AU2001249558A1

AU2001249558A1 - CDMA system which uses pre-rotation before transmission

Info

Publication number: AU2001249558A1
Application number: AU2001249558A
Authority: AU
Inventors: John D. Kaewell
Original assignee: InterDigital Technology Corp
Current assignee: InterDigital Technology Corp
Priority date: 2000-03-28
Filing date: 2001-03-28
Publication date: 2001-12-20
Anticipated expiration: 2021-03-28

Description

[0001] CDMA SYSTEM WHICH USES

PRE-ROTATION BEFORE TRANSMISSION

[0002] BACKGROUND

[0003] The present invention relates generally to digital communications. More

specifically, the invention relates to a system and method for pre-rotating a digital

spread spectrum signal prior to transmission in order to improve receiver accuracy and

recovery of the phase and frequency information by the receiver.

[0004] Many current communication systems use digital spread spectrum

modulation or code divisional multiple access (CDMA) technology. Digital spread

spectrum is a communication technique in which data is transmitted with a broadened

band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise

signal. CDMA can transmit data without being affected by signal distortion or an

interfering frequency in the transmission path.

[0005] Shown in Figure 1 is a simplified CDMA communication system that

involves a single communication channel of a given bandwidth which is mixed by a

spreading code which repeats a predetermined pattern generated by a pseudo-noise

(pn) sequence generator. A data signal is modulated with the pn sequence to produce

digital spread spectrum signal. A carrier signal is modulated with the digital spread

spectrum signal to establish a forward link and is then transmitted. A receiver

demodulates the transmission to extract the digital spread spectrum signal. The same

process is repeated to establish a reverse link. [0006] During terrestrial communication, a transmitted signal is typically

disturbed by reflections due to varying terrain and environmental conditions and man-

made obstructions. Thus, a single transmitted signal produces a plurality of received

signals with differing time delays at the receiver, an effect which is commonly known

as multipath distortion. During multipath distortion, the signal from each different

path arrives delayed at the receiver with a unique amplitude and carrier phase.

[0007] In the prior art, the error associated with multipath distortion is typically

corrected at the receiver after the signal has been correlated with the matching pn

sequence and the transmitted data has been reproduced. Thus, the correlation is

completed with error incorporated in the signal. Similar multipath distortion affects

the reverse link transmission.

[0008] Accordingly, there exists a need for a system that corrects a signal for

errors encountered during transmission.

[0009] SUMMARY

[0010] The present invention relates to a digital spread spectrum

communication system that calculates phase and frequency error on a received signal

from a communicating entity during a wireless communication and pre-corrects a

signal for phase and frequency error prior to transmission to that entity. [0011] BRIEF DESCRIPTION OF THE DRAWING(S)

[0012] Figure 1 is a simplified block diagram of a prior art CDMA

communication system.

[0013] Figure 2 is a detailed block diagram of a B-CDMA™ communication

system.

[0014] Figure 3 A is a detailed block diagram of the present invention using one

pseudo-pilot signal, with carrier-offset correction implemented at the chip level.

[0015] Figure 3B is a block diagram of a rake receiver.

[0016] Figure 4 is a diagram of a received symbol p₀ on the QPSK constellation

showing a hard decision.

[0017] Figure 5 is a diagram of the angle of correction corresponding to the

assigned symbol.

[0018] Figure 6 is a diagram of the resultant symbol error after applying the

correction corresponding to the assigned symbol.

[0019] Figure 7 is a block diagram of a conventional phase-locked loop.

[0020] Figure 8 A is a simple block diagram of a transmitter in accordance with

the preferred embodiment of the present invention.

[0021 ] Figure 8B is a simple block diagram of a transmitter in accordance with

an alternative embodiment of the present invention.

[0022] Figure 8 C is a simple block diagram of a transmitter in accordance with

an alternative embodiment of the present invention. [0023]DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The preferred embodiment will be described with reference to the

drawing figures where like numerals represent like elements throughout.

[0025] A CDMA communication system 25 as shown in Figure 2 includes a

transmitter 27 and a receiver 29, which may reside in either a base station or a mobile

user receiver. The transmitter 27 includes a signal processor 31 which encodes voice

and nonvoice signals 33 into data at various rates, e.g. data rates of 8 kbps, 16 kbps,

32 kbps, or 64 kbps. The signal processor 31 selects a specific data rate depending

upon the type of signal, or in response to a set data rate.

[0026] By way of background, two steps are involved in the generation of a

transmitted signal in a multiple access environment. First, the input data 33 which can

be considered a bi-phase modulated signal is encoded using forward error-correction

(FEC) coding 35. For example, if a convolution code is used, the single bi¬

phase modulated data signal becomes bivariate or two bi-phase modulated signals.

One signal is designated the in-phase (I) channel 41a. The other signal is designated

the quadrature (Q) channel 41b. A complex number is in the form a+bj, where a and

b are real numbers and j²=- 1. Bi-phase modulated I and Q signals are usually referred

to as quadrature phase shift keying (QPSK). In the preferred embodiment, the tap

generator polynomials for a constraint length of K=7 and a convolutional code rate of

[0027] In the second step, the two bi-phase modulated data or symbols 41 a, 4 lb

are spread with a complex pseudo-noise (pn) sequence. The resulting 145a and Q 45b spread signals are combined 53 with other spread signals (channels) having different

spreading codes, mixed with a carrier signal 51 and then transmitted 55. The

transmission 55 may contain a plurality of individual channels having different data

rates.

[0028] The receiver 29 includes a demodulator 57a, 57b which downconverts

the transmitted broadband signal 55 into an intermediate frequency signal 59a, 59b.

A second downconversion reduces the signal to baseband. The QPSK signal is then

filtered 61 and mixed 63a, 63b with the locally generated complex pn sequence 43a,

43b which matches the conjugate of the transmitted complex code. Only the original

waveforms which were spread by the same code at the transmitter 27 will be

effectively despread. Others will appear as noise to the receiver 29. The data 65a,

65b is then passed onto a signal processor 67 where FEC decoding is performed on

the convolutionally encoded data.

[0029] When the signal is received and demodulated, the baseband signal is at

the chip level. Both the I and Q components of the signal are despread using the

conjugate of the pn sequence used during spreading, returning the signal to the symbol

level. However, due to carrier offset, phase corruption experienced during

transmission manifests itself by distorting the individual chip waveforms. If carrier

offset correction is performed at the chip level overall accuracy increases due to the

inherent resolution of the chip-level signal. Carrier offset correction may also be

performed at the symbol level but with less overall accuracy. However, since the symbol rate is much less than the chip rate, a lower overall processing speed is

required when the correction is done at the symbol level.

[0030] As shown in Figure 3 A, a receiver using the system 75 and method of

the present invention is shown. A complex baseband digital spread spectrum signal

77 comprised of in-phase and quadrature phase components is input and filtered using

an adaptive matched filter (AMF) 79 or other adaptive filtering means. The AMF 79

is a transversal filter (finite impulse response) which uses filter coefficients 81 to

overlay delayed replicas of the received signal 77 onto each other to provide a filtered

signal output 83 having an increased signal-to-noise ratio (SNR). The output 83 of

the AMF 79 is coupled to a plurality of channel despreaders 85,, 85₂, 85_n and a pilot

despreader 87. The pilot signal 89 is despread with a separate despreader 87 and pn

sequence 91 contemporaneous with the transmitted data 77 assigned to channels

which are despread 85,, 85₂, 85_n with pn sequences 93,, 93₂, 93_n of their own. After

the data channels are despread 85„ 85₂, 85_n, the data bit streams 95,, 95₂, 95_n are

coupled to Viterbi decoders 97,, 97₂, 97_n and output 99 , 99₂, 99_n.

[0031 ] The filter coefficients 81 , or weights, used in adjusting the AMF 79 are

obtained by the demodulation of the individual multipath propagation paths. This

operation is performed by a rake receiver 101. The use of a rake receiver 101 to

compensate for multipath distortion is well known to those skilled in the

communication arts.

[0032] As shown in Figure 3B, the rake receiver 101 consists of a parallel

combination of path demodulators "fingers" 103₀, 103,, 103₂, 103_n which demodulate a particular multipath component. The pilot sequence tracking loop of a particular

demodulator is initiated by the timing estimation of a given path as determined by a

pn sequence 105. In the prior art, a pilot signal is used for despreading the individual

signals of the rake. In the present invention, the pn sequence 105 may belong to any

channel 93₁ of the communication system. Typically, the channel with the largest

received signal is used.

[0033] Each path demodulator includes a complex mixer 107₀, 107,, 107₂, 107_n,

and summer and latch 109₀, 109,, 109₂, 109_n. For each rake element, the pn sequence

105 is delayed τ l l l_l5 111₂, l l l_n by one chip and mixed 107_l5 107₂, 107_n with the

baseband spread spectrum signal 113 thereby despreading each signal. Each

multiplication product is input into an accumulator 109₀, 109_l5 109₂, 109_n where it is

added to the previous product and latched out after the next symbol-clock cycle. The

rake receiver 101 provides relative path values for each multipath component. The

plurality of n-dimension outputs 115₀, 115_l5 115₂, 115_n provide estimates of the

sampled channel impulse response that contain a relative phase error of either 0 ° , 90 ° ,

180°, or 270°.

[0034] Referring back to Figure 3A, the plurality of outputs from the rake

receiver are coupled to an n-dimensional complex mixer 117. Mixed with each rake

receiver 101 output 115 is a correction to remove the relative phase error contained

in the rake output.

[0035] A pilot signal is also a complex QPSK signal, but with the quadrature

component set at zero. The error correction 119 signal of the present invention is derived from the despread channel 95 , by first performing a hard decision 121 on each

of the symbols of the despread signal 95j. A hard decision processor 121 determines

the QPSK constellation position that is closest to the despread symbol value.

[0036] As shown in Figure 4, the Euclidean distance processor compares a

received symbol p₀ of channel 1 to the four QPSK constellation points x, _l5 x_, ,, x_,

.,, x_x .,. It is necessary to examine each received symbol p₀ due to corruption during

transmission 55 by noise and distortion, whether multipath or radio frequency. The

hard decision processor 121 computes the four distances d,, d₂, d₃, d₄ to each quadrant

from the received symbol p₀ and chooses the shortest distance d₂ and assigns that

symbol location x_, ,. The original symbol coordinates ρ₀ are discarded.

[0037] Referring back to Figure 3 A, after undergoing each hard symbol

decision 121, the complex conjugates 123 for each symbol output 125 are determined.

A complex conjugate is one of a pair of complex numbers with identical real parts and

with imaginary parts differing only in sign. As shown in Figure 5, a symbol is

demodulated or de-rotated by first determining the complex conjugate of the assigned

symbol coordinates x.^, forming the correction signal 119 which is used to remove

the relative phase error contained in the rake output. Thus, the rake output is

effectively de-rotated by the angle associated with the hard decision, removing the

relative phase error. This operation effectively provides a rake that is driven by a pilot

signal, but without an absolute phase reference.

[0038] Referring back to Figure 3 A, the output 119 from the complex conjugate

123 is coupled to a complex n-dimensional mixer 117 where each output of the rake receiver 101 is mixed with the correction signal 119. The resulting products 127 are

noisy estimates of the channel impulse response p_j as shown in Figure 6. The error

shown in Figure 6 is indicated by a radian distance of π/6 from the in-phase axis.

[0039] Referring back to Figure 3 A, the outputs 115 of the complex n-

dimensional channel mixer 117 are coupled to an n-dimensional estimator 131. The

channel estimator 131 is a plurality of low-pass filters, each for filtering a multipath

component. The outputs 81 of the n-dimensional estimator 131 are coupled to the

AMF 79. These outputs 81 act as the AMF 79 filter weights. The AMF 79 filters the

baseband signal to compensate for channel distortion due to multipath without

requiring a large magnitude pilot signal.

[0040] - The rake receiver 101 is used in conjunction with the phase-locked loop

(PLL) 133 circuits to remove carrier offset. Carrier offset occurs as a result of

transmitter/receiver component mismatches and other RF distortion. The present

invention 75 uses a low level pilot signal 135 which is produced by despreading 87

the pilot from the baseband signal 77 with a pilot pn sequence 91. The pilot signal is

coupled to a single input PLL 133, shown in Figure 7. The PLL 133 measures the

phase difference between the pilot signal 135 and a reference phase of 0. The

despread pilot signal 135 is the actual error signal coupled to the PLL 133.

[0041] The PLL 133 includes an arctangent analyzer 136, complex filter 137,

an integrator 139 and a phase-to-complex-number converter 141. The pilot signal 135

is the error signal input to the PLL 133 and is coupled to the complex filter 137. The

complex filter 137 includes two gain stages, an integrator 145 and a summer 147. The output from the complex filter 137 is coupled to the integrator 139. The integral of

frequency is phase, which is output 140 to the converter 141. The phase output 140

is coupled to a converter 141 which converts the phase signal into a complex signal

for mixing 151 with the baseband signal 77. Since the upstream operations are

commutative, the output 149 of the PLL 133 is also the feedback loop into the system

75.

[0042] The correction signal 119 of the complex conjugate 123 and the output

signal 149 of the PLL 133 are each coupled to mixers located within the transmitter

181, in order to correct the signal before transmission as shown in Figure 8 A. The

transmitter 181 shown in Figure 8 A operates in a similar manner to the transmitter 27

shown in Figure 2, except that the signal ready for transmission is pre-rotated prior to

transmission. Referring to Figure 8 A, data 164,, 164₂, 164₃ is encoded using forward

correcting coding (FEC) 35. The two bi-phase modulated data or symbols 41a, 41b

are spread with a complex pseudo-noise (pn) sequence and the resulting 145a and Q

45b spread signals are mixed with the correction signal 119, upconverted with the

carrier signal 51, and combined 53 with other spread signals having different

spreading codes. The resulting signal 55 is again corrected using the signal 149 from

the receiver PLL 133. The signal 56 which has been pre-corrected for phase and

frequency is then transmitted. In this manner, the present invention utilizes the signals

119 , 149 generated by the receiver 71 to pre-correct the transmitted signal and reduce

the phase and frequency errors in the signals as received at the receiving unit. [0043] Referring to Figure 8B, a transmitter 183 made in accordance with an

alternative embodiment of the present invention is shown. This embodiment is similar

to the embodiment shown in Figure 8 A, except that the correction signal 119 is mixed

with the baseband data signal via a mixer 157. Thus, the baseband data is pre-

corrected prior to encoding and spreading. Of course, those of skill in the art should

realize that other processing steps maybe introduced before the correction signal 119

is mixed with the data signal.

[0044] Referring to Figure 8C, a transmitter 188 made in accordance with

another alternative embodiment of the present invention is shown. In this

embodiment, the correction signal 119 and the carrier offset signal 149 are input into

a combiner, which combines the signal into a single pre-correction signal, and mixed

using the mixer 169 with the output of the summer 53 prior to transmission.

[0045] Finally, it should be noted that the carrier offset correction and the pre-

rotation correction are separate corrections. Each maybe utilized independently of the

other. For example, the system may pre-correct only for carrier offset error and may

not perform pre-rotation. Alternatively, the system may perform pre-rotation but may

not correct for carrier offset error.

[0046] 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.

Claims

CLAIMSWhat is claimed is:

1. A method for reducing transmission errors in a CDMA communication

system having at least two communication units, comprising:

receiving at a first communication unit a CDMA communication signal sent

from a second communication unit;

analyzing said received signal for phase errors;

correcting said received signal with a correction signal based upon said

analysis;

using said correction signal to pre-rotate a signal prior to transmission from

said first communication unit to said second communication unit.

2. A method for reducing transmission errors in a CDMA communication

system having at least two communication units, comprising:

receiving at a first communication unit a CDMA communication signal sent

from a second communication unit;

analyzing said received signal for errors;

generating a correction signal based upon said analysis; and

correcting an information signal, including voice or data, with said correction

signal prior to transmission of said information signal from said first communication

unit to said second communication unit.

3. The method of claim 2 wherein said errors include phase errors and said

correction signal corrects for said phase errors.

4. The method of claim 2 wherein said errors include frequency errors and

said correction signal corrects for said frequency errors.

5. A CDMA communication system for reducing transmission errors

during communications between at least two communication units, each

communication unit comprising:

a receiver for receiving a CDMA communication signal sent from another

communication unit;

an analyzer for analyzing said received signal for errors and for generating a

correction signal; and

a correction unit correcting said received signal with a correction signal based

upon said analysis;

a transmitter for using said correction signal to pre-correct a signal prior to

transmission to another communication unit.

6. A communication station of a CDMA system having a plurality of

communication stations which communicate with each other over a CDMA air

interface using a plurality of channels and a pilot signal for carrier offset recovery

during reception; each communication station including a receiver and a transmitter; the receiving comprising:

an adaptive matched filter for receiving demodulated CDMA

communication signals producing a filtered signal by using a weighting signal;

a rake receiver for receiving demodulated CDMA communication

signals and a pseudo-noise signal generated for a selected channel and producing a

filter weighting signal;

means for the filter weighting signal with a correction signal, said correction

signal for producing the weighting signal used by said adaptive matched filter;

at least one despreader coupled to said adaptive matched filter output for

despreading said filtered signal using the pseudo-noise signal generator for said

selected channel to produce a despread signal; and

the transmitter comprising:

a data input for providing an information signal;

at least one spreader for spreading said information signal;

a mixer for mixing the spread signal with said correction signal prior to

upconversion and transmission;

whereby a transmitted signal is pre-corrected with said correction signal prior

to transmission.