Data transmission method, transmitter, and receiver

A receiver, and a data transmission method in a system wherein the CDMA multiple access method in a system wherein the CDMA multiple access method is utilized and wherein each data signal to be transmitted is multiplied by a pseudorandom code of a certain length, having a bit rate that is higher than that of the data signal to be transmitted. In order to ensure fast and inexpensive reception, the signal multiplied by the pseudorandom code is further modulated by a group of waveforms (f.sub.1 . . . f.sub.N) the number of which equals the number of bits in the pseudorandom code, the frequency domain given. The signal is converted in the receiver into a digital form and multiplied by the pseudorandom code which has been subjected to an inverse Fourier transform, and the multiplied signal is subjected to a Fourier transform.

BACKGROUND OF THE INVENTION 
The invention relates to a data transmission method in a system wherein the 
CDMA multiple access method is utilized and wherein each data signal to be 
transmitted is multiplied by a pseudorandom code of a certain length, 
having a bit rate that is higher than that of the data signal to be 
transmitted. 
CDMA (Code Division Multiple Access) is a multiple access method, which is 
based on the spread spectrum technique and which has been applied recently 
in cellular radio systems, in addition to the prior FDMA (Frequency 
Division Multiple Access) and TDMA (Time Division Multiple Access) 
methods. CDMA has several advantages over the prior methods, for example 
spectral efficiency, the simplicity of frequency planning and traffic 
capacity. 
In the CDMA method, the narrow-band data signal of the user is 
conventionally multiplied to a relatively wide band of a traffic channel 
by a spreading code having a considerably broader band than the data 
signal. In known cellular network test systems, traffic channel bandwidths 
such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with 
multiplying, the data signal spreads to the entire band to be used. All 
users transmit by using the same frequency band, or traffic channel, 
simultaneously. A separate spreading code is used over each connection 
between a base station and a subscriber terminal, and the signals of the 
users can be distinguished from one another in the receivers on the basis 
of the spreading code of each connection. 
Correlators provided in the conventional CDMA receivers are synchronized 
with a desired signal, which they recognize on the basis of the spreading 
code in the signal. The data signal is restored in the receiver to the 
original band by multiplying it again by the same spreading code as during 
the transmitting stage. Signals multiplied by some other spreading code 
during the transmitting stage do not correlate in an ideal case with the 
spreading code used in the receiver, and they are therefore not restored 
to the narrow band. They appear thus as noise with respect to the desired 
signal. The spreading codes of the system are preferably selected in such 
a way that the codes used in each system cell are mutually orthogonal, 
i.e. they do not correlate with each other. 
The most time-consuming process in a CDMA transceiver implemented in known 
manners is correlation, and, simultaneously, the most expensive component 
is the correlator situated in the receiver. In the correlator, a received 
signal is compared bit by bit with a known spreading code, and the 
comparison produces a correlation value. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide a data transmission 
method which utilizes CDMA and in which the use of correlation can be 
replaced with a faster and more effective method. 
Instead, the signal multiplied by the pseudorandom code is further 
modulated by a group of waveforms the number of which equals the number of 
bits in the pseudorandom code, the frequency of each waveform being 
included in the frequency domain given. 
The invention also relates to a transmitter comprising means for 
multiplying each data signal to be transmitted by a pseudorandom code of a 
certain length, having a bit rate that is higher than that of the data 
signal to be transmitted. The transmitter according to the invention 
comprises means for modulating the signal multiplied by the pseudorandom 
code by a group of waveforms the number of which equals the number of bits 
in the pseudorandom code, the frequency of each waveform being included in 
the frequency domain given. 
The invention also relates to a receiver comprising means for converting a 
received analog signal into a digital form, and means for generating a 
desired pseudorandom code. The receiver according to the invention 
comprises means for subjecting the desired pseudorandom code to an inverse 
Fourier transform, and means for multiplying the digitized received signal 
by the converted pseudorandom code, and means for subjecting the 
multiplied signal to a Fourier transform. 
In the method according to the invention wherein the signal to be 
transmitted is not only multiplied by a spreading code but also modulated 
by a number of waveforms, the signal to be transmitted is subjected to a 
kind of inverse Fourier transform to the frequency domain. As a result of 
modulation, the spreading code is distributed in the frequency domain at 
desired intervals. The actual signal to be transmitted is thus the sum of 
the waveforms, which are dependent on the spreading code used over the 
connection. In practice the transmitter can thus utilize a certain number 
of waveforms on given frequencies, and the bits of the spreading code of 
each connection determine the waveforms that are sent over each 
connection. The frequencies of the waveforms can be selected on the basis 
of the properties of the radio path. 
In a receiver according to the invention, a received signal can be 
multiplied by the spreading code that has been subjected to an inverse 
Fourier transform. This can replace the conventional correlation 
operation, wherefore the process becomes considerably faster and simpler 
to implement.

DETAILED DESCRIPTION 
In the following, the method, transmitter and receiver according to the 
invention will be described in greater detail utilizing the cellular 
system as an example of a telecommunication system wherein the method 
according to the invention is applied. However, the invention can also be 
applied in several other types of systems besides those utilizing cellular 
technology, for example in telecommunication systems utilizing power 
lines. 
FIG. 1 illustrates an example of a cellular system wherein the method 
according to the invention can be applied. The system comprises a base 
station 10, which has a bidirectional connection 11-13 with each 
subscriber terminal 14-16 engaged in a call in the area. According to CDMA 
principles, the traffic of all terminal equipment in each transmission 
direction occurs in the same frequency domain, and every connection 
utilizes its own spreading code unique to that connection. 
In the method according to the invention, a narrow-band data signal of a 
user is first multiplied, according to conventional CDMA principles, by a 
spreading code which is unique to each user within the same coverage area. 
The length of the spreading code, i.e. the number of the bits in the code, 
is denoted by N. As a result of multiplication, the narrow-band data 
signal spreads to the given frequency band that is determined by the bit 
rate of the spreading code. The thus obtained broad-band signal is 
supplied for modulation by a group of orthogonal frequencies f.sub.1 . . . 
f.sub.N, the number of which equals the number N of the bits in the 
spreading code. As a result of modulation, the signal to be transmitted 
consists of the sum signal of the orthogonal frequency components, which 
depend on the spreading code used. 
The mutual intervals between the aforementioned frequencies f.sub.1 . . . 
f.sub.N in the frequency domain can be selected freely. The intervals 
between the frequencies does not have to be constant. In some embodiments, 
it may be preferable to center more frequencies in a section of the band 
and to decrease the number of frequencies in other sections of the band, 
for example according to the properties of the transmission channel used. 
It is possible, for example, to place several frequencies in the middle of 
the frequency band and to reduce the density of frequency placement near 
the edges of the band. 
FIG. 2 is a general block diagram illustrating the structure of a CDMA 
transmitter according to the invention. The transmitter comprises means 20 
for performing speech coding on the signal to be transmitted, and means 21 
for performing channel coding on the speech-coded signal. The 
channel-coded signal is then supplied to means 22 wherein the data signal 
to be transmitted is subjected to multiplication by the spreading code of 
the user. The transmitter according to the invention further comprises 
means 22 performing the modulation of the signal, which was multiplied by 
the spreading code, by a number of waveforms, the obtained modulated 
waveforms being summed up in the means. The obtained sum signal is 
supplied via radio-frequency means 23 for transmission by an antenna 24. 
FIG. 3 illustrates in greater detail the structure of a transmitter 
implementing the method according to the invention. The transmitter 
comprises means 31 for generating the spreading code 40 of the user, 
characteristic of the connection, and means 32 for multiplying the data 
signal 30 to be transmitted by this spreading code. Assume for example 
that the spreading code used is 1101011. The length N of the spreading 
code is thus 7. In the actual system, the spreading codes are naturally 
considerably longer. In a multiplier 32, each bit of the data signal 30 is 
multiplied by the spreading code 40. The possible shape of the signal in 
the time domain after the multiplication by the spreading code is 
illustrated in FIG. 4a. The multiplied signal forms a bit sequence having 
the bit rate of the spreading code. 
The transmitter further comprises N generators 33a to 33c having outputs 
that contain waveforms f.sub.1 . . . f.sub.N. The waveforms can be, for 
example, sinusoidal frequencies. The transmitter also comprises means 34 
the operation of which is controlled by a bit sequence obtained from the 
output of the multiplier 32, the input of the means 34 consisting of the 
output signals of the aforementioned generators 33a to 33c. The means 34 
can be realized, for example, by means of N switches 41a to 41c, so that a 
corresponding waveform f.sub.1 is supplied to each switch 41i, i=1, . . . 
N, as the input, and that each switch is controlled by a corresponding 
number i bit in the bit sequence obtained from the output of the 
multiplier 32. If the number i bit of the bit sequence has the value `1` 
or some other corresponding value, the corresponding switch is opened for 
the duration of the bit. Correspondingly, if the number i bit of the bit 
sequence has the value `0` or some other corresponding value, the switch 
is closed for the duration of the bit. The transmitter further comprises 
means 36 for summing the output signals 35 of the switching means 34, and 
the obtained summed signal forms the signal of the user to be transmitted. 
The described method can be considered as an inverse Fourier transform, 
which is performed on the signal to be transmitted. 
A possible shape of the signal in the frequency domain is illustrated in 
FIG. 4b, assuming that the spreading code used is the aforementioned 
1101011 and that the modulating waveforms are sinusoidal signals. In this 
case, the signal consists of a number of signal components having, in this 
example, the frequencies of f.sub.1, f.sub.2, f.sub.4, f.sub.6 and 
f.sub.7, i.e. the frequencies for which the bits of the bit sequence have 
the value `1`. 
If the transmitter is such that it transmits signals of several users 
simultaneously, for example as in a base station transmitter, it comprises 
means 37 for adding the signals 38 of other users, formed in a similar 
manner, to the signal to be transmitted. The obtained sum signal 39 is 
supplied further to radio-frequency parts. The same group of orthogonal 
waveforms f.sub.1 . . . f.sub.N is used in the composition of signals of 
all users, but the waveform components of the composed signal of each user 
vary since the spreading codes of the users differ from one another. 
In the method according to the invention, in addition to sinusoidal 
signals, the modulating waveforms may also be other kinds of waveforms, 
for example signals generated by means of a binary orthogonal function, 
such as the Walsh function. When the Walsh functions is used, the 
transmitter operates as described above except that instead of the 
frequency generators 33a to 33c, means generating orthogonal signals 
according to the Walsh function W.sub.0 . . . W.sub.N-1 are used. The 
summed output signal of the switching means 34 thus comprises a 
combination of the set of the Walsh function. 
When the data transmission method according to the invention is applied in 
the receiver, the received digitized signal can be subjected to a Fourier 
transform according to the length of the spreading code used. This 
converts the received CDMA signal into a normal shape, whereafter the 
signal can be correlated with the spreading code used in the transmission 
by use of conventional methods. 
The method according to the invention is most preferably applied in such a 
way that the correlation is performed before the Fourier transform. The 
correlation then becomes multiplication, which is considerably easier to 
perform. This is performed in such a way that the received digitized 
signal is multiplied by the spreading code which has been subjected to an 
inverse Fourier transform. The signal thus obtained is subjected to a 
Fourier transform, whereafter the original data signal is obtained. The 
advantage of this method is that it is fast and inexpensive to implement, 
compared to the use of a correlator. 
FIG. 5 is a general block diagram illustrating the structure of a CDMA 
receiver according to the invention. The receiver comprises an antenna 50 
receiving a signal that is supplied via radio-frequency parts 51 to 
converter means 52 wherein the received signal is converted into a digital 
form. The digitized signal is further supplied to demodulation means 53, 
wherein the signal is correlated with the spreading code used and wherein 
the required Fourier transforms are performed. The output signal of the 
demodulation means 53 that is restored to the original narrow band is 
supplied to a channel decoder 54 and from there to other parts of the 
receiver, for example to a speech decoder 55. 
FIG. 6 illustrates in greater detail the structure of a receiver 
implementing the method according to the invention. The receiver comprises 
means 52 for converting a received analog signal into a digital form. The 
digitized signal is supplied to converter means 60 wherein the signal is 
subjected to a Fourier transform. The converted signal is supplied further 
to correlation means 62 wherein the signal is correlated with a spreading 
code generated in means 61. The spreading code used in the correlation is 
the same that was used in the transmission of the signal. The correlated 
signal 63, which has been restored to the original band during 
correlation, is further supplied to other parts of the receiver. 
FIG. 7 illustrates a possible structure of a receiver according to a 
preferred embodiment of the invention. The receiver comprises means 52 for 
converting a received analog signal into a digital form. The receiver 
comprises means 61 for generating the required spreading code that is the 
same that was used in the transmission of the signal. The spreading code 
is supplied to first converter means 70 wherein the spreading code is 
subjected to an inverse Fourier transform. The receiver comprises a 
multiplier 71 wherein the received digitized signal is multiplied by the 
output signal of the first converter means 70. The output signal of the 
multiplier 71 is supplied to second converter means 72 wherein the signal 
is subjected to a Fourier transform. The obtained signal 73 is then 
supplied to other parts of the receiver, for example to a channel decoder. 
Even though the invention is described above with reference to the example 
according to the accompanying drawings, it is clear that the invention is 
not limited thereto, but it can be varied in many ways within the scope of 
the inventive idea disclosed in the appended claims.