FIELD OF THE INVENTION
This application claims priority under 35 U.S.C. § 119(e)(1) of provisional application Ser. No. 60/114,346, filed Dec. 29, 1998.
- BACKGROUND OF THE INVENTION
This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to a pseudorandom noise generator for generating a Long Code having an arbitrary delay.
Present wideband code division multiple access (WCDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. Base stations in adjacent cells or transmit areas also have a unique pseudorandom noise (PN) code associated with transmitted data. This PN code or Long Code is typically generated by a Linear Feedback Shift Register (LFSR), also known as a Linear Sequence Shift Register, and enables mobile stations within the cell to distinguish between intended signals and interference signals from other base stations. Identification of a PN code requires the mobile station to correctly identify an arbitrary part of the received PN sequence. The identification is frequently accomplished by a sliding window comparison of a locally generated PN sequence with the received part of the PN sequence. The sliding window algorithm often requires the mobile station to efficiently calculate multiple offsets from the LFSR to match the received sequence.
In another application of an LFSR, the base station typically generates a PN sequence for the forward link by a combination of one or more LFSRs 100, 120 as in FIG. 1. The mobile unit is also generates a PN sequence for the reverse link with LFSR circuits 200, 220 as in FIG. 2. This PN sequence is used for quadrature phase shift keyed (QPSK) reverse link transmission. This transmission requires that the PN sequence be arbitrarily shifted by the number of chips equivalent to 250 microseconds for transmitting the in-phase component and the quadrature component. This arbitrary shift may vary with data rate.
- SUMMARY OF THE INVENTION
Another application of an arbitrary offset LFSR arises for spreading and despreading transmitted signals as disclosed in U.S. Pat. No. 5,228,054 by Timothy I. Rueth and incorporated herein by reference. Rueth discloses an advantage of modulating each data bit at a constant chip rate for various transmit data rates. For example, a constant chip rate produces 128 chips for each bit at 9600 bits per second and 256 chips for each bit at 4800 bits per second. Thus, the chip rate may remain constant while the transmitted data rate may vary in response to rate information from a base station. Rueth further teaches that synchronization of base and mobile stations is simplified by inserting a zero in the PN sequence, thereby increasing the number of states from 2N−1 to 2N. Synchronization is further simplified by including an arbitrary offset circuit for the LFSR. Rueth teaches a mask circuit 30 in combination with an N-bit LFSR 10 (FIG. 2) for producing a PN offset with respect to the LFSR state. The mask circuit 30 produces the desired offset in response to a mask signal MASK on bus 32. Rueth gives a specific example of a particular mask signal for a 10-chip offset for an exemplary 4-bit LFSR (col. 7, lines 37-40). Rueth, however, fails to teach or suggest how the mask signal is generated for this specific case or how the mask signal might be generated for an LFSR of arbitrary length. Rueth states that “it would be simplest to implement if the paired values of OFFSET and MASK were pre-computed and stored in a Read Only memory (ROM) not shown.” (col. 8, lines 63-66). For a 15-bit LFSR, however, this would require 2N−2 (32,722) 15-bit masks. A particular problem with generation of this mask signal, therefore, is the need for a simple circuit to generate states with an arbitrary offset from an LFSR state. Other problems include the practical memory limitation of mobile handsets, calculation complexity of offset determination and speed and power requirements to generate the offset.
These problems are resolved by a circuit designed with a plurality of logic circuits for producing an offset state matrix. The circuit includes a first logic circuit coupled to receive N elements of a respective row of a transition matrix and N elements of column of an input state matrix. The first logic circuit produces a multi-bit logical combination of corresponding bits of the respective row and the column. A second logic circuit is coupled to receive the multi-bit logical combination and produces a respective element of the offset state matrix.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention produces a state vector with an arbitrary offset from an initial state vector with minimal power and gate delay. Memory storage requirements for transition matrices are minimized.
A more complete understanding of the invention may be gained by reading the subsequent detailed description with reference to the drawings wherein:
FIG. 1 is a simplified block diagram of a linear feedback shift register of the prior art;
FIG. 2 is a simplified block diagram of another linear feedback shift register of the prior art;
FIG. 3A is a block diagram of a PN generator circuit of the present invention;
FIG. 3B is a schematic diagram of an embodiment of a matrix multiplication circuit of FIG. 3A of the present invention;
FIG. 4 is a block diagram of a state generator circuit of the present invention for producing a plurality of state matrices separated by a predetermined offset;
FIG. 5 is a block diagram of another embodiment of a PN generator of the present invention;
FIG. 6 is a schematic diagram of yet another embodiment of a PN generator of the present invention that may be used for truncated state or zero insertion sequences;
FIG. 7A is a diagram of elements of a PN sequence generated by the circuit of FIG. 6;
FIG. 7B is a diagram of a vector of an exemplary PN sequence generated by the circuit of FIG. 6; and
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 7C is a diagram of another vector of the exemplary PN sequence generated by the circuit of FIG. 6.
Referring to FIG. 3A, there is a block diagram of a PN generator circuit of the present invention that may be used to generate an N-bit PN sequence corresponding to the LFSR 220 of FIG. 2. The N-stage PN generator circuit has 2 N−1 or approximately 2.2×1012 states. The PN generator circuit includes plural matrix generator circuits connected in series. The first matrix generator circuit receives an input state matrix S0 on bus 300. The last matrix generator circuit in the series produces an output state matrix Sn on bus 340. Each matrix generator circuit, for example the first matrix generator circuit, includes a matrix multiplication circuit 302 and a multiplex circuit 306. The matrix multiplication circuit is arranged to produce a matrix product of the respective input matrix and a respective stored matrix. The multiplex circuit produces one of the input state matrix and the matrix product in response to a respective count signal on lead 308.
In operation, the output state matrix Sn on bus 340 of the PN generator circuit is a matrix multiplication product of the initial state matrix S0 on bus 300 and a stored transition matrix. Alternatively, the stored transition matrix may be included in the matrix multiplication circuit as hard-wired combinatorial logic. This matrix multiplication is preferably a modulo-2 matrix multiplication for producing a state matrix or vector that is offset or delayed from the initial state matrix by the offset value. In general, this new state Sn is determined by equation .
S n =M n S 0 (1)
The state matrix Sn is offset or delayed from initial state matrix S0 by n states of the PN sequence. The transition matrix Mn is an initial transition matrix M1 raised to the power n. This transition matrix has a form determined by the PN sequence polynomial as will be explained in detail. A maximum length of the offset value is determined by a practical length of the total PN sequence as will be described in detail. The concept of the present invention, however, may be extended to any N-bit offset value for a corresponding N-stage LFSR. The offset value c on leads 308, 318, 328 and 338 may be represented in binary form as shown in equation .
n=c n−12n−1 +c n−22n−2 +. . . +c 121 +c 020 (2)
A transition matrix for producing an arbitrary offset n from initial state S0 is then represented by equation .
M n=(M2 n−1 )c n−2 x . . . x(M2 2 )c 2 ×(M 2 0 )c 0 (3)
Any transition matrix having an arbitrary n exponent, therefore, may be calculated by storing the matrices of equation  in memory circuits of matrix multiplication circuits 302, 312, 322 and 332. Any zero-value bit of the offset value, for example bit c0 on lead 308, produces the input state matrix S0 at the respective output bus 310. Alternatively, a one-value bit c0 of the delay value on lead 308 applies the matrix product on bus 304 of the respective transition matrix and the input matrix to the respective output matrix bus 310. This selective matrix multiplication continues at each matrix generator stage in response to the value of each respective bit of the offset signal. The final state matrix Sn at bus 340 may be any arbitrary offset with respect to the input state matrix So in response to the offset value.
This circuit is highly advantageous for efficiently producing a state vector having an arbitrary offset with respect to an initial state vector. Memory requirements are greatly reduced by storing only exponentially weighted matrices rather than the matrices for each desired offset. Moreover, computation time and power are minimized by use of combinatorial logic for modulo-2 matrix multiplication.
Referring now to FIG. 3B, there is a matrix multiplication circuit of the present invention that may be used with the matrix generator circuits of FIG. 3A. The matrix multiplication circuit includes n logic circuits 370-374 corresponding to elements of the state vector s11—s1n. Each logic circuit, for example logic circuit 370, receives row elements m11—m1n of a respective transition matrix and column elements s01—s0n of a respective input state matrix. The matrix multiplication circuit includes a first logic circuit 380-383 that performs a logical AND of corresponding row and column elements of the transition and state matrices, respectively. A second logic circuit 390 produces a logical exclusive-OR (XOR) of the multi-bit logical AND signal for each respective state matrix element sil. This circuit is highly advantageous in producing a matrix product with minimal gate propagation delay. The transition matrix may be stored in a memory circuit (not shown) as previously described, thereby providing programmability.
Alternatively, each element of the state output matrix might be generated by Boolean minimization. For example, the 18-bit LFSR 100 of the prior art (FIG. 1) produces a PN polynomial as in equation  where offset value C7 represents feedback tap 106.
G(x)=x 18 +c 7 x 7+1 (4)
An initial transition matrix M1
for this PN polynomial has the form of equation . The left column of the initial transition matrix includes zero elements m0,0
and a one in element m18,0
. The I of equation  indicates a 17×17 square identity matrix having ones from the upper-left m0,1
element along the diagonal to the lower-right m17,18
element and zeros elsewhere. The 18-element vector c corresponds to coefficients of the PN polynomial of equation  in elements m18,1
. Only element m18
corresponding to coefficient C7
, has a non-zero value.
Logic equations for each element of the matrix multiplication product of FIG. 3B have the general form of equation .
The predetermined form of the sparse transition matrix of equation , therefore, provides a highly advantageous matrix multiplication circuit. A first element of the offset state vector for the PN polynomial of equation , for example, is simply column element s01, since row element m01 is the only non-zero element in the first row of the initial transition matrix. Other matrix products are also realized with minimal logic due to the relatively sparse characteristic of each transition matrix. Thus, transition matrix storage as a hard-wired combinatorial logic circuit offers significant advantages in speed and simplicity and eliminates the need for programmable memory.
Turning now to FIG. 4, there is a block diagram of a state generator circuit of the present invention for producing a plurality of state matrices separated by a predetermined count or offset. The circuit includes a state matrix generator circuit as previously described in FIG. 3A. The state matrix generator circuit receives a state-input matrix S0 on bus 402 and a count signal Cki+j on bus 404. The state matrix generator circuit produces a k-bit state matrix Sn on bus 410 that is delayed from the state input matrix S0 by a number of states in the count signal. A plurality of m transition matrix multiplication circuits 406-408, similar to circuits 302, 312, 322 and 332 (FIG. 3A), are connected in series. Each matrix multiplication matrix circuit 406-408 includes a transition matrix multiplication circuit having a predetermined order n. Each matrix multiplication circuit 406-408 produces a respective state matrix delayed from a respective state input matrix by this predetermined order.
In operation, the count signal cki+j on bus 404 is initialized at a desired offset j. This initial count signal produces m output state matrices at buses 410, 412 and 414. Each output state matrix is delayed from the respective input state matrix by the predetermined order n of the transition matrix Mn. Index i is incremented to produce a count signal that is incremented in multiples of k from the initial offset j, where k is less than n. Thus, a sequence of m sets of state matrices are produced in parallel, each set having a predetermined offset from an adjacent set according to the order of the transition matrix Mn. Each set of the sequence further includes a sequence of k-bit state matrices. This circuit is highly advantageous in producing multiple PN sequences for matching with a received signal. Minimal logic is required and parallel sets are generated in a single clock cycle.
Referring now to FIG. 5, there is a block diagram of another embodiment of a PN generator of the present invention that may be used in lieu of the prior art circuit of FIG. 1 or FIG. 2. The PN generator circuit includes a first and a second series of matrix generator circuits arranged in parallel. Each series of matrix generator circuits includes preferably N stages having 2N−1 unique states. Each matrix generator circuit is similar to those previously described in FIG. 3A. Each matrix generator circuit includes, for example, a matrix multiplication circuit 502 and a multiplex circuit 506. The matrix multiplication circuit is arranged to produce a matrix product of the respective input matrix and a respective stored matrix. The multiplex circuit produces one of the input state matrix So and the matrix product in response to a respective count signal c0 on lead 535.
In operation, each matrix generator circuit of the first series includes a respective exponentially weighted transition matrix N1—N2n—1 having a form determined by a respective PN polynomial according to equation . Likewise, the second corresponding series includes a respective exponentially weighted transition matrix M1—M2n—1 having a form determined by a respective PN polynomial according to equation . The input state matrix S0 for the first series of matrix multiplication circuits is preferably an all-one vector. The input state matrix G0 for the second series of matrix multiplication circuits is preferably a Gold code sequence assigned to a specific base station. Corresponding matrix multiplication circuits from each of the first and second series receive a respective weighted count signal. This respective weighted count signal produces one of the respective input state matrix and the matrix product at a respective output bus as previously described. Each of the first and second series of matrix generator circuits produce respective output state matrices at buses 532 and 572, respectively, according to the count signal on leads 535-538. These respective output state matrices at buses 532 and 572 are applied to XOR circuit 576 to produce a PN sequence on bus 576 that is unique to the assigned Gold code and distinguished from that of other base stations in the receiving area. Thus, the PN generator circuit of the present invention is highly advantageous in producing a PN state in response to each count signal in a single clock cycle. Furthermore, speed and power are optimal with hard-wired transition matrix circuits.
A practical application of the previously described embodiments creates a need to generate modified PN sequences. These modified sequences include generation of truncated PN sequences and zero insertion in PN sequences as will be described in detail. The need to generate a truncated sequence arises when an application requires a periodic PN sequence that is a subset of a maximal length PN sequence. For example, an N-bit LFSR of the prior art has a maximal length of 2N—1, as previously described. Some applications, however, may require a periodic subset of length L, where L is less than 2N—1. In particular, WCDMA applications require generation of a periodic PN subset of length 40,960 from an 18-bit LFSR having a maximal length of 262,143. If the required PN sequence is generated one number at a time, then the circuit of the present invention simply counts to length L and restarts the count at zero, thereby producing the required PN subset. When the PN sequence is generated as a k-bit vector, however, the PN sequence may require truncation to preclude vector elements from exceeding the periodic length L.
Referring now to FIG. 6, there is a block diagram of a circuit of the present invention for generating a truncated sequence. The circuit receives an input state matrix S0 on bus 600. The PN generators 602 and 604 are the same as previously described in FIG. 3A, except that PN generator 602 receives offset or count signal zero at lead 603 and PN generator 618 receives offset of M at lead 619. The output of PN generator 602 is applied to shift register 606 via bus 604. The output of shift register 606 is subsequently applied to OR gate 610. The other input of OR gate 610 on bus 626 is an output of AND gate 624 produced by a combination of signal MASK on bus 622 and the output of PN generator 618 on bus 620. This output on bus 620 is also applied to multiplex circuit 614 together with the output of OR gate 610 on bus 612. Comparator circuit 632 compares offset M with the quantity L-k on lead 630 to produce a multiplex select signal on lead 634. If the offset M is greater than the quantity L-k, therefore, the signal on lead 634 is a logical one and the state matrix on bus 612 is produced as output state matrix Sn on bus 616. Alternatively, if the signal on lead 634 is a logical zero, the state matrix on bus 620 is produced as output state matrix Sn on bus 616.
Turning to FIG. 7A-7C, operation of the circuit of FIG. 6 will be explained in detail for a simplified example where L=20, k=8 and offset M=15. PN generator 602 receives a zero offset on lead 603 and produces state matrix A on bus 604, including 8 bits extending from bit 0 through bit 7. A value R=M−L+k or, for this specific example, R=15−20+8=3, is calculated. Shift register 606 then produces a logical left shift of k-R or 5 bits of the matrix A on bus 604 to produce the matrix C (FIG. 7B) on bus 608.
Offset M on lead 619 produces a state matrix B at the output of PN generator 618 on bus 620, including 8 bits extending from bit 15 through bit 22. The three most significant bits 20-22, however, are outside the permissible periodic range zero through L-1. The value R applies zeros to the three most significant bits of signal MASK on bus 622 and ones elsewhere. Thus, AND gate 624 produces state matrix D on bus 626. OR gate 610 combines input state matrices on buses 608 and 626 to produce a modified state matrix on bus 612. This modified state matrix includes original bits 15-19 as the least significant bits and original bits 0-2 as the most significant bits. Comparator circuit 632 determines if offset M is greater than L-k. If so, multiplex circuit 614 applies the modified state matrix on bus 612 to output bus 616. Otherwise, multiplex circuit 614 applies the unmodified PN sequence on bus 620 to output bus 616. The resulting output state matrix Sn on bus 616, therefore, is the same as the state matrix on bus 620 if the PN sequence is within the allowed range of values. Alternatively, when the PN sequence exceeds the allowed range of values, the output state matrix Sn on bus 616 wraps around to continue the sequence at the beginning of the allowed PN sequence.
Although the invention has been described in detail with reference to its preferred embodiment, it is to be understood that this description is by way of example only and is not to be construed in a limiting sense. For example, the circuit of FIG. 6 may also be used to advantageously insert a zero into the PN sequence, thereby producing a sequence having 2N states rather than 2N—1 states. Shifted and masked state matrices are produced on buses 608 and 626, respectively, as previously described. Comparator circuit 632 compares offset M to a predetermined offset for which zero insertion is desired. During normal operation, the state matrix on bus 620 is applied to output bus 616 via multiplex circuit 614. When a match is detected, multiplex circuit 614 applies a modified state matrix with inserted zero from bus 612 to output bus 616. Furthermore, any of the previously described embodiments may be implemented in software by a digital processor as will be appreciated by those of ordinary skill in the art having access to the instant specification. It is to be further understood that the inventive concepts of the present invention may be embodied in a mobile communication system as well as circuits within the mobile communication system. Moreover, numerous changes in the details of the embodiments of the invention will be apparent to persons of ordinary skill in the art having reference to this description. It is contemplated that such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.