WO1995016310A1

WO1995016310A1 - A data transmission method, cdma transmitter, and cdma receiver

Info

Publication number: WO1995016310A1
Application number: PCT/FI1994/000560
Authority: WO
Inventors: Kari Rikkinen; Ari Hottinen
Original assignee: Nokia Telommunications Oy; Nokia Mobile Phones Ltd.
Priority date: 1993-12-10
Filing date: 1994-12-09
Publication date: 1995-06-15
Also published as: AU1244295A; FI97661C; FI935560A0; FI935560A; FI97661B

Abstract

The invention relates to a CDMA transmitter and CDMA receiver and a data transmission method for transmitting a digital signal, the method comprising interleaving the signal to be transmitted prior to transmission and deinterleaving the received signal. To reduce the effects of interference caused by the transmission path, in signal interleaving in the method of the invention the symbols are divided in the time domain into elements substantially smaller in duration than the symbols, and a given number of such elements are regrouped in a new order, and the symbols to be tramsitted are formed of these rearranged elements, and in deinterleaving the symbols received are correspondingly divided into elements, said elements being regrouped in the original order.

Description

A data transmission method, CDMA transmitter, and CDMA receiver

The present invention relates to a data trans- mission method for transmitting a digital signal, the method comprising interleaving the signal to be trans¬ mitted prior to transmission and deinterleaving the received signal.

In telecommunication connections it is known that the transmission path employed for transmitting signals causes interference in telecommunication. This will happen irrespective of the physical form of the transmission path, no matter whether the transmission path is a radiocommunication channel, optical fiber link, or copper wireline.

To reduce transmission path interference and its effects, the digital signal is coded to improve the reliability of the transmission connection. A typical coding method employed in cellular radio applications is convolution coding, which is well suited to a channel having Gaussian noise. In a typical radio channel between a base station and a mobile station, errors appear in bursts, i.e., they are unequally distributed in the time domain. For this reason, cellular radio systems employ interleaving by which an attempt is made to convert the channel-induced errors appearing in bursts into independent random errors. For example, in the prior art interleaving method used in CDMA systems, the coded binary symbols are grouped into blocks in which the bit order is changed prior to transmission. This method may be termed bit inter¬ leaving.

Of systems employing multilevel coding/ modulation, such as orthogonal or biorthogonal modu- lation, the following two interleaving methods are known.

EIA/TIA Interim Standard: Mobile Station - Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS- 95, July 1993, discloses an interleaving method in which the data bits to be transmitted are interleaved by bit interleaving subsequent to channel coding, the interleaved bits determining the multilevel orthogonal symbols to be transmitted further without any other interleaving.

U.S. Patent 5 159 608 discloses a method in which code bits determine the multilevel orthogonal symbol. Such symbols, which are made up by an ortho¬ gonal signal sequence, are grouped into blocks in which the order of the symbols is changed prior to transmis¬ sion. This method may be termed symbol interleaving.

In bit interleaving, errors caused by long- term channel impairment, i.e. interference having a duration of several successive symbols transmitted, may be distributed so that after deinterleaving the errors are no longer located in successive symbols. However, bit interleaving is of no avail in a situation where the channel-induced errors have a shorter duration than that of the symbol transmitted. Such- interference is due to fast fading on account of multipath propagation, for instance.

If bit interleaving is employed in orthogonal or biorthogonal modulation, only part of the informa¬ tion contained in the demodulator can be utilized in the decoding. This will impair transmission quality, which can be measured by the bit error rate, for example. This phenomenon may be avoided by employing symbol interleaving, but only at the cost of increased system complexity. Errors caused by short-term inter- ference cannot be remedied by either of the prior art interleaving methods, i.e. bit or symbol interleaving. It is an object of the present invention to realize an interleaving method which is capable of reducing the effects of channel impairments better than heretofore and which thereby enables higher transmis¬ sion quality particularly in cellular radio systems.

This is achieved with the data transmission method of the kind set forth in the preamble, which is characterized in that in signal interleaving, the sym- bols are divided in the time domain into elements sub¬ stantially smaller in duration than the symbols, and that a given number of such elements are regrouped in a new order, and the symbols to be transmitted are formed of these rearranged elements, and that the sym- bols received in deinterleaving are correspondingly divided into elements, said elements being regrouped in the original order.

The invention also relates to a CDMA trans¬ mitter, comprising means for generating a baseband waveform of the signal to be transmitted, means for interleaving the signal, and means for multiplying the signal by a spreading code and transforming to a fre¬ quency employed in the transmission. The transmitter of the invention is characterized in that it comprises means for dividing the generated symbols in the time domain into elements substantially smaller in duration than the symbols, and means for grouping a given number of elements in a new order, and means for generating of these rearranged elements the symbols to be multiplied by a spreading code.

The invention also relates to a CDMA receiver, comprising means for transforming the received signal to baseband and multiplying by a spreading code, means for deinterleaving the signal, and means for de- modulating the signal. The receiver of the invention is characterized in that it comprises means for dividing the symbols multiplied by a spreading code in the time domain into elements substantially smaller in duration than the symbols, and means for grouping the elements in the original order for demodulation.

Thus, in the novel solution of the invention the generated symbols are divided into smaller elements that are grouped into blocks in which the order of the elements is changed. Correspondingly, at the receiving end the symbols received are divided into elements and regrouped in the original order for demodulation. Thus errors appearing in bursts are now distributed over a longer time span. Also short error bursts, shorter than the symbols to be transmitted, are equally distributed. The method of the invention can be implemented in digital data transmission systems of several kinds, such as cellular radio systems. The method is particu¬ larly suitable for use in the CDMA system. The CDMA is a multiple access system in which the signals of sev- eral users are transmitted over the same frequency band which is much wider than the original frequency band of the users' data signal. Prior to transmission, the users' data signal is multiplied by a user unique spreading code by which the users can be separated from one another. The bit rate of the spreading code is much higher than the user data speed. In the receiver, the signal is multiplied by the same spreading code, and thus the data signal reassumes its original frequency band. In the following the invention will be ex¬ plained in greater detail with reference to embodiments in accordance with the accompanying drawings, in which Figure 1 shows the structure of a transmitter of the invention, Figure 2 illustrates the structure of a receiver of the invention.

Figure 3 illustrates more closely the inter¬ leaving method of the invention, and

Figure 4 illustrates deinterleaving in accord- ance with the invention.

In the following the method of the invention will be described in the context of the CDMA system, but the disclosure is not to be construed as being so limited. The method of the invention can be likewise implemented in systems applying other multiple access methods.

Figure 1 is a block diagram representation of the structure of a CDMA transmitter realizing the method of the invention. The transmitter comprises a microphone 10, a vocoder 11, means 12 for coding and generating a baseband waveform, interleaving means 13, means 14 for multiplying the signal by a spreading code and transforming to a frequency employed in the trans¬ mission, and an antenna 15. The transmitter to be real- ized naturally also comprises other components, such as A/D converters and filters, but since these are un¬ essential to the present invention, for clarity they are not shown in the figure.

The signal derived from the microphone is voice coded in block 11. Thereafter the signal is applied to the encoding and signal representation block 12 in which the signal is channel coded and represented as baseband waveform. The duration of the symbols obtained from the output of block 12 can be indicated as Ts. In the method of the invention,* Ts_β is divided in the interleaving block 13 in the time domain into M elements (time slots), the duration of each element being T_c. Hence, T_B = M*T_C. The ith symbol can be expressed as the vector ^Xl ~ ( ^Xi,0 ' ^Xi,l ' * * * ^{, X}i,M-l ) where the format of elements x_{A 0},x_{i x}, ... ,x_{i M}_₁ is depend¬ ent on the format of the generated symbol, i.e. the signal representation applied in the system. In the case of binary symbols, all elements receive the same value, which may be either +1 or -1. In the case of multilevel orthogonal/biorthogonal symbols, each element may have the value ±1, depending on the multilevel symbol. The time slot employed for interleaving may be greater than or equal to the bit length of the spreading code employed in CDMA.

The interleaving length is denoted by T, which corresponds to the length of, for instance, L succes¬ sive symbols (T = L*M elements). The means for inter- leaving may be contemplated of as a memory circuit that can accommodate L*M elements. L successive symbols to be interleaved are stored in the memory one by one. After this, the content of the memory is read out one element at a time, so that the successive elements in the symbol X_i to be interleaved are no longer adjacent, but come in a different order. Once the memory is empty, the next L consecutive symbols to be interleaved are stored in the memory for interleaving. The inter¬ leaved signal is applied as an input to the spreading block 14, in which the signal is multiplied by a user unique spreading code, and thus the signal band is spread over the available frequency band. Subsequent to multiplication, the signal is transformed to the fre¬ quency employed for transmission and provided to the antenna 15 for transmission.

Figure 2 is a block diagram representation of the structure of a CDMA receiver realizing the method of the invention. The receiver comprises means 20 for transforming the signal received by antenna 15 to base- band and multiplying by a spreading code, a deinter¬ leaving block 21, means 22 for signal demodulation and decoding, a vocoder 23, and a loudspeaker 24. The re¬ ceiver to be realized naturally also comprises other components, such as D/A converters and filters, but since these are unessential to the present invention, for clarity they are not shown in the figure.

The signal received by antenna 15 is applied to block 20, in which it is transformed to baseband and multiplied by a connection-specific spreading code. The received signal is applied as an input to means 21, in which deinterleaving is performed. Deinterleaving may be implemented similarly as interleaving by means of a memory circuit. L*M samples of the received signal are input into the memory circuit. Thereafter L vectors Y_t are read out from the memory circuit in a different order than that in which the samples were written into the memory, so that the received symbols Y_± correspond to the transmitted, channel-corrupted symbols X . The symbols generated are applied to means 22 in which they are demodulated and decoded. After this, the signal is applied to the loudspeaker via vocoder 23.

The transmitter and receiver shown in Figures 1 and 3 are exemplary of a mobile station. When the method of the invention is applied to a base station, the equipment is naturally somewhat different from that described above, for example as concerns the loud¬ speaker and microphone, such equipment not being pro¬ vided at the base station. However, the equipment that is essential to performing the interleaving and de¬ interleaving is similar.

Figure 3 is an exemplary illustration of the interleaving method of the invention. The interleaving is performed by means of a matrix having the dimensions M*L; the matrix may be implemented with a memory cir¬ cuit. L successive symbol vectors X_{i f} X_i+1 , ... ,Xu._L to be interleaved are read into the interleaving matrix, so that each vector X_± = (x_{± 0},x_{i :} • • • Xi_,M-_ι) constitutes one column in the matrix. Successive symbols X_{l f} X_i+1 do not necessarily form adjacent columns in the matrix, but the order can be random. When the matrix is full, the elements of the matrix are read out row by row. In the example of Figure 3, the output of the interleaving block now contains ...,x_i(1,x_i+Lr0,.. •

the symbols to be transmitted are spread equally in the interleaving time window T = L*M. The interleaving may also be realized in such a way that the matrix is filled up in rows and read out in columns. Figure 4 is an exemplary illustration of de¬ interleaving in accordance with the invention. Deinter¬ leaving is correspondingly performed by means of a matrix having the dimensions L*M; the matrix may be implemented with a memory circuit. L*M received samples are read into the interleaving matrix, so that L suc¬ cessive samples constitute one column in the L*M matrix. When the matrix is full, the elements of the matrix are read out row by row. The output of the deinterleaving block now contains the original symbols, added with channel degradation. When the matrix is empty, the next L*M samples are read in. The deinter¬ leaving may also be realized in such a way that the matrix is filled up in rows and read out in columns. Interleaving and deinterleaving shall naturally employ corresponding matrix dimensions.

Although the invention is described herein with reference to the example in accordance with the accompanying drawings, it will be appreciated that the invention is not to be so limited, but the invention may be modified in a variety of ways within the scope of the inventive idea disclosed in the appended claims.

Claims

Claims :

1. A data transmission method for transmitting a digital signal, the method comprising interleaving the signal to be transmitted prior to transmission and deinterleaving the received signal, c h a r a c ¬ t e r i z e d in that in signal interleaving, the sym¬ bols are divided in the time domain into elements sub¬ stantially smaller in duration than the symbols, and that a given number of such elements are regrouped in a new order, and the symbols to be transmitted are formed of these rearranged elements, and that the sym¬ bols received in deinterleaving are correspondingly divided into elements, said elements being regrouped in the original order.

2. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the symbols to be inter¬ leaved are multilevel orthogonal or biorthogonal symbols.

3. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the symbols to be inter¬ leaved are binary symbols.

4. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that a given number of elements derived from symbols to be interleaved are successively written at a time into a storage element, and that the elements are read out from the storage element in a different order than in which they were written into the storage element, and that the symbols to be pro- cessed further are formed of the elements read out from the storage element.

5. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that in deinterleaving performed in the reception, a given number of elements formed of received symbols are successively written at a time into a storage element, and that the elements are read out from the storage element in a different order than in which they were written into the storage element, and that the symbols to be processed further are formed of the elements read out from the storage element.

6. A method as claimed in any one of the pre¬ ceding claims, c h a r a c t e r i z e d in that the bits to be transmitted are coded, interleaved and mul¬ tiplied by a spreading code prior to transmission, and in the interleaving the symbols are divided in the time domain into elements substantially smaller in duration than the symbols, the duration of said symbols being equal to or greater than the bit length of the spread¬ ing code employed, and that a given number of such elements are regrouped in a new order, and the symbols to be multiplied by the spreading code are formed of these rearranged elements.

7. A CDMA transmitter, comprising means (12) for generating a baseband waveform of the signal to be transmitted, means (13) for interleaving the signal, and means (14) for multiplying the signal by a spread¬ ing code and transforming to a frequency employed in the transmission, c h a r a c t e r i z e d in that it comprises means (13) for dividing the generated symbols in the time domain into elements substantially smaller in duration than the symbols, and means (13) for grouping a given number of elements in a new order, and means (13) for generating of these rearranged elements the symbols to be multiplied by a spreading code.

8. A CDMA receiver, comprising means (20) for transforming the received signal to baseband and multi¬ plying by a spreading code, means (21) for deinter¬ leaving the signal, and means (22) for demodulating the signal, c h a r a c t e r i z e d in that it com- prises means (22) for dividing the symbols multiplied by a spreading code in the time domain into elements substantially smaller in duration than the symbols, and means (22) for grouping the elements in the original order for demodulation.

9. A transmitter as claimed in claim 6, c h a r a c t e r i z e d in that the grouping means (13) are implemented by means of a memory circuit.

10. A received as claimed in claim 7, c h a r a c t e r i z e d in that the grouping means

(22) are implemented by means of a memory circuit.