WO2001001617A1

WO2001001617A1 - Cdma communication system

Info

Publication number: WO2001001617A1
Application number: PCT/EP2000/005907
Authority: WO
Inventors: Wilhelmus J. Van Houtum; Carel J. L. Van Driel
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 1999-06-29
Filing date: 2000-06-26
Publication date: 2001-01-04

Abstract

The CDMA communication system according to the invention comprises at least one primary station (2) and a plurality of secondary stations (4). The primary station (2) and the secondary stations (4) can exchange CDMA signals (18) at a basic data rate via a communication medium (6). The secondary stations (4) each comprise a modulator (10) for modulating a respective data signal (16) with a respective code word (14), which corresponds to a row or a column in a Walsh-Hadamard matrix Hn, in order to obtain a respective CDMA signal (18). The secondary stations (4) are arranged for communicating at a data rate which is higher than the basic data rate by supplying the respective data signals (16) at the higher data rate to the modulators (10). Due to the modulation of the respective code word (14) with the higher data rate data signal the resulting respective CDMA signal (18) appears to be modulated by other Walsh-Hadamard code words as well as the assigned code word (14). The primary station (2) does not assign already assigned code words nor these 'virtual' other code words to other secondary stations (4). In this way, the primary station (2) can prevent that more than one secondary station (4) use or appear to use the same code word, which would make it impossible for the primary station (2) to distinguish between the CDMA signals generated by those secondary stations (4). In this way different data rates can be assigned to different secondary stations (4).

Description

CDMA communication system

The invention relates to a CDMA communication system comprising at least one pπmary station and a plurality of secondary stations, the pπmary station and the secondary stations exchanging CDMA signals at a basic data rate via a communication medium, the secondary stations each compπsing a modulator for modulating a respective data signal with a respective code word in order to obtain a respective CDMA signal, the respective code word corresponding to a row or a column in a Walsh-Hadamard matπx H_n

The invention further relates to a secondary station for exchanging CDMA signals via a communication medium with at least one pπmary station

A CDMA communication system according to the preamble is known from United States Patent Number 5 499 236. Code Division Multiple Access (CDMA) is a multiplexing technique which permits a number of users to simultaneously access a transmission channel For this purpose a data signal to be transmitted is modulated with a code word, i.e. a pseudorandom binary sequence, in order to spread the spectrum of the waveform In a receiver the oπginal data signal can be detected by correlating the received CDMA signal with the corresponding code word This correlation despreads the spectrum Other CDMA signals are not despread by the correlator because their code words do not match CDMA can be used, for example, m mobile communication systems and in interactive cable television networks

The system capacity, l e. the total sum of the bit rates of the users, of a synchronised CDMA communication system is limited by the maximum number of different code words, whereas the system capacity of an asynchronous CDMA communication system is limited by the interference noise Hence, the system capacity of a synchronised CDMA communication system is generally much higher than that of an asynchronous CDMA communication system. The system capacity of a synchronised CDMA communication system can be further increased by using Walsh-Hadamard code words Walsh-Hadamard code words have ideal cross correlation properties because all the Walsh-Hadamard code words are mutually orthogonal. In the known CDMA communication system all stations conduct communications at the basic data rate There are no provisions for one or more stations to use other data rates than the basic data rate

An object of the invention is to provide a CDMA communication system, wherein communications can be conducted by the stations at several data rates This object is achieved in the CDMA communication system according to the invention, which is characteπzed in that the secondary stations are arranged for communicating at a data rate which is higher than the basic data rate by supplying the respective data signals at the higher data rate to the modulators. The invention is based upon the recognition that a secondary station can transmit a data signal at a higher data rate simply by supplying that data signal at the higher data rate to the modulator The modulator then produces a CDMA signal which appears to be modulated with more than one code word: the respective code word and other, 'virtual ' code words.

A first embodiment of the CDMA communication system according to the invention is characteπzed in that the higher data rate is 2^P higher than the basic data rate, p being a positive integer number Due to the symmetπcal properties of the Walsh-Hadamard code words the 'virtual ' code words which appear to have been used by the modulator are orthogonal to the set of Walsh-Hadamard code words. In this way, the communications conducted at the higher data rate do not interfere with the communications conducted at the basic data rate

A second embodiment of the CDMA communication system according to the invention is characteπzed in that the pπmary station assigns the respective code words to secondary stations wanting to start communications. In this way, the pπmary station can prevent that more than one secondary station use the same respective code word If more than one secondary station use the same respective code word, the pπmary station would not be able to distinguish between the CDMA signals generated by these secondary stations

A third embodiment of the CDMA communication system according to the invention is characteπzed in that the respective CDMA signal of a secondary station transmitting at a higher data rate appears to be modulated by other Walsh-Hadamard code words than the assigned code word, the pπmary station being embodied so as to not assign the other code words to other secondary stations. Some of the 'virtual ' code words apparently used by a secondary station are part of the set of Walsh-Hadamard code words, others are not By the measures of the third embodiment the primary station can prevent that those 'virtual' code words that are part of the set of Walsh-Hadamard code words are assigned to other secondary stations.

The above object and features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the drawings, wherein:

Figure 1 shows a block diagram of an embodiment of a CDMA communication system according to the invention,

Figure 2 shows a block diagram of part of an embodiment of a secondary station,

Figure 3 shows a Walsh-Hadamard matrix H 4 ^■

Figure 1 shows a block diagram of an embodiment of a CDMA communication system according to the invention. In such a CDMA communication system CDMA signals are exchanged at a basic data rate via a communication medium 6 between a number of stations 2 and 4. These CDMA communication stations 2 and 4 comprise at least one primary station 2, which is here a head end, and a plurality of secondary stations 4. The CDMA communication system, which may comprise further primary stations 2 and secondary stations 4, is a partly synchronised CDMA communication system. This means that some of the secondary stations 4 are synchronised to the primary station 2, while other secondary stations 4 are not yet synchronised to the primary station 2. Figure 2 shows a block diagram of a part of a secondary station 4 according to the invention. The part that is shown is that relating to the modulation of an input data signal 16. Operational parameters of all blocks shown are controlled by a controller (not shown). The secondary stations 4 each comprise a modulator 10 for modulating the input data signals 16 with code words 14 in order to obtain the CDMA signals 18. These code words 14 may be generated by a generator 12. This generator 12 may be embodied so as to generate code words 14 which correspond to rows or columns in a Walsh-Hadamard matrix H_n .

Figure 3 shows a Walsh-Hadamard matrix H₄. A Walsh-Ηadamard matrix H„ is defined inductively and can be calculated from a given Walsh-Ηadamard matrix H, . The rows R₀..R_{r ι} and columns C₀..C _n of a Walsh-Hadamard matrix H_n are orthogonal.

The code words 14 may be based on the rows R₀..R or the columns C^..C_τ__χ of such a

Walsh-Hadamard matrix H_n . A code word based upon the first row R₀ or the first column C₀ of the Walsh-Hadamard matrix H₄ (in general: of the Walsh-Ηadamard matrix H _n ) has symbol values which are all equal to each other.

A secondary station 4 which wants to communicate at a data rate which is higher than the basic data rate must ask permission to do so from the primary station 2. If the primary station 2 grants this request it assigns a Walsh-Ηadamard code word to the secondary station 4. This secondary station 4 then supplies its data signal 16 at the higher data rate to the modulator 10, which modulates the data signal 16 with the assigned code word 14. Due to the modulation performed by the modulator 10 the resulting CDMA signal 18 appears to be modulated not only by the assigned code word 14 but also by other, 'virtual ' code words. For example, if a secondary station 4 wants to transmit a data signal 16 to the primary station 2 at a data rate which is two times higher than the basic data rate the primary station 2 may assign a code word from the matrix H₄ , e.g. the code word corresponding to the second row R_x or the second column C, , to that secondary station 4. Let's assume that binary data symbols have to be transmitted and that a first bit value, e.g. a logical zero, does not change the symbol values of the assigned code word, whereas a second bit value, e.g. a logical one, inverts the symbol values of the assigned code word. In this case the following CDMA signals 18 correspond to all possible sets of two data symbols:

The first CDMA signal 18, which corresponds to the 00 set of data symbols, is modulated with the assigned code word (R, or C, ). The second CDMA signal 18, which corresponds to the 01 set of data symbols, appears to be modulated with the Walsh-Ηadamard code word corresponding to the tenth row R_g or the tenth column C₉. The third CDMA signal 18, which corresponds to the 10 set of data symbols, appears to be modulated with a code word which corresponds to the inverse of the tenth row R_g or the inverse of the tenth column

C₉ . The fourth CDMA signal 18, which corresponds to the 11 set of data symbols, appears to be modulated with a code word which corresponds to the inverse of the second row /?, or the inverse of the second column C, . In this way, two data symbols can be transmitted by means of a single assigned code word Λ, or C, . Hence, these data symbols are transferred at a data rate which is two times the basic data rate. In order to be able to distinguish between the CDMA signals 18 generated by the secondary stations 4 the primary station 2 cannot assign the code word R₉ or C₉ (and off course the already assigned code word /?, or C, ) to another secondary station 4.

Another example: if a secondary station 4 wants to transmit a data signal 16 to the primary station 2 at a data rate which is four ( 2^P = 4 , thus p = 2) times higher than the basic data rate, the primary station 2 may assign a code word from the matrix H₄ , e.g. the code word corresponding to the third row R₂ or the third column C₂ , to that secondary station 4. Let's again assume that binary data symbols have to be transmitted and that a first bit value, e.g. a logical zero, does not change the symbol values of the assigned code word, whereas a second bit value, e.g. a logical one, inverts the symbol values of the assigned code word. In this case the following CDMA signals 18 correspond to all possible sets of four data symbols:

In this way, four data symbols can be transmitted by means of a single assigned code word R₂ or C₂ . Hence, these data symbols are transferred at a data rate which is four times the basic data rate. In the above table the 'virtual ' code words which are not part of the set of Walsh-Hadamard code words are indicated with 'virtual '. In order to be able to distinguish between the CDMA signals 18 generated by the secondary stations 4 the primary station 2 cannot assign the code words R₆/C₆ , R_w/C_l0 and R IC_γi (and off course the already assigned code wordR₂/C₂ ) to another secondary station 4.

The scope of the invention is not limited to the embodiments explicitly disclosed. The invention is embodied in each new characteristic and each combination of characteristics. Any reference signs do not limit the scope of the claims. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. Use of the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A CDMA communication system comprising at least one primary station (2) and a plurality of secondary stations (4), the primary station (2) and the secondary stations (4) exchanging CDMA signals (18) at a basic data rate via a communication medium (6), the secondary stations (4) each comprising a modulator (10) for modulating a respective data signal (16) with a respective code word (14) in order to obtain a respective CDMA signal (18), the respective code word (14) corresponding to a row or a column in a Walsh-Hadamard matrix H _n , characterized in that the secondary stations (4) are arranged for communicating at a data rate which is higher than the basic data rate by supplying the respective data signals (16) at the higher data rate to the modulators (10).

2. A CDMA communication system according to Claim 1, characterized in that the higher data rate is 2^P higher than the basic data rate, p being a positive integer number.

3. A CDMA communication system according to Claim 1 or 2, characterized in that the primary station (2) assigns the respective code words (14) to secondary stations (4) wanting to start communications.

4. A CDMA communication system according to Claim 3, characterized in that the respective CDMA signal (18) of a secondary station (4) transmitting at a higher data rate appears to be modulated by other Walsh-Hadamard code words than the assigned code word (14), the primary station (2) being embodied so as to not assign the other code words to other secondary stations (4).

5. A secondary station (4) for exchanging CDMA signals (18) at a basic data rate via a communication medium (6) with at least one primary station (2), at a basic data rate via a communication medium (6), the secondary station (4) comprising a modulator (10) for modulating a data signal (16) with a code word (14) in order to obtain a CDMA signal (18), the code word (14) corresponding to a row or a column in a Walsh-Hadamard matrix H_n , characterized in that the secondary station (4) is arranged for communicating at a data rate which is higher than the basic data rate by supplying the data signal (16) at the higher data rate to the modulator (10).

6. A secondary station (4) according to Claim 5, characterized in that the higher data rate is 2^P higher than the basic data rate, p being a positive integer number.