Spread spectrum interference canceler system and method

IS 2 137 140 T3

DESCRIPTION

Spread spectrum interference elimination device and procedure.

Background of the invention

This invention relates to spread spectrum communications, and more in particular to an interference canceller and a method for reducing interference in a direct sequence cocode division multiple access receiver.

Description of the prior art

Spread spectrum communication systems, with multiple access by direct sequence code division, have a limited capacity due to interference caused by other simultaneous users. This is compounded if adaptive power control is not used or is used but not perfect.

Multiple access by code division was limited by interference. The more users transmitting simultaneously, the higher the bit error rate (BER). Higher capacity requires return channelless error correction (FEC) encoding, which in turn increases data rates and limits capacity.

A receiver for receiving a spread spectrum signal is described by S. Tachikawa in IEICE Transactions on Communications E76-B (1993), August, No. 8, Tokyo, pages 941-946. As described there, the spread spectrum signal has multiple data signals sent using different channelization codes. The receiver wants to retrieve data from one of the multiple channels. The received signal is reconcentrated using each of the channelization codes other than the channelization code of the data desired by the receiver. Each reconcentrated channel is integrated and then limited. The integrated and limited channel signals are again widened using the channelization code. The received signal is delayed. Each of the channelization codes of the re-spread channels is subtracted from the delayed received signal. The subtracted signal is reconcentrated using the receiver's data channelization code. The reconcentrated data is later integrated and constrained to retrieve the data from that channel.

The present invention provides a remote unit for receiving a selected CDMA spread spectrum channel signal according to claim 1 and a method for using a remote unit for receiving a selected CDMA spread spectrum channel signal according to claim. 9. Other aspects of the invention are presented in accordance with the dependent claims.

In the description that follows, further objectives and advantages of the invention are set forth in part, and in part are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention can also be understood and achieved by means and combinations especially pointed out in the appended claims.

Brief description of the drawings

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention, and together with the description serve to explain the principles of the invention.

Figure 1 is a block diagram of the spread spectrum CDMA interference canceller using correlators;

Figure 2 is a block diagram of the spread spectrum CDMA interference canceller for processing multiple channels using correlators;

Figure 3 is a block diagram of the spread spectrum CDMA interference canceller using matched filters;

Figure 4 is a block diagram of the spread spectrum CDMA interference canceller for processing multiple channels using matched filters;

Figure 5 is a block diagram of the spread spectrum CDMA interference canceller having multiple iterations to process multiple channels;

Figure 6 illustrates the theoretical operating characteristic for E_b/ n = 6 dB;

Figure 7 illustrates the theoretical operating characteristic for E_b/ n = 10 dB;

Figure 8 illustrates the theoretical operating characteristic for E_b/ n = 15 dB;

Figure 9 illustrates the theoretical operating characteristic for E_b/ n = 20 dB;

Figure 10 illustrates the theoretical operating characteristic for E_b/ n = 25 dB;

Figure 11 illustrates the theoretical operating characteristic for E_b/ n = 30 dB;

Figure 12 is a block diagram of interconnected interference cancellers according to the invention;

Figure 13 is a block diagram combining the outputs of the interference cancellers of Figure 12;

Figure 14 illustrates the simulated operating characteristics for asynchronous channel, PG = 100, equal powers, EbN = 30dB;

Figure 15 illustrates the simulated operating characteristics for asynchronous channel, PG = 100, equal powers, EbN = 30dB;

Figure 16 illustrates the simulated operating characteristics for asynchronous channel, PG = 100, equal powers, EbN = 30dB; and Figure 17 illustrates the simulated operating characteristics for asynchronous channel, PG = 100, equal powers, EbN = 30dB.

Detailed description of the preferred embodiments

Reference is now made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings, in which like reference numerals indicate similar elements throughout the various figures.

Figures 1 to 11 illustrate the interference cancellation principles incorporated in an interference canceller of the invention illustrated in Figures 12 to 17.

In the example scheme shown in Figure 1, a spread spectrum code division multiple access (CDMA) interference canceller is presented to reduce interference in a CDMA receiver having N channels. The present invention also works with a spread spectrum code division multiplex (CDM) system. Accordingly, without loss of generality, the term spread spectrum CDMA signal, as used herein, includes spread spectrum CDMA signals and spread spectrum CDM signals. In a personal communications service, the jammer canceller can be used in a base station or in a remote unit such as a handset.

Figure 1 illustrates the interference canceller for the first channel, defined by the first segment code signal. The interference canceller includes a plurality of reconcentration means, a plurality of timing means, a plurality of spectrum spread processing means, a subtractor means, and a first channel reconcentration means.

Using a plurality of segment code signals, the plurality of reconcentration means reconcentrates the received spread spectrum CDMA signals as a plurality of reconcentrated signals, respectively. In Figure 1 the plurality of reconcentration means is shown as a first reconcentration means, a second reconcentration means, etc. up to a nth half of concentration. The first reconcentration means includes a first correlator, which is embodied, for example, as a first mixer 51, a first generator of signals of

ES 2 137 140 T3 segment codes 52 and a first integrator 54. The first integrator 54 may alternatively be a first low-pass filter or a first band-pass filter. The first mixer 51 is coupled between the input 41 and the first segment code signal generator 52 and the first integrator 54.

The second reconcentration means includes a second correlator, which is embodied, for example, as a second mixer 61, a second segment code signal generator 62, and a second integrator 64. The second integrator 64 may alternatively be a second signal filter. low pass or a second band pass filter. The second mixer 61 is coupled between the input 41, the second segment code signal generator 62, and the second integrator 64.

The nth reconcentration mean is represented as an nth correlator shown, as an example, as an nth mixer 71, an nth segment code signal generator 72 and an nth integrator 74. The nth integrator 74 can alternatively be a nth filter low pass or nth band pass filter. The nth mixer 71 is coupled between input 41, the nth segment code signal generator 72, and the nth integrator 74.

As is well known in the art, the first to nth reconcentration means can be embodied as any device that can reconcentrate a channel into a spread spectrum signal.

The plurality of timing means can be embodied as a plurality of delay devices 53, 63, 73. A first delay device 53 has a delay time T, which is approximately the same as the integration time Tb of the first integrator 54, or the time constant of the first low-pass filter or the first band-pass filter. A second delay device 63 has a delay T, which is approximately the same as the integration time Tb of the second integrator 64, or the time constant of the second low-pass filter or the second band-pass filter. Similarly, the nth delay device 73 has a delay T, which is approximately the same as the integration time Tb of the nth integrator 74, or the time constant of the nth low-pass filter or the nth band-pass filter. . Typically, the integration times of the first integrator 64, the second integrator 64, etc. and the nth integrator 74 are the same. If low-pass filters are used, the time constants of the first low-pass filter, the second low-pass filter, etc. and the nth low pass filter are typically the same. If using band-pass filters, the time constants of the first band-pass filter, the second band-pass filter, etc. and the nth band pass filter are the same.

The plurality of spread spectrum processing means regenerates each of the plurality of reconcentrated signals as a plurality of spread spectrum signals. The plurality of spread spectrum processing means uses a timed version, that is to say a delayed version, of the plurality of segment code signals, to spread the spectrum of the plurality of reconcentrated signals, respectively, with a code signal of segment corresponding to the respective concentrated signal. The plurality of spread spectrum processing means is shown, by way of example, as a first processing mixer 55, a second processing mixer 65, and so on. and an nth processing mixer 75. The first processing mixer 55 is coupled to the first integrator 54 and, through a first delay device 53, to the first segment code signal generator 52. The second processing mixer 65 is coupled to the second integrator 64 and, through the second delay device 63, to the second segment code signal generator 62. The nth processing mixer 65, and so on. and an nth processing mixer 75 is coupled to the nth integrator 74 and, through the delay device 73, to the nth segment code signal generator 72.

To reduce interference on a channel that uses a segment code signal i from the spread spectrum CDMA signal, the subtractor half subtracts, from the spread spectrum CDMA signal, each of the N-1 reconcentrated and processed signals to broaden the spectrum that do not correspond to channel i. The subtractor means thus generates a subtracted signal. The half subtractor is shown as a first subtractor 150. The first subtractor 150 is shown coupled to the output of the second processing mixer 65, etc. and the nth processing mixer 75. In addition, the first subtractor 150 is coupled through a main delay device 48 to input 41.

The channel i reconcentration means reconcentrates the subtracted signal with the segment code signal i as channel i. The first channel reconcentration means is shown as a first channel mixer 147. The first channel mixer 147 is coupled to the first delay device 53 and the first subtractor 150. The first channel integrator 146 is coupled to the first channel mixer. 147.

IS 2 137 140 T3

The first segment code signal generator 52, the second segment code signal generator 62, and so on. and the nth segment code signal generator 72 generates a first segment code signal, a second segment code signal, and so on. and one nth segment code signal, respectively. The term "segment code signal" is used herein to denote the spread signal of a spread spectrum signal, as is well known in the art. Typically the segment code signal is generated from a pseudo-random sequence (PN). The first segment code signal, the second segment code signal, etc. and the nth segment code signal could be generated from a first PN sequence, a second PN sequence, etc. and one nth PN sequence, respectively. The first PN sequence is defined or generated by a first segment code word, the second PN sequence is defined or generated by a second segment code word, and so on. and the nth PN sequence is defined or generated by an nth segment code word. Each of the first segment code word, the second segment code word, etc. and of the nth segment code word, it is different, that is, they are different from each other. In general, a segment code word can be the same sequence as a PN sequence, or used to define settings to generate the PN sequence. Settings can be shift registers delay taps, for example.

A first channel of a spread spectrum CDMA signal received at input 41 is reconcentrated by the first mixer 51 as a first reconcentrated signal, using the first segment code signal generated by the first segment code signal generator 52. The first concentrated signal from the first mixer 51 is filtered by the first integrator 54. The first integrator 54 integrates for a time Tb, which is the duration time of a symbol such as a bit. At the same time, the first segment code signal is delayed by a time T by the delay device 53. The delay time T is approximately equal to the integration time Tb plus the system or component delays. System or component delays are normally small compared to the integration time T_b.

The delayed version of the first segment code signal is processed with the first reconcentrated signal from the output of the first integrator 54 using the first spreading mixer 55. The output of the first spreading mixer 55 is applied to subtractors other than the first subtractor 150 to process the second to nth channels of the spread spectrum CDMA signal.

To reduce interference in the first channel of the spread spectrum CDMA signal, the received spread spectrum CDMA signal is processed by the reconcentrators second to nth as follows. The second channel of the spread spectrum CDMA signal is reconcentrated by the second reconcentration means. In the second mixer 61, a second segment code signal, generated by the second segment code signal generator 62, reconcentrates the second channel of the spread spectrum CDMA signal. The second reconcentrated channel is filtered through the second integrator 64. The output of the second integrator 64 is the second reconcentrated signal. The reconcentrated second signal is processed to spread the spectrum by a second processing mixer 65 with a delayed version of the second segment code signal. The second segment code signal is delayed through delay device 63. Delay device 63 delays the second segment code signal by a time T. The second channel mixer 65 does spectrum spread processing of a timed version, ie a delayed version, of the second segment code signal with the filtered version of the second spread spectrum channel from the second integrator 64. The term "spread spectrum processing" as used herein includes any method of generating a spread spectrum signal by mixing or modulating a signal with a segment code signal. Spread spectrum processing can be done by product devices, gates OR EXCLUSIVE, tailored filters or any other device or circuit as is well known in the art.

Similarly, the nth channel of the spread spectrum CDMA signal is reconcentrated by the nth means of reconcentration. Consequently, in the received spread spectrum CDMA signal the nth channel is reconcentrated by the nth mixer 71, mixing the spread spectrum CDMA signal with the nth segment code signal from the nth segment code signal generator 72. The output of the nth mixer 71 is filtered by the nth integrator 74. The output of the nth integrator 74, which is the nth reconcentrated signal, is a concentrated and filtered version of the nth channel of the spread spectrum CDMA signal. The nth reconcentrated signal is processed to spread the spectrum by a delayed version of the nth segment code signal. The nth segment code signal is delayed through the nth delay device

IS 2 137 140 T3

73. The nth processing mixer 75 does the spectrum spread processing of the timed version, ie a delayed version, of the nth segment code signal with the nth reconcentrated signal.

In the first subtractor 150 each of the outputs of the second processing mixer 65, etc. and from the nth processing mixer 75 a timed version, ie a delayed version, of the spread spectrum CDMA signal is subtracted from input 41. The delay of the spread spectrum CDMA signal is timed through the first device main delay 48. Typically, the delay of the first main delay device 48 is a time T, which is approximately equal to the integration time of the first integrator 54, etc. and the nth integrator 74.

At the output of the first subtractor 150 a first subtracted signal is generated. The first signal subtracted for the first channel of the spread spectrum CDMA signal is here defined as the outputs of the second processing mixer 65, etc. and from the nth processing mixer 75, subtracted from the delayed version of the spread spectrum CDMA signal. The second signal subtracted, etc. and the subtracted nth signal are defined similarly.

The delayed version of the first segment code signal from the output of the first delay device 53 is used to reconcentrate the output of the first subtractor 150. Consequently, the first subtracted signal is reconcentrated by the first channel's mixer 147 with the first segment code signal. The first channel 147 mixer output is filtered by the first channel integrator 146. This produces an output estimate d₁ of the first channel of the spread spectrum CDMA signal.

As shown in the illustration of Figure 2, a plurality of subtractors 150, 250, 350, 450 can be appropriately coupled to the input 41 and to a first widening mixer 55, a second widening mixer 65, a third widening mixer, etc. and a very much broadening mixer 75 of Figure 1. The plurality of subtractors 150, 250, 350, 450 are also coupled to the input 41 through the main delay device 48. This arrangement can generate a first subtracted signal from the first subtractor 150, a second subtracted signal from the second subtractor 250, a third subtracted signal from the third subtractor 350, and so on. and a subtracted nth signal from a 450 subtractor.

The outputs of the first receiver 150, the second receiver 250, the third receiver 350, etc. and the nth subtractor 450 are each coupled to a first channel mixer 147, a second channel mixer 247 to a second channel mixer 347, and so on. and to a mixer on the nth channel 447, respectively. Each of the channel mixers is coupled to a delayed version of the first segment code signal, g₁ (tT), from the second segment code signal, g₂ (tT), of the third G segment code signal₃ (tT), etc. and the nth segment code signal, G_N(tT). The outputs of each of the first channel mixer 147, the second channel mixer 247, the third channel mixer 347, and so on. and the nth channel mixer 447, respectively, are coupled to a first channel integrator 146, a second channel integrator 246, a third channel integrator 346, and so on. and an integrator of the nth channel 446, respectively. At the output of each of the channel integrators, an estimate of the first channel d1, the second channel d2, the third channel d3, etc. is obtained. and of the nth channel dN, respectively.

Referring to Figure 1, the use of the present invention for the first channel of the spread spectrum CDMA signal is illustrated, with the understanding that the second to enosyme CDMA channels function in a similar manner. A spread spectrum CDMA signal received at input 41 is delayed by delay device 48 and applied to first subtractor 150. In the spread spectrum CDMA signal the second through nth channels are reconcentrated by the second mixer 61 using the second segment code signal, etc. and by the nth mixer 71 using the nth segment code signal. The second segment code signal, etc. and the nth segment code signal are respectively generated by the second segment code signal generator 62, etc. and the nth segment code signal generator 72. The second channel, and so on. and the nth channel are reconcentrated and filtered through the second integrator 64, etc. and the nth integrator 74, respectively. The reconcentration eliminates, partially or totally, the non-reconcentrated channels at each of the outputs of the second integrator 64, etc. and the nth integrator 74.

In a preferred embodiment each segment code signal used for the first segment code signal generator 52, the second segment code signal generator 62, and so on. and the nth segment code signal generator 72 are orthogonal to each other. However, the use of orthogonal segment code signals is not required for the operation of this

ES 2 137 140 T3 invention. When using orthogonal segment code signals, the reconcentrated signals have the respective channel plus noise at the output of each of the integrators. With orthogonal segment code signals, the mixers theoretically eliminate channels orthogonal to the reconcentrated channel. The respective channel is processed to spread the spectrum by the respective processing mixer.

At the outlet of the second processing mixer 65, etc. and the nth processing mixer 75 has a re-widened version of the second channel, etc. and of the ninth channel, more noise components contained in it. Each of the second through nth channels is then subtracted from the spread spectrum CDMA signal received by the first subtractor 150. The first subtractor 150 produces the first subtracted signal. The first subtracted signal is reconcentrated with a delayed version of the first segment code signal by the first channel mixer 147, and filtered by the first channel filter 146. Consequently, before reconcentrating the first channel of the CDMA signal From spread spectrum, the second through nth channels, plus the noise components aligned with these channels, are subtracted from the received spread spectrum CDMA signal. As shown in the illustration of Figure 3, an alternative embodiment of the spread spectrum CDMA interference canceller includes a plurality of first reconcentration means, a plurality of spread spectrum processing means, subtraction means, and second reconcentration means. . In Figure 3, the plurality of reconcentration means is shown as a first reconcentration means, a second reconcentration means, and so on. and a nth half of concentration. The first reconcentration means was embodied as a first matched filter 154. The first matched filter 154 has an impulse response matched to the first segment code signal, which is used for the spectrum spreading process and to define the first channel. of the spread spectrum CDMA signal. The first matched filter 154 was coupled to input 41.

The second reconcentration means is shown as a second matched filter 164. The second matched filter 164 has an impulse response matched to the second segment code signal, which is used for the spectrum spreading process and to define the second channel. of the spread spectrum CDMA signal. The second matched filter 164 was coupled to the input 41.

The nth mean of reconcentration is shown as an nth matched filter 174. The nth matched filter 174 has an impulse response matched to the nth segment code signal, which is used for the spectrum broadening process and to define the nth channel. of the spread spectrum CDMA signal. The nth matched filter was coupled to input 41.

The term "matched filter", as used herein, includes any type of matched filter that can accommodate a segment code signal. The matched filter may be a digital matched filter or an analog matched filter. An acoustic surface wave (SAW) device can be used at a radio frequency (RF) or an intermediate frequency (IF). Digital signal processors and application-specific integrated circuits (ASICs) that have filters tailored to RF, IF, or baseband frequencies can be used.

In Figure 3 the plurality of spectrum spreading processing means is shown, such as the first processing mixer 55, the second processing mixer 65, and so on. and the nth processing mixer 75. The first processing mixer 55 may be coupled through the first adjusting device 97 to the first segment code signal generator 52. The second processing mixer 65 may be coupled through the second setting device 98 to the second segment code signal generator 62. The nth processing mixer 75 may be coupled through the nth setting device 73 to the nth processor generator. segment code signals 72. The first setting device 97, the second setting device 98, and so on. and the nth adjusting device 99 are optional and used as an adjuster to align the first segment code signal, the second segment code signal, etc. and the nth segment code signal with the first reconcentrated signal, with the second reconcentrated signal, etc. and with the nth reconcentrated signal, coming out of the first matched filter 154, the second matched filter 164, etc. and the nth matched filter 174, respectively.

The half subtractor is shown as the first subtractor 150. The first subtractor 150 was coupled to the output of the second processing mixer 65, and so on. and the nth processing mixer 75. In addition, the first subtractor 150 was coupled through the main delay device 48 to input 41.

The reconcentration medium of the first channel is shown as a filter matched to the first channel 126.

IS 2 137 140 T3

The first channel matched filter 126 was coupled to the first subtractor 150. The first channel matched filter 126 has a pulse response matched to the first segment code signal.

A first channel of a spread spectrum CDMA signal received at input 41 is reconcentrated by the first matched filter 154. The first matched filter 154 has a pulse response matched to the first segment code signal. The first segment code signal defines the first channel of the spread spectrum CDMA signal and is used by the first segment code signal generator 52. The first segment code signal may be delayed by an adjustment time τ by the adjustment device 97. The output of the first matched filter 154 is processed to spread the spectrum by the first processing mixer 55 with the first segment code signal. . The output of the first processing mixer 55 is applied to subtractors other than the first subtractor 150 to process the second channel, etc. and the nth channel of the spread spectrum CDMA signal.

To reduce interference in the first spread spectrum channel, the received spread spectrum CDMA signal is processed by the second reconcentration means, etc. and by the nth means of concentration as follows. The second matched filter 164 has an impulse response matched to the second segment code signal. The second segment code signal defines the second channel of the spread spectrum CDMA signal and is used by the second segment code signal generator 62. The second matched filter 164 reconcentrates the second channel of the spectrum CDMA signal. widened. The output of the second matched filter 164 is the second concentrated signal. The second reconcentrated signal triggers the second segment code signal generator 62. The second concentrated signal is also processed to spread the spectrum by the second processing mixer 65 with a timed version of the second segment code signal. The timing of the second segment code signal triggers the second reconcentrated signal from the second matched filter 164.

Similarly, the nth channel of the spread spectrum CDMA signal is reconcentrated by the nth reconcentration medium. Consequently, in the received spread spectrum CDMA signal the nth channel is reconcentrated by the nth matched filter 174. The output of the nth matched filter 174 is the nth reconcentrated signal, that is, a concentrated and filtered version of the nth matched channel. spread spectrum CDMA. The nth reconcentrated signal is processed to process the spectrum by a timed version of the nth segment code signal. The timing of the nth segment code signal is driven by the nth reconcentrated signal from the nth matched filter 174. The nth processing mixer 75 does the spectrum spreading processing of the timed version of the nth segment code signal with the most concentrated signal.

In the first subtractor 150 each of the outputs of the second processing mixer 65, etc. and the nth processing mixer 75 are subtracted from a delayed version of the spread spectrum CDMA signal from input 41. The delay of the spread spectrum CDMA signal is timed through delay device 48. The delay device 48 is timed to align the reconcentrated and processed signals to spread the second to nth spectrum, to subtract them from the spread spectrum CDMA signal. This generates at the output of the first subtractor 150 a first subtracted signal. The first subtracted signal is reconcentrated by the filter matched to the first channel 126. This produces an output estimate d₁ of the first channel of the spread spectrum CDMA signal.

As illustrated in Figure 4, a plurality of subtractors 150, 250, 350, 450 may be appropriately coupled to the output of a first processing mixer, a second processing mixer, a third processing mixer, etc. and from an umpteenth processing mixer, and input through a main delay device. A first subtracted signal leaves the first subtractor 150, a second subtracted signal leaves the second subtractor 250, a third subtracted signal leaves the third subtractor 350, and so on. and a subtracted nth signal comes out of the nth subtractor 450.

Each of the outputs of the first receiver 150, the second receiver 250, the third receiver 350, etc. and the nth subtractor 450 is respectively coupled to a filter matched to the first channel 126, to a filter matched to the second channel 226, to a filter matched to the third channel 326, and to a filter matched to the nth channel 426. The filter matched to the first channel 126, the second channel matched filter 226, the third channel matched filter 326, etc. and the ninth channel matched filter 426 have a pulse response matched to the first segment code signal, the second segment code signal, the third segment code signal, and so on. and to the nth segment code signal, which define the first channel, the second channel, the third channel, etc. and the nth channel, respectively, of the signal

Spread spectrum CDMA ES 2 137 140 T3. At each of the respective outputs of the filter matched to the first channel 126, the filter matched to the second channel 226, the filter matched to the third channel 326, etc. and the filter matched to the nth channel 426, an estimate of the first channel d is produced₁, of the second channel d₂, of the third channel d₃, etc. and the nth channel d_N, respectively.

The present invention is illustrated in use for the first channel of the spread spectrum CDMA signal, with the understanding that the second channel, etc. and the nth channel work similarly. A spread spectrum CDMA signal received at input 41 is delayed by delay device 48 and applied to subtractor 150. In the same spread spectrum CDMA signal the second channel, etc. and the nth channel are reconcentrated by the second matched filter 164, and the nth matched filter 174. This reconcentration removes the other CDMA channels from the respective reconcentrated channel. In a preferred embodiment, each of the segment code signals used for the first channel, the second channel, etc. and the nth channel are orthogonal to the other segment code signals. At the outlet of the first matched filter 154, the second matched filter 164, etc. and from the nth matched filter 174 you have the first reconcentrated signal, the second reconcentrated signal, and so on. and the umpteenth signal concentrated, more noise.

The respective channels are processed to spread the spectrum by the processing mixers. Consequently, at the outlet of the second processing mixer 65, etc. and of the nth processing mixer 75 there is a widened version of the second reconcentrated signal, etc. and the umpteenth reconcentrated signal, plus the noise components contained in them. Each of the signals concentrated and processed to process the spectrum is then subtracted from the spread spectrum CDMA signal received by the first subtractor 150. This produces the first subtracted signal. The first subtracted signal is refocused by the first channel matched filter 126. Consequently, before refocusing the first channel of the spread spectrum CDMA signal, the second channel, etc. is subtracted. and the nth channel, plus the noise components aligned with these channels of the received spread spectrum CDMA signal.

As is well known in the art, correlators and filters adapted to perform the same function can be interchanged. Figures 1 and 3 show alternative embodiments using adapted correlators or filters. Schemes can be varied. For example, the plurality of reconcentration means can be embodied as a plurality of matched filters, while the channel reconcentration means can be embodied as a correlator. Alternatively, the plurality of reconcentration means may be a combination of matched filters and correlators. Also the spectrum spreading processing means can be embodied as a matched filter or SAW device, or as EXCLUSIVE OR gates or other devices for mixing a concentrated signal with a segment code signal. As is well known in the art, any spread spectrum demodulator or reconcentrator can reconcentrate the spread spectrum CDMA signal. The particular circuits shown in Figures 1 to 4 illustrate the invention by way of example.

The concepts taught in Figures 1 to 4 can be repeated as shown in Figure 5. Figure 5 illustrates a first plurality of interference cancellers 511, 512, 513, a second plurality of interference cancellers 521, 522, 523, etc. . and an nth plurality of interference cancellers 531, 532, 533. Each plurality of interference cancellers includes appropriate elements as already described with reference to Figures 1 to 4. The input is delayed through a delay device in each interference canceller.

In the received spread spectrum CDMA signal the interferences are initially canceled by the first plurality of interference cancellers 511, 512, 513, thereby producing a first set of estimates, i.e. a first estimate d11, a second estimate d12, etc. . and an umpteenth estimate of_1N, the first channel, the second channel, etc. and the nth channel of the spread spectrum CDMA signal. Interferences from the first set of estimates can be canceled by the second plurality of interference cancellers 521, 522, 523. The first set of estimates d11, d12, ... d1N, from the first channel, the second channel, and so on. and the nth channel are applied at the inputs of the second plurality of interference cancellers, interference canceller 521, interference canceller 522, and so on. and the nth interference canceller 523 of the second plurality of interference cancellers. The second plurality of interference cancellers thus produces a second set of estimates, ie d21, d22, ... d2N, of the first channel, the second channel, etc. and the umpteenth channel. Similarly, the second set of estimates can pass through a third plurality of interference cancellers and finally through a nth set of interference cancellers 531, 532, 533, respectively.

IS 2 137 140 T3

The present invention also includes a method of reducing interference in a spread spectrum CDMA receiver having N segment code channels. Each of the N channels is identified by a different segment code signal. The method comprises the steps of re-concentrating, using a plurality of segment code signals, the spread spectrum CDMA signal as a plurality of re-concentrating signals, respectively. Using a timed version of the plurality of segment code signals, the plurality of concentrated signals are processed to spread the spectrum with a segment code signal corresponding to a respective concentrated signal. Each of the N-1 signals reconcentrated and processed to spread the spectrum is subtracted from the spread-spectrum CDMA signal, the N-1 signals concentrated and processed to spread the spectrum do not include a signal processed to spread the spectrum of the signal. reconcentrated i, thus generating a subtracted signal. The subtracted signal is reconcentrated to generate channel i.

The error probability Pe for a direct sequence spread spectrum CDMA system is:

P_and = 2 erfc (aSNR)^1/2 where erfc is the complementary error function, SNR is the signal-to-noise ratio, and 1 <α <12. The value of α depends on how a given interference canceller system is designed.

The SNR after the cancellation of the method interferences is given by:

(PG / N)^r+¹

SNR

1+ (PG / N)^{R + 1} 'n / n í- (N / PG)^r+¹

1-N / PG where N is the number of channels, PG is the processing gain, R is the number of repetitions of the interference canceller, Eb is the energy per bit of information and η is the spectral density of the noise power .

Figure 6 illustrates the theoretical performance characteristic of the interference canceller and the method for the case of E_b/ r¡ = 6 dB. The performance characteristic is plotted for SNR outside the interference canceller as a function of PG / N. The lower curve, for R = 0, is the operating characteristic without the interference canceller. The curves for R = 1 and R = 2 illustrate a better performance characteristic using one and two iterations of the interference canceller as shown in Figure 5. When PG / N 1, there is insufficient SNR to work. If PG> N, the interference canceller output SNR approaches E_b/ r. Also, yes (N / PG)^R+¹ 1,

SNR (E_b/ r) (1 - N / PG)

Figure 7 illustrates the performance characteristic in the case of Eb / r = 10 dB. Figure 7 shows that three iterations of the interference canceller can give a 4 dB improvement with PG / N = 2.

Figure 8 illustrates the performance characteristic in the case of Eb / r = 15 dB. With this ratio of energy per bit to noise, two iterations of the interference canceller can give a 6 dB improvement with PG / N = 2.

Figure 9 illustrates the performance characteristic for the case of Eb / r = 20 dB. With this relationship of energy per bit to noise, two iterations of the interference canceller can give a 6 dB improvement with PG / N = 2. Similarly, Figures 10 and 11 show that one iteration of the interference canceller can give more than 10 dB improvement for PG / N = 2.

The present invention can be extended to a plurality of interference cancellers. As shown in Figure 12, a received spread spectrum signal, R (t), is reconcentrated and detected by the CDMA / DS detector 611. Each of the channels is represented as outputs O01, O02, O₀₃, ... or_0M. Thus, each output is a reconcentrated spread spectrum channel of a received spread spectrum signal, R (t).

Each of the CDMA / DS detector outputs 611 is passed through a plurality of interference cancellers 612, 613 ... 614, which are connected in series. Each of the spectrum channels

The spread ES 2 137 140 T3 goes through the interference cancellation processes as previously discussed. The input to each interference canceller is obtained by sampling and holding the output of the previous stage once for each bit length. For channel i, the first interference canceller shows the output of the CDMA / DS detector at time t = T + T_i. This value is kept constant as input until t = 2T + t_i, at which point the value of the next bit is displayed. Thus, the input waveforms to the interference canceller are estimates, di (tr_i) of the original data waveform, d_i(tT_i), and the outputs are second estimates, d ~ i (tT_i). The M spread spectrum channel outputs O0i with i = 1, 2, ..., M, are passed through the interference canceller 612 to obtain a new corresponding set of channel outputs O1i, with i = 1, 2, ... M.

As shown in Figure 13, the outputs of a given spread spectrum channel can be combined, which will be at the output of each of the interference cancellers. Consequently, the combiner 615 can combine the output of the first channel from the CDMA / DS detector 611, the output O11 of the first interference canceller 612, the output O21 of the second interference canceller 613, and so on. and the ON1 output of the nth interference canceller 614. Each output to be combined is of the corresponding bit. Consequently, delays of the length of "s" bits are introduced for each Os1. The combined outputs are then passed through decision device 616. This can be done for each spread spectrum channel and therefore designate the outputs of each of the combiners 615, 617, 619 as the average output O1 for the channel. one, the average O2 output for channel two, and the average OM output for channel M. Each of the middle outputs are sequentially passed through decision device 616, decision device 618, and decision device 620. Preferably, the average outputs have a multiplying factor cj that can vary according to a given design. In a preferred embodiment cj = 1/2^j. This allows the outputs of the various interference cancellers to be combined in a certain way.

Figures 14 to 17 illustrate the simulated operating characteristics for the schemes of Figures 12 and 13. Figures 14 to 17 correspond to asynchronous channels (the relative delays are evenly distributed between 0 and the bit duration, T), a gain of 100 processing, all users have equal powers, and the signal-to-noise ratio and a signal-to-thermal noise ratio (Eb / N of 30 dB). Gold codes of length 8191 are used for PN sequences.

Figure 14 shows the operating characteristics of each of the output stages of Figure 12. Asai, S0 represents the resulting BER at the output of the CDMA / DS 611 detector, S1 represents the resulting BER at the output of the interference canceller 612, S2 represents the resulting BER at the output of interference canceller 613, etc. No combination of the interference canceller outputs is used to determine the performance characteristic shown in Figure 14. Instead, the performance characteristic corresponds to the repetitive use of interference cancellers. As a guide, in each of the following Figures, the output for each characteristic of the CDMA / DS 611 detector is shown.

Figure 15 shows the performance characteristic when the outputs of the rear interference cancellers are combined. This is displayed for a given channel. Thus, the curve S0 is the output of the CDMA / DS 611 detector. The curve S1 represents the BER resulting from the average of the outputs of the CDMA / DS 611 detector and the interference canceller 612. AquiC0 = C1 = 1/2 Cj = 0, j different from zero, one. The curve S2 represents the BER resulting from the average of the outputs of the interference canceller 613 and the interference canceller 612. The curve S2 is determined using the combiner shown in Figure 13. Aqui, C1 and C2 are equal to 1/2 and all other Cj are set equal to zero. Similarly, the curve S3 is the result of the average of the outputs of a second and a third interference canceller. Thus, the curve S3 is the performance characteristic of the average of the outputs of a second and a third interference cancellers. The curve S4 is the performance characteristic of the average output of a third and a fourth interference cancellers. Only two interference cancellers are taken at any one time to determine a performance characteristic of an average output of those interference cancellers in question. Figure 16 shows the normal outputs for the CDMA / DS detector 611 and a first and second interference cancellers 612, 613. In addition, the average output of the CDMA / DS detector 611 and the first interference canceller is shown as S1 AVG. interference 612. The BER resulting from the average of the outputs of the first interference canceller 612 and the second interference canceller 613 is shown as the average output S2 AVG.

Figure 17 shows operating characteristics corresponding to those of Figure 16, but

ES 2 137 140 T3 referring to the ratio of the signal to noise in decibels (dB).

It will be clear to those skilled in the art that various modifications can be made to the spread spectrum CDMA interference canceller and the method of the present invention.