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 <b>Figure 1</b> 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

<ul id="ul0001" list-style="none" compact="compact"><li><b>Figure 1</b> is a simplified block diagram of a prior art, CDMA communication system.</li><li><b>Figure 2A</b> is a detailed block diagram of a B-CDMA™ communication system.</li><li><b>Figure 2B</b> is a detailed system diagram of a complex number multiplier.</li><li><b>Figure 3A</b> is a plot of an in-phase bit stream.</li><li><b>Figure 3B</b> is a plot of a quadrature bit stream.</li><li><b>Figure 3C</b> is a plot of a pseudo-noise (pn) bit sequence.</li><li><b>Figure 4</b> is a block diagram of a global pilot signal cancellation system according to the present invention.</li><li><b>Figure 5</b> is a block diagram of an unwanted traffic signal (s) cancellation system according to the present invention.</li><li><b>Figure 6</b> is a diagram of a received symbol p_o on the QPSK constellation showing a hard decision.</li><li><b>Figure 7</b> is a block diagram of a combined pilot and unwanted traffic signal cancellation system according to the present invention.</li></ul>

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 <b>17</b> as shown in <b>Figure 2</b> includes a transmitter <b>19</b> and a receiver <b>21</b>, 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) <b>27</b>. One signal is designated the in-phase channel <i>I</i><b>33x</b>. The other signal is designated the quadrature channel <i>Q</i><b>33y</b>. Bi-phase modulated <i>I and Q</i> signals are usually referred to as quadrature phase shift keying (QPSK).

In the second step, the two bi-phase modulated data or symbols <b>33x, 33y</b> are spread with a complex, pseudo-noise (pn) sequence <b>35I, 35Q</b> using a complex number multiplier <b>39</b>. The operation of a complex number multiplier <b>39</b> is shown in <b>Figure 2B</b> and is well understood in the art. The spreading operation can be represented as:<maths id="math0001" num="Equation (1)"><math display="block"><mrow><mtext>(</mtext><mtext mathvariant="italic">x+jy</mtext><mtext>) </mtext><mtext mathvariant="italic">×</mtext><mtext> (</mtext><mtext mathvariant="italic">I+jQ</mtext><mtext>) </mtext><mtext mathvariant="italic">=</mtext><mtext> (</mtext><mtext mathvariant="italic">xI-yQ</mtext><mtext>) </mtext><mtext mathvariant="italic">+ j</mtext><mtext>(</mtext><mtext mathvariant="italic">xQ+yI</mtext><mtext>)</mtext><mspace linebreak="newline" /><mtext>       </mtext><mtext mathvariant="italic">= a+jb.</mtext></mrow></math><img file="EP1376889A2_D0001.tif" /></maths>

A complex number is in the form <i>a+jb,</i> where <i>a</i> and <i>b</i> are real numbers <i>and j</i>^<i>2</i><i>=-</i>1 Referring back to <b>Figure 2a</b>, the resulting <i>I</i><b>37a</b> and <i>Q</i><b>37b</b> spread signals are combined <b>45a, 45b</b> with other spread signals (channels) having different spreading codes, multiplied (mixed) with a carrier signal <b>43</b>, and transmitted <b>47</b>. The transmission <b>47</b> may contain a plurality of individual signals.

The receiver <b>21</b> includes a demodulator <b>49a</b>, <b>49b</b> which mixes down the transmitted broadband signal <b>47</b> with the transmitting carrier <b>43</b> into an intermediate carrier frequency <b>51a, 51b</b>. A second down conversion reduces the signal to baseband. The QPSK signal <b>55a, 55b</b> is then filtered <b>53</b> and mixed <b>56</b> with the locally generated complex pn sequence <b>35I</b>, <b>35Q</b> 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 <b>21</b>. The data <b>57x, 57y</b> is coupled to a signal processor <b>59</b> where FEC decoding is performed on the convolutionally encoded data.

As shown in <b>Figures 3A</b> and <b>3B</b>, a QPSK symbol consists of one bit each from both the in-phase <i>(I)</i> and quadrature <i>(Q)</i> signals. The bits may represent a quantized version of an analog sample or digital data. It can be seen that symbol duration <i>t</i>_<i>6</i> is equal to bit duration.

The transmitted symbols are spread by multiplying the QPSK symbol stream by the complex pn sequence. Both the <i>I</i> and <i>Q</i> 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 <b>Figure 3C</b>. 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 <i>t</i>_<i>c</i>.

When the signal is received and demodulated, the baseband signal is at the chip level. When the <i>I</i> and <i>Q components</i> 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 <b>Figures 4, 5</b> and <b>7</b>. The global pilot signal cancellation system 61 embodiment is shown in <b>Figure 4</b>. A received signal <i>r</i> is expressed as:<maths id="math0002" num="Equation (2)"><math display="block"><mrow><mtext mathvariant="italic">r</mtext><mtext> = ∝ </mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><mtext> + β</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><mtext> + </mtext><mtext mathvariant="italic">n</mtext></mrow></math><img file="EP1376889A2_D0002.tif" /></maths> where the received signal <i>r</i> is a complex number and is comprised of the pilot strength ∝ multiplied with the pilot code <i>c</i>_<i>p</i>, summed with the traffic strength <i>β</i> multiplied with the traffic code <i>c</i>_<i>t</i><i>,</i> summed with random noise <i>n.</i> The noise <i>n</i> includes all received noise and interference including all other traffic signals. To cancel the global pilot signal from the received signal <i>r,</i> the system <b>61</b> must derive the signal strength of the pilot code ∝ where:<maths id="math0003" num="Equation (3)"><math display="block"><mrow><mtext>∝ ≠ β</mtext></mrow></math><img file="EP1376889A2_D0003.tif" /></maths> since the global pilot is transmitted at a higher power level than a traffic signal.

When the received signal <i>r</i> is summed over time, Equation (2) becomes:<maths id="math0004" num="Equation (4)"><math display="block"><mrow><mtext mathvariant="italic">Σr</mtext><mtext> = ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><mtext> + β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><mtext> + ∑</mtext><mtext mathvariant="italic">n</mtext><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0004.tif" /></maths>

Referring to <b>Figure 4</b>, the received baseband signal <i>r</i> is input <b>63</b> into the pilot signal cancellation system <b>61</b> and into a pilot despreader <b>65</b> which despreads the pilot signal from the received signal <i>r.</i> First mixer <b>67</b> despreads the received signal <i>r by</i> multiplying with the complex conjugate <i>c</i>_<i>P</i>^<i>*</i><b>69</b> of the pilot pn code used during spreading yielding:<maths id="math0005" num="Equation (5)"><math display="block"><mrow><mtext>∑</mtext><msub><mrow><mtext mathvariant="italic">rc</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>= ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup></mrow></math><img file="EP1376889A2_D0005.tif" /></maths> 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 <b>71</b> is coupled to a first sum and dump processor <b>73</b> where it is summed over time. The first sum and dump <b>73</b> output <i>O</i>_<i>sd1</i> is:<maths id="math0006" num="Esuation (6)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">sd1</mtext></mrow></msub><mtext mathvariant="italic"> ∝L</mtext><mtext> + β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup></mrow></math><img file="EP1376889A2_D0006.tif" /></maths> where <i>L</i> is the product of the pilot spreading code c_p and the complex conjugate of the pilot spreading code c_p^* summed over <i>L</i> chips.

The sum and dump <b>73</b> output <i>O</i>_<i>sd1</i> is coupled to a low pass filter <b>75.</b> The low pass filter <b>75</b> 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 <i>n.</i> Therefore, after filtering <b>75</b>, the second and third terms in <b>Equation (6)</b> become zero. The low pass filter <b>75</b> output <i>O</i>_<i>lpf</i> over time is:<maths id="math0007" num="Equation (7)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">lpf</mtext></mrow></msub><mtext> = ∝</mtext><mtext mathvariant="italic">L</mtext><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0007.tif" /></maths>

The low pass filter <b>75</b> output <i>O</i>_<i>lpf</i> is coupled to a processing means <b>77</b> to derive the pilot code strength ∝. The processing means <b>77</b> calculates ∝ by dividing the low pass filter <b>79</b> output <i>O</i>_<i>lpf</i> by <i>L.</i> Thus, the processing means <b>77</b> output <i>O</i>_<i>pm</i> is:<maths id="math0008" num=" Equation (8)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">pm</mtext></mrow></msub><mtext> = ∝.</mtext></mrow></math><img file="EP1376889A2_D0008.tif" /></maths>

The pilot spreading code <i>c</i>_<i>p</i><i>*</i> complex conjugate generator <b>69</b> is coupled to a complex conjugate processor <b>79</b> yielding the pilot spreading code <i>c</i>_<i>p</i>. The pilot spreading code <i>c</i>_<i>p</i> is input to a second mixer <b>81</b> and mixed with the output of a traffic spreading code <i>c</i>_<i>t</i>^<i>*</i> complex conjugate generator <b>83.</b> The resulting product from the second mixer <b>81</b> output is coupled to a second sum and dump processor <b>85</b>. The output <i>O</i>_<i>sd2</i> of the second sum and dump processor <b>85</b> is ∑<i>c</i>_<i>p</i><i>c</i>_<i>t</i>^<i>*</i> and is combined with ∝ at a third mixer <b>87</b>. The third mixer <b>87</b> output <b>89</b> is ∝∑<i>c</i>_<i>p</i><i>c</i>_<i>t</i>^<i>*</i>

The received signal <i>r</i> is also despread by traffic despreader <b>91</b>. The traffic despreader <b>91</b> despreads the received signal r by mixing the received signal <i>r</i> with the traffic code <i>c</i>_<i>t</i>^<i>*</i> complex conjugate generator <b>83</b> using a fourth mixer <b>93</b> yielding:<maths id="math0009" num=" Equation (9)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">∑rc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> = ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0009.tif" /></maths> The traffic despreader <b>91</b> output <b>95</b> is coupled to a third sum and dump <b>97</b>. The third sum and dump <b>97</b> output <i>O</i>_<i>sd3</i> over time is:<maths id="math0010" num="Equation (10)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">sd3</mtext></mrow></msub><mtext> = ∑</mtext><msub><mrow><mtext mathvariant="italic">rc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> = β</mtext><mtext mathvariant="italic">L</mtext><mtext> + ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup></mrow></math><img file="EP1376889A2_D0010.tif" /></maths> where <i>L</i> is the product of the traffic spreading code <i>c</i>_<i>t</i> and the complex conjugate of the traffic spreading code <i>c</i>_<i>t</i>^<i>*</i> summed over <i>L</i> chips.

The third sum and dump <b>97</b> output <i>O</i>_<i>sd3</i> is coupled to an adder <b>99</b> which subtracts the third mixer <b>87</b> output <b>89</b>. The adder <b>99</b> output <i>O</i>_<i>add</i> is:<maths id="math0011" num="Equation (11)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">add</mtext></mrow></msub><mtext> = </mtext><mtext mathvariant="italic">βL</mtext><mtext> + ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><msub><mrow><mtext mathvariant="italic">, + ∑nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> - ∝∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext mathvariant="italic">.</mtext></mrow></math><img file="EP1376889A2_D0011.tif" /></maths>

Thus, the pilot canceler 61 output <i>O</i>_<i>add</i> is equal to the received signal <i>r</i> minus the pilot signal simplified below:<maths id="math0012" num="Equation (12)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">add</mtext></mrow></msub><mtext> = β</mtext><mtext mathvariant="italic">L</mtext><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0012.tif" /></maths>

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 <b>101</b> embodiment is shown in <b>Figure 5</b>. As above, a received signal <i>r</i> is input <b>103</b> to the system:<maths id="math0013" num="Equation (13)"><math display="block"><mrow><mtext mathvariant="italic">r</mtext><mtext> = ψ</mtext><msub><mrow><mtext mathvariant="italic">dc</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext></mrow></msub><mtext> + β</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><mtext mathvariant="italic"> + n</mtext></mrow></math><img file="EP1376889A2_D0013.tif" /></maths> where the received signal <i>r</i> is a complex number and is comprised of the traffic code signal strength ψ multiplied with the traffic signal data <i>d</i> and the traffic code <i>c</i>_<i>d</i> for the unwanted traffic signal to be canceled, summed with the desired traffic code strength β multiplied with the desired traffic code <i>c</i>_<i>t</i>, summed with noise <i>n.</i> The noise <i>n</i> 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 <i>r</i>, the system <b>101</b> must derive the signal strength of the unwanted traffic code <i>ψ</i> to be subtracted and estimate the data <i>d</i>, where:<maths id="math0014" num="Equation (14)"><math display="block"><mrow><mtext mathvariant="italic">ψ</mtext><mtext> ≠ </mtext><mtext mathvariant="italic">d</mtext><mtext> ≠ β.</mtext></mrow></math><img file="EP1376889A2_D0014.tif" /></maths>

When the received signal <i>r</i> is summed over time, Equation <b>13</b> can be expressed as:<maths id="math0015" num="Equation (15)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">Σr = ψdΣc</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext></mrow></msub><mtext mathvariant="italic">+</mtext><mtext> β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><mtext>+ </mtext><mtext mathvariant="italic">∑n.</mtext></mrow></math><img file="EP1376889A2_D0015.tif" /></maths>

Referring to <b>Figure 5</b>, the received baseband <i>signal r</i> is input <b>103</b> into the desired traffic signal despreader <b>91</b> which despreads the desired traffic signal from the received signal <i>r</i>. Desired traffic signal mixer <b>93</b> mixes the received signal <i>r</i> with the complex conjugate <i>c</i>_<i>t</i>^<i>*</i> of the desired traffic pn code used during spreading. The despread traffic signal is coupled to a sum and dump processor <b>97</b> and summed over time. The sum and dump <b>97</b> output <i>O</i>_<i>sd3</i> is:<maths id="math0016" num="Equation (16)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">sd3</mtext></mrow></msub><mtext> = Σ</mtext><msub><mrow><mtext mathvariant="italic">rc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>= β</mtext><mtext mathvariant="italic">L</mtext><mtext> + ψ</mtext><mtext mathvariant="italic">d</mtext><mtext>∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0016.tif" /></maths>

The traffic signal canceler system <b>101</b> shown in <b>Figure 5</b> includes <i>n</i> unwanted traffic signal cancelers <b>115</b>_<b>1</b><b>-115</b>_<b>n</b>. An exemplary embodiment includes 10 (where <i>n</i>=10) unwanted traffic signal cancelers <b>115</b>_<b>1</b>-<b>115</b>_<b>10</b>.

Each unwanted traffic signal canceler <b>115</b>_<b>1</b><b>-115</b>_<b>n</b> comprises: an unwanted traffic signal despreader <b>139</b>_<b>1</b><b>-139</b>_<b>n</b> that includes a first mixer <b>117</b>_<b>1</b><b>-117</b>_<b>n</b> and an unwanted traffic signal code generator <b>119</b>_<b>1</b><b>-119</b>_<b>n</b>; second <b>133</b>_<b>1</b><b>-133</b>_<b>n</b> mixer, first <b>121</b>_<b>1</b><b>-121</b>_<b>n</b> and second <b>123</b>_<b>1</b><b>-123</b>_<b>n</b> sum and dump processors, a hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b>, a low pass filter <b>127</b>_<b>1</b><b>-127</b>_<b>n</b>, a processing means <b>129</b>_<b>1</b><b>-129</b>_<b>n</b>, third mixer <b>131</b>_<b>1</b><b>-131</b>_<b>n</b>, a conjugate processor <b>135</b>_<b>1</b><b>-135</b>_<b>n</b>, an adjustable amplifier <b>137</b>_<b>1</b><b>-137</b>_<b>n</b>, and a desired traffic signal code generator <b>83</b>.

As above, the received signal <i>r</i> is input <b>103</b> into each unwanted traffic canceler <b>115</b>_<b>1</b><b>-115</b>_<b>n</b>. The unwanted traffic signal despreader <b>139</b>_<b>1</b><b>-139</b>_<b>n</b> is coupled to the input <b>103</b> where the received signal <i>r</i> is mixed <b>117</b>_<b>1</b><b>-117</b>_<b>n</b> with the complex conjugate <i>c</i>_<i>d1</i>^<i>*</i><i>-c</i>_<i>dn</i>^<i>*</i> of the traffic pn sequence for each respective unwanted signal. The despread <b>139</b>_<b>1</b><b>-139</b>_<b>n</b> traffic signal is coupled to a first sum and dump processor <b>121</b>_<b>1</b><b>-121</b>_<b>n</b> where it is summed over time. The first sum and dump <b>121</b>_<b>1</b><b>-121</b>_<b>n</b> output <i>O</i>_<i>edln</i> is:<maths id="math0017" num="Equation (17)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">sd1n</mtext></mrow></msub><mtext>= ∑</mtext><msubsup><mrow><mtext mathvariant="italic">rc</mtext></mrow><mrow><mtext mathvariant="italic">dn</mtext></mrow><mrow><mtext>*</mtext></mrow></msubsup><mtext></mtext><mtext mathvariant="italic">= ψdL +</mtext><mtext> β∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msubsup><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">dn</mtext></mrow><mrow><mtext>*</mtext></mrow></msubsup><mtext> + ∑</mtext><msubsup><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">dn</mtext></mrow><mrow><mtext>*</mtext></mrow></msubsup><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0017.tif" /></maths> where <i>L</i> is the product of the unwanted traffic signal spreading code <i>c</i>_<i>dn</i> and <i>c</i>_<i>dn</i>^<i>*</i> is the complex conjugate of the unwanted traffic signal spreading code.

The first sum and dump <b>121</b>_<b>1</b><b>-121</b>_<b>n</b> output <i>O</i>_<i>sd1n,</i> is coupled to the hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b>. The hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b> determines the phase shift φ in the data due to modulation. The hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b> also determines the QPSK constellation position <i>d</i> that is closest to the despread symbol value.

As shown in <b>Figure 6</b>, the hard decision processor 125₁-<b>125</b>_<b>n</b>compares a received symbol <i>p</i>_<i>o</i> of a signal to the four QPSK constellation points x_{1, 1'} x_{-1, 1'} x_{-1, -1'} x_{1, 1}. It is necessary to examine each received symbol <i>p</i>_<i>o</i> due to corruption during transmission <b>47</b> by noise and distortion, whether multipath or radio frequency. The hard decision processor computes the four distances <i>d</i>_<i>1</i><i>, d</i>_<i>2</i>, <i>d</i>_<i>3</i><i>, d</i>_<i>4</i> to each quadrant from the received symbol <i>p</i>_<i>o</i> and chooses the shortest distance <i>d</i>₂ and assigns that symbol d location <i>x</i>_{-1, 1}. The hard decision processor also derotates (rotates back) the original signal coordinate <i>p</i>_<i>o</i> by a phase amount φ that is equal to the phase corresponding to the selected symbol location <i>x</i>_<i>-1</i>, ₁. The original symbol coordinate <i>p</i>_<i>o</i> is discarded.

The hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b> phase output φ is coupled to a low pass filter <b>127</b>_<b>1</b><b>-127</b>_<b>n</b>. Over time, the low pass filter <b>127</b>_<b>1</b><b>-127</b>_<b>n</b> 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 <b>127</b>_<b>1</b><b>-127</b>_<b>n</b> output <i>O</i>_<i>lpfn</i> over time is:<maths id="math0018" num="Equation (18)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">O</mtext></mrow><mrow><mtext mathvariant="italic">lpfn</mtext></mrow></msub><mtext> = ψ</mtext><mtext mathvariant="italic">L.</mtext></mrow></math><img file="EP1376889A2_D0018.tif" /></maths>

The low pass filter <b>127</b>_<b>l</b><b>-127</b>_<b>n</b> output <i>O</i>_<i>lpfn</i> is coupled to the processing means <b>129</b>_<b>1</b><b>-129</b>_<b>n</b> to derive the unwanted traffic signal code strength <i>ψ.</i> The processing means <b>129</b>_<b>1</b><b>-129</b>_<b>n</b> estimates φ by dividing the filter <b>127</b>_<b>l</b><b>-127</b>_<b>n</b> output <i>O</i>_<i>lpfn,</i> by <i>L</i>.

The other hard decision processor <b>125</b>_<b>1</b><b>-125</b>_<b>n</b> output is data <i>d</i>. This is the data point <i>d</i> corresponding to the smallest of the distances <i>d</i>_<i>1</i><i>, d</i>_<i>2</i><i>, d</i>_<i>3</i><i>,</i> or <i>d</i>_<i>4</i> as shown in <b>Figure 6</b>. Third mixer <b>131</b>_<b>1</b><b>-131</b>_<b>n</b> mixes the unwanted traffic signal strength <i>ψ</i> with each date value <i>d.</i>

The unwanted traffic signal spreading code complex conjugate generator <i>c</i>_<i>dl</i><i>*-c</i>_<i>dn</i><i>*</i> is coupled to the complex conjugate processor <b>135</b>_<b>1</b><b>-135</b>_<b>n</b> yielding the unwanted traffic signal spreading code c_dl-c_dn and is input to the second mixer <b>133</b>_<b>1</b><b>-133</b>_<b>n</b> and mixed with the output of desired traffic signal spreading code complex conjugate generator <i>c</i><maths id="math0019" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>*</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></mfrac></mrow></math><img file="EP1376889A2_D0019.tif" /></maths><i>.</i> The product is coupled to the second sum and dump processor <b>123</b>_<b>1</b><b>-123</b>_<b>n</b><b>.</b> The second sum and dump processor <b>123</b>_<b>1</b><b>-123</b>_<b>n</b> output <i>O</i>_<i>sd2n</i> is ∑<i>cd</i>_<i>n</i><i>c</i>_<i>t</i><i>*</i> and is coupled to variable amplifier <b>137</b>_<b>1</b><b>-137</b>_<b>n</b>. Variable amplifier <b>137</b>_<b>1</b><b>-137</b>_<b>n</b> amplifies the second sum and dump processor <b>123</b>_<b>1</b><b>-123</b>_<b>n</b> output <i>O</i>_<i>sd2n</i> in accorance with the third mixer <b>131</b>_<b>1</b><b>-131</b>_<b>n</b> output which is the determined gain.

The variable amplifier <b>137</b>_<b>1</b><b>-137</b>_<b>n</b> output <b>141</b>_<b>1</b><b>-141</b>_<b>n</b> is coupled to an adder <b>143</b> which subtracts the output from each variable amplifier <b>137</b>_<b>1</b><b>-137</b>_<b>n</b> from the output of the desired traffic signal despreader <b>105</b>. The output <i>O</i> is:<maths id="math0020" num="Equation (19)"><math display="block"><mrow><mtext mathvariant="italic">O</mtext><mtext> = </mtext><mtext mathvariant="italic">βL</mtext><mtext> +ψ</mtext><mtext mathvariant="italic">d</mtext><mtext>∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>- ψ</mtext><mtext mathvariant="italic">d</mtext><mtext>∑</mtext><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">c</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>.</mtext></mrow></math><img file="EP1376889A2_D0020.tif" /></maths> The adder <b>143</b> output <i>O</i> (also the unwanted traffic canceler system <b>101</b> output) is equal to the received signal <i>r</i> minus the unwanted traffic signals simplified below:<maths id="math0021" num="Equation (20)"><math display="block"><mrow><mtext mathvariant="italic">O</mtext><mtext> = β</mtext><mtext mathvariant="italic">L</mtext><mtext> + ∑</mtext><msub><mrow><mtext mathvariant="italic">nc</mtext></mrow><mrow><mtext mathvariant="italic">t</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>*</mtext></mrow></msup></mrow></math><img file="EP1376889A2_D0021.tif" /></maths> where the noise <i>n</i> varies depending on the amount of traffic signals subtracted from the received signal.

Another embodiment <b>145</b> cancelling the global pilot signal and unwanted traffic signals is shown in <b>Figure 7.</b> As previously discussed, the unwanted traffic cancellation system <b>101</b> includes the desired traffic signal despreader <b>91</b> and a plurality of unwanted traffic signal cancelers <b>115</b>_<b>1</b><b>-115</b>_<b>n</b><b>.</b> The traffic cancellation system is coupled in parallel with the pilot cancellation system <b>61</b> previously described, but without a desired traffic singal despreader. A common input <b>147</b> is coupled to both systems <b>101, 61</b> with a common adder <b>149</b> which is coupled to the outputs <i>O</i>, <i>O</i>_<i>add</i> from both systems <b>101, 61</b>. The pilot and unwanted traffic signals are subtracted from the desired traffic signal yielding an output <b>151</b> 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.