HK1066658A - Parallel spread spectrum communication system and method - Google Patents

Description

Parallel spread spectrum communication system and method

Related application patent

The present invention claims priority from U.S. provisional patent application 60/268.942, filed on day 16, 2/2001, and is incorporated herein by reference.

Background

1. Field of the invention

The present invention relates to digital communications, and more particularly, to systems and methods for providing spread-spectrum related communications.

2. Description of the related Art

Spread spectrum communication techniques have a wide range of applications. For example, spread spectrum originates from military communications that are extremely sensitive to spy/interception and susceptible to intentional intervention interference/jamming losses. However, hosts have been developed that are suitable for spread spectrum commercial applications, particularly in the field of wireless communications, e.g., cellular mobile communications.

The basic concept of spread spectrum is different from long-term standard communication practices. In particular, conventional practice has focused on reducing the frequency bandwidth of information-bearing signals in order to adapt more signals to a communication link (channel). The goal of spread spectrum, in contrast, is to substantially increase the bandwidth of the information-bearing signal. Indeed, spread spectrum communication links occupy a much larger bandwidth than the minimum bandwidth required by standard communication links. That is, spread spectrum signals typically occupy a bandwidth that far exceeds the bandwidth required to transmit digital data in accordance with nyquist's theorem. As will be discussed in more detail below, this increase in bandwidth helps to reduce the adverse effects of various forms of interference.

In spread spectrum systems, a transmitter spreads (increases) the bandwidth of an information-bearing signal prior to transmission. The receiver, upon receiving the signal, despreads (reduces) the bandwidth by substantially the same amount. Ideally, the despread received signal is identical to the transmitted signal before spreading. However, the communication channel typically introduces some form of narrowband (as opposed to extended bandwidth) interference.

One popular type of spread spectrum system is the direct sequence spread spectrum system ("DSSS"). With DSSS systems, spread spectrum is obtained by multiplying digital data by a pseudo-noise sequence "PN-sequence" or "PN-code", also known as a pseudorandom sequence or chip code (chipping code), whose symbol rate is many times the code rate of binary data. The symbol rate of the spreading sequence is again referred to as the fraction rate. The chip code is data independent and includes a redundant bit pattern for each bit to be transmitted. The code in effect provides interference immunity to the transmitted signal. If one or more bits in the pattern are lost during transmission, the original data can still be recovered due to redundancy in the transmission. The pseudo-noise sequence is a sequence of fractional values of-1 or 1 (polar), or 0 and 1 (non-polar), which also has other relevant properties.

Fig. 1 illustrates a conventional direct sequence ("DS") spread spectrum spreading technique. There are several well-known classes of pseudo-noise sequences that can be applied to DSSS systems, such as M-sequences, Gold (Gold) codes, and Kasami codes; each type of sequence or code has its own particular properties. The number of segments in a code is referred to as the period (N) of the code. For example, if a complete PN sequence is multiplied by a single data bit (using N-7 as shown in fig. 1), the bandwidth of the signal is multiplied by a factor N, which is also referred to as the gain of the processing. In other words, the processing gain in spread spectrum communications is directly related to the length of the sequence. Referring to fig. 2A, if an M-sequence code is used, the effect in the power spectrum is to have a sinc²(x) The power spreading density of the function.

The benefits of using spread spectrum techniques can be quickly seen through the necessity of interference suppression. Interference affecting a signal is mainly of three types: jamming, multiple access, and multipath. Jamming can occur when another signal is pre-prepared (e.g., using a military jammer) or inadvertently superimposed on the signal. Multiple access interference occurs when a signal shares the same frequency spectrum of other signals. Multipath interference methods occur when the signal itself is delayed.

With jamming related techniques, the hostile party or "jammer" may have different times on the spread spectrum signal. In fact, after spreading the spread spectrum signal is scrambled with noise, see fig. 2B, the jammer signal is limited to only a small portion of the spectrum, after despreading (spectrum reduction) the jammer is attenuated to the noise level, see fig. 2C, and the information can be recovered, see fig. 2D. In commercial applications, the main advantage of spread spectrum communications is the elimination of the associated interference from another transmitter.

The spread spectrum benefits associated with multiple access have greater commercial application value. From a commercial application point of view, spread spectrum communication allows multiple users to communicate in the same frequency band. When this approach is used, it changes frequency division multiple access ("FDMA") or time division multiple access ("TDMA") and is commonly referred to as code division multiple access ("CDMA") or spread spectrum multiple access ("SSMA"). When using CDMA, each signal in the shuffling gives its own spreading sequence. FDMA requires that all users use mutually disjoint frequency bands and transmit synchronously in time. TDMA requires that all users use the same bandwidth by assigning separate time windows to each user in each channel. CDMA, in contrast, distinguishes waveforms that differ from each other at the receiver by the assigned spreading code they employ.

CDMA has particular interest in wireless communications. These applications may include cellular communications, personal communication services ("PCS"), and wireless local area networks. The main reason for its popularity is due to the performance exhibited by spread-spectrum waveforms when transmitted over multipath input channels. To be able to illustrate this concept, the signaling of the DS is considered. The use of DS waveforms provides the system designer with one of two options as long as the process of spreading a single segment of the sequence is a spreading that is less than the multipath delay. Multipath can be treated as a form of interference, which means that the receiver should attenuate it as much as possible. Indeed, under such conditions, all multipath returns arrive at the receiver with a time delay greater than the fraction time (usually the first return) in the multipath return process that arrives at the receiver synchronously, and all get attenuated, due to the processing gain of the system. In addition, during the return from the main path, the multipath separated by the multiple fragments all represent the "independence" seen in the received signal and may constructively enhance the overall performance of the receiver. That is, because all multipath returns contain information about the data to be transmitted, the information can be acquired by a properly designed receiver.

That is, spread spectrum communications have the benefit that different spreading codes can be used so that multiple links can operate on the same frequency at the same time. Another benefit that can be achieved by this technique is that the processing gain allows the spread spectrum communication link to operate at a much lower signal level than conventional radio links.

However, conventional spread spectrum systems also have some disadvantages. One problem with conventional wireless systems is that the system has the requisite requirement of considerable RF transmitter power. Particularly in portable handheld cellular devices, it is believed that such power conditions and associated strong electromagnetic signals of the device can negatively impact human physiology. Another related disadvantage of conventional systems is the short battery life of portable devices in certain applications. Thus, conventional spread spectrum systems require large communication bandwidths and employ many spreading codes to limit the number of users on each bandwidth.

Another disadvantage is that the spread spectrum is able to withstand the near ar far effect. This problem is due to the fact that the receiver can receive multiple signals from multiple transmitters with different powers. In general, the power of the signal transmitted by the non-interfering transmitter can be suppressed in the receiver by the cross-correlation properties of the interfering codes. However, if the non-interfering transmitter is in close proximity to the interfering transmitter, the received signal power of the non-interfering transmitter may constitute much higher than the signal power of the interfering transmitter. In this case, the PN correlator in the receiver will have difficulty detecting and despreading the weak interfering transmissions.

Another significant disadvantage is that conventional systems do not provide enhanced processing gain in a practical and efficient manner. Currently, spread spectrum techniques cannot support large PN sequence lengths that can improve processing gain. In addition, conventional systems have difficulty employing optimized processing gains for forward error correction.

Disclosure of Invention

The invention teaches a method and system for dual-order parallel spread spectrum. The invention advantageously combines a series of code sequences to produce an enhanced and robust communication technique that can be implemented in a wide variety of applications, which may include point-to-point or point-to-multipoint wireless communication systems.

In one embodiment of the invention, a wireless communication system includes a transmitter and a receiver station. A double-ordered parallel spectrum spreading method including a basic code sequence and an auxiliary code sequence is employed. According to the invention, the steps performed by the transmitter station comprise: encoding the digital data signal using a base coding scheme (e.g., a quadrature Walsh coding scheme); spreading the encoded signal with an auxiliary sequence (e.g., a PN sequence); modulating the spread coded signal using, for example, a DBPSK modulation technique; and transmitting the modulated signal. According to the preferred embodiment, the receiver station performs the steps comprising: despreading the received signal using the stored auxiliary sequence, demodulating the despread signal; and decoding the demodulated signal using the base coding scheme.

The use of multiple short spreading sequences in parallel can fundamentally enhance processing gain and multiple access properties.

The present invention also provides for the use of forward error correction while enhancing processing gain.

Another significant advantage of the present invention is that the enhanced processing gain allows for reduced transmitted power conditions. For example, a processing gain of 18dB theoretically means that only 1/8 for the RF transmit power condition is needed to meet the needs of the communication link. The lower power condition of the present invention may reduce health concerns and allow longer battery usage in certain applications.

Another advantage of the present invention is that independent spreading sequences are employed in the in-phase and quadrature channels, allowing for increased link security.

A further advantage of the invention is that the bandwidth efficiency is improved. For example, the invention generally provides greater than 5 times greater bandwidth efficiency than conventional spread spectrum techniques that employ the same processing gain attribute.

Another advantage of the present invention is that implementing a forward error correction algorithm at the receiver advantageously improves the performance of the code error rate.

Yet another advantage of the present invention is the reduced acquisition period due to the short PN sequences employed.

The above and other features and advantages of the present invention will become more apparent from the following more detailed discussion of the embodiments of the invention, the accompanying drawings, and the claims.

Drawings

For a more complete understanding of the present invention, and for further objects and advantages thereof, reference is now made to the following discussion, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional direct sequence spread spectrum spreading technique;

FIGS. 2A-2D illustrate frequency spectra in a conventional direct-sequence spread spectrum communication system;

fig. 3 illustrates a parallel spread spectrum communication system according to an embodiment of the present invention;

FIG. 4 illustrates a process for transmitting parallel spread-spectrum signals according to an embodiment of the present invention;

FIG. 5 illustrates a process for receiving a parallel spread-spectrum signal according to an embodiment of the invention;

FIG. 6 illustrates a signal diagram for parallel spreading of data according to an embodiment of the present invention;

fig. 7 illustrates a signal channel parallel spread spectrum transmitter system in accordance with an embodiment of the present invention;

fig. 8 illustrates a hardware component diagram of a QPSK distinct encoder according to an embodiment of the present invention;

FIG. 9 illustrates a parallel spread-spectrum receiver system according to an embodiment of the present invention;

fig. 10 illustrates Walsh code correlation and decoding circuitry in accordance with an embodiment of the present invention;

fig. 11 illustrates a hardware component diagram of a different PSK demodulator in accordance with an embodiment of the present invention;

fig. 12 illustrates a dual-channel parallel spreading system according to an embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention will now be discussed with reference to fig. 3-12, in which like reference numerals refer to like elements and the left-most numerical values of the various numerals correspond to the first-used numerical values in the figures.

These preferred embodiments are discussed in the context of a wireless telephone communication system. However, the present invention may be implemented in a wide range of applications, such as broadband wireless point-to-point and point-to-multipoint digital communication links; low power wireless applications; telemetry applications using CDMA; a WLAN application; and a secure communication channel. The preferred embodiment relates to a parallel double sequence spread spectrum ("PBSS") technique for spreading code data onto a predetermined sequence according to the present invention. Thus, the present invention also provides other processing gains, data forward error correction ("FEC") benefits, and other benefits and advantages.

The present invention can also be applied to any existing digital communication channel to essentially create a pseudo-direct sequence spread spectrum communication link that employs bit-by-bit (B x B) or parallel spreading of multiple bit-by-bit (MB x MB) of the input digital data. When combined with a DSSS communication channel, a two-layer parallel spreading of the data stream results. The invention widens the bandwidth condition and improves the processing gain of the link.

Referring to fig. 3, a spread spectrum communication system 300 is depicted in accordance with an embodiment of the present invention. System 300 includes a transmitting station 310 and a receiving station 320. The transmitting station 310 communicates a parallel spread-spectrum signal 330 with the receiving station 320. To facilitate bi-directional communication, receiving station 320 may also act as a transmitter to transmit parallel spread-spectrum signal 340 to transmitting station 310 acting as a receiver. It will be appreciated by those of ordinary skill in the art that the parallel spread-spectrum signals 330 and 340 may be transmitted over a wireless network (not shown), such as a cellular telephone service network and a personal communications services ("PCS") network. For example, transmitting station 310 and receiving station 320 may be within the same cell or different cells in a cellular network or within cells of two different networks. The cellular network may include one or more base stations, which may operate individually in respective cells, and the telephone exchange office may be considered a mobile telephone switching office ("MTSO"). Each base station may include one or more transmitters and/or receivers that may forward parallel spread-spectrum signals 330 and 340 so that the cellular network may communicate with transmitting station 310 and/or receiving station 320. In such embodiments, the MTSP handles all telephones connected to the handheld telephone system and other cellular networks, and controls all base stations in the designated area. The parallel spread spectrum signals 330 and 340 may be converted at the base station or MTSO to signals of a different format, which may be a format as desired depending on the requirements of the terrestrial-based communication system or other cellular network.

In a preferred embodiment, the parallel spread-spectrum signal 330 is generated according to the process 400 described in fig. 4. In an embodiment of the invention, the launch pad 310 encodes the digital data signal using a base encoding scheme (step 410). The basic coding scheme employs length 2n orthogonal codes such as orthogonal Walsh functions. For example, the base code may be a 4, 8, or 16-bit Walsh code. The extended primary coded data is secondary encoded with a secondary code (step 420). The auxiliary code may be any type of even ordered code, such as M-sequence, Barker, Gold, Kasami, and others, but is preferably a PN sequence. The auxiliary code can be multiplied simultaneously with the complete base sequence as required, so that the auxiliary sequence must be an integer multiple of the length of the base sequence. For example, if the primary code is an 8-bit Walsh code, the secondary code must be an integer multiple of 8, e.g., a bit PN sequence that can be 16, 24, 32, 48, or 64, etc. Once the auxiliary encoding is complete, the signal is modulated (step 430) and transmitted to the receiving station 320 (step 440).

Fig. 5 illustrates a process 500 suitable for receiving a parallel spread-spectrum signal 330 in accordance with the preferred embodiment of the present invention. The parallel spread-spectrum signal 330 is first received at the receiving station 320 (step 510). Parallel spread-spectrum signal 330 is digitized (step 520) and then despread (step 530) using the stored auxiliary sequence corresponding to the auxiliary sequence used by transmitting station 310. Once despreading is complete, the signal is demodulated (step 540) and then decoded using the scheme employed in the transmit spread 310 (step 550).

With this embodiment, a potential processing gain of 18.4dB can be achieved if 8-bit Walsh codes are used as the base sequence and 48-bit PN sequences are used as the auxiliary sequences (as will be described in more detail below). A higher degree of processing gain can be achieved by using longer primary and/or secondary codes. However, the complexity of the electronics in the re-receiving station 320 is directly proportional to the code length and may therefore limit the specific application of larger codes. In comparison, to achieve the 18.4dB processing gain in conventional DSSS systems, more than 69 unused spreading codes must be employed, which is difficult to achieve for high data rate applications using current technology.

Fig. 6 illustrates a signal diagram 600 of parallel spread data according to an embodiment of the present invention. As shown, the 8-bit orthogonal code 610 may be spread with a parallel PN sequence 620 of 48 bits to produce a parallel spread-spectrum data signal 630. As explained above, the parallel sequences must be integer multiples of the selected orthogonal code length. Each data symbol 640 is spread by a 6-bit parallel spreading sequence 650 and yields a potential processing gain of 7.78dB (10 log 6). Once the appropriate orthogonal codes and parallel PN sequences are selected, they are fixed during the communication. CDMA communications are possible when each receiver assigns orthogonal PN sequences, which may or may not be of variable length.

In essence, long parallel spreading sequences are often used over multiple data bits. The spreading sequence used may be, for example, an M-sequence, Barker, Gold, Kasami, and any type of PN sequence. The parallel spreading in accordance with the present invention may employ different coding of the data streams in the transmit path to simplify data recovery at the receiver. If the parallel spreading scheme is applied to the M-ary modulation link, in-phase (I) and quadrature (Q) channels can be spread using different PN sequences to improve channel security.

M-sequence modulation systems can emit more information during each transition (symbol) of the transmitted signal than binary systems. Because log is required₂(M) bits to select one of M possibilities so that each waveform can be converted to log₂(M) bit information. Each transformed waveform represents log₂A (M) bit symbol.

Table 1 illustrates an example of an M sequence scheme.

Table 1: m-ary scheme

M sequence	Modulation scheme
		4	QPSK
8	8PSK
		16	16QAM
64	64QAM

In an embodiment of the present invention, Walsh encoding of the base data provides initial spreading and coding gain. An 8-bit Walsh encoder would provide a potential processing gain of 9dB and a coding gain of 1.6 dB. The link uses advanced protocols and converts data into a packetized data format. The preamble indicates the beginning of the transition to enable initialization of the receiver acquisition. For sequential data packet conversion, differential binary phase shift keying ("DBPSK") modulation may initialize the preamble to QDPSK. The difference is transformed in a way that the data is shifted by a discrete phase shift Δ θ, where the phase reference is the originally transformed signal phase. This approach reduces the complexity of the modulation process because it does not require an absolute phase reference.

Fig. 7 illustrates a parallel spread-spectrum system 700 with a single channel in accordance with an embodiment of the present invention. The input data 772 is mixed with mixer 710 to make it strangely white and remove any DC bias in the data. In this embodiment of the invention, a Walsh encoder 720 is used to encode and spread the data stream with orthogonal Walsh functions. The resulting data is divided into 4-bit nibbles with 3 bits defining amplitude and the remaining bits are designed into symbols. The magnitude bit determines one of the 8-bit Walsh codes, while the sign bit defines whether the true or inverse Walsh code is selected. This introduces system processing gain in the form of spreading and coding. The spreading gain is 9dB (10 log 8) while the higher quadrature Walsh function can provide 1.6dB coding gain. Thus, the use of Walsh codes provides an effective system gain of 10.6 dB. However, the present invention may also use another digital modulation scheme involving in-phase (I) and quadrature (Q) channels. Therefore, in another embodiment, each channel uses different parallel spreading sequences to greatly improve the security of the channel.

M-sequence biorthogonal keying ("MBOK") modulation is block coding of data using orthogonal codes and may be usedTechniques implemented in binary ("BMBOK") or quadrature ("QMBOK") formats. The technique may produce coding gain that improves link bit error rate ("BER") performance by implementing an FEC algorithm at the receiver. Thus, MBOK modulation is more efficient than BPSK, e.g., at le10^-5BER，E_b/N_oIs 8dB instead of 9.6 dB.

It should be noted that Walsh coding can be implemented as part of the preferred embodiments, which can have the benefits and advantages outlined above, but in another embodiment it also encompasses other processing gains obtained directly from parallel spreading. Because the orthogonality and FEC properties of the codes can be obtained, Walsh coding is recommended. Walsh codes exhibit zero-crossing correlation only at zero phase offset and better synchronization. When this offset is present, the Walsh codes exhibit much larger cross-correlation values and much poorer auto-correlation than the PN sequences. Therefore, in order to coherently decode the Walsh sequence at the receiver portion, overlapping parallel PN spreading sequences are often used to obtain the required phase and timing information. The start may send some preambles that cannot be decoded in order to obtain initial acquisition at the receiver part. Preamble generator 740 generates a preamble that is then used to transmit the packetized data via signal 774 of a signal medium access controller ("MAC") (not shown) for Walsh encoding. The MAC controls the flow of data between the host system and the radio section.

In order to simplify the phase determination required during demodulation, a differential encoding of the data stream is produced. The differential encoder 730 uses the original symbol as a phase reference to determine the result of the current symbol. This ignores the prerequisite of a constant phase reference transmission in a coherence detection system. Differential encoding for BPSK can be achieved by simply xoring the values of the current and original symbols. However, the differential coding for QPSK is more complex, and there are 16 possible states as shown in table 2.

Fig. 8 illustrates a QPSK differential encoder circuit 800 according to an embodiment of the present invention. The hardware includes four two-input exclusive or gates 810 and 820 connected to a two-bit adder 830. The operation of the circuit 800 will be apparent to each of skill in the art.

Table 2: differential coding sequence QPSK

New input IN (I, Q)k

Original code OUT (I, Q)k-100 0 1 1 1 1 0

0 00 11 11 0

0 00 11 11 0

0 11 11 00 0

1 11 00 00 1

1 00 00 11 0

Referring again to fig. 7, data buffer 750 holds the data bits before parallel spreading and ensures that it can be synchronized with the PN sequence. For example, Walsh encoder 720 provides a synchronization pulse to synchronizer 732. To ensure that the Walsh codes and PN sequences are aligned in time, synchronizer 732 provides timing information to data buffer 750, PN sequence generator 760, and parallel spreader 770. The PN generator is programmable to generate PN sequences from short to very long. The PN sequence spreads the data in a parallel manner by a parallel controller 770 using multiple PN bits for each data symbol. The output data stream 776 is modulated using a digital modulation scheme such as BPSK or QPSK.

Fig. 9 and 10 illustrate the major elements of a parallel spread-spectrum system (receiver) 900 according to an embodiment of the present invention. Fig. 9 illustrates I902 and Q904 channels in which the modulation scheme of DPSK is employed. Fig. 10 illustrates a circuit 100 for Walsh code correlation and decoding for FEC; for ease of illustration, only the in-phase [ I ] channel is illustrated, but other channels may be used. The operation of the circuit 1000 will be apparent to each skilled in the art.

Referring to fig. 9, a receiver 900 despreads parallel spread sequences, according to an embodiment of the present invention. In particular, the IF signal is downconverted to baseband where it is digitized by a dual 4-bit analog-to-digital converter ("ADC") 910. A sampling rate of 4 times the chip rate may be used. A carrier tracking digital phase locked loop ("DPLL") is formed by carrier phase monitor 930, lead/lag filter 940, numerically controlled oscillator ("NCO") 950, and complex multiplier 920. An NCO is an oscillator that generates digital sample values corresponding to a sinusoidal or other waveform. The purpose of the DPLL is to remove any carrier bias, which is a property that can be redundant in the RF down-conversion process. The quadrature NCO multiplies the received samples to remove the carrier offset before correlation. The demodulation section outputs an auxiliary DPLL error signal. This phase is aligned or synchronized to the samples introduced to the PN matched filtered correlator 960 to optimize the performance of the receiver.

The PN matched filter 960 comprises a single programmable multi-stage cascaded variable correlator. In operation, the PN matched filter 960 computes the cross correlation between the input and the programmable PN maximum sequence. The peak of the correlation can be used to initialize the sequence of parallel accumulation, integration, and unloading, and to extract the information of the multi-bit samples and bit timing sequence in turn. The results of the various bit accumulators in the PN matched filter 960 are input in parallel to a correlation and symbol tracking processor 970, which determines the correlation of the bits and extracts symbol timing information from the extracted data samples. By using the formula: max [ abs (I) ((Q)) ] +1/2Min [ abs (I) ((Q)) ], and the correlation is obtained by approximating the magnitude of the sum of the I and Q channel correlation sums. The calculated value may be used to generate a multi-bit tracking reference clock signal.

Programmable thresholds and intelligent tracking are implemented to ignore false detections and automatically insert missing correlation pulses. The multi-bit detection pulse initializes a parallel correlation that can extract the timing of a symbol by calculating the magnitude of the power of the symbol correlation, which also forms a reference suitable for the symbol tracking process. The extracted spread symbol samples forming the symbol tracking process are sent to the DPSK demodulator 980 along with associated timing information.

The DPSK demodulator transmits each symbol by performing "dot product" and "cross product" calculations on each despread information from the current and previous parallel correlation processes. For BPSK demodulation, the "dot product" only allows determination of the phase shift between successive samples. For QPSK demodulation, both "dot products" and "cross products" are necessary to determine the phase shift. Mathematically, the dot product and cross product are:

dot(k)＝I_K·I_K-1+Q_K·Q_K-1and

cross(k)＝Q_K·I_K-1-I_K·Q_K-1

where I and Q are the in-phase and quadrature samples of the current K and previous K-1 symbols. The results of these products on the complex plane show that the method can effectively demodulate differentially encoded QPSK signals in the format shown in table 2.

Fig. 11 illustrates a hardware implementation of a differential PSK demodulator 1100 in accordance with an embodiment of the present invention. The operation of demodulator 1100 will be apparent to those skilled in the art.

Dot and cross products may be used to generate other error signals for the function of the initial DPLL. The automatic frequency control ("AFC") error signal reflects the sine of the phase difference between the current and previous symbols after correction of the phase shift increment evaluated between symbols due to PSK modulation. Mathematical analysis can produce approximate approximations, which can be used using dot products and cross products. The operation is as follows:

AFC_Error_BPSK＝Cross·Sign[Dot]and

AFC_Error_QPSK＝(Cross·Sign[Dot])-(Dot·Sign[Cross])

may be applicable to BPSK and QPSK modulation schemes, respectively. The error signals of the individual parallel processing channels can be combined and averaged before being input to the NCO by the loop filter. This function can substantially remove small frequency errors and thus ensure optimal receiver performance.

The recovered I and Q data may be latched in parallel and input to a serial converter. In another embodiment of the present invention, other signal processing methods may be required to complete the interface with the existing Walsh decoder. The data samples are output in parallel I1202 and Q1204 buses to the Walsh code FEC 1210 of the two-channel parallel spread spectrum system 1200, as shown in fig. 12.

Walsh correlation, demodulation, and FEC processing depend on parallel despreading sections to correctly remove carrier frequency and phase offsets. The symbol timing processor of the parallel despreading section also provides the phase reference needed to coherently correlate and decode the Walsh code sequence.

FEC processor 1210 examines the I1202 and Q1204 data buses and compares the received bits with one bit in a 16 possible bit pattern. The intelligent processing is used to correct bit errors in the received I and Q symbols. FEC 1210 operates with Walsh decoder 1220 to ensure optimal performance. The orthogonal nature of the Walsh codes enhances their FEC characteristics and thus reduces the BER between links.

FEC processing the outputs are applied to blocks of 16-bit correlators (not shown), 8 bits each for I and Q channels, which may be multiplied by the inputs using corresponding Walsh codes, accumulated, integrated, and dumped throughout the bit period. The "max selector" 1230 for the I channel and the "max selector" 1235 for the Q channel analyze the correlation peaks of the respective 8-bit correlators and output corresponding data to determine the symbol corrections and Walsh codes of the data sequence 1240. The Walsh decoded information is returned to the FEC processor 1210 to form a Walsh decoder and FEC processing. Irregularities between processes may result in a double reprocessing of the input samples. A fault in this process can cause the generation of an error signal, which can be used with the link protocol to initialize the algorithm for retransmission. Once the Walsh codes are successively decoded, the I and Q data can be determined and combined into a signal data stream.

The data stream may be described using polynomial division and periodic redundancy detection (CRC) may be performed on the data packets using data description and CRC detection. The data is then serially output to the MAC to complete the received operation.

Most critical processing regions involve the requirements for parallel processing in the receiver. A typical processing period from PN sequence to data recovery should be achieved in 0.4 × Q, where Q is equal to the sample time. For the illustrated E1 data stream with the 48-bit parallel expansion example, the receive processing needs to be completed within 1.5 us.

The present invention is a novel parallel spread spectrum system and method that combines Walsh code orthogonal properties with approximate correlation properties of PN sequences to produce robust communication techniques that can be implemented over point-to-point or point-to-multipoint communication links. Independent parallel spreading sequences may be applied in the network to implement CDMA. In an embodiment of the present invention, the parallel spreading is dynamic, in which the Walsh encoder is programmable and the code length of the parallel spreading is varied. The user may determine the maximum processing gain for a fixed data rate in the allocated bandwidth.

The example illustrated in the above discussion and drawings is the use of an 8-bit Walsh encoder and 48-bit PN sequence to achieve a system processing gain of 18.4dB (9+1.6+ 7.8), which potentially increases the effective range of the PSS link over the entire conventional link represented by the 8 folded sheets. Another embodiment of the present invention may have different sizes of Walsh encoder and PN sequences. It is desirable to be able to use codes of smaller length in order to maximize acquisition speed and minimize design complexity.

In another embodiment of the present invention, further layer spreading sequences may be implemented to improve processing gain and CDMA characteristics. For example, in addition to the auxiliary spreading sequence, the third sequence may also be used in parallel with the base and auxiliary sequences.

In another embodiment of the invention, coherent demodulation may also be used to offset the need for differential encoding. In another embodiment, QAM-based and coded orthogonal frequency division multiple access techniques may also be used as the modulation scheme.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (52)

1. A method of encoding data for spread spectrum data communications, the method comprising the steps of:

encoding data by using n-bit orthogonal codes;

the m-bit spreading sequence is multiplied with the encoded data, where m is an integer multiple of n.

2. The method of claim 1, wherein the orthogonal codes are Walsh codes.

3. The method of claim 2, wherein n is 8.

4. The method of claim 1, wherein the spreading sequence is an even ordered code.

5. The method of claim 4, wherein the even-ordered code is selected from a group consisting of an M-sequence, a Barker code, a Gold code, a Kasami code, a pseudo-noise sequence, or a combination thereof.

6. The method of claim 1, wherein the encoded data is one or more orthogonal codes.

7. A method for spreading data in a spread spectrum communication system, the method comprising the steps of:

encoding the data stream according to a basic encoding scheme employing a basic code; and

the primary encoded data is extended with an auxiliary sequence, wherein the bit length of the auxiliary sequence is an integer multiple of the bit length of the primary code.

8. The method of claim 7, further comprising the step of:

differentially encoding the data stream; and

scrambling said data stream prior to said encoding and spreading steps.

9. The method of claim 7, wherein the base codes are orthogonal Walsh codes.

10. The method of claim 9 further comprising dividing said data stream into multiple-bit data packets representing one of a series of true or inverse Walsh codes.

11. The method of claim 9, further comprising:

providing a synchronization pulse to synchronize the Walsh code with the auxiliary sequence; and

maintaining the data stream in a data storage buffer prior to expanding the data stream using the auxiliary sequence.

12. The method of claim 8, wherein the differential encoding is differential encoding for BPSK modulation.

13. The method of claim 8, wherein the differential encoding is differential encoding for QPSK modulation.

14. The method of claim 7, wherein the auxiliary sequence is selected from a group consisting of an M-sequence, a Barker code, a Gold code, a Kasami code, a pseudo-noise sequence, or a combination thereof.

15. The method of claim 7, further comprising the step of:

modulating the spread data stream; and

and transmitting the modulated data stream.

16. A method for data exchange in a parallel spread spectrum communication system, the method comprising the steps of:

receiving a communication signal of a parallel spread spectrum; and

recovering a data stream from the parallel spread spectrum communication signal.

17. The method of claim 16, wherein the step of recovering the data stream from the parallel spread spectrum communication signal comprises:

converting the received signal into a digitized data stream;

calculating a cross-correlation between the digitized data stream and a programmable sequence;

extracting multi-bit samples and bit timing information using the cross-correlation;

extracting symbol timing information from said extracted multi-bit samples; and

demodulating the extracted multi-bit samples.

18. The method of claim 17, wherein the programmable sequence is a pseudo-noise sequence.

19. The method of claim 16, further comprising generating said parallel spread spectrum communication signal comprising the steps of:

encoding data by using n-bit orthogonal codes;

multiplying the encoded data with an m-bit spreading sequence, where m is an integer multiple of n.

20. A method for exchanging parallel spread spectrum communication signals in a cellular network, the method comprising:

receiving a parallel spread spectrum communication signal at a first receiver; and

forwarding the received parallel spread spectrum communication signal to a second receiver.

21. The method of claim 20, wherein the first receiver is a base station.

22. The method of claim 20, wherein the first receiver is a mobile telephone switching system.

23. The method of claim 20, wherein the forwarding step comprises:

transmitting the received parallel spread spectrum communication signal to the second receiver.

24. The method of claim 22, wherein the second receiver is a cellular device.

25. The method of claim 20, wherein the forwarding step comprises:

converting the received parallel spread spectrum communication signal into a communication signal;

transmitting the converted communication signal to the second receiver.

26. The method of claim 25, wherein the second receiver is a cellular device or a land-based telephony device or a network.

27. The method of claim 20 wherein said parallel spread spectrum communication signal is generated by a generation method comprising:

encoding data by using n-bit orthogonal codes;

multiplying the m-bit spreading sequence with a one-or multi-bit orthogonal code encoding the data, wherein m is an integer multiple of n.

28. A parallel spread spectrum communication device, comprising:

an encoder for encoding a data stream according to a base coding scheme; and

a spreader for spreading the encoded data stream with an auxiliary sequence.

29. The apparatus of claim 28, wherein the base coding scheme employs n-bit orthogonal Walsh codes.

30. The apparatus of claim 29, wherein the spreading sequence is an m-bit pseudo-noise sequence.

31. The apparatus of claim 30, wherein m is an integer multiple of n.

32. The apparatus as recited in claim 28, further comprising:

a modulator; and

a transmitter.

33. A parallel spread spectrum communication device, comprising:

an encoder for encoding a data stream according to an orthogonal coding scheme;

a spreading sequence generator for generating a spreading sequence; and

a spreader for spreading the orthogonally encoded data streams with the spreading sequence.

34. The apparatus as recited in claim 33, further comprising:

a synchronization module that synchronizes the orthogonally encoded data stream with the spreading sequence; and

a data buffer for temporarily storing the orthogonally encoded data streams.

35. The apparatus as recited in claim 33, further comprising:

a differential encoder for differentially encoding the orthogonally encoded data streams prior to spreading with the spreading sequence.

36. The apparatus as recited in claim 33, further comprising:

a scrambler that whitens the spectrum and removes Direct Current (DC) bias from the data stream.

37. The apparatus of claim 33, wherein the spreading sequence is selected from a group consisting of an M-sequence, a Barker code, a Gold code, a Kasami code, a pseudo-noise sequence, or a combination thereof.

38. The apparatus of claim 33, wherein the orthogonal coding scheme employs orthogonal Walsh codes.

39. A parallel spread spectrum communication device, comprising:

a receiver for receiving a parallel spread spectrum communication signal; and

means for recovering a data stream from the parallel spread spectrum communication signal.

40. The apparatus of claim 39, wherein the means for recovering comprises:

a digitizer to convert the received signal into a digitized data stream;

means for calculating a cross-correlation between said digital data stream and a programmable sequence, using said cross-correlation to extract multi-byte samples and byte timing information, and extracting symbol timing information from said extracted multi-byte samples; and

a demodulator for demodulating the extracted multi-byte samples.

41. The device of claim 40, wherein the programmable sequence is a pseudo-noise sequence.

42. The apparatus of claim 39, wherein said parallel spread spectrum communication signal is generated by a generation method comprising:

encoding data by using n-bit orthogonal codes;

multiplying the m-bit spreading sequence with one or more orthogonal codes in encoding the data, wherein m is an integer multiple of n.

43. A system for exchanging parallel spread-spectrum data, the system comprising:

means for encoding and expanding a data stream according to a first coding scheme;

a differential encoder;

means for generating a spreading sequence;

means for synchronizing the differentially encoded data stream with the spreading sequence;

means for spreading said differentially encoded data stream using said spreading sequence;

a phase shift keying modulator;

a transmitter;

a receiver; and

means for recovering said data stream from said received data stream.

44. The system of claim 43, further comprising a scrambler that whitens the spectrum and removes all DC bias from the data stream.

45. The system of claim 43, wherein the means for encoding and spreading a data stream according to a first coding scheme is a quadrature Walsh encoder.

46. The system of claim 45, further comprising:

means for providing synchronization pulses to ensure that the Walsh encoder and the spreading sequence are aligned in time; and

a data storage buffer.

47. The system of claim 43, wherein the spreading sequence is a pseudo-noise sequence.

48. The system of claim 43, further comprising:

a preamble is generated containing timing information for each packet, and means are provided for inserting the preamble into each packet.

49. The system of claim 43, wherein the spreading sequence is selected from the group consisting of an M-sequence, a Barker code, a Gold code, a Kasami code, a pseudo-noise sequence, or a combination thereof.

50. The system of claim 43, further comprising:

means for converting said received data stream into a digital data stream;

means for calculating a cross-correlation between said digital data stream and a programmable sequence stored at said remote location;

means for extracting multi-byte samples and byte timing information using the cross-correlations;

means for extracting symbol timing information from said extracted multi-byte samples; and

means for demodulating said extracted multi-byte samples.

51. The system of claim 43, further comprising means for removing carrier offset from the received samples.

52. The system of claim 43, wherein the programmable sequence is a pseudo-noise sequence.