CDMA communication method and group spreading modulator

Abstract: A CDMA communication method capable of providing a multi-rate user with high quality transmission using a spreading sequence selected from sequences which consist of multilayer sequences arranged in a hierarchical tree structure. A higher level sequence C 16 (1) and its inverted sequence C 16 (1) are included in its lower level sequences {C 32 (1), C 32 (2)} and {C 64 (1), C 64 (2), C 64 (3), C 64 (4)}, for example, and a sequence from a higher level is selected to transmit a higher rate data: The C 16 (1) is used to transmit data whose peak rate is twice the peak rate of the data transmitted using its lower level sequences {C 32 (1), C 32 (2)}, and four times (= 2 2 ) that of the data transmitted using the sequences {C 64 (1), C 64 (2), C 64 (3), C 64 (4)}. The C 16 (1), however, cannot be selected if any one of its lower level sequences {C 32 (1), C 32 (2)} and {C 64 (1), C 64 (2), C 64 (3), C 64 (4)} are already being used. This makes it possible for all the spreading sequences of simultaneous multi-rate users to be orthogonal at any transmission rate.

Description: The present invention relates to a CDMA (Code Division Multiple Access) communication method and a group spreading modulator applicable to the method. Conventionally, spreading sequences of the same length have been used in the CDMA communication system. Those spreading sequences are orthogonal to each other when multiple users communicate simultaneously through forward links (from a base station to mobile stations). This is because all the simultaneous users share the same radio band in the CDMA, and hence the interference between the users must be minimized. Applying this method, however, causes a problem in that transmission rates become equal for the entire simultaneous users. For example, when using spreading sequences with a period of 1024 chips at a bandwidth of about 1 MHz, the transmission rate will be 9.6 kbps at the peak. The peak transmission rate, however, generally varies according to transmission types: For example, although voice communications requires only about 8 kbps, picture transmission necessitates at least 64 kbps, and modem data transmission needs 28.8 kbps. In such an environment, a plurality of simultaneous users have conventionally communicated using spreading sequences with different periods which are not orthogonal to each other. This presents a problem in that interference between the simultaneous users increases when communicating through the same radio band at different transmission rate, resulting in the degradation in the transmission quality. WO-A-95/03652 discloses a simultaneous multi-user communication method in which each user channel is assigned as a spreading code, a unique orthogonal Walsh sequence of a length determined by the channel data rate. In the case of voice channels, the digital symbol stream for each voice signal is multiplied by its assigned Walsh sequence. The length of the assigned Walsh sequence may change while a call is in progress to enhance efficiency. The article by McTiffin et al, "Mobile Access to an ATM Network Using a CDMA Air Interface", IEEE Journal on Selected Areas in Communications, June 1994, Vol. 12, No. 5, New York, US, Pages 900-908 , discloses an integrated system concept using Asynchronous Transfer Mode (ADM) transmission in a CDMA system in which the bit rate is variable. On the downlink, a variable spreading factor is used, with the bit rates being multiples of a minimum base bit rate in order to achieve code orthogonality. On the uplink, a variable spreading factor is used, although code orthogonality cannot be achieved. It is therefore an object of the present invention to provide a CDMA communication method capable of achieving generation and selection of spreading sequences for implementing multi-rate CDMA communications without interference. Another object of the present invention is to provide a group spreading modulator suitable for the spreading modulation using the spreading sequences thus generated. According to an aspect of the present invention, there is provided a CDMA communication method for spreading modulated transmitting data using a spreading sequence selected from one of the row vectors of 2 N x 2 N dimensional matrices, where N is an integer greater than zero, generated from a smaller dimensional matrix under a predetermined rule, each of said matrices including row vectors orthogonal to each other, wherein said transmitting data in said CDMA communication method includes a plurality of data with differential peak transmission rates, and wherein the method comprises the step of: dividing said transmitting data into a plurality of groups based on the peak rate of transmission rates, the method being characterized by comprising the steps of selecting, when transmitting data at a peak rate of 2 Q times the lowest peak rate, one of 2 Q row vectors in a maximum dimensional 2 N x 2 N matrix as the spreading sequence, said 2 Q row vectors being generated from one of the row vectors in the 2 N-Q x 2 N-Q matrix, where Q is an integer greater than zero and equal to or less than N, such that the other 2 Q -1 row vectors are not selected whilst said spreading sequence is being used. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing a transmitting end of a CDMA transmission system in accordance with one embodiment of the present invention; Fig. 2 is a diagram illustrating a generation rule of spreading sequences in accordance with a comparative example of the present invention; Fig. 3 is a diagram illustrating a hierarchical structure of the spreading sequences of a comparative example; Fig. 4 is a block diagram showing a receiving end of the CDMA transmission system in accordance with one embodiment of the present invention; Fig. 5 is a diagram illustrating a radio transmission data sequence of a base station and a mobile station; Fig. 6 is a circuit diagram showing a basic modulator element employed in the embodiments in accordance with the present invention; Fig. 7 is a block diagram showing an embodiment 1 of a group spreading modulator in accordance with the present invention, which spreads data of 2 N channels; Fig. 8 is a waveform diagram showing spreading signals input to basic modulator elements of respective layers; Fig. 9 is a block diagram showing an embodiment 2 of a group spreading modulator in accordance with the present invention, which spreads data of channels of different transmission rates; Fig. 10 is a block diagram showing an embodiment 3 of a group spreading modulator in accordance with the present invention, which spreads data of 2 N channels; Fig. 11 is a block diagram showing another transmitting end of the CDMA transmission system in accordance with an embodiment of the present invention; Fig. 12 is a diagram illustrating another generation rule of spreading sequences in accordance with one embodiment of the present invention; and Fig. 13 is a diagram showing time relationships between low rate spreading code sequences. EMBODIMENT 1 Fig. 1 is a block diagram showing a transmitting end, and Figs. 2 and 3 are schematic diagrams illustrating a generation and assignment rule of spreading sequences in accordance with an embodiment of the present invention. As shown in Fig. 1, transmitted data of users are each modulated by data modulators 1 into narrowband modulated signals. The narrowband modulated signals each undergo spreading modulation into wideband signals using short period spreading sequences selected by a short period spreading sequence generator 2 in a manner as will be described later, followed by addition by an adder 3, and followed by spreading using a long period spreading sequence, thereby being output as a wideband spread signal. Here, the long period spreading sequence, having a period longer than that of the short period spreading sequences, is generated by a long period spreading sequence generator 4. A controller 5 controls the data modulators 1, short period spreading sequence generator 2 and long period spreading sequence generator 4. The spreading sequences are generated under a predetermined rule as illustrated in Fig. 2 which shows a comparative example. In Fig. 2, the matrix C 2 consists of C 2 (1)=(1,1) and C 2 (2) =(1,0). In Fig. 2, C̅ 2 (1) and C̅ 2 (2) denote C̅ 2 (1) = (0,0) and C̅ 2 (2)=(0,1), in which 1 and 0 are exchanged as those of C 2 (1) and C 2 (2). In this way, C 2n 's are defined as shown in Fig. 2. The row vectors of the matrices generated in this example become Walsh functions. They are described in Fig. 3 in the form of a multilayer structure. Suffixes attached to symbols C represent the dimension of the matrices. An example is shown in which the maximum dimension is 64. This indicates that the period of the short period spreading sequence is 64 chips. At the peak transmission rate at the lowest layer, one of the 64 row vectors {C 64 (1), ..., C 64 (64)} is assigned as a spreading sequence. Let us assume that the peak transmission rate of the lowest layer is 9.6 kbps. Then, at its double rate, one of the 32 row vectors {C 32 (1), ..., C 32 (32)} is selected. If the peak rate is at 2 Q of that, one of the 2 (6-Q) row vectors {C 2 6-Q (1), ..., C 2 6-Q (2 6-Q ) } is assigned as a spreading sequence. The values Q corresponding to the individual layers are represented at the far right of Fig. 3. For example, assume that Q = 2 and C 16 (1) sequence is used. The row vectors (called sequences from now on) {C 32 (1), C 32 (2)} and {C 64 (1), C 64 (2), C 64 (3), C 64 (4)}, which are lower in rank than C 16 (1) sequence, contain C 16 (1) sequence or its inverted sequence C̅ 16 (1), where the mark "-" denotes inversion. Thus, if the sequences {C 32 (1), C 32 (2)} or {C 64 (1), C 64 (2), C 64 (3), C 64 (4)} which are lower in rank than C 16 (1) sequence are already being used, the C 16 (1) cannot be assigned as the spreading sequence for transmission with a peak rate of 2 2 times the lowest transmission rate. In other words, the spreading sequence is selected and is used for different peak transmission rates in such a way that no sequences are used that are generated from the sequence to be selected and are lower in rank and correspond to. Thus, the spreading sequences of the entire simultaneous users can be made orthogonal at all transmission rate, as can be seen from the generation rule of the spreading sequences. Fig. 4 is a block diagram showing a receiving end in accordance with the invention. A received signal is despread using a long period spreading sequence selected by a long period spreading sequence generator 11, is further despread using a short period spreading sequence selected by a short period spreading sequence generator 12, passes through an integration and dump filter 13, and undergoes data decision by a data decision circuit 14 to be output as received data. A controller 15 supplies the long period spreading sequence generator 11 and short period spreading sequence generator 12 with data required for selecting the spreading sequences, provides a frequency divider 16 with the output of a clock generator 17, and supplies the integration and dump filter 13 with the integration and dump timing. A despreading method at the receiving end will now be described in the case where the spreading sequence for the data transmission rate with a peak of 2 Q times the lowest peak rate is selected from among the 2 Q row vectors in the maximum dimensional 2 N ×2 N matrix, which are generated from one of the row vectors in the 2 N-Q ×2 N-Q matrix. More specifically, the despreading method at the receiving end will be explained taking an example in which the peak data transmission rate is 2 Q= 4 times (Q=2) the lowest peak rate. In this case, the transmitting end does not use C 16 (1) but selects C 64 (2), for example, from among the sequences {C 64 (1), C 64 (2), C 64 (3), C 64 (4)} in the maximum dimensional matrix, which include the sequence C 16 (1) as their sub-sequence. Accordingly, although the period of the sequence is 64 chips, the number of chips per bit of the transmitted data is 16. In this case, none of the {C 64 (1), C 64 (3), C 64 (4)} can be used for other users. The receiving end despreads using the spreading sequence C 64 (2), and decides the received data every 16 chip interval. The transmitted data will be decided correctly because the sequence C 64 (2) consists of the sequence C 16 (1) and its inverted sequence C̅ 16 (1), which are arranged in regular orders at every 16 chip interval. This makes it possible to handle the spreading sequences at any desired transmission rates as though they were the spreading sequences for the lowest transmission rate. In selecting the spreading sequence, however, it is inhibited for other users to employ any of the lowest layer spreading sequences {C 64 }'s which belong to the sequence (that is, C 16 (1) in this case) reached by tracing back the code tree structure of Fig. 3 from C 64 (2) up to the second layer (Q = 2). Next, a method will be described for generating vacancies in the transmission time in accordance with the transmission rate when the data transmission rate falls below 2Q times the lowest peak rate during the CDMA communications. As an example of this, Fig. 5 illustrates a radio transmitted data sequence of a base station and a mobile station. The transmitted data is divided at every fixed interval (one frame time), is converted into frame data with a rate of R bit/sec corresponding to the peak of the data transmission rate regardless of the current data transmission rate, and is multiplied by the spreading sequence (that is, being spread). Here, R equals 2 Q times the lowest peak rate, where Q is any integer equal to or less than N. Accordingly, when the current transmission rate is R×C, where C is equal to or less than one, the number of the transmitted data in the frame becomes C times that at the peak rate. Thus, C is referred to as a transmission time ratio. Adjusting the transmission time ratio in this way makes it possible for the radio transmission rate to be kept constant (at the peak rate R) even if the data transmission rate changes during the communication. A method for assigning spreading sequence in the case where Q = 2 will now be described referring to Fig. 3. Considering that the peak of the data transmission rate is 2Q=4 times the lowest peak, let us assume that the spreading sequence C 16 (1) is assigned. When the data transmission rate changes in the course of the communications, the transmitted data is decided at every 16 chips at the receiving end without changing the spreading sequence. Thus, once the peak of the data transmission rate has been determined, the same spreading sequence is continually used in spite of the change of the data transmission rate during the communications, in which case it may occur that the transmission time ratio approaches zero. In view of this, when the data transmission rate falls, reassignment of a lower layer spreading sequence is possible in response to that. If the data transmission rate falls in the range from 2 P-1 to 2 P times the lowest peak rate, where P is any integer equal to or less than Q, the transmission time ratio C is at least 50%. A method for assigning the spreading sequence when Q=2 will be described. Let us assume that the sequence C 16 (1) as shown in Fig. 3 is initially assigned. When the transmission rate falls below 1/2 of the peak during the communications, one of the spreading sequences {C 32 (1), C 32 (2)} is reassigned which belongs to the layer following the C 16 (1) layer. If the transmission rate falls below 1/4, one of the spreading sequences {C 64 (1), C 64 (2), C 64 (3), C 64 (4)} is reassigned which belongs to the one more lower layer. On the other hand, when assigning the spreading sequence, one of the spreading sequences {C 64 (1), C 64 (2), C 64 (3), C 64 (4)} in the lowest layer is assigned from the beginning. In this case, even if the transmission rate changes during the communications, the spreading sequence is not changed unless it falls below 1/2 of the peak (corresponding to Q = 2), thus to continue the transmission by adjusting the transmission time ratio, causing spaces in the transmission time. When despreading the received data using this sequence, the receiving end changes the decision period of the transmitted data in such a way that it decides the transmitted data at every 32 chip interval when the transmission rate falls below 1/2 of the peak, and every 64 chip interval when it falls below 1/4 of the peak. A configuration of a group spreading modulator will now be described which has one to one correspondence with the above-described spreading code generation method with a tree structure. Fig. 6 shows a basic modulator element with three inputs and a single output as the basic component of the group spreading modulator. In Fig. 6, the basic modulator element has its two input terminals 61 and 62 to which modulation signals are input, and its input terminal 63 to which a spreading signal is input, wherein one of the two modulation signals (that is, the signal fed to the terminal 62) is multiplied by the spreading signal by a multiplier 65. The modulation signal fed to the terminal 61 is added to the output of the multiplier 65 by an adder 64, and the resultant sum is output from an output terminal 66. Fig. 7 shows the group spreading modulator composed of hierarchically connected, N-layer basic modulator elements 60 as shown in Fig. 6. In the arrangement of Fig. 7, the number of channels is the N-th power of two. The primary layer of the group modulator as shown in Fig. 7 includes 2 (N-1) elements, and the number of the elements is halved as the layer rises each step. The spreading rectangular signals input to respective layers are a periodic square waveform signal, and their frequencies are doubled as the layer rises each step, such as the frequency of the primary layer is 1/2 N of the clock frequency fc, and that of the secondary layer is fc/2 (N-1) , where the clock frequency fc equals the chip rate. The frequency of the spreading signal input to the highest layer (N-th layer) is fc/2. Fig. 8 illustrates the relationships between the periodic rectangular signals of respective layers. The configuration of Fig. 7 can achieve the modulation when the data rate of the entire channels are equal. In this case, the ratio of the rate of the spreading sequence (chip rate fc) to the modulation rate of the narrowband modulation signal (symbol rate) is 2 N . For example, when the chip rate fc=4.096 Mcps and N=6, the symbol rate becomes 4.096 Mcps/64 = 64k symbols/sec, and the number of channels is 2 N = 64 channels. EMBODIMENT 2 Fig. 9 shows a configuration of a modulator that can implement multi-rate modulation using the spreading sequences of the tree structure as shown in Fig. 3. The modulator as shown in Fig. 9 has, besides the configuration as shown in Fig. 7, a structure that enables the narrowband modulation signals to be input directly to the input terminals of upper layers. To achieve this, each of the two inputs of the basic modulator element is provided with a switch for switching the inputs. For example, a modulation signal of a channel of twice the symbol rate can be directly input to one of the two inputs of the element at the secondary layer, which includes that channel. Likewise, a modulation signal of a channel of four times the symbol rate can be directly input to one of the two input terminals of the element at the third layer, the one of the two input terminals including that channel, and a modulation signal of a channel of the 2P times the symbol rate can be directly input to one of the two input terminals of the element at the (p+1)-th layer, the one of the two input terminals including that channel. This modulation method has strict one-to-one correspondence with the generation rule of the spreading code sequences of the tree structure as shown in Fig. 3. EMBODIMENT 3 Fig. 10 shows a group modulator constructed using a smaller number of basic modulator elements than that of Fig. 7. In Fig. 10, the basic modulator elements are hierarchically connected in R layers, where R is less than N, and the output of the topmost element is multiplied by an orthogonal code sequence with a period of 2 (N-R) chip intervals, thus constituting a group modulator unit. At the final layer, the outputs of the 2 (N-R) group modulator units are summed up. The configuration as shown in Fig. 10 is a case where N=6 and R=3. In the configuration as shown in Fig. 10, the elements are hierarchically connected in R layers (R<N) rather than connected in N layers as shown in Fig. 7, and the output of the topmost element is multiplied by an orthogonal spreading code sequence with the 2 (N-R) chip intervals by a multiplier 103. The total of 2 (N-R) thus constructed group modulator units 102 are used so that their outputs are summed up by the adder 101, thereby producing a spread signal obtained by the spread modulation of the 2 N channels. With this arrangement connecting in parallel a plurality of the group modulator units 102 with a smaller number of channels makes it possible to expand to a group spreading modulator having a larger number of channels. Furthermore, the configuration as shown in Fig. 10 can be altered to a modulator that can achieve the multi-rate modulation corresponding to the tree-structure as shown in Fig. 3. To achieve this, it is necessary to provide each of the basic modulator elements in the upper layers with a switch to enable the narrowband signals to be input directly to their input terminals as shown in Fig. 9. This makes it possible to construct a modulator that can achieve the modulation using the spreading codes in the tree structure corresponding to the multi-rates. EMBODIMENT 4 Fig. 11 shows another configuration of an 2 R channel group modulator corresponding to that enclosed by the broken lines in Fig. 10. In Fig. 11, the user data of 2 R channels are each input to data modulators 112 to obtain narrowband modulated signals. The 2 R data modulated signal outputs from the data modulators 112 are multiplied by spread sequences fed from a low rate orthogonal periodic spreading sequence generator 114 by multipliers 116, and combined by an adder 117. Subsequently, the output of the adder 117 is multiplied by a orthogonal spreading code sequence with a period of an 2 (N-R) chip interval as in Fig. 10. The spreading sequences to be multiplied by the modulated signal outputs will now be described below. The spread modulation as shown in Fig. 11 carries out multiplication by the orthogonal codes through two steps. First, the spreading codes generated by the low rate orthogonal periodic spreading sequence generator 114 are multiplied which constitute Walsh functions with a rate of a 1/2 (N-R) of the spread chip rate. The spreading sequences generated by the low rate orthogonal periodic spreading sequence generator 114 will be described with reference to Figs. 12 and 13. In Fig. 12, a matrix D 1 = 1, and two adjacent matrices have relationships as shown in this figure. The low rate orthogonal periodic spreading sequences are generated from the row vectors of the matrices related by equations in Fig. 12. Fig. 13 shows an example of time relations between the row vectors and the spreading sequences, in which N=6 and R=3. As seen from this figure, the low rate orthogonal periodic spreading sequences are well-known Walsh functions. Second, the orthogonal spreading code sequence with a period of the 2 (N-R) chip interval generated by the generator 115 is multiplied. It is obtained by successively generating greater dimensional matrices from lower dimensional matrices as described above in connection with Figs. 2 and 3, and by selecting one of the row vectors in the matrices. Or one of the components of a set of orthogonal sequences can be used (for example, orthogonal Gold sequences). Thus, the spreading sequences for the spreading modulation can be obtained. It is obvious that the spreading sequences obtained by means of the low rate orthogonal periodic signal generation have a hierarchical structure as shown in Fig. 3. It is also true that if a corresponding spreading code sequence in a lower layer in this hierarchical structure has already been assigned to a user, the spreading sequence of the layer that generates the corresponding spreading code sequence cannot be used. In addition, it is also possible for the low rate orthogonal periodic signal generation to carry out control in such a manner that vacancies are generated in the transmission time without changing the spreading sequence as described above in connection with Fig. 5, even if the lowest peak rate of the data transmission varies. Furthermore, it is also possible for the low rate orthogonal periodic signal generation to perform control such that the spreading sequence is reassigned when the data transmission rate reduces by 50% or more. The spread modulation as described above in connection with Figs. 11-13 can be achieved using the group modulators as described in connection with Figs. 6-10. The present invention has been described in detail with respect to various embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention as claimed in the appended claims.

Claims: A CDMA communication method for spreading modulated transmitting data using a spreading sequence selected from one of the row vectors of 2 N x 2 N dimensional matrices, where N is an integer greater than zero, generated from a smaller dimensional matrix under a predetermined rule, each of said matrices including row vectors orthogonal to each other, wherein said transmitting data in said CDMA communication method includes a plurality of data with differential peak transmission rates, and wherein the method comprises the step of: dividing said transmitting data into a plurality of groups based on the peak rate of transmission rates, the method being characterized by comprising the steps of selecting, when transmitting data at a peak rate of 2 Q times the lowest peak rate, one of 2 Q row vectors in a maximum dimensional 2 N x 2 N matrix as the spreading sequence, said 2 Q row vectors being generated from one of the row vectors in the 2 N-Q x 2 N-Q matrix, where Q is an integer greater than zero and equal to or less than N, such that the other 2 Q -1 row vectors are not selected whilst said spreading sequence is being used. The CDMA communication method as claimed in claim 1, further comprising the step of generating vacancies in a transmission time in accordance with the transmission rate if the data transmission rate falls below 2 Q times the lowest peak rate during communications. The CDMA communication method as claimed in claim 1, further comprising the step of reassigning the newly selected spreading sequence if the data transmission rate falls in a range from 2 P-1 to 2 P times the lowest peak rate during communications, where P is an integer equal to or less than Q. The CDMA communication method as claimed in claim 1, further comprising the step of generating vacancies in a transmission time in accordance with the transmission rate if the data transmission rate is in a range from 2 P-1 to 2 P times the lowest peak rate, where P is an integer equal to or less than Q.

Inventors: ADACHI FUMIYUKI, ADACHI, FUMIYUKI
Applicants: NTT DOCOMO INC, NTT DOCOMO, INC.
Application year: 1997
Main classification: H04J11
IPC codes: H04J11, H04J13, H04B1, H04B7