Efficient multichannel filtering for CDMA modems

[0002] This application is being filed concurrently with an application entitled Code Division Multiple Access (CDMA) Communication System, Ser. No. 08/669,775, filed Jun. 27, 1996, which is herein incorporated by reference as if fully set forth.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to digital filtering techniques for code division multiple access telecommunication. More specifically, the invention relates to a high-speed, multichannel, finite impulse response filter architecture which obviates multipliers throughout the filter structure.

[0005] 2. Description of the Related Art

[0006] Communications technology today includes the use of spread spectrum modulation or CDMA (code division multiple access) for point-to-multipoint telecommunications. CDMA has long been used in military applications due to the difficulty to detect and jam the transmission. This attribute is due to a wireless communication technique that uses a transmission bandwidth much greater than the information bandwidth of a given user. All users communicate with each other or a common receiver over the same bandwidth and are identified by a particular code. Multiple access is provided through the sharing of a large common bandwidth thereby increasing overall system performance.

[0007] High tolerance to intentional or unintentional interference and the ability to communicate with a large population of users in a common geographical area make CDMA communication techniques attractive for commercial applications. Since each user in a CDMA communication system transmits and receives data or communication signals over the same frequency bandwidth, guard band requirements are lessened and the capacity of the communication system increases.

[0008] Each communication channel within the communication system typically uses DSP (digital signal processing) hardware and software to filter, weight, and combine each signal prior to transmission. The weighting, filtering and combining of multiple signal channels is performed in the transmit circuitry of a CDMA communication system base station.

[0009] Prior art CDMA modems require many multipliers and binary adders for channel weighting and combining. The filter operation used is equivalent to that of a FIR (finite impulse response or transversal) structure. Each individual FIR filter used also requires many multipliers and adders.

[0010] A multiplier implemented in digital form is inefficient and expensive. The expense is directly related to logic gate count. Binary adders are less costly than binary multipliers, however, their use should be minimized. To implement a design using binary multiplication and addition into an ASIC (application specific integrated circuit) would be expensive to manufacture and would result in a more inefficient and slower signal throughput.

[0011] One disadvantage of FIR filters is the computational complexity required for each output sample. For example, for each output sample, N multiply-accumulate (MAC) operations need to be performed. To those knowledgeable in the state of the art, disclosed in U.S. Pat. No. 4,811,262 (White) and U.S. Pat. No. 4,862,402 (Shaw et al.) are digital filter structures obviating multipliers. Both referenced patents disclose a reduction or elimination of multipliers in digital FIR filters by storing the weighting coefficients in memory. However, neither referenced filter structure, or the prior art has been optimized for multichannel operation.

[0012] The disadvantage with prior art CDMA modems is the ability to weight, filter, and combine a plurality of single bit valued signal channels efficiently and accurately. When a multiplicity of signal processing channels are involved, the consistency between channels becomes important and the cost of hardware per channel escalates.

[0013] In a CDMA communication system, it is necessary to use the minimum amount of power to achieve the minimum required BER (bit error rate) for maximum user capacity. Since CDMA communication systems allocate the same transmission bandwidth to all users, controlling the transmitted power of each user to the minimum required to maintain a given signal-to-noise ratio is paramount. Since each user employs a wide band signal occupying the entire frequency bandwidth for a finite duration, each user contributes to the overall background noise that effects all users. Therefore, the lack of power control will increase user-to-user interference.

[0014] Each channel must have appropriate individual weights applied so that the same relative amplitudes are transmitted. After the weighting operation, each data stream is represented by multibit values. These are typically summed together in a large digital summing circuit that consists of a tree of numerous two input adders.

[0015] The weighted and summed digital values are then filtered in a conventional FIR filter. The multipliers in the FIR process the multibit data and weighting coefficients to the desired precision.

[0016] A multichannel filter for a CDMA modem constructed according to the teachings of the prior art would require separate FIR integrated circuits rather than total integration onto an economical ASIC (application specific integrated circuit).

[0017] Accordingly, there exists a need for a multichannel CDMA modem FIR filter architecture which uses weighting coefficients, either fixed or variable through adaptation, operating with the accuracy and speed of multiplierless filters.

[0018] The efficient, multichannel filter for CDMA modems of the present invention allows multiple channels consisting of serial, digital bit streams to be filtered by digital signal processing techniques performing sample weighting and summing functions. Each individual channel may have custom weighting coefficients or weighting coefficients common for all channels. If the weighting coefficients are by adaption, the same approach may be taken.

[0019] The multichannel FIR filter presented is implemented with no multipliers and a reduction in the number of adders. To increase the speed of operation, the filter structure utilizes LUTs (look-up tables) storing the weighting coefficients. The invention can be constructed either as a FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The use of LUTs save significant chip resources and manufacturing costs.

[0020] Accordingly, it is an object of the present invention to provide an efficient CDMA FIR filter structure for multichannel applications.

[0021] It is a further object of the invention to provide a multichannel FIR filter structure of reduced complexity and increased performance.

[0022] If is a further object of the invention to provide a multiplierless, multichannel FIR filter.

[0023] Other objects and advantages of the system and method will become apparent to those skilled in the art after reading the detailed description of the preferred embodiment.

[0024] FIG. 1 is a block diagram of a typical, prior art, single input FIR filter.

[0025] FIG. 2 is a block diagram of a typical, prior art, single input FIR filter structure.

[0026] FIG. 3 is a block diagram of an alternative implementation of a prior art, single input FIR filter structure.

[0027] FIG. 4A is a block diagram of a single channel of a multichannel FIR filter.

[0028] FIG. 4B is a detailed block diagram of a multichannel FIR filter.

[0029] FIG. 5 is a block diagram showing a first refinement.

[0030] FIG. 6 is a block diagram showing a second refinement.

[0031] FIG. 7 is a block diagram of the multichannel processing element.

[0032] FIG. 8A is a global block diagram of a LUT table.

[0033] FIG. 8B is a detailed block diagram showing the multichannel LUT input of the present invention.

[0034] FIG. 9 is a detailed block diagram of the preferred embodiment.

[0035] A multichannel FIR filter for CDMA modems is described with reference to the drawing figures where like numerals represent like elements throughout. Such modems are used in multichannel wireless communication stations in conjunction with the transmission and reception of communication signals.

[0036] By way of background, many systems have the property of having their outputs at a given instant of time depend not only on the input at the time, but on the entire, or immediate history of the input. Such systems are said to have memory, averaging past and present samples in arriving at an output. It is necessary to separate systems with memory into the classes of discrete and continuous systems. A discrete system is one whose inputs and outputs are sequences of numerical values rather than continuous functions of time.

[0037] A sequence of discrete values can be represented as x_k, where the value x is a quantity such as voltage. The subscript k represents the sequence number. Very often in digital signal processing, x_krepresents a sampled waveform or signal where the subscript specifies the point in time at which the sample was taken. However, the subscript can represent an alternative meaning such as distance in a spatially sampled application. For a system to be physically realizable, the output must depend only on the present and past history of the input. No real system can have an output that depends on the future of the input. The dependence of the output of any physically realizable system on the input is indicated by

[0038] y_k=f(x_k, x_k−1, x_k−2, . . . , x_k−n),  (1)

[0039] where the input variables are x_k, the output variable is y_k, and f(*) is any arbitrary function of n+1 variables. Although this function is too broadly defined to be analyzed in general, the subset of linear operations becomes very useful for a plurality of signal processing applications. These functions also prove to be much more tractable in analysis.

[0040] If the output depends on the previous n samples of the input (a system having a finite memory) in a linear fashion, Equation (1) can be written as $\begin{array}{cc}{y}_k=\sum _{j=0}^N{a}_j{x}_{k-j}+b.& \left(2\right)\end{array}$

[0041] Such a linear system is characterized by the N+1 weighting variables a_j, and by the bias b. An unbiased, discrete linear system is characterized by the weighting variables (a₀, a₁, . . . , a_n). If the input x_kis a delta function (unity for one sample and zero for all others), it can be seen that the output of Equation (2) is the sequence of weighting variables ao, a₀, a₁, . . . , a_n. Therefore, the response to the input completely characterizes an unbiased, linear system.

[0042] There are certain types of linear systems with memory that can be analyzed using linear techniques. Even though digital signal processing is discrete by nature, if the input is samples of a continuous input and is sampled sufficiently fast, it is possible to simulate a continuous system using the samples as the input variables. The output then appears as a linear system with a long memory. One such system is a FIR filter 20. A fixed coefficient FIR filter is characterized by the input/output equation $\begin{array}{cc}{y}_k=\sum _{j=0}^{N-1}{c}_i{x}_{k-j},& \left(3\right)\end{array}$

[0043] as shown in FIG. 1, or expanded as

[0044] y_k=c₀x_k+c₁x_k−1+ . . . +c_k−1x_k−(N−1),  (4)

[0045] where the FIR filter has an impulse response c₀, c₁, . . . , x_krepresents the discrete input signal samples at time k, c_iare the filter coefficient weights, N are the number of taps, and y_krepresents the output at time k. As shown in FIG. 1, the block diagram forms a tapped delay line with the coefficients being known as tap weights.

[0046] Digital filters are presently a common requirement for digital signal processing systems. In the field of discrete systems, the most popular type of digital filter using convolution is the FIR. FIR filters have two advantages. The first is that FIR filters are inherently stable. The finite length of the impulse response guarantees that the output will go to zero within N samples. The second advantage is that FIR filters can be designed and implemented. The FIR filter 20 can be physically realized by using digital shift registers 22, multipliers 24 and summers 26 as shown in FIG. 2. The discrete signals 28 are shifted into registers 22 by a sampling clock pulse 30. The registers 22 hold past values 32 of the sampled signal 28 as well as present values 34 required for mathematical convolution. The past 32 and present 34 values are multiplied 24 by filter weighting coefficients 36, summed 26 and then output 38.

[0047] Another way of representing a FIR filter structure 20 is shown in FIG. 3. The operation described can be shown to be the equivalent of FIG. 2 since

[0048] A=c₃x_k−1,  (5)

[0049] B=c₃x_k−1+c₂x_k,  (6)

[0050] C=c₃x_k−2+c₂x_k−1,  (7)

[0051] resulting in $\begin{array}{cc}\begin{array}{c}D=y_k=c_3{x}_{k-3}+c_2{x}_{k-2}+c_1{x}_{k-1}+c_0{x}_k\\ =\sum _{j=0}^3{c}_j{x}_{k-j}\\ =c_k*x_k.\end{array}& \left(8\right)\end{array}$

[0052] As can be seen in FIGS. 2 and 3 the weighting 36 of the discrete input samples 28 relies upon many multipliers 24.

[0053] A single channel of a multichannel FIR filter 40 for CDMA modems is shown in simplified form in FIG. 4A. The multichannel FIR filter 40 is shown as a single element with a multichannel input sequence x_(i)kentering the filter 40 and the filtered result y_(i)kexiting. The subscript “i” identifies which channel from a plurality channels is being filtered.

[0054] The multiple single bit data/signal streams represent serial data streams that have been modulated with a pseudo noise (PN) code sequence. Each channel could represent user traffic channels at various data rates. Various types of signaling data might comprise other channels.

[0055] A typical example of an ISDN (integrated service digital network) CDMA modem would require five channels. Two channels would be 64 Kbps traffic channels (B1 and B2), a 16 Kbps auxiliary signaling and packet channel (D), an order wire channel (OW), and a reverse automatic power control channel (APC).

[0056] For maximum user capacity in a CDMA system it is necessary to use the minimum amount of power to achieve the required BER. Each channel must have the appropriate individual weight applied so that the correct relative amplitudes are transmitted. After the weighting operation the individual data streams become multibit values. The data streams are summed together in a large digital summing circuit that consists of a tree of numerous two input adders.

[0057] The weighted and summed digital values are then filtered in a conventional FIR filter. The FIR filter is required to pulse shape the input waveforms while suppressing out-of-band emissions. The multipliers in the FIR must handle the multibit data and coefficients to the desired precision.

[0058] In FIG. 4B, four signal channels are input individually into separate FIR filters 20 (the clock signal has been omitted for clarity). The individually filtered signals are then weighted using multipliers 24 with a channel specific weighting coefficient 37 w_(i)for power control (equalizing the power or gain between individual channels) before being input to a multichannel summer 46. Since all users occupy the same frequency spectrum and time allocation in spread spectrum communication systems, it is desired that each user is received with the same power level. The result, y_(i)k44, is a weighted sum of the individually FIR filtered multiple signal channels.

[0059] A CDMA transmitter combines many channels of varying types of digital signals (serial digital voice, power control, ISDN data). Typically, each channel is modulated with a different spreading code. The spreading code allows a CDMA receiver to recover the combined signals by use of the proper code during demodulation. Alternatively, any set of orthogonal functions could be combined with the preferred embodiment and later separated by correlation.

[0060] The output 44 of the multichannel FIR filter 40 is a weighted and filtered average. Although each channel has been described as a single bit valued serial data stream, multi-bit values or levels may be processed with the identical multichannel filter structure.

[0061] Referencing FIG. 5, the multichannel FIR filter 40 is shown using four tap FIR filters 48. The weighting of the discrete samples is performed by conventional multipliers 24. Each FIR structure is comprised of shift registers 22 and summers 26 for past 32 and present 34 sampled signals. Each tap weight coefficient 36 is multiplied by the respective channel power control weighting factor 37. The result is the same as shown in FIG. 4B, but with the external multipliers inside the FIR 48 structures.

[0062] Hardware reduction is accomplished by sharing FIR registers and adders as shown in FIG. 6. Each multichannel processing element 52 performs part of the channel weighting 37, the FIR tap coefficient 36 multiply 24, and the summing 26 of the multiple channels for that tap. The partitioning of the discrete functions reveals the preferred embodiment.

[0063] FIG. 7 shows the multichannel processing element 52 as a processing block with “N” single bit input signals x_(o)k, x_(1)k, . . . , x(_N)k. The computed output z_k54 contains “W” bits of resolution. The discrete input signals 28 form a vector. This vector can be assigned an overall value by weighting each bit with an increasing power of two. In the alternative, the multichannel signal bits are treated as a binary valued word. The output of the processing block is a “W” bit wide function of the N bit binary input argument. The block performs the equivalent logical function of a memory device where the input signal bits form an address and the computed values are contents of the selected memory word. A memory based Look-Up-Table (LUT) 56 can perform an arbitrary function quickly and efficiently as shown in FIG. 8A.

[0064] A mathematical function f of an argument x with a result of y is expressed as y=f(x). The function performs a mapping of all values of x into another space of y values. A LUT performs this mapping for the values of interest in the preferred embodiment. The LUT memory device is presented with an address of a location within the memory circuit. The value previously stored at that location is delivered to the memory output data bus. The values of interest of x, which are discrete, are mapped into a binary number. Since the multichannel signals are represented by zero or one logic levels, they are used as bits to form a binary number. Every possible combination of channel values is therefore assigned a state number. This operation is represented as $\begin{array}{cc}\begin{array}{c}A=\sum _{j=0}^{M-1}{x}_j{2}^j=x_{M-1}{2}^{M-1}+\dots x_3{2}^3+x_2{2}^2+x_1{2}^1+x_0{2}^0\\ =x_{M-1}{2}^{M-1}+\dots x_{3}8+x_{2}4+x_{1}2+x_0.\end{array}& \left(9\right)\end{array}$

[0065] Each state is a binary number that references an address in the LUT. The output value from the LUT is the precomputed value of the function resultant that would occur given the argument corresponding to that address. This is illustrated as a tabular representation of the LUT contents. The function to be performed is the weighted sum of the multiple channels for a given single tap of the FIR structure.

[0066] For example, in an application using 4 channels (M=4), the LUT contents located at the 2nd tap of the multichannel FIR (j=2) would be as shown in Table 1.

[0067] The LUT 56 memory words contain precomputed values corresponding to the current input address value as shown in FIG. 8B. The memory can be implemented in either ROM or RAM, depending upon the application.

[0068] In the preferred embodiment, ROM (read only memory) is used to store permanent LUT values. This is implemented efficiently as an integrated circuit. ROM is appropriate for time invariant systems where the required channel weights and filter coefficients are known a priori. RAM (random access memory) allows new values to be written over old. LUT values can be computed and loaded to achieve adaptivity. RAM is not as space efficient as ROM but is still efficient considering the increased flexibility.

[0069] The preferred embodiment of the multichannel FIR filter 40 for CDMA modems according to the present invention is shown in FIG. 9. The filter structure uses LUTs 56 rather than the inefficient multichannel processing elements 52 which require a plurality of multipliers 24 and summers 26.

[0070] The signal bits form the address word which is applied to the LUT 56. There is a LUT 56 for each filter tap required. The contents of each LUT 56 is computed $\begin{array}{cc}{L}_j\left(D_N,D_{N-1}\dots ,D_2,D_1\right)=C_j\sum _{j=1}^N{D}_i{W}_i.& \left(10\right)\end{array}$

[0071] As shown, any combination of signal values has its weighted sum precomputed. The multiplication of each tap coefficient of the FIR function is included in the precomputed table.

[0072] The weighted and filtered single channel operation of FIG. 4A with and N tap FIR can be expressed as $\begin{array}{cc}{y}_{\left(i\right)k}=w_i\sum _{j=0}^{N-1}{c}_{\left(i\right)j}{x}_{\left(i\right)k-j}=w_i\left[c_{\left(i\right)j}*x_{\left(i\right)j}\right].& \left(11\right)\end{array}$

[0073] An M channel multichannel version of this is shown in FIG. 4B and can be expressed as $\begin{array}{cc}{y}_{\left(i\right)k}=\sum _{i=0}^{M-1}{y}_{\left(i\right)k}=\sum _{i=0}^{M-1}\left(w_i\sum _{i=0}^{M-1}{c}_{\left(i\right)j}{x}_{\left(i\right)k-j}\right),& \left(12\right)\\ y_{\left(i\right)k}=\sum _{i=0}^{M-1}{w}_i\left[c_{\left(i\right)j}*x_{\left(i\right)j}\right].& \left(13\right)\end{array}$

[0074] This is the desired weighted sum of convolutions or FIR filtering operations. The convolution is performed in FIR filters 20, the weighting in multipliers 24 and the summation in adders 46. The convolution achieved is identical to that originally presented in Equation 3. The summation and weights are a result of the extension to a multichannel process.

[0075] The preferred embodiment shows an improved filter for multichannel CDMA FIR filtering modem applications. It has been shown that the signal processing operation over multiple channels, as shown in FIG. 4, can be implemented using no multipliers and a reduced number of adders.

[0076] While specific embodiments of the present invention have been shown and described, many modifications and variations could be made by one skilled in the art without departing from the spirit and scope of the invention. The above description serves to illustrate and not limit the particular form in any way.