Subtractive multi-user interference cancellation in a CDMA communications system

Abstract: In a DS-CDMA multiple user serial interference canceler unit, the sum of other user interference replica signals that have been generated and synthesized is transmitted to each user at each stage. An interference replica signal of a present user generated at a stage previous to a present stage is subtracted from the synthesized replica signal. The remaining interference replica signal is subtracted from the received signal, thus cancelling the other user interference.

Description: BACKGROUND OF THE INVENTION The present invention relates to a Direct Sequence Code Division Multiple Access (DS-CDMA) multiple user serial interference canceler and, more particularly to a method of transmitting an interference replica signal. DS-CDMA is used for example in digital mobile radio communications. A multiple user serial interference canceler has been proposed for reducing interference within a cell of a DS-CDMA, see Technical Report of The Institute of Electronics, Information and Communication Engineers (Japan) No. SAT95-14, RCS95-50 (July 1995), "Serial Canceler Using Recursive Channel Estimation by Pilot Symbols for DS-CDMA" by M. Sawahashi, Y. Miki, H. Andoh, and K. Higuchi and in document 'Pibt symbol-assisted coherent multistage interference canceller using recursive channel estimation for DS-CDMA mobile radio', by Sawahashi M.; Miki Y.; Andoh H.; Higochi K., Electronic Letters Online, Japan, 19. December 1996, vol. 32, no. 4, pages 301 to 302, a coherent multistage interference canceller with recursive channel estimation for DS-CDMA mobile radio is disclosed. In the above multiple user serial interference canceler, an interference replica signal of a user (other than a user k) (1≤ k ≤ K) that has been demodulated previous to demodulation of the user k is generated. The generated interference replica signal is subtracted from the received signal and the above process repeats N times (N : natural number ≥ 1, representing the number of stages) so that the influence owing to the other user's interference can be reduced. In a usual manner of transmitting the interference replica signal of other user, a bus line that can be shared and accessed by all panels is provided so that the interference replica signal from each panel is notified and synthesized with that of other panels. Fig. 3 is an example of the construction of a known interference canceler for the interference replica transmission through the common bus line. A reference numeral 31 is a received signal line, 32 is a common bus line for interference replica transmission, 33 is an interference canceler unit (including decoder), 34 is a delay memory for matching the timing between the received signal and the interference replica signal, 35 is a subtracter for subtracting the interference replica signal from the received signal, 36 is a processor for synthesizing the interference replica signal of other user, and 37 is a decoder. The common bus line 32 is connected to all users at all stages so that the interference replica signal generated by each user at each stage is transmitted to other users. Each user synthesizes the transmitted interference replica in the processor 36, which is subjected to interference cancellation through subtraction from the received signal in the subtracter 35. The resultant signal is transmitted to the interference canceler unit (hereinafter referred to as an ICU) 33. The decoder 37 at the last stage (third stage in Fig. 3) outputs the final decoding result. The parameter defining the transmission rate of the signal running through the common bus line 32 is spreading chip rate (spread spectrum coding is used), n sampling times, K users and N stages. The transmission rate obtained by (spreading chip rate x n sampling times x K user x N stage) is required. In a known method, the common bus line has been used to send the interference replica signal of each user to all users other than the present user. In the above method, a signal at a transmission rate KxN times higher than that of the interference replica signal equivalent to one user runs through the common bus line. For example, assuming that the transmission rate of the interference replica signal is 120 Mbps, the number of users is 16 and the number of stages is 3, respectively, the transmission rate required for the common bus line becomes 5.76 Gbps. Therefore it is practically difficult to design the circuit conforming to the above condition. In the above-described prior art, as generation of the interference replica signal proceeds to the latter phase (the number of ICU#1-K increases), a delay is accumulated. As a result, the received signal delay holder memory D for delaying and holding the received signal has to be increased. The whole system, thus, becomes unnecessarily large. For further background relative to DS-CDMA, reference may be made to European Patent Applications EP-A-696856 and 641102 and International Patent Application WO95/34140. SUMMARY OF THE INVENTION The invention is defined by the independent claims to which reference should now be made. Advantageous features are set forth in the dependent claims. The present invention uses no common bus line for transmission of other user's signal required for canceling interference. Instead the signal is transmitted through a parallel bus line or the like connecting adjacent users for successive processing and transferring the sum. The interference signal component of a present user is transferred and subtracted through a different transmission line. As a result, the signal can be transmitted practically in good condition without changing the principle on interference canceling operation, thus facilitating system construction. Also in the present invention, a signal containing the received signal that should be interference removed is first input by a first user at a first stage and then transmitted sequentially to subsequent phases. So the received signal does not have to be held by the respective users through a different line. Therefore a size of the memory for holding the reception signal in the latter stages of processing does not have to be increased, thus facilitating construction of the system without changing the principle on the interference cancel operation. BRIEF DESCRIPTION OF THE DRAWINGS This and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and drawings, in which: Fig. 1 is a view for explaining a method of transmitting an interference replica signal of an interference canceler of the present invention; Fig. 2 is a view showing an embodiment of an interference canceler unit of the present invention; and Fig. 3 is a view for explaining a method of transmitting an interference replica signal of an interference canceler of the prior art. DESCRIPTION OF THE EMBODIMENTS Next a first embodiment of the present invention is described in detail referring to the drawings. Fig. 1 shows an interference canceler according to the preferred embodiment of the present invention. A reference numeral 11 is a received signal transmission line, 12 is an interference replica signal transmission line for transmitting the interference replica signal to the adjacent user, 13 is an interference cancel unit (hereinafter referred to as ICU), 14 is a delay memory, 16 is an interference replica signal transmission line for transmitting the interference replica signal of each user at the nth stage to the present user at the next stage (the (n+1)th stage), 17 is a subtractor for subtracting the interference replica signal of the present user at a previous stage from the interference replica signal that has been transmitted in a form of a signal indicating the sum of each user's signals, 18 is an adder for synthesizing the present user's signal to the synthesized interference replica signal up to the previous user, 19 is a subtractor for subtracting the interference replica signal output by the previous user, and 20 is a decoder. Each user at each stage removes the signal component of the present user at the previous stage from the interference replica signal that has been previously generated. By subtracting the remaining synthetic interference replica signal from the received signal, the interference conponent of the interference replica signal can be cancelled. In the present invention, the interference replica signal of each user is not transmitted through the common bus line. The interference replica signal is transmitted through a parallel bus line or the like directly connecting the adjacent users at each stage. Immediately after the processing, the interference replica signal is transmitted to the next user sequentially. Interference replica signals of a plurality of users are not directly transmitted. The signal as the sum of the interference replica signals that have been generated is transmitted so as to reduce the transmission rate. Next the operation of the embodiment of the present invention is described referring to Fig. 1. In the ICU 13 (in this embodiment, ICU#1-1, ICU#1-2, ... ICU#1-K) of each user in the first stage, an interference replica signal generated by the ICU#1-1 is transmitted to the next adjacent ICU#1-2. The interference replica signal_is further transmitted to the next adjacent ICU#1-3 together with the interference replica signal generated by the ICU#1-2. The similar processing is continued to the last ICU#1-K in the stage. The sum of the interference replica signals generated by the ICU#1-1, ICU#1-2, ... and ICU#1-K is transmitted to the next stage. Concurrently the interference replica signal generated by the ICU 13 of each user is transmitted to the subtractor 17 connected to each interference cancel unit 13 (in this embodiment, #2-1, #2-2, ... #2-K) corresponding to the present user in the next stage (the second stage) through the transmission line 16. Any medium means allowing for high rate transmission, such as direct connection of parallel bus, direct connection of serial bus, optical fiber cable, twisted wiring or the like can be used as the transmission lines 12 and 16. The ICU 13 of each user at the second stage (ICU#2-1, ICU#2-2, ... ICU#2-K) is described with respect to the ICU#2-2. A signal input to the ICU#2-1 is a signal obtained by subtracting an interference replica signal generated by the ICU#1-1 from the sum of the interference replica signals generated by the respective ICU 13 (ICU#1-1, ICU#1-2, ... ICU#1-K) at the previous stage, i.e., first stage and further subtracting the remaining signal from the received signal. The signal output from the ICU#2-1 is the interference replica signal generated by the ICU#2-1. A signal input to the ICU#2-2 is a signal obtained by subtracting the interference replica signals generated by ICU#1-1 and ICU#1-2 from the sum of the interference replica signals generated by the respective ICU 13 at the previous stage, i.e., first stage (ICU#1-1, ICU#1-2 ... ICU#1-k) and further subtracting the remaining signal from the received signal. The signal output from the ICU#2-2 is the interference replica signal generated by the ICU#2-2. As described above, the similar signal processing is executed in the ICU 13 at the adjacent subsequent phases. The adder 18 connected to the last ICU#1-K in the second stage transmits the sum of the interference replica signals generated by the respective ICU 13 at the second stage to the next stage, i.e., third stage. In the third stage, the signal is input to the interference cancel unit (#3-1, #3-2) in the same manner as described above. The decoder 20 provides the final decoding result. In this method, the transmission rate of the interference replica signal transferred to each user becomes the same (n sampling times the spreading chip rate) as that of the interference replica signal for one user no matter how the number of the user K and the number of stages N are increased. The transmission rate of the signal transmitted through the transmission line 12 is the same as that of the interference replica signal of one user (spreading chip rate x n sampling times). Therefore the transmission rate is 1/(KxI) of that of the prior art, leading to easy circuit construction. The resent invention reduces data transmission rate between panels. As a result, data can be transferred at practically high rate, facilitating construction of the unit. Conventionally the interference replica signal is transmitted as a sum of bit amount of the interference replica signals generated by each ICU. As the present invention uses the adder, transmission can be executed at the same or slightly more bit amount compared with the bit amount of the interference replica signal generated by a single ICU. Next, an interference canceler according to a second embodiment of the present invention is described in detail referring to Figure 2 of the drawings. A reference numeral 21 is a reception signal, 22 is a received signal transmission line for transmitting the interference cancelled received signal to an adjacent panel, 23 is an interference cancel unit, 24 is a delay holder memory for delaying and holding the received signal (shown as D in Fig. 2), 25 is a subtractor, 26 is an interference replica signal transmission line for transmitting the interference replica signal of each user to a,present user at the next stage, 27 is an adder for adding a(n) (interference) replica signal of the present user at a previous stage, 28 is a delay holder memory for delaying and holding the present user's interference replica signal at the previous stage (shown as DD in Fig. 2) and 29 is a decoder for executing final decoding. The operation of the interference canceler unit of the second embodiment is now briefly described. A kth user at the nth stage receives an input of a signal obtained by subtracting an interference replica signal that has been previously generated from the received signal and further generates a new interference replica signal of the kth user for outputting. The input signal is allowed to delay for a period equivalent to the processing of one user. The interference replica signal generated by the kth user is subtracted from the input signal, which is transmitted to the next user, i.e., the (k+1)th user at the nth stage. As the interference replica signal of the (k+1)th user generated at the stage has been already subtracted from the transmitted signal, the signal at a previous stage where another line transmission is executed, i.e., the signal of the (k+1)th user at the (n-1) stage, is added before inputting to the next ICU for removing the signal. Then the signal is input to the ICU 23 of the (k+1)th user. In this method, the signal containing the received signal that has been first input to the first user at the first stage is transmitted sequentially to the subsequent phases. The received signal, thus, does not have to be held by each user through a different line, resulting in preventing the memory for holding the received signal from being enlarged in the latter phase of processing. Referring to Fig. 2, the above operation is further described. A reception signal r is first input to the ICU#1-1 of the first user at the first stage. An interference replica signal S 1,1 is generated and output. While the thus generated interference replica signal S 1,1 is subtracted from the reception signal r. That is the signal r-S 1,1 is transmitted to the ICU#1-2 of the next user, i.e., the second user at the first stage. The adder 27 connected to the ICU#n-k (not shown) of the kth user at the n (n: natural number ≥ 1)th stage receives the signal as shown in the following [number 1]. [Number 1] r-S n,1 -S n,2 - ... -S n,k-1 -S n-1,k -S n-1,k+1 .. -S n-1,k As the above [Number 1] shows, the reception signal r is always contained. So the reception signal does not have to be transmitted and held through a different line. The interference replica signal S n-1,k generated by the ICU 23 of the present user at a previous stage transmitted through the interference replica signal transmission line can be added to the signal shown in the [Number 1] and input to the ICU 23 of the present user at the stage. At a final stage, the decoder 29 set for each user provides the final decoding result. The above method eliminates the delay holder memory for holding the reception signal at an initial phase of the processing of each user at each stage. While this method requires the memory for holding the interference replica signal from the present user at the previous stage for a size equivalent to one stage. However the size of the memory requires only 1/N (N: number of stage) of the capacity irrespective of the number of the stage of the system compared with the conventional art. Therefore the circuit can be easily constructed. The present invention eliminates the memory for holding the reception signal, thus facilitating construction of the system.

Claims: A method of transmitting an interference replica signal of a DS-CDMA multiple user serial interference cancel unit comprising 1 st to Nth stages (N: natural number ≥ 2), and K users (K: natural number ≥1), the method comprising: transmitting to a first user at the nth (1≤n≤N) stage, a signal (S) comprising the sum of the interference replica signals of each user generated in the previous (n-1)th stage; for the first user in the nth stage, subtracting the interference replica signal from the first user in the (n-1)th stage from the signal (S) to produce a remainder signal; for the kth user in the nth stage, subtracting the interference replica signal from the kth user in the (n-1)th stage from a signal (S') received from the (k-1)th user in the nth stage to produce a remainder signal; for each user, subtracting said remainder signal from a received signal such that the interference from other users is cancelled. A method according to claim 1, comprising inputting the result of the subtraction of the received signal (r) and said remainder signal into an interference cancel unit (13) for each user. A method according to claim 1 or 2, comprising, adding an interference replica signal of each user at the nth stage (1≤n≤N) sequentially and transmitting the sum of the interference replica signals (S) to the next (n+1)th stage. A DS-CDMA multiple user serial interference cancel unit provided with 1st to Nth (N : natural number ≥ 2) stages, comprising: K (K: natural number ≥ 1) interference cancel units (13) provided in each stage; the first interference cancel unit in each stage receiving a signal (S) comprising the sum of the interference replica signals of each interference cancel unit generated in the previous (n-1)th stage; the kth interference cancel unit in each stage receiving a signal (S') from the (k-1)th interference cancel unit in the nth stage; a first subtractor (17) provided for each interference cancel unit (13), that subtracts the interference replica signal output from the corresponding interference cancel unit (13) in the previous (n-1)th stage from the signal (S') for the first unit, or the signal (S') for the kth unit to produce a remainder signal; a second subtractor (19) provided at an input side of each interference cancel unit (13), other than the first interference cancel unit in the first stage, that subtracts said remainder signal from a received signal, and outputs the result to the interference cancel unit (13); and an adder (18) provided at the output side of each interference cancel unit at each stage, other than the first interference cancel unit in the first stage, that adds the output of the interference cancel unit to said remainder signal. The DS-CDMA multiple user serial interference cancel unit of claim 4, further comprising: a first transmission line (12) for transmitting said signal (S) comprising the sum of interference replica signals added by the adder (18) to the first unit in the next stage; and a second transmission line (16) for transmitting the interference replica signal output from an interference cancel unit (13) in a previous (n-1)th stage, to the first subtractor (17) of a corresponding interference cancel unit (13) in the next nth stage. The DS-CDMA multiple user serial interference cancel unit (13) of claim 5, wherein each of said first (12) and said second (16) transmission lines comprise one of a parallel bus, a serial bus, an optical fibre cable or twisted wiring. A method of transmitting an interference replica signal of a DS-CDMA multiple user serial interference cancel unit provided with 1 st to Nth stages (N : natural number ≥ 2), each provided with K units (K : natural number ≥ 1) of interference replica signal generation means for generating an interference replica signal based on a received signal at each stage, the method comprising: transmitting a signal S, obtained by subtracting the interference replica signals from each user in the (n-1)th stage from a received signal, to the first user in the nth stage; transmitting an interference replica signal generated by each interference replica signal generation means (23) in the (n-1)th stage to a corresponding user in the nth stage; generating for each user in the nth stage a signal S" by adding the interference replica signal received from the corresponding user in the (n-1)th stage to the signal (S') received from the previous user in the nth stage; generating an interference replica signal based on the signal S" for each user; transmitting the interference replica signal for each user to the corresponding user in the (n+1)th stage. A method according to claim 7, comprising: transmitting the signal, obtained by subtracting the interference replica signal of each user in the nth stage from the received signal, to the first user in the (n+1)th stage. A method according to claim 7 or 8, comprising, for each user in the nth stage, decoding the received signal. A DC-CDMA multiple user serial interference cancel unit having 1st to Nth (N: natural number ≥ 2) stages, each stage comprising: K (K: natural number ≥ 1) interference replica signal generation means (23), connected in series, for generating an interference replica signal from a received signal; the last interference replica signal generation means (23) in one stage being connected to the first of the next; respective received signal delay holder memories (24), connected to each of the interference replica signal generation means (23), for delay and holding of a received signal (r) received from the signal during generation of the interference reception signal; means (21) for receiving a reception signal (r) at the first interference replica signal generation means (23); respective subtractors (25) for subtracting the interference replica signal generated by each interference replica signal generation means from the signal received from the previous interference generation means in the stage; respective adders (27) associated with each of the interference replica signal generation means (23) for receiving an interference replica signal (S') from the corresponding interference replica signal generating means (23) in the previous (n-1)th stage and adding this to the signal received from the (k-l)th interference signal generating means (23) in the present stage. A DS-CDMA multiple user serial interference cancel unit according to claim 10 comprising delay means (28) coupled to the respective adders (27) for delaying the interference replica signal generated by the interference replica signal generation means in the previous stage.

Inventors: SUZUKI HIDETO, SUZUKI, HIDETO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: H04B1
IPC codes: H04B1