WO1997050173A1 - Efficient multichannel filtering for cdma modems - Google Patents
Efficient multichannel filtering for cdma modems Download PDFInfo
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- WO1997050173A1 WO1997050173A1 PCT/US1997/010909 US9710909W WO9750173A1 WO 1997050173 A1 WO1997050173 A1 WO 1997050173A1 US 9710909 W US9710909 W US 9710909W WO 9750173 A1 WO9750173 A1 WO 9750173A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/0223—Computation saving measures; Accelerating measures
- H03H17/0225—Measures concerning the multipliers
- H03H17/0226—Measures concerning the multipliers comprising look-up tables
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
- H04B1/70751—Synchronisation aspects with code phase acquisition using partial detection
- H04B1/70753—Partial phase search
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
- H04B1/70755—Setting of lock conditions, e.g. threshold
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
- H04B1/70758—Multimode search, i.e. using multiple search strategies
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
- H04B1/708—Parallel implementation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/709—Correlator structure
- H04B1/7093—Matched filter type
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/06—Non-recursive filters
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
- H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
- H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
- H04B2201/70702—Intercell-related aspects
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
- H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
- H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
- H04B2201/70707—Efficiency-related aspects
Definitions
- CDMA Code Division Multiple Access
- 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.
- CDMA code division multiple access
- 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. 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.
- 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.
- 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.
- 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.
- FIR filters 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.
- MAC multiply-accumulate
- U.S. Patent No. 4,811,262 White
- U.S. Patent No. 4,862,402 Shaw et al.
- Both referenced patents disclose a reduction or elimination of multipliers in digital FIR filters by storing the weighting coefficients in memory.
- neither referenced filter structure, or the prior art has been optimized for multichannel operation.
- 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.
- each channel must have appropriate individual weights applied so that the same relative amplitudes are transmitted.
- 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.
- 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.
- 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) .
- 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.
- the multichannel FIR filter presented is implemented with no multipliers and a reduction in the number of adders.
- the filter structure utilizes LUTs (look-up tables) storing the weighting coefficients.
- LUTs look-up tables
- 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.
- Figure 1 is a block diagram of a typical, prior art, single input FIR filter.
- Figure 2 is a block diagram of a typical, prior art, single input FIR filter structure.
- Figure 3 is a block diagram of an alternative implementation of a prior art, single input FIR filter structure.
- Figure 4A is a block diagram of a single channel of a multichannel FIR filter.
- Figure 4B is a detailed block diagram of a multichannel FIR filter.
- Figure 5 is a block diagram showing a first refinement .
- Figure 6 is a block diagram showing a second refinement .
- Figure 7 is a block diagram of the multichannel processing element.
- Figure 8A is a global block diagram of a LUT table.
- Figure 8B is a detailed block diagram showing the multichannel LUT input of the present invention.
- Figure 9 is a detailed block diagram of the preferred embodiment .
- 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.
- 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.
- x k represents a sampled waveform or signal where the subscript specifies the point in time at which the sample was taken.
- the subscript can represent an alternative meaning such as distance in a spatially sampled application.
- 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
- the input variables are x k
- the output variable is y k
- f(*) is any arbitrary function of n+1 variables.
- Equation (1) can be written as
- Such a linear system is characterized by the N+l weighting variables a j t and by the bias Jb.
- An unbiased, discrete linear system is characterized by the weighting variables (a 0 , a l t . .. , a n ) . If the input x k is 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 a 0 , a ⁇ , . . . , a réelle . Therefore, the response to the input completely characterizes an unbiased, linear system.
- the FIR filter has an impulse response c 0 , c l t ... , x k represents the discrete input signal samples at time k, c t are the filter coefficient weights, N are the number of taps, and y k represents the output at time k.
- the block diagram forms a tapped delay line with the coefficients being known as tap weights.
- 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 Figure 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.
- a single channel of a multichannel FIR filter 40 for CDMA modems is shown in simplified form in Figure 4A.
- the multichannel FIR filter 40 is shown as a single element with a multichannel input sequence x U)k entering the filter 40 and the filtered result y ajjc exiting.
- the subscript "i" identifies which channel from a plurality channels is being filtered.
- the multiple single bit data/signal streams represent serial data streams that have been modulated with a pseudo noise (PN) code sequence.
- PN pseudo noise
- Each channel could represent user traffic channels at various data rates.
- Various types of signaling data might comprise other channels.
- a typical example of an ISDN (integrated service digital network) CDMA modem would require five channels. Two channels would be 64 Kbps traffic channels (Bl and B2) , a 16 Kbps auxiliary signaling and packet channel (D) , an order wire channel (OW) , and a reverse automatic power control channel
- 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.
- 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.
- 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 (ij 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)k 44, is a weighted sum of the individually FIR filtered multiple signal channels.
- a CDMA transmitter combines many channels of varying types of digital signals (serial digital voice, power control, ISDN data) .
- 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.
- any set of orthogonal functions could be combined with the preferred embodiment and later separated by correlation.
- the output 44 of the multichannel FIR filter 40 is a weighted and filtered average.
- 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.
- Hardware reduction is accomplished by sharing FIR registers and adders as shown in Figure 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.
- Figure 7 shows the multichannel processing element 52 as a processing block with n N" single bit input signals x lo!k , x (1)k , ... , X (N)k -
- the computed output z k 54 contains "IV" 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.
- the multichannel signal bits are treated as a binary valued word.
- the output of the processing block is a *IV" bit wide function of the -V 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 Figure 8A.
- LUT Look-Up- Table
- 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
- 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.
- the LUT 56 memory words contain precomputed values corresponding to the current input address value as shown in Figure 8B.
- the memory can be implemented in either ROM or RAM, depending upon the application.
- ROM read only memory
- RAM random access memory
- FIG. 40 for CDMA modems according to the present invention is shown in Figure 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.
- the signal bits form the address word which is applied to the LUT 56.
- 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.
- 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 Figure 4, can be implemented using no multipliers and a reduced number of adders.
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- Signal Processing (AREA)
- Physics & Mathematics (AREA)
- Computer Hardware Design (AREA)
- Mathematical Physics (AREA)
- Computing Systems (AREA)
- Theoretical Computer Science (AREA)
- Filters That Use Time-Delay Elements (AREA)
- Communication Control (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
- Input Circuits Of Receivers And Coupling Of Receivers And Audio Equipment (AREA)
- Telephonic Communication Services (AREA)
- Networks Using Active Elements (AREA)
- Circuits Of Receivers In General (AREA)
- Mobile Radio Communication Systems (AREA)
- Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
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Priority Applications (9)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
CA002259261A CA2259261C (en) | 1996-06-27 | 1997-06-24 | Efficient multichannel filtering for cdma modems |
EP97931346A EP0908008B1 (en) | 1996-06-27 | 1997-06-24 | Efficient multichannel filtering for cdma modems |
AT97931346T ATE240611T1 (en) | 1996-06-27 | 1997-06-24 | EFFICIENT MULTI-CHANNEL FILTERING FOR CDMA MODEMS |
AU34996/97A AU3499697A (en) | 1996-06-27 | 1997-06-24 | Efficient multichannel filtering for cdma modems |
JP50345398A JP3550598B2 (en) | 1996-06-27 | 1997-06-24 | Efficient multi-channel filtering for CDMA modems |
DE0908008T DE908008T1 (en) | 1996-06-27 | 1997-06-24 | EFFICIENT MULTI-CHANNEL FILTERING FOR CDMA MODEMS |
DK97931346T DK0908008T3 (en) | 1996-06-27 | 1997-06-24 | Effective multi-channel filtration for CDMA modems |
DE69721991T DE69721991T2 (en) | 1996-06-27 | 1997-06-24 | EFFICIENT MULTI-CHANNEL FILTERING FOR CDMA MODEMS |
HK99102017A HK1017175A1 (en) | 1996-06-27 | 1999-05-05 | Efficient multichannel filtering for cdma modems |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US08/670,160 US6487190B1 (en) | 1996-06-27 | 1996-06-27 | Efficient multichannel filtering for CDMA modems |
US08/670,160 | 1996-06-27 |
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WO1997050173A1 true WO1997050173A1 (en) | 1997-12-31 |
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PCT/US1997/010909 WO1997050173A1 (en) | 1996-06-27 | 1997-06-24 | Efficient multichannel filtering for cdma modems |
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US (3) | US6487190B1 (en) |
EP (3) | EP1337040B1 (en) |
JP (1) | JP3550598B2 (en) |
KR (1) | KR100383266B1 (en) |
CN (1) | CN1117431C (en) |
AT (2) | ATE335304T1 (en) |
AU (1) | AU3499697A (en) |
CA (2) | CA2434522A1 (en) |
DE (3) | DE908008T1 (en) |
DK (1) | DK0908008T3 (en) |
ES (2) | ES2268207T3 (en) |
HK (2) | HK1017175A1 (en) |
WO (1) | WO1997050173A1 (en) |
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EP1039635A1 (en) * | 1999-03-20 | 2000-09-27 | Motorola, Inc. | Multi channel filtering device and method |
EP1113577A2 (en) * | 1999-12-28 | 2001-07-04 | Nec Corporation | Variable-gain digital filter |
EP1443646A2 (en) | 2003-01-28 | 2004-08-04 | Agere Systems Inc. | Multi-dimensional hybrid and transpose form finite impulse response filters |
KR100585644B1 (en) * | 1999-12-01 | 2006-06-02 | 엘지전자 주식회사 | Low power finite impulse response filter |
EP1983656A1 (en) * | 2007-04-19 | 2008-10-22 | MediaTek Inc. | Shared filter design for pilot symbol averaging in rake fingers in WCDMA systems |
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US6487190B1 (en) * | 1996-06-27 | 2002-11-26 | Interdigital Technology Corporation | Efficient multichannel filtering for CDMA modems |
US6885652B1 (en) | 1995-06-30 | 2005-04-26 | Interdigital Technology Corporation | Code division multiple access (CDMA) communication system |
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- 1997-06-24 KR KR10-1998-0710703A patent/KR100383266B1/en not_active IP Right Cessation
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EP0935189A2 (en) * | 1998-02-04 | 1999-08-11 | Texas Instruments Incorporated | Reconfigurable co-processor with multiple multiply-accumulate units |
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US6658440B1 (en) | 1999-03-20 | 2003-12-02 | Motorola, Inc. | Multi channel filtering device and method |
KR100585644B1 (en) * | 1999-12-01 | 2006-06-02 | 엘지전자 주식회사 | Low power finite impulse response filter |
EP1113577A2 (en) * | 1999-12-28 | 2001-07-04 | Nec Corporation | Variable-gain digital filter |
EP1113577A3 (en) * | 1999-12-28 | 2004-09-22 | Nec Corporation | Variable-gain digital filter |
US6985522B2 (en) | 1999-12-28 | 2006-01-10 | Nec Corporation | Variable-gain digital filter |
EP1443646A2 (en) | 2003-01-28 | 2004-08-04 | Agere Systems Inc. | Multi-dimensional hybrid and transpose form finite impulse response filters |
EP1983656A1 (en) * | 2007-04-19 | 2008-10-22 | MediaTek Inc. | Shared filter design for pilot symbol averaging in rake fingers in WCDMA systems |
US8027375B2 (en) | 2007-04-19 | 2011-09-27 | Mediatek Inc. | Shared filter design for pilot symbol averaging in rake fingers in WCDMA systems |
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