Radio with peak power and bandwidth efficient modulation

A communication device (300) includes a digital modulator (301), a Digital Signal Processor (306), and an amplifier (312). The digital modulator (301) includes an information generator (304) and a peak suppression device (402). The peak suppression device (402) includes a symbol mapper (404) and a symbol scaler (406). The information generated by the generator (401) are mapped on a constellation diagram via the symbol mapper (404). The mapped information is then scaled at the scaler (406) in order to reduce the peak-to-average ratio of the signal at the input to the amplifier (312).

TECHNICAL FIELD OF THE INVENTION 
This invention is generally related to communication devices and more 
particularly to communication devices with efficient modulation. 
BACKGROUND OF THE INVENTION 
In one dimensional digital communication systems the transmitted waveform 
is formed by adding time-shifted versions of a basic pulse shape. The 
amplitude of this pulse is adjusted according to the data being sent (e.g. 
binary phase shift keyed). In multi-dimensional digital communication 
systems (e.g. Quadrature Amplitude Modulated) multiple pulse streams are 
generated according to the data. To minimize the bandwidth of the 
transmitted waveform and thereby secure that the transmitted waveform does 
not interfere with other systems operating in a nearby (frequency) 
channel, the pulse shape used must have a time duration which spans 
several symbol intervals. That is the pulse associated with one data 
symbol will overlap pulses associated with adjacent data symbols. Certain 
data sequences will cause these overlapping pulses to add constructively 
producing large peaks in the transmitted waveform, while other data 
sequences will cause these overlapping pulses to cancel one another 
producing small values of the transmitted waveform. Amplifiers that are 
used to boost the power of the transmitted signal just prior to 
transmission work best when the signal remains at a fairly constant level. 
Large peaks in the transmitted signal lead to inefficient usage of the 
power amplifier which in turns wastes precious battery life. 
Battery operated communication devices employ a variety of techniques to 
save battery energy in order to prolong the operating life of the battery. 
Increasing the efficiency of power amplifiers is one technique that 
designers utilize to prolong the operating life of a communication device. 
Another scheme by which battery energy may be saved is the use of another 
power-efficient modulation technique. Various modulation techniques have 
different associated peak-to-average power ratios. In general, it is 
highly desirable to have a peak-to-average ratio as close to zero dB as 
possible. However, many existing modulation formats result in relatively 
high peak-to-average power ratios. Two commonly used modulation formats 
are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). 
The former uses a signal constellation where all data symbols have the 
same magnitude while the latter varies both the phase and magnitude of the 
individual data symbols. Binary signaling is a special case of PSK (i.e. 
BPSK). In both modulation formats, the peak-to-average ratio depends upon 
the pulse shape used. 
Quadrature Amplitude Modulation (QAM) utilizes both the phase and amplitude 
of a carrier to transmit information and hence has the potential to 
generate a higher peak-to-average power ratio. Indeed, experiments have 
demonstrated that, for example, a sixteen symbol PSK constellation enjoys 
a 3-4 dB improvement in peak-to-average power ratio over a 16 QAM signal. 
However, this gain in efficiency improvement is accompanied with a 4 dB 
loss in sensitivity. Due to this loss of sensitivity, many system 
designers prefer to use the QAM modulation format despite its degraded 
peak-to-average power ratio. 
Referring to FIG. 1, a communication device is shown as is presently 
available. FIG. 2 shows a phase and magnitude trajectory of a complex 
baseband 8 PSK signal. In other words, this figure represents the 
transition from one symbol to the next as the generated data changes 
state. A filter that is used to limit the sideband noise produces 
undesirable overshoot as shown by reference 202. This overshoot 202 
contributes to an increase in peak power which results in an increase in 
the peak-to-average power ratio. This increase in the peak-to-average 
power ratio forces a designer to design an amplifier that can tolerate the 
maximum peak power which in turn renders the power amplifier more 
expensive to produce. In addition, the increase in peak-to-average ratio 
reduces the power efficiency of the power amplifier. 
In the design of portable communication devices, the aim of a designer is 
to utilize efficient components at the lowest possible price. Power 
amplifiers have traditionally been some of the most expensive components 
of a communication device and have often resisted attempts aimed at 
lowering their cost. One parameter that is directly related to the cost of 
amplifiers is the peak-to-average power ratio. This is because the 
designer is forced to employ an amplifier that can handle peak powers 
significantly larger than the average power. It has therefore been the 
goal of designers to reduce peak-to-average power ratios as much as 
possible without degrading other performance parameters. There is 
therefore a need for a modulation scheme that would have minimum 
peak-to-average power ratio without suffering other performance 
degradation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 3, relevant components of a communication device 300 in 
accordance with the present invention are shown. A microphone 302 produces 
an analog signal which is coupled to a vo-coder 304 where it is converted 
to a digital signal. The vo-coder 304 generates a digital information 
signal and applies it to a Digital Signal Processor (DSP) 306. The 
combination of the vo-coder 304 and the DSP 306 form a digital modulator 
301. The DSP 306 manipulates this digital information signal in accordance 
with the principles of the present invention. In addition to making peak 
and instantaneous power measurements, keeping track of the time that such 
peaks occur and combining the I and Q components; which methods are known 
in the art, the DSP 306 performs signal scaling. More details of the 
operation of the DSP 306 will be discussed in association with FIG. 4. The 
processed signal at the output of the DSP 306 is coupled to a digital to 
analog converter 308 where the signal is converted back to analog before 
being applied to an RF mixer 310. This mixer 310, which could be a 
quadrature mixer, mixes the analog signal with a locally generated 
oscillator signal (LO). The output of the mixer is coupled to an amplifier 
312 which amplifies the mixed signal before it is transmitted via an 
antenna 314. 
Referring to FIG. 4, essential elements of the DSP 306 in accordance with 
the present invention are shown. In essence, a random binary data 
generator 401 is shown coupled to a peak suppression algorithm 402. The 
generator 401 may be any source of digital data such as the vo-coder 304. 
The peak suppression algorithm includes a symbol mapping section 404 and a 
symbol scaling section 406. The digital information generated at 401 is 
mapped onto a constellation diagram 404 to produce data symbols each 
having a symbol interval and an onset. These data symbols are represented 
via vectors 405 each having I and Q signal components. In other words, the 
data symbols are represented by vector components with orthogonal 
relationships. It is noted that the peak suppression algorithm can also 
operate on one-dimensional signals (e.g. BPSK). The I and Q signal 
components collectively represent the magnitude and the phase of the 
vector 405. Each vector represents a symbol interval whose content is 
determined by the number of bits that are processed at each instant of 
time. Indeed, the data symbols are processed at a rate which separates 
them at their respective onset via a symbol interval. For instance, in a 
three bit system, the vector 405 represents three bits with eight distinct 
possibilities. In a four bit system, a vector represents four bits and the 
signal constellation has sixteen symbol locations thereon. In the 
preferred embodiment and in order to facilitate the understanding of the 
principles of the present invention, a three bit symbol interval is 
assumed. 
Once the symbols have been mapped a symbol scaling process is embarked 
upon. As part of this process the magnitudes of the I and Q components are 
altered according to an algorithm that would minimize overshoot at the 
subsequent filtering step. This step is performed via a pulse shape filter 
408. The aim of this filter is to reduce high frequency components of the 
symbols before they are transmitted. Due to its characteristics, however, 
this filter tends to produce signal peaks during transitions from one 
symbol to another. These signal peaks translate into additional peak power 
demanded from the amplifier 312. The magnitude of these peaks depends on 
both the sequence of the symbols and the filter characteristics. The 
present invention seeks to adjust or scale these vectors (i.e. 405) in 
such a fashion as to compensate or reduce the signal peaks. This 
compensation relieves the amplifier from having to operate at unnecessary 
peaks while maintaining system integrity. 
The scaling of the data symbols may be implemented solely in the magnitude 
or both magnitude and phase. In other words, the amplitude of the I and Q 
components may be altered in such a way to maintain the phase of the 
vector 405 constant. Alternatively, the amplitude of the I and Q 
components may be altered independently thereby resulting in changes to 
both the magnitude and the phase of the vector 405. 
The magnitude of the unscaled symbols on FIG. 5 are shown via the dotted 
line circle 502. This circle represents the magnitude of the symbols as 
they are generated by the random binary data generator 401 and symbol 
mapper 404. Ideally, the amplifier 312 will have to amplify these constant 
magnitude signals. But due to the pulse shape filter 408 these signal 
magnitudes are increased to the point where the concentric circle 504 is 
formed at the output of the pulse shape filter. This external circle 504 
shows the extent of the overhead placed on the amplifier 312. Indeed, the 
diametric distance between the two concentric circles 502 and 504 
represents the magnitude difference between unfiltered and filtered 
symbols. This difference is directly translated in undesirable peak power. 
The scaling of the symbols amounts to a shrinking of the diameter of this 
circle hence a lower peak demand on the amplifier 312. 
The scaling algorithm looks at the sequence of symbols and determines the 
alteration needed on each of the symbols as they are generated by the data 
generator 401. The algorithm utilizes the filter characteristics during 
this determination. FIG. 5 shows a phase and magnitude trajectory of 
several symbols after they have been scaled. Unscaled symbols are 
represented by 506 while their scaled counterparts are shown via 508. In 
this example we assume that five symbols are transmitted. The first symbol 
501 is unaltered for there is no peak generated. The next symbol is scaled 
radially downward to prevent the signal peak which would normally result 
due to the interaction between the time delayed filtered symbols. The 
third symbol is similarly scaled down to avoid a signal peak magnitude. 
The fourth is similarly scaled down. The fifth symbol is scaled up due to 
the small signal magnitude that occurs during the transition from the 
fourth symbol thereto. The symbol scaling is accomplished in a manner that 
maintains the symbol integrity and prevent the loss of information. 
The peak suppression algorithm determines the instantaneous power of the 
baseband signal during each symbol interval. The scaling of the signal 
will directly follow from the determination of the peak power and its time 
location on the symbol interval. Under these circumstances, the average 
power associated with the baseband signal is also determined. With the 
peak power information available the algorithm determines the time at 
which the peak power of the composite baseband signal occurs. Next, the I 
and Q components of the symbol associated with adjacent symbol intervals 
are altered. The amplitude of these components may be radially scaled 
equally at which time only the magnitude of the composite signal is 
varied. Independent and unequal scaling of the I and Q signal is also 
possible which would result in scaling of the phase and magnitude of the 
composite signal. 
In summary, digital data symbols generated by the vo-coder 304, and the 
symbol mapper 404 are processed through the peak suppression algorithm 402 
in order to take benefit of the principles of the present invention. Data 
symbols generated as the result of this mapping are represented via their 
I (In phase) and Q (Quadrature) components. The I and Q components are 
then dynamically scaled via the symbol scaling portion 406 of the peak 
suppression algorithm block 402. The scaling of the I and Q components is 
in anticipation of the filtering action that takes place via the pulse 
shape filter 408. The symbol scaling simply keeps track of the magnitude 
and phase trajectory of the baseband signal (constituted by the I and Q 
components). As was discussed, the problem with the prior art is that the 
pulse shape filter produces signal peaks during symbol transitions. The 
present invention provides for a method to minimize this peak signal 
problem. By scaling the I and Q components of the data symbols, the 
present invention aims at minimizing the magnitude of the signal peaks, 
hence reducing the peak power demand on the amplifier 312. 
The algorithm used in the preferred embodiment, accepts data symbols that 
have been produced by the constellation mapper 404, processes the symbols 
and outputs them to the pulse shape filter 408. Specifically, the 
algorithm sequentially loads the data symbols into an input data block for 
iterative processing. Following the completion of processing, the input 
data block is copied into an output data block and the scaled symbols are 
sequentially output to the pulse shape filter 408. To maintain a constant 
symbol rate, newly arriving data symbols are shifted into the vacated 
input data block while the scaled symbols are being shifted out of the 
output block. Hence, if the processing time is assumed to be 
insignificant, the transmission delay created by the algorithm is 
approximately equal to (block size)/(symbol rate) seconds. The block size 
must be large enough to guarantee that the symbols within the block 
accurately represent the statistical characteristics of the total 
transmitted data symbol sequence. 
Upon successful population of the input data symbol block, the algorithm 
proceeds to determine several values for each symbol interval defined by 
the input symbol block. These values are: (1) the peak transmitted signal 
magnitude, (2) the time location of the peak, and (3) the peak scale 
factor for the peak magnitude. The algorithm determines these values on a 
symbol interval by applying the pulse shape filter function to the 
appropriate data symbols. The number of data symbols utilized to calculate 
the signal over a particular symbol interval depends upon the impulse 
response of the pulse shape filter function. All symbols that combine with 
the pulse shape to produce significant signal magnitude within the symbol 
interval of interest must be included in these calculations. The impulse 
response of the pulse shape filter 408 also determines how much symbol 
overlap there must be between successive symbol blocks. 
The algorithm utilizes the peak transmitted signal magnitude on a 
particular symbol interval to determine the peak scale factor for that 
interval. A peak scale function is applied to the peak signal value. The 
peak scale function is defined such that it produces a negative peak scale 
factor if the peak magnitude is greater than some reference value and a 
positive scale factor if it is less than the reference value. The 
magnitude of this scale factor increases with the difference between the 
peak magnitude and the reference value. The reference value is usually set 
equal to the desired peak magnitude. The algorithm stores the peak scale 
factor and the corresponding peak time location for each symbol interval 
in two separate vectors. These values will be subsequently utilized to 
determine the symbol scale factor for the symbols in the block. 
Following the successful determination of the peak scale factors and their 
associated time locations, the algorithm calculates the symbol scale 
factor for each of the data symbols. To determine a particular symbol 
scale factor the algorithm utilizes the peak information from the two 
symbol intervals that are immediately adjacent to a particular symbol. 
These two intervals will be referred to as the left-hand and right-hand 
intervals. The symbol scale function weights the left-hand peak scale 
factor by the relative time distance that the peak is located from the 
particular symbol. Likewise, the right-hand peak scale factor is weighted 
by the relative distance that it is from the particular symbol. The two 
weighted scale factors are than summed together with a unity value to 
determine the symbol scale factor. In this manner, signal peaks that are 
located close to a particular symbol have a greater impact on the scale 
factor for that symbol. 
After each of the symbol scale factors have been determined, the algorithm 
normalizes the symbol scale factors to maintain the desired average power. 
Assuming that the pulse shape has unity average energy and that the 
individual symbols are independent and identically distributed, the 
average power (Ps), is calculated by simply averaging the squared scaled 
symbol magnitudes. The desired average power is usually equal to the 
average power of the unscaled transmitted signal (Pu). Hence, the 
normalizing factor is equal to Sqrt(Pu/Ps). In the case of a circular PSK 
constellation of unity symbol magnitude Ps is simply equal to the average 
of the symbol scale factors. 
The algorithm repeats the symbol processing steps described above for a 
specified number of iterations or until some target peak-to-average power 
ratio is attained. After one of these conditions has been met, the 
algorithm scales the data symbols by the appropriate final symbol scale 
factors and copies the scaled symbols to the output block. The algorithm 
then proceeds to sequentially output the scaled symbols to the pulse shape 
filter while simultaneously loading the input block with the new, unscaled 
symbols from the constellation mapper. 
In an alternative embodiment, the peak suppression algorithm produces an 
imaginary sphere around each data symbol in order to create a boundary for 
scaling them. This spherical boundary helps in establishing limits for 
phase and magnitude movement and scaling. Once again this scaling helps in 
minimizing the peak power requirement on the amplifier 312. 
Simply stated, the scaling algorithm looks at the phase and magnitude of 
symbols as they are generated by the vo-coder 304 and symbol mapper 404 
and estimates the magnitude of the signal peaks (extent of overshoot) that 
will be at the output of the filter 408. This estimate of the signal peaks 
is considered in determining the level and direction of scaling that must 
be implemented on each symbol. In so doing, the I and Q components are 
presented to the filter 408 with sufficient compensation to minimize the 
effect of the unavoidable signal peaks. This compensation minimizes the 
peak power requirement on the amplifier 312. It is appreciated that 
without the benefit of the present invention, the amplifier 312 must be 
able to handle the peak power demands as represented by circle 504. This 
additional requirement greatly increases the cost of the amplifier 312. 
The increase in the peak power vis-a-vis the average power adversely 
affects the efficiency of the amplifier 312. Portable radio devices are 
particularly at a disadvantage in view of this additional degradation in 
efficiency. 
The principles of the present invention provide a general method to 
suppress peaks in the transmitted waveform before it is amplified. The 
magnitude of a data symbol is slightly adjusted according to the values of 
the neighboring symbols and the pulse shape filter response. The result is 
a transmitted waveform which retains a much more constant magnitude level. 
The algorithm works on a block of data (usually about 50 to 500 symbols at 
a time works best). The peak suppression algorithm can be briefly 
described as follows: 
STEP 1: Based on the data symbols for the block, and the pulse shape to be 
used, construct the transmitted waveform. 
STEP 2: For each symbol interval in the transmitted waveform, calculate the 
peak value of the waveform in that interval, the position of that peak, 
and the peak scale factor. 
STEP 3: Based on the peak scale factors and their positions, rescale the 
heights of each data symbol. 
STEP 4: Repeat steps 1-3 using the scaled data symbols. Continue repeating 
this procedure until no more (or very little) further peak suppression can 
be achieved. Use of this peak suppression algorithm can in some cases 
double the efficiency of the power amplifier or equivalently double the 
battery life in a portable radio. 
Referring once again to FIG. 5, dots 506 on the inner circle represent the 
magnitude of the unfiltered symbols. To avoid these symbols from 
overshooting all the way to the boundaries shown by the outer circle 504, 
they are scaled as shown by 508. As can be seen, some of the symbols are 
scaled down while others are scaled up in order to minimize error and peak 
signal magnitude. 
The scaled symbols reduce the peak power demand and hence improve the 
amplifier efficiency. In addition, the peak-to-average power requirement 
of the power amplifier is reduced. This reduction is directly translated 
in to lower cost for the amplifier 312. 
The improvement in the performance of the system is accomplished with 
minimum impact on the accuracy of the modulation. FIG. 6 shows an eye 
diagram of a demodulated signal having its peaks suppressed. The eye 
opening 602 is shown to have a sufficiently wide opening to maintain error 
performance. This is highly significant as a modulation technique is only 
desirable when demodulation techniques available therefor are highly 
accurate. In addition to modulations utilizing phase or amplitude, the 
principles of the present invention are applicable to QAM system which 
utilize both the phase and the amplitude of a signal to carry information.