Cancellation of pilot and unwanted traffic signals in a cdma system

Description of equivalent WO 0024135 A1

CANCELLATION OF PILOT AND UNWANTED TRAFFIC SIGNALS IN A CDMA SYSTEM

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to digital communications. More specifically, the invention relates to a system and method which cancels the global pilot signal and unwanted- traffic signals from a received code division multiple access signal thereby removing them as interferers prior to decoding.

Description of the Prior Art

Advanced communication technology today makes use of a communication technique in which data is transmitted with a broadened band by modulating the data to be transmitted with a pseudo-noise (pn) signal. The technology is known as digital spread spectrum or code divisional multiple access (CDMA) . By transmitting a signal with a bandwidth much greater than the signal bandwidth, CDMA can transmit data without being affected by signal distortion or an interfering frequency in the transmission path. Shown in Figure 1 is a simplified, single channel CDMA communication system. A data signal with a given bandwidth is mixed with a spreading code generated by a pn sequence generator producing a digital spread spectrum signal. The signal which carries data for a specific channel is known as a traffic signal. Upon reception, the data is reproduced after correlation with the same pn sequence used to transmit the data. Every other signal within the transmission bandwidth appears as noise to the signal being despread. For timing synchronization with a receiver, an unmodulated traffic signal known as a pilot signal is required for every transmitter. The pilot signal allows respective receivers to synchronize with a given transmitter, allowing despreading of a traffic signal at the receiver.

In a typical communication system, a base station communicates with a plurality of individual subscribers fixed or mobile. The base station which transmits many signals, transmits a global pilot signal common to the plurality of users serviced by that particular base station at a higher power level. The global pilot is used for the initial acquisition of an individual user and for the user to obtain signal-estimates for coherent reception and for the combining of multipath components during reception. Similarly, in a reverse direction, each subscriber transmits a unique assigned pilot for communicating with the base station.

Only by having a matching pn sequence can a signal be decoded, however, all signals act as noise and interference. The global pilot and traffic signals are noise to a traffic signal being despread. If the global pilot and all unwanted traffic signals could be removed prior to despreading a desired signal, much of the overall noise would be reduced, decreasing the bit error rate and in turn, improve the signal- to-noise ratio (SNR) of the despread signal. Some attempts have been made to subtract the pilot signal from the received signal based on the relative strength of the pilot signal at the receiver. However, the strength value is not an accurate characteristic for calculating interference due to the plurality of received signals with different time delays caused by reflections due to terrain. Multipath propagation makes power level estimates unreliable.

There is a need to improve overall system performance by removing multiple noise contributors from a signal prior to decoding .

SUMMARY OF THE INVENTION

The present invention reduces the contributive noise effects of the global pilot signal and unwanted traffic signals transmitted in a spread spectrum communication system. The present invention effectively cancels the global pilot and unwanted traffic signal (s) from a desired traffic signal at a receiver prior to decoding. The resulting signal has an increased signal-to-noise ratio. Accordingly, it is an object of the present invention to provide a code division multiple access communication system receiver which reduces the contributive noise effects from the pilot and active, unwanted traffic signals.

It is another object of the present invention to improve the desired traffic signal SNR by eliminating the noise effects of the global pilot and active traffic signals.

Other objects and advantages of the system and method will become apparent to those skilled in the art of advanced telecommunications after reading the detailed description of the preferred embodiment. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified block diagram of a prior art, CDMA communication system.

Figure 2A is a detailed block diagram of a B-CDMA™ communication system.

Figure 2B is a detailed system diagram of a complex number multiplier.

Figure 3A is a plot of an in-phase bit stream. Figure 3B is a plot of a quadrature bit stream. Figure 3C is a plot of a pseudo-noise (pn) bit sequence.

Figure 4 is a block diagram of a global pilot signal cancellation system according to the present invention.

Figure 5 is a block diagram of an unwanted traffic signal (s) cancellation system according to the present invention.

Figure 6 is a diagram of a received symbol p₀ on the QPSK constellation showing a hard decision.

Figure 7 is a block diagram of a combined pilot and unwanted traffic signal cancellation system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.

A B-CDMA™ communication system 17 as shown in Figure 2 includes a transmitter 19 and a receiver 21, which may reside in either a base station or a mobile user receiver. The transmitter 19 includes a signal processor 23 which encodes voice and nonvoice signals 25 into data at various bit rates. By way of background, two steps are involved in the generation of a transmitted signal in a multiple access environment. First, the input data which can be considered a bi-phase modulated signal is encoded using forward error- correcting coding (FEC) 27. One signal is designated the in- phase channel I 33x. The other signal is designated the

quadrature channel Q 33y. Bi-phase modulated 7 and Q signals are usually referred to as quadrature phase shift keying

(QPSK) .

In the second step, the two bi-phase modulated data or symbols 33x, 33y are spread with a complex, pseudo-noise (pn) sequence 351, 35Q using a complex number multiplier 39. The operation of a complex number multiplier 39 is shown in Figure

2B and is well understood in the art . The spreading operation can be represented as :

(x+jy) X (I+jQ) = (xI-yQ) + j(xQ+yI) Equation (1)

<img file="WO0024135A1_D0001.tif" />

A complex number is in the form a+jb , where a and are

real numbers and —-l - Referring back to Figure 2a, the

resulting I 37a and Q 37b spread signals are combined 45a,

45b with other spread signals (channels) having different spreading codes, multiplied (mixed) with a carrier signal 43, and transmitted 47. The transmission 47 may contain a plurality of individual signals.

The receiver 21 includes a demodulator 49a, 49b which mixes down the transmitted broadband signal 47 with the transmitting carrier 43 into an intermediate carrier frequency 51a, 51b. A second down conversion reduces the signal to baseband. The QPSK signal 55a, 55b is then filtered 53 and mixed 56 with the locally generated complex pn sequence 351, 35Q which matches the conjugate of the transmitted complex code. Only the original signals which were spread by the same code will be despread. All other signals will appear as noise to the receiver 21. The data 57x, 57y is coupled to a signal processor 59 where FEC decoding is performed on the convolutionally encoded data. As shown in Figures 3A and 3B, a QPSK symbol consists of one bit each from both the in-phase (I) and quadrature ( ) signals. The bits may represent a quantized version of an analog sample or digital data. It can be seen that symbol duration t_s is equal to bit duration. The transmitted symbols are spread by multiplying the

QPSK symbol stream by the complex pn sequence . Both the I and

Q pn sequences are comprised of a bit stream generated at a much higher frequency, typically 100 to 200 times the symbol rate. One such pn sequence is shown in Figure 3C. The complex pn sequence is mixed with the symbol bit stream producing the digital spread signal (as previously discussed) . The components of the spread signal are known as chips having a much smaller duration t_c .

When the signal is received and demodulated, the baseband signal is at the chip level . When the I and Q components of the signal are despread using the conjugate of the pn sequence used during spreading, the signal returns to the symbol level .

The embodiments of the present invention are shown in

Figures 4, 5 and 7. The global pilot signal cancellation system 61 embodiment is shown in Figure 4. A received signal r is expressed as :

Equation (2) r = ^K c p + β^rc&lt;, + n

where the received signal r is a complex number and is

comprised of the pilot strength «^■ multiplied with the pilot

codec_p, summed with the traffic strength β multiplied with the

traffic code c_t, summed with random noise n . The noise n includes all received noise and interference including all other traffic signals . To cancel the global pilot signal from the received signal r, the system 61 must derive the signal

strength of the pilot code «^• where:

β Equation (3)

since the global pilot is transmitted at a higher power level than a traffic signal. When the received signal r is summed over time , Equation

(2 ) becomes :

∑r - «∑c + B∑c + ∑n Equation (4 ) p ^r *

Referring to Figure 4, the received baseband signal r is input 63 into the pilot signal cancellation system 61 and into a pilot despreader 65 which despreads the pilot signal from the received signal r. First mixer 67 despreads the received

signal r by multiplying with the complex conjugate c * 69 of

the pilot pn code used during spreading yielding: Equation (5) <img file="WO0024135A1_D0002.tif" />

A complex conjugate is one of a pair of complex numbers with identical real parts and with imaginary parts differing only in sign.

The despread pilot signal 71 is coupled to a first sum and dump processor 73 where it is summed over time . The first

sum and dump 73 output O_sJl is:

Equation ( 6 ) <img file="WO0024135A1_D0003.tif" />

where is the product of the pilot spreading code c_p and the

complex conjugate of the pilot spreading code c * summed over

chips. The sum and dump 73 output O_sJl is coupled to a low pass filter 75. The low pass filter 75 determines the mean value for each signal component. The mean value for pilot-traffic cross-correlation is zero and so is the mean value of the noise n . Therefore, after filtering 75, the second and third terms in Equation (6) become zero. The low pass filter 75 output 0_pyθver time is:

Equation (7) <img file="WO0024135A1_D0004.tif" />

The low pass filter 75 output O^ is coupled to a

processing means 77 to derive the pilot code strength &lt;^. The

processing means 77 calculates «^■ by dividing the low pass

filter 79 output Oy by X. Thus, the processing means 77

output O_pm is:

O _m = «. Equation (8)

The pilot spreading code c * complex conjugate generator

69 is coupled to a complex conjugate processor 79 yielding the pilot spreading code c_p . The pilot spreading code c is input to a second mixer 81 and mixed with the output of a traffic spreading code c ^* complex conjugate generator 83. The resulting product from the second mixer 81 output is coupled to a second sum and dump processor 85. The output O_sj2 of the second sum and dump processor 85 is c_pc_t^' and is combined with

° at a third mixer 87. The third mixer 87 output 89 is ^cc,^'

The received signal r is also despread by traffic despreader 91. The traffic despreader 91 despreads the received signal r by mixing the received signal r with the

traffic code c^* complex conjugate generator 83 using a fourth mixer 93 yielding:

Equation (9)

∑rc_{ = °^cJc_ιc_t + β∑c_tc₍ + ∑«c_f .

The traffic despreader 91 output 95 is coupled to a third sum and dump 97. The third sum and dump 97 output O_sj3 over time is:

Equation (10)

⁰ J3 ^{= ∑rc} = β^ ⁺ " ^{+ ∑nc}t^*

where is the product of the traffic spreading code c_t and the

complex conjugate of the traffic spreading code c* summed over

chips.

The third sum and dump 97 output O_sj3 is coupled to an adder 99 which subtracts the third mixer 87 output 89. The adder 99 output O_aιU is:

„ Equation (11)

^_add ^~ βi ⁺ °^C∑^{C c}t ⁺ nc_t^~ ^∑c c_t ■ Thus, the pilot canceler 61 output O_acU is equal to the

received signal r minus the pilot signal simplified below:

O_aJJ = βl + ∑nc ^*. Equation (12)

The invention uses a similar approach to cancel unwanted traffic signal (s) from a desired traffic signal. While traffic signals are interference to other traffic signals just as the global pilot signal is, unwanted traffic signal cancellation differs from global pilot signal cancellation since a traffic signal is modulated by the data and is therefore dynamic in nature. A global pilot signal has a constant phase, whereas a traffic signal constantly changes phase due to data modulation.

The traffic signal canceler system 101 embodiment is shown in Figure 5. As above, a received signal r is input 103 to the system:

Equation (13) r = dc_j + βc_f + n

where the received signal r is a complex number and is

comprised of the traffic code signal strength ψ multiplied

with the traffic signal data J and the traffic code c_j for the unwanted traffic signal to be canceled, summed with the desired traffic code strength β multiplied with the desired

traffic code c_t, summed with noise n . The noise n includes all received noise and interference including all other traffic signals and the global pilot signal. To cancel the unwanted traffic signal (s) from the received signal r, the system 101 must derive the signal strength of the unwanted traffic code ψ to be subtracted and estimate the data d , where:

ψ ≠ d ≠ β. Equation (14)

When the received signal r is summed over time , Equation

13 can be expressed as :

Equation (15) ∑r = J∑c_j + β∑c_f + ∑_M.

Referring to Figure 5, the received baseband signal r is

input 103 into the desired traffic signal despreader 91 which despreads the desired traffic signal from the received signal r . Desired traffic signal mixer 93 mixes the received signal

r with the complex conjugate c* of the desired traffic pn code

used during spreading. The despread traffic signal is coupled to a sum and dump processor 97 and summed over time. The sum and dump 97 output O_sj3 is :

° <img file="WO0024135A1_D0005.tif" />^{+ ∑nc}t^*' Equation (16)

The traffic signal canceler system 101 shown in Figure 5 includes n unwanted traffic signal cancelers llδi-llS,,. An exemplary embodiment includes 10 (where n=10) unwanted traffic signal cancelers <img file="WO0024135A1_D0006.tif" />

Each unwanted traffic signal canceler HS_j-llS,, comprises: an unwanted traffic signal despreader 139₁-139_n that includes a first mixer ll?_!-!!?_^ and an unwanted traffic signal code generator l^-l^^ second 133-_L-133,, mixer, first 121₁-121_n and second 123_x-123_n sum and dump processors, a hard decision processor 125-_L-125_J,, a low pass filter 127₁-127_n, a processing means 129₁-129_n, third mixer 131-_L-131.,, a conjugate processor 135_J.-135.,, an adjustable amplifier 137₁-137_n, and a desired traffic signal code generator 83.

As above, the received signal r is input 103 into each unwanted traffic canceler llSi-llS-_j. The unwanted traffic signal despreader 139₁-139_n is coupled to the input 103 where the received signal r is mixed 117₁-117_n with the complex

conjugate c^ ^*-c_d^* of the traffic pn sequence for each respective unwanted signal. The despread 139₁-139_n traffic signal is coupled to a first sum and dump processor 121₁-121_n where it is summed over time. The first sum and dump 121_x- 12l_n output O_sdln is:

O_sJln - ∑rci = ψ &amp; ₊ β∑c^ ₊ nc Equation ⁽17⁾

where L is the product of the unwanted traffic signal

spreading code c_Jn and c_Jn^* is the complex conjugate of the unwanted traffic signal spreading code. The first sum and dump 121₁-121_n output O_sAln is coupled to the hard decision processor 125₁-125_n. The hard decision processor 125₁-125_n determines the phase shift φ in the data due to modulation. The hard decision processor 125₁-125_n also determines the QPSK constellation position d that is closest to the despread symbol value.

As shown in Figure 6, the hard decision processor 125_x-

125_n compares a received symbol p_o of a signal to the four QPSK

constellation points *_π, x_{Λ l7} x_{Λ Λ} , χ_{x Λ} . It is necessary to

examine each received symbol p₀ due to corruption during transmission 47 by noise and distortion, whether multipath or radio frequency. The hard decision processor computes the four distances d_υ d^ d_Α d₄ to each quadrant from the received

symbol p₀ and chooses the shortest distance d₂ and assigns that

symbol d location x_Λ . The hard decision processor also

derotates (rotates back) the original signal coordinate p_o by a phase amount φ that is equal to the phase corresponding to the selected symbol location x_{Λ x}. The original symbol

coordinate p_o is discarded.

The hard decision processor 125₁-125_n phase output φ is coupled to a low pass filter 127₁-127_n. Over time, the low pass filter 127₁-127_n determines the mean value for each signal component. The mean value of the traffic-to-traffic cross-correlation and also the mean value of the noise n are zero. Therefore, the low pass filter 127₁-127_n output O_/ r over time is:

O_{j fn} = ψ L. Equation (18)

The low pass filter 127₁-127_n output O_lp^ is coupled to the processing means 129₁-129_n to derive the unwanted traffic signal code strength ψ. The processing means 129₁-129_n

estimates φ by dividing the filter 127_x-127_n output CA byZ.

The other hard decision processor 125₁-125_n output is data d . This is the data point d corresponding to the

smallest of the distances d_v d^ c„ or d₄ as shown in Figure 6. Third mixer 1Zl₁- ^' i' 'L_a mixes the unwanted traffic signal strength ψ with each date value d.

The unwanted traffic signal spreading code complex conjugate generator CJX * is coupled to the complex conjugate processor 135-_L-135,_! yielding the unwanted traffic signal spreading code c_Jrcj_n and is input to the second mixer 133_x-133_n and mixed with the output of desired traffic signal spreading code complex conjugate generator c *. The product is coupled to the second sum and dump processor 123₁-123_n. The second sum and dump processor 123₁-123_n output O_sj2n is

∑c _nci and is coupled to variable amplifier 137_x-137_n. Variable amplifier 137₁-137_n amplifies the second sum and dump processor 123₁-123_n output O_sj2n in accorance with the third mixer lSl^lS^ output which is the determined gain.

The variable amplifier 137₁-137_n output 141 _r141 _n is coupled to an adder 143 which subtracts the output from each variable amplifier 137₁-137_n from the output of the desired traffic signal despreader 105. The output O is:

O = βi + ψc/∑_Λ^* + nc ^* - d∑cj: ^*. Equation (19)

The adder 143 output O (also the unwanted traffic canceler

system 101 output) is equal to the received signal r minus the unwanted traffic signals simplified below:

O - βi ₊ n_c Equation ⁽20⁾

where the noise n varies depending on the amount of traffic signals subtracted from the received signal.

Another embodiment 145 cancelling the global pilot signal and unwanted traffic signals is shown in Figure 7. As previously discussed, the unwanted traffic cancellation system 101 includes the desired traffic signal despreader 91 and a plurality of unwanted traffic signal cancelers 115-_L-115-_J. The traffic cancellation system is coupled in parallel with the pilot cancellation system 61 previously described, but without a desired traffic singal despreader. A common input 147 is coupled to both systems 101, 61 with a common adder 149 which is coupled to the outputs O O_acU from both systems 101, 61.

The pilot and unwanted traffic signals are subtracted from the desired traffic signal yielding an output 151 free of interference contributions by the pilot and plurality of transmitted traffic signals.

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.