Radio receiver comprising analog dynamic compression and digital expansion

A radio receiver for an RF signal which is phase or frequency modulated by an information signal. The receiver has an analog receiving section which provides non-linear dynamic compression. The resulting analog signal is sampled and the samples are digitized by an A/D converter. The non-linear dynamic compression reduces the necessary number of quantizing steps in the A/D converter, which is less expensive, but thereby introduces non-linear distortion. To compensate such distortion, in the digital processing section of the receiver an expansion section is provided which processes the digital signal samples in accordance with the inverse of the non-linear dynamic compression characteristic. The resulting expanded digital sample values equalize the analog non-linear dynamic compression. The information signal is recovered from the expanded digital signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a radio receiver for a phase or frequency 
modulated information signal, comprising an analog receiving section which 
provides non-linear dynamic compression, an analog-to-digital converter 
and a digital signal processing section. 
Radio receivers of this type can be used, for example, in the Pan-European 
mobile radio system GSM. In this mobile radio system speech signals in 
digitized form are transmitted together with other digital signals in a 
Time-Division Multiple Access mode (TDMA). 
2. Description of the Related Art 
DE 39 25 305 A1 has disclosed a radio receiver of the type mentioned in the 
opening paragraph. In that document the frequency of a received 
information signal is selected in an analog receiving section of the radio 
receiver and converted into an intermediate frequency by means of a mixer. 
An IF amplifier which performs non-linear dynamic compression is connected 
to the IF mixer. As a result, the output signal of the IF amplifier is 
either compressed or limited, depending on the input signal level. The 
output signal of the IF amplifier is applied to a baseband converter which 
produces therefrom two quadrature components. These quadrature components 
are sampled and applied to an analog-to-digital converter. Subsequently, 
the sampled digital values are buffered in a digital signal processing 
section, a channel estimate is made and an equalization and a decoding is 
performed. The non-linear dynamic compression and limitation achieves that 
a more cost-effective analog-to-digital converter can be used, having a 
relatively small number of quantization steps. Even at the largest 
possible levels of the input information signal there is no overload of 
the analog-to-digital converter, and, nevertheless, even at the smallest 
possible input levels usable sample values are still produced. However, in 
practice is found that the output signal of the digital signal processing 
section has bit errors. 
SUMMARY OF THE INVENTION 
It is an object of the invention to improve a radio receiver of the type 
mentioned in the opening paragraph so that the bit error rate of the 
output signal of the digital signal processing section is small. 
This object is achieved in a radio receiver of the aforesaid type, in that 
the digital signal processing section comprises an expansion section to 
compensate for the non-linear dynamic compression of the analog receiving 
section. 
The invention is based on the recognition that on account of the dynamic 
compression the radio receiver is capable of processing information 
signals which have a large dynamic range, but that the dynamic compression 
also causes non-linear distortions to occur, as a result of which bit 
errors may be produced in the output signal of the digital signal 
processing section. These non-linear distortions may be compensated by 
including an expansion section in the digital signal processing section, 
so that bit errors due to the non-linear distortions are compensated in 
the output signal of the digital signal processing section. 
The analog receiving section comprises, for example, an input circuit which 
selects the frequency of the received information signal, and which 
includes one or more intermediate frequency mixers for converting the 
information signal to one or more intermediate frequencies. The analog 
receiving section further includes an arrangement for non-linear dynamic 
compression. The input signal to the dynamic compression arrangement is 
thereby either compressed or limited, depending on the signal level. Such 
a compression characteristic for dynamic compression is represented and 
described in, for example, DE 39 25 305 A1. The compression characteristic 
therein has a logarithmic behaviour in a first region, and a limiting 
behavior in a succeeding second region. Then there follows a quadrature 
conversion to baseband, two quadrature components being produced. These 
components are then subjected to analog-to-digital conversion, and the 
thus obtained digitized quadrature components are processed in the digital 
signal processing section. In order to compensate for the non-linear 
distortions caused by the dynamic compression, in accordance with the 
invention the analog-to-digital conversion is followed by an expansion 
section which performs an expansion of the two digitized quadrature 
components. The thus produced expanded quadrature components are 
subsequently processed in known fashion by the digital signal processing 
section, which, for example, includes a channel estimator and an 
equalizer. Consequently, it is possible to use a cost-effective 
analog-to-digital converter to process information signals which have a 
large dynamic range, and without causing disturbing non-linear 
distortions. As a result of the expansion, the distortions caused by the 
dynamic compression are cancelled and thus also bit errors due to such 
distortions are prevented from occurring in the output signal of the 
digital signal processing section, which bit errors would have resulted in 
the absence of an expansion. 
In an advantageous embodiment, pairs of inverse function values which are 
inversely proportional to the characteristic of the non-dynamic 
compression are determined in the expansion section for each pair of 
sample values of digitized quadrature components of the information 
signal, and are used for linear equalization of the digitized quadrature 
components. The non-linear characteristic of the analog dynamic 
compression is described, from example, by a mathematic function for which 
also the inverse function may be determined. In the expansion section the 
pairs of inverse function values may thus, for example, be calculated on 
the basis of the pairs of sample values of the digital quadrature 
components. With the aid of these pairs of inverse function values a 
linear equalization of the pairs of sample values of the digitized 
quadrature components may be effected in the expansion section, so that 
the non-linear distortions of the dynamic be effected in the expansion 
section, so that the non-linear distortions of the dynamic compression are 
cancelled. 
In an advantageous embodiment the expansion section comprises a Table in 
which a pair of inverse function values is stored for each pair of 
possible sample values of the digitized quadrature components resulting 
from the non-linear dynamic compression. This is advantageous in that for 
each pair of sample values the inverse function need not be calculated 
each time, because the pair of inverse function values for each pair of 
sample values of the digitized quadrature components is already stored in 
the Table of the expansion section. However, this requires a relatively 
large storage capacity for the Table. 
In another embodiment, therefore, the expansion section comprises a Table 
in which an expansion factor is stored for each pair of sample values of 
the digitized quadrature components. Since the non-linear dynamic 
compression affects the value of the information signal, for each pair of 
sample values only one expansion factor need be stored in the Table 
instead of a pair of inverse function values. The values of the expanded 
quadrature components at the output of the expansion section are then 
derived as the product of the relevant expansion factor and each digitized 
quadrature component. Consequently, the memory capacity of the Table may 
be considerably reduced. 
In a further embodiment the memory cells of the Table are addressed by a 
pair of sample values of the digitized quadrature components. The pair of 
sample values of the digitized quadrature components are thus 
simultaneously used for addressing each memory cell of the Table, in which 
either the pairs of inverse function values themselves or the 
corresponding expansion factors are stored. 
In a further advantageous embodiment the memory cells of the Table are 
addressed by only a predetermined number of most significant bits of a 
pair of sample values of the digitized quadrature components. The address 
of a specific memory cell thus does not require the full word size of the 
pair of sample values of the digitized quadrature components, but only the 
most significant bits of each digitized quadrature component value. As a 
result, the size of the Table is considerably reduced without resulting in 
appreciable distortions of the information signal occurring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The radio receiver represented in FIG. 1 comprises an analog receiving 
section 10, an analog-to-digital converter 25 and a digital signal 
processing section 30. A received phase or frequency modulated RF signal, 
modulated by an e, is applied to the analog receiving section 10. The 
analog receiving section 10 comprises an RF input section 20 for frequency 
selection, an IF mixer 21, an IF amplifier 22 having non-linear dynamic 
compression, quadrature baseband converter 23, and a sample-and-hold 
circuit 24. Quadrature signal components I and Q produced at the output of 
the sample-and-hold circuit 24 are applied to the analog-to-digital 
converter 25, at whose outputs digitized quadrature components Id, Qd are 
available. The digitized quadrature components Id and Qd are applied to 
the digital signal processing section 30, which comprises an expansion 
section 29, a random-access memory 26, a channel estimator 27 as well as 
an equalizer 28. At the output of the expansion section 29 expanded 
quadrature components Iex and Qex are available, whereas the output of the 
equalizer 28 presents a decoded information signal. 
In the RF input circuit 20 the received signal is first frequency selected 
and then converted to an intermediate frequency by means of the IF mixer 
21. The signal thus converted is subsequently subjected to a non-linear 
dynamic compression in the IF amplifier 22. The of such a dynamic 
compression characteristic is denoted in the block representing the IF 
amplifier 22. Such amplifier has in a first region I of its power 
transmission characteristic a dynamic compression, and in a subsequent 
region II a limitation, of the input signal. Such a dynamic compression 
characteristic has been shown and described in DE 39 25 305 A1 already 
mentioned above, and also in FIG. 4. hereof. As a result non-linear 
dynamic compression of the it is possible to utilize a cost-effective 
analog-to-digital converter while, nevertheless, processing signals which 
have a large dynamic range, but at the cost introducing non-linear 
distortions. In the analog receiving section 10, subsequent to the dynamic 
compression the information signal is subdivided by baseband converter 23 
into two quadrature components I and Q, which are then sampled in the 
sample-and-hold circuit 24 and applied to the analog-to-digital converter 
25. 
For equalizing the distortions caused by the dynamic compression in 
amplifier 22, the digitized quadrature components Id and Qd are expanded 
in the expansion section 29. A first possibility is that in the expansion 
section pairs of inverse functions can be determined for each pair of 
sample values of the digitized quadrature components Id, Qd which will 
linearly equalize the digitized quadrature components Id, Qd. The pairs of 
inverse function values can be calculated from the inverse of the 
non-linear characteristic of the IF amplifier 22, as described by a 
mathematical function. The expansion is then effected in accordance with 
the calculated pairs of inverse function values. Since the dynamic 
compression of the amplifier 22 affects the values of the quadrature 
components I, Q, the inverse of the dynamic compression characteristic and 
thus also the pairs of inverse function values depend on the values of the 
quadrature components I and Q. 
A recalculation of each the inverse function values for each pair of sample 
values Id, Qd of the quadrature components would require much calculation 
circuitry and consequent high cost. For this reason, in a second option, 
the inverse function is not calculated again for each pair of sample 
values, but instead the expanded quadrature component values Iex, Qex for 
each possible pair of sample values of a digitized quadrature components 
Id, Qd are stored in the Table in the expansion section 29. The memory 
cells of the Table are then addressable by the pair of sample values of 
the digitized quadrature components Id, Qd. The disadvantage of this 
solution is the need for an enormous memory capacity. For example, for an 
8-bit word size of the analog-to-digital converter 25 and a 16-bit word 
size of the expanded values Iex, Qex, the Table has a size of: 
EQU 2.sup.8 *2.sup.8 *2*16 bits=2 097 152 bits. 
This need for memory capacity may be reduced if instead of storing the 
actual for the expanded values expanded signal values for the expanded 
values of quadrature components Iex, the Table stores Qex, only an 
expansion factor F(Id, Qd) which depends on the values of the quadrature 
components Id and Qd. The expanded quadrature component values Iex, Qex 
are then calculated according to the following instruction: 
EQU Iex=F(Id, Qd)*Id 
EQU Qex=F(Id, Qd)*Qd. 
Consequently, for example, for an 8-bit word size the analog-to-digital 
converter 25 and an 8-bit word size of the expansion factor, the size of 
the Table is: 
EQU 2.sup.8 *2.sup.8 *8 bits=524 288 bits. 
A further reduction of the Table size may be achieved in that the address 
of a memory cell of the Table is not based on the full word size of the 
digitized quadrature components but only the most significant bits of a 
pair of sample values of the digitized quadrature component values Id, Qd 
are used for the addressing. The calculation of the expanded signal values 
is then represented as follows: 
EQU Iex=F(Im, Qm)*Id 
EQU Qex=F(Im, Qm)*Qd. 
In this representation Im and Qm represent the most significant bits of a 
pair of sample values of the quadrature components I.alpha. and Q.alpha.. 
A detailed representation of such a form of instruction is described and 
represented in FIG. 3. For an 8-bit word size of the analog-to-digital 
converter 25, and with calculation of the expansion factor on the basis of 
only the 5 most significant bits (m=5), the size of the Table will then 
only: 
EQU 2.sup.5 *2.sup.5 *8 bits=8192 bits. 
This will leave very small residual distortions of the information signal 
which, however, do not have a disturbing effect on the further digital 
signal processing. The further digital signal processing of the thus 
expanded quadrature component values Iex, Qex is effected in known fashion 
by means of buffering in the random-access memory 26 and subsequent 
channel estimation 27 and equalization 28 as described, for example, in DE 
39 25 305 A1. 
FIG. 2 shows a digital radio transmission system in which information in 
the form of a binary signal b is transmitted by modulating an RF carrier 
via a transmitter 1. Between the transmitter 1 and the receiver 2 there is 
a radio transmission path denoted by an arrow. The transmitted modulated 
signal is received by a radio receiver 2. The radio receiver 2 comprises 
an analog receiving section as in FIG. 1 having an analog-to-digital 
converter 25 and a digital signal processing section 30. 
The functions of the individual components of the such digital transmission 
system have, in essence, already been described in connection with FIG. 1 
and are also contained in DE 39 25 305 A1. In contradistinction to the 
digital transmission system described in therein, however, the digital 
signal processing section 30 here comprises an expansion section 29. This 
effects a compensation for the non-linear dynamic compression 22 performed 
by the analog receiving section, as has already been described in 
connection with FIG. 1. 
FIGS. 3a, 3b each show the expansion section 29 used in the digital signal 
processing section 30 of a radio receiver as in FIGS. 1 and 2. The 
expansion section 29 derives expanded quadrature component values Iex, Qex 
for a pair of sample values of the digitized quadrature components Id, Qd. 
For this purpose, FIG. 3a shows that the digitized quadrature component 
values Id, Qd are applied to the expansion section 29 which, in essence, 
is constituted by a Table. At the output of the expansion section 29 the 
expanded quadrature component values Iex, Qex are available. As already 
described in connection with FIG. 1, the Table of the expansion section 29 
stores for each pair of sample values of the digitized quadrature 
components Id, Qd a pair of inverse function values, i.e., the expanded 
quadrature component values Iex, Qex, or alternatively an expansion 
factor. For addressing the individual memory cells of the Table T, each 
time a pair of values Id, Qd is used. 
FIG. 3b shows an embodiment of the expansion section 29 wherein the 
digitized quadrature component values Id, Qd each have an 8-bit word size; 
i.e., in the radio receiver an analog-to-digital converter with an 8-bit 
word size is used. This is symbolized in FIG. 3b by means of /8 at the 
individual signal arrows. To reduce the Table size, only the most 
significant bits, for example, the 5 most significant bits, are used for 
addressing the Table T to obtain the expanded quadrature components Iex, 
Qex from the 8 bits of each of the digitized quadrature components Id, Qd, 
of a signal sample the remaining least significant bits being discarded. 
This is illustrated in FIG. 3b by means of the signal arrows denoted by 
/5. For each pair of individual digitized quadrature components Id, Qd an 
expansion factor is stored in a memory cell in the Table, which factor can 
be selected by using the digitized quadrature components Id, Qd as memory 
cell addresses. The expansion factor may only have positive values. The 
expansion factor is used as a multiplier for the digitized quadrature 
components Id, Qd, thus producing the expanded quadrature components Iex, 
Qex. The maximum word size of the expansion factor may be selected at 
random and determines the total word size of the expanded quadrature 
components Iex, Qex. In this manner, with an analog-to-digital converter 
having an 8-bit word size and with a maximum expansion factor of 255 there 
is a final word size of the expanded quadrature components Iex, Qex of 16 
bits each. As a result, in the embodiment shown in FIG. 3b the Table T has 
a size of: 
EQU 2.sup.5 *2.sup.5 *8 bits=8192 bits. 
FIG. 4 gives shows a compression characteristic which has a logarithmic 
behaviour in a specific region for non-linear dynamic compression, as has 
already been explained with respect to the IF amplifier 22 described in 
FIGS. 1 and 2 and also in DE 39 25 305 A1. The characteristic shows the 
output signal Ua of the IF amplifier 22 shown in the FIGS. 1 and 2, in mV, 
plotted against the input signal level denoted Pe, in dBm. The 
characteristic has a linear region III, a logarithmic region I and a 
limitation region II. On the basis of the selected semi-logarithmic 
representation, the region II is correctly shown. The characteristic shown 
in FIG. 4 is used for calculating the inverse function values or expansion 
values respectively, which are stored in the Table shown in the FIG. 3b. 
For this purpose, first an equation of the characteristic has to be 
determined. This corresponds to calculation of the compression values of 
the IF amplifier 22 (FIGS. 1, 2). Subsequently, the inverse function is 
then calculated, which is digitally approximated and quantized, as 
required.