least one complex 
symbol 620 based on the phase representation of the at least one symbol 
618. The at least one complex symbol 620 is a typically a representation 
of the at least one symbol 614 as in-phase (I) and quadrature (Q) 
components, as known in the art. 
At step 705, a gain-modifiable amplifier 609 scales the at least one 
complex symbol 620 according to the scalar gain value 616. The 
gain-modifiable amplifier 609 may comprise a physical amplifier device, of 
the sort well known in the art, in which case that scalar gain value 616 
is a control voltage that determines the gain value of the amplifier 
device. Alternatively, the gain-modifiable amplifier 609 may comprise a 
complex multiplier function implemented in software, in which case the 
scalar gain value 616 is a multiplicand. Regardless of the implementation, 
the resulting at least one scaled complex symbol 622 reflects the relative 
weight given to the at least one symbol 614 according to the symbol 
weighting function. 
At step 706, the splatter filter 611 filters the at least one scaled 
complex symbol 622. In practice, the splatter filter may be implemented in 
either hardware, as a filter network, or as a software algorithm, as known 
in the art. Although not a requirement for the physical operation of the 
RF communication device 600--the scaled complex symbol could be 
transmitted without further filtering--the splatter filter 611 is 
typically used to limit adjacent-channel interference. It has been 
demonstrated that the present invention exhibits substantially identical 
spectral performance as prior art techniques (assuming the same average 
transmit power). 
At step 707, the at least one splatter-filtered scaled complex symbol 624 
produced by the splatter filter 611 is sent to the RF transmitter 613 
which, in turn, modulates an RF carrier 626 based on the at least one 
splatter-filtered scaled complex symbol 624. In the preferred embodiment, 
the RF transmitter 613 produces phase variations in the RF carrier 626 
based on the at least one splatter-filtered scaled complex symbol 624. The 
energy of the RF carrier 626 will reflect the scaling inherent to the at 
least one splatter-filtered scaled complex symbol 624. That is, if the at 
least one splatter-filtered scaled complex symbol 624 reflects a decrease 
in amplitude relative to other symbols, the power used to transmit it will 
similarly be decreased. In this manner, power conservation could be 
achieved, for example, in a portable radio having a limited battery life. 
At the other extreme, if the at least one splatter-filtered scaled complex 
symbol 624 reflects an increase in amplitude relative to other symbols, 
the power used to transmit it will be similarly increased. In this manner, 
the received reliability of the at least one splatter-filtered scaled 
complex symbol 624 can be improved. Additionally, if it is desired to 
maintain the average transmitted power at the same level as would occur 
without the use of the present invention (i.e., the modulator and 
constellation pattern of FIGS. 1 and 2, respectively), the energy saved 
from decreased amplitude symbols could be applied to increased amplitude 
symbols, thereby improving overall sensitivity. Overall sensitivity would 
be improved in the sense that the most important symbols will have 
improved reliability, and the decreased reliability of the less important 
symbols would likely not affect the successful completion of the message. 
It is noted that the present invention, because it alters the amplitude of 
the RF carrier 626, is not applicable to RF communication devices 
utilizing amplitude-modulation techniques. 
FIG. 8 is a block diagram of an RF communication device 800 that 
incorporates a second embodiment of a modulator. In particular, the RF 
communication device 800 comprises a symbol source 801, a symbol weight 
determiner 803, an integrator 805, a complex symbol generator 807, a 
gain-modifiable splatter filter 809, and an RF transmitter 811 operably 
coupled as shown. The difference between the embodiment shown in FIG. 6 
and that shown in FIG. 8 is the effective combination of the 
gain-modifiable amplifier 609 and splatter filter 611 as the 
gain-modifiable splatter filter 809. In all other respects, the symbol 
source 801, the symbol weight determiner 803, the integrator 805, the 
complex symbol generator 807, and the RF transmitter 811 are functionally 
equivalent to their counterparts presented with respect to FIGS. 6 and 7. 
Operation of the RF communication device 800 is further described with 
reference to FIG. 9. 
Because the RF communication device 800 is structurally similar to that 
shown in FIG. 6, steps 901 through 904 are functionally equivalent to 
steps 701 through 704, discussed above. However, at step 905, the 
gain-modifiable splatter filter 809 both splatter filters the at least one 
complex symbol 820 and effects a gain change to the at least one complex 
symbol 820 based on the scalar gain value 816. In this embodiment, the 
gain-modifiable splatter filter 809 is a linear function. As a result of 
this linearity, the scalar gain value 816 can be applied to the at least 
one complex symbol 820 prior to actual processing by the filter (this 
would be functionally equivalent to the embodiment described in FIG. 6), 
or the scalar gain value 816 can be incorporated into the filter itself, 
modifying filter taps as the at least one complex symbol 820 progresses 
through the filter. 
At step 906, the resulting at least one splatter-filtered scaled complex 
symbol 824 is then sent to the RF transmitter 811. Operation of the RF 
transmitter, and the effect upon the RF carrier 826, is equivalent to that 
described above with respect to FIGS. 6 and 7. The benefits achieved by 
the embodiment of FIG. 8 is equivalent to those achieved by the embodiment 
of FIG. 6. 
The present invention provides an apparatus and method for modifying at 
least one symbol based in part upon its relative importance to a given 
communication. Prior art techniques restricted the transmission of all 
symbols to a single level of energy. By scaling symbols in accordance with 
their relative importance to a given message, as determined by a symbol 
weighting function, the present invention allows the average energy used 
to transmit the symbols to be increased, decreased, or maintained. 
Additionally, by allocating energy from relatively unimportant (but 
otherwise required) symbols to relatively important symbols, the overall 
sensitivity of a communication can be improved for a given average power. 
For example, if a lower gain is applied to low priority symbols and a 
higher gain is applied to high priority symbols, the overall sensitivity 
of the message is improved to the extent that the reliability of those 
symbols crucially related to the successful completion of the message is 
improved.