MXPA97009331A - Use of orthogonal wave forms to allow multiple transmitters to share a single channel - Google Patents

Abstract

The present invention relates to a system and method for enabling multiple transmitters (400) to share a single multiplexed code division (CDM) or multiple code division (CDMA) access channels using orthogonal waveforms. A set of channelized orthogonal codes W1 (1) is generated, and each transmitter (400) is distributed in the pipelined orthogonal codes and the pseudo-noise polynomials in a predetermined manner. The transmitters channel each user signal using a channelized orthogonal code W1 (t), and distribute each user signal using a broadcast pseudo-noise (PN) code. Each transmitter uses the same PN broadcast codes and the compensation time. Additionally, no pipelined orthogonal code is assigned to more than one transmitter during the time period that a CDM channel is sharing. The broadcast signals are added to each transmitter (404) before transmission as a composite signal. The compensations are pre-corrections of time (406, 510) to ensure the alignment of time in the receivers. The frequencies of the signals are pre-corrected (408, 512) to ensure the alignment of the frequency in the receiver

Description

USE OF ORTHOGONAL WAVE FORMS TO ALLOW MULTIPLE TRANSMITTERS TO SHARE A SINGLE CDM I CHANNEL. Field of the Invention The present invention relates generally to broadcast spectrum communication systems, and more particularly to the enabling of multiple transmitters to share a single channel multiplexed by code division (CDM) or multiple access by division of code (CDMA), as a shared resource in such systems. II. Description of the Related Art In a multiplexed system by code division (CDM), the proposed signals for one or more receivers are transmitted from a single site using a single frequency band, or CDM channel, through the appropriate assignment of channelization codes to create code channels. Such systems include, for example, paging systems, message or information dissemination systems, and position or position determination systems in which the information is transferred to various target receivers. Some CDM systems, such as broadcast spectrum wide access (CDMA) communication systems (CDMA), obtain code channels by assigning orthogonal channelization codes, such as Walsh codes, or broadcast codes with low correlation to each user. of the system. A variety of multiple access communication systems and techniques for transferring information among a large number of system users have been developed. However, the spread spectrum modulation techniques, such as those used in communication systems (CDMA) provide significant advantages over other modulation schemes, especially when they provide service to a large number of users of the communication system. Such techniques are disclosed in the teachings of U.S. Patent No. 4,901,307, issued February 13, 1990 under the title "Broadcast Access Multiple Spread Communication System Utilizing Terrestrial or Satellite Repeaters", and the Patent Application. American Series No. 08 / 368,570, presented under the title "Method and Apparatus for Using Total Spectrum Transmitted Power in a Broadcast Spectrum Communication System to Track Time and Energy of the Individual Receptor Phase", which are assigned to the transferee of the present invention and are incorporated herein by reference. The aforementioned patents disclose multiple access communication systems in which a large number of users of the generally remote or mobile system each employ at least one transceiver to communicate with other users of the system or users of other connected systems, such as a network of public telephone switching. The transceivers communicate through accesses and satellites, or land base stations (also sometimes referred to as cell sites or cells). The base stations cover cells, while the satellites have drop zones on the surface of the Earth. In any system, capacity gains can be achieved by segmenting, or subdividing, the geographic regions to be covered. The cells can be divided into "sectors" by the use of directional antennas in the base station. Similarly, a fall area of a satellite can be geographically divided into "beams", through the use of beamforming antenna systems. These techniques for subdividing a coverage region can be understood as the creation of an isolation by the use of relative antenna directionality or space division multiplexing. In addition, taking into account that there is available bandwidth, each of these subdivisions, be they sectors or beams, can be assigned to multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite systems, each CDMA channel is referred to as a "sub-beam", because there are several of these by "beam".

In a typical spread spectrum communication system, one or more preselected pseudo-noise code (PN) sequences are used to modulate or "broadcast" user information signals over a predetermined spectral band before modulation on a carrier signal for its transmission as communication signals. The diffusion of PN, a widespread spectrum transmission method that is well known in the art, produces a signal for its transmission that has a band amplitude much greater than that of the digital signal. In the base station or user access communication link, the PN broadcast codes or binary sequences are used to discriminate between the signals transmitted by different base stations or on different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell or sub-beam. In a typical CDMA broadcast spectrum communication system, channelization codes are used to discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link (i.e. the signal path from the base station or access to the user transceiver). That is, each user transceiver has its own orthogonal channel provided in the forward link by using an orthogonal code of * single channeling. Walsh functions are generally used to implement channeling codes, with a typical code length so that the forward link is in the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. In general, a CDMA satellite system allocates system resources to the many accesses. The simplest allocation scheme is to divide the resources into the resolution of complete CDMA channels, or sub-beams. The system assigns complete sub-beams of individual satellites to individual accesses for specific periods of time. However, when there are many more accesses than sub-beams available, the allocation of full CDMA channels becomes potentially inefficient in the use of system resources. In such situations, it may be useful to share a sub-beam between the accesses. This increases the resolution of the system resources available for allocation. Therefore, it is desirable that multiple accesses share a CDMA channel or sub-beam as a shared resource. However, according to conventional wisdom, multiple assignment of a CDMA or CDM channel to multiple transmitters results in signal interference at the receivers. For one skilled in the art, it will be apparent that this discussion also applies to terrestrial communications systems (eg, cellular) that employ base stations instead of accesses, and various types of message or information dissemination systems. Therefore, what is needed is a method to allow multiple transmitters (eg, accesses, base stations) to share a single CDM channel without the creation of interference. SUMMARY OF THE INVENTION The present invention is a system and method for allowing multiple transmitters to share a single channel of CDM or CDMA using orthogonal waveforms. Applicants have found that, contrary to conventional wisdom, multiple transmitters can share a single CDM channel using orthogonal waveforms when certain aspects of transmitter operation are constrained according to the present invention. Furthermore, according to conventional knowledge, it is impractical to control the carrier wave phase from multiple transmitters in order to align to one, or each of several, mobile receivers. Applicants have found that when the methodology of the present invention is employed, certain parameters of operation such as the relative carrier phase do not need to be controlled or adjusted over the period of interest. By requiring that certain operating characteristics of the transmitter be controlled, the present invention makes the multiple assignment of a CDM channel by multiple transmitters quite practical and useful. According to a preferred embodiment of the invention, each transmitter sharing a single CDMA channel is assigned a portion of a predefined set of Walsh codes, which are used to channel the user's information signals. In addition, all multiple assignment transmitters broadcast the channeled user signals by using the same offset and pseudo-noise (PN) broadcast code. The transmitters can then share a single frequency band (CDM or CDMA channel) without mutual interference when the following constraints of transmitter operation are observed: each transmitter employs the same PN broadcast code or PN code sequence pair. of quadrature and displacement of time; time shifts are pre-clocked to ensure time alignment in the receiver; the frequencies of the signals are pre-corrected to ensure alignment of the frequency in the receiver; and no orthogonal channelization code is assigned to more than one transmitter at a time. One purpose of the preferred embodiment of the present invention is to allow multiple transmitters to share a single CDMA channel without creating mutual interference. Likewise, the present invention allows multiple transmitters to share a single CDM channel without creating mutual interference. An advantage of the present invention is that it improves the signal-to-noise ratio of the specific communication signals and system. Another advantage of the present invention is that it allows an improved search of time and phase for the signals. A further advantage of the present invention is that it allows an improved frequency search. Still another advantage of the present invention is that it allows better signal acquisition during acquisition. Another purpose of the present invention is to allow the use of multiple pilot signals for the frequency search. Because each transmitter sharing a CDMA channel according to the present invention provides a pilot signal, multiple pilot signals are available in a receiver for use in the frequency search. An advantage of the use of multiple pilot signals for frequency estimation is that this technique allows for faster frequency capture. Another advantage of this technique is that it allows the search of frequency at lower signal-to-noise ratios. Still another advantage of this technique is that it allows a better overall demodulation performance in a fading channel; When a pilot signal is fading, its energy can be supplemented by that of pilot signals from other transmitters to maintain the carrier closure. Yet another advantage of this technique is that it allows the use of a pilot signal of lower power. The additional features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, become more apparent from the detailed description set forth below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The invention is better understood with reference to the drawings in which like reference numbers indicate identical or functionally similar elements. In addition, the digits to the left of the reference numbers refer to the figure in which the first reference number appears in the accompanying drawings.

Figure 1 illustrates a typical multiple access communication system; Figure 2a illustrates a circuit block diagram representing a signal modulator of conventional design; Figure 2b illustrates a circuit block diagram representing an alternative signal modulator of conventional design; Figure 3 illustrates a circuit block diagram representing a QPSK separator of conventional design; Figure 4 illustrates a circuit block diagram representing a preferred embodiment of the present invention; Figure 5 illustrates a flow chart depicting the operation of a preferred embodiment of the present invention; Figure 6 illustrates a circuit block diagram of an automatic frequency control cycle that employs multiple pilot signals to obtain an estimate of the carrier frequency of a received QPSK signal; and Figure 7 illustrates a flow chart illustrating the operation of the automatic frequency control cycle of Figure 6.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES I. Introduction The present invention is a system and method for allowing multiple transmitters to share a single CDM channel, or a single broadband signal resource in common. It is a preferred embodiment. However, first a number of aspects of the invention necessary for its understanding are discussed. Although specific stages, configurations and facilities are treated, it should be understood that it is done for illustrative purposes only. A person skilled in the relevant art will recognize that other steps, configurations and facilities may be used without departing from the spirit and scope of the invention. As described above, a typical CDMA wireless communication system employs at least one frequency band for the transmission of signals using broadcast spectrum CDMA techniques; Each frequency band is known as a CDMA Channel. Different CDMA channels are used to transfer different communication signals to different sets of users. CDMA Channels can also be reassigned to other systems for reuse under various plans of the Federal Communications Commission (FCC), or separated by intermediate bands used by other services. The geographical coverage area for different CDMA channels can partially or completely cover depending on the design of the selected communication system. Users can switch between CDMA channels for purposes of satellite capacity, coverage or position, signal strength, interference and the like. In a CDMA communication system, multiple users transmitting from a single site can share a single frequency band (CDMA channel) through the appropriate allocation to each of the orthogonal channelization codes, such as Walsh codes . In a typical CDMA system, the available spectrum is divided into a number of frequency bands, each of which represents a CDMA channel. Then, each CDMA channel is analyzed in a number of code channels by applying channelization codes to the signals to be transmitted. Each code channel is a separate communication channel, capable of transporting voice, data, etc. In a preferred embodiment of the invention, each code channel within a CDMA channel has been created by modulating a data signal with a different Walsh code selected from a set of Walsh codes. An exemplary set of known codes is specified in the IS-95 system specification entitled "Mobile Station Standard Compatibility - Base Station for a Broadband Dual-Spread Broadband Spectrum Cell System" which is incorporated herein by reference. The resulting communication signals are mutually orthogonal. II. The Walsh Codes Generation One type of orthogonal channelization code is the Walsh code, which is employed in a preferred embodiment of the present invention. A discussion of the generation and use of Walsh codes is found in Pat. No. 5,103,459 entitled "System and Method for Generating Signal Wave Forms in a CDMA Cell Phone System", Pat. No. 5,103,459 is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. A short description is provided below for the convenience of the reader. It is well known in the art that a set of n orthogonal binary sequences can be constructed, each of length n, where n is a power of 2. In fact, orthogonal binary sequence sets are known for many lengths that are multiples of four and less than two hundred. A class of orthogonal binary sequences that is useful for orthogonal channelization codes and that is also relatively easy to generate is called Walsh functions. Walsh functions are derived from Walsh function matrices, also known as Hadamard matrices. A Hadamard matrix of order n can be defined repetitively as: where H denotes the additive inverse of H, and on the real field H ^ l (ie, H -_ = - l). Therefore, the first two Hadamard matrices of orders 2 and 4 can be represented as: 1 1 H2 = y 1 -1 So, a Walsh function, Wn, is simply one of the rows of a Walsh matrix (Hadamard matrix), and a Walsh function matrix of order? P 'is a square matrix containing n functions or sequences, each being n micromicroplaquetas (bits) of length. A Walsh function of order n (as well as all other orthogonal functions) has the property that over the range of n code symbols, the cross-correlation between all the different sequences within the set is zero, provided that the sequences are align in time with each other. This can be seen by noting that each sequence differs from every other sequence in exactly half of its bits. It should also be noted that there is always a sequence that contains all (real) and that all other sequences contain halves and less than half. The above-described properties of Walsh codes make them useful in CDMA communication systems. As will be described below, when two user signals are modulated using two different Walsh sequences from the same set, respectively, the resulting signals do not interfere with each other. III. A Wireless Information System As discussed above, the present invention could find use in a variety of wireless information and communication systems. Such systems include information dissemination systems such as that typically used for paging or position determination. Other systems include wireless communication systems, such as satellite and terrestrial cellular telephone systems. A preferred application is found in CDMA broadcast spectrum communication systems for mobile or portable telephone service.

An exemplary wireless communication system in which the present invention is used, is illustrated in Figure 1. The portion of a communication system 110 illustrated in Figure 1 uses two base stations 112 and 114, a satellite 116, and two accesses or associated terminals 120 and 122. These elements of the communication system are shown by establishing communications with two subscriber units 124 and 126. Typically base stations and satellites / accesses are components of separate communication systems, terrestrial and satellite based, but this It is not necessary. The subscriber units 124 and 126 each have or comprise a wireless communication device such as, but not limited to, a cellular phone, a data transceiver, or a position or paging receiver, and can be portable or mounted on the carrier as desired. Here, the subscriber units are illustrated as portable telephones. However, it is also understood that the teachings of the invention are applicable to fixed units where remote wireless service is desired, including indoor and outdoor 'as well as' locations. In general, the multiple beams from satellite 116 at different frequencies, also referred to as CDMA channels or 'sub-beams', can be addressed to cover the same region. It is also easily understood by those skilled in the art that the beam coverage or service areas for the multiple satellites or antenna patterns for the multiple base stations could be designed to completely or partially cover a given region depending on the design of the system. communication and the type of service offered, and if the diversity of space is achieved. A variety of communication systems of multiple satellites have been proposed, such as the use of orbital planes in Low Earth Orbit (LEO), to serve a large number of subscriber units. Those skilled in the art will readily understand how the teachings of the present invention are applicable to a variety of access and satellite system configurations., including other orbital distances and constellations. At the same time, the invention is equally applicable to land based systems of various base station configurations. Some possible signal paths are illustrated in FIG. 1 for communications occurring between subscriber units 124 and 126 and base stations 112 and 114, or via satellite 116 with ports 120 and 122. The base station communication links- subscriber unit are illustrated by lines 130, 132, 134, and 136. Satellite access communication links, between accesses 120 and 122, and satellite 116, are illustrated by lines 140 and 142, respectively. The satellite communication links-subscriber unit between the satellite 116 and the subscriber units 124 and 126, are illustrated by lines 144 and 146, respectively. As stated above, the accesses 120 and 122, and the base stations 112 and 114, can be used as part of one or two way communication systems or simply to transfer messages or data to the subscriber units 124 and 126. In any In this case, accesses 120 and 122, or base stations 112 and 114, may wish to share the same CDM or CDMA channels. This is especially true when base stations 112 and 114 are located close to each other, or when accesses 120 and 122 currently have unequal resource demands, or have messages for common user groups. IV. Diffusion and Coverage Before information signals are transferred to system subscribers, they are first digitized, as necessary, and coded and interleaved as desired, to create a basic digital communication signal. These operations use techniques well known in the art. The signals addressed to specific users are also modulated by a different orthogonal function or code broadcast sequence assigned to that user forward link. That is, a single coverage orthogonal code, typically a Walsh code, is used to distinguish between different users or subscriber signals within a cell or beam. This coding in the forward link of a given carrier frequency produces subscriber signals also referred to as channels. Such orthogonal functions are sometimes referred to as channelization codes. An exemplary block diagram of transmitting circuitry for implementing data coverage and broadcast signals is illustrated in Figures 2a and 2b. A transmission modulator 200 in Figure 2a uses a first multiplier 202, a second multiplier 204, an orthogonal code or function generator 206, and a PN 208 generator. Alternatively, as explained below, the modulator 200 may employ a multiplier 210. The transmit modulator 200 receives data or data symbols previously encoded and orthogonally codes or covers them with an assigned orthogonal code sequence, Walsh code, and then broadcasts the data covered before transmission. Referring now to Figure 2a, an information signal S (t) is channeled through multiplication with a Walsh function W (t). An orthogonal function or Walsh code generator 206 generates the desired orthogonal coverage code for channeling the signal, using the apparatus known in the art. The code W ^ t) of the generator 206 is multiplied by or combined with the symbol data in a logic element 202, which is generally a multiplier. In the exemplary embodiment, the orthogonal function is typically clocked at a rate of 1.2288 MHz, although other known speeds may be used. The orthogonally covered data signal S (t) W (t) emitted by the multiplier 202 is input to the logic element or multiplier 204, which multiplies the signal by a PN diffusion code. The orthogonally encoded and broadcast output signal of the resulting PN is then typically filtered by bandpass, it is transferred to an appropriate power control and amplification circuitry, and is modulated on an RF carrier. Alternatively, the PN channel orthogonal and diffusion channel codes can be multiplied together or combined before they are combined with the data. This is illustrated in Figure 2b where a transmission modulator 201 has the outputs of the orthogonal code generator 206 and the PN generator 208 transferred to a multiplier 210. The multiplier 210 produces a combined code which is then combined with the data signal. S (t) W (t), again using the multiplier 204. The resulting signals can also be amplified and filtered before being added with other forward link signals and irradiated by an antenna. The operations of filtration, amplification and modulation are well understood in the matter. As is known, alternate modes can exchange the order of some of these operations to form a transmitted signal. Further details on the operation of this type of transmission apparatus are found in the above-mentioned US Patent No. 5,103,459. The PN generator 208 generates one or more different PN diffusion codes to be used in this process. This generator could be shared in time between several transmitters using appropriate interface elements. An exemplary generation circuit for these sequences is set forth in U.S. Patent No. 5,228,054 entitled "Pseudo-Sequence Generator Power of Double Length with Fast Displacement Adjustments", issued July 13, 1993 and assigned to transferee of the present invention, and incorporated herein by reference. Alternatively, the PN codes may be pre-stored in the memory elements such as a ROM or RAM circuit. The PN generator 208 can output a sequence of real value or complex value, as desired. These PN broadcast codes can also be the same code applied 90 ° out of phase in some applications. Each PN sequence consists of a series of 'chips' that appear over a period of pre-selected PN code at a frequency much greater than the baseband communication signal that is broadcasting. A typical chip rate is around 1.2288 MHz with a PN code sequence length or 1024 chip period. However, this code length can be adjusted to increase code separation, or decrease search times, as would be apparent to those skilled in the art. Each system design specifies the dissemination of PN dissemination codes within a communication system according to the factors included in the subject. A known clock source is used to provide synchronization information, and time shifts or shift values are typically provided by one or more control processors to affect the synchronization of these operations.

V. A QPSK Separator A preferred embodiment of the invention described hereinafter employs a conventional quadrature phase shift manipulation (QPSK) separator. After reading the following discussion, it will be apparent to a person skilled in the relevant art how other schemes of diffusion could be employed in the present invention. In Figure 3 a block diagram of a QPSK separator is illustrated. The QPSK 300 separator is comprised of in-phase, first and second multipliers, 302 and 304, quadrature multipliers, first and second, 306 and 308, two filters 310 and 312, and an analog summing or summing element 314. They are used two PN generators 316 and 318 to provide phase and quadrature diffusion codes, PNS and PNQ, respectively, which are the same as the PN generator 208 described above. Referring now to Figure 3, an information signal S (t) has been channeled through a multiplication with a Walsh function W (t) to produce a channelized information signal S (t) W (t). The channelized information signal S (t) W (t) is applied to an input of each of the multipliers 302 and 306. In general, the same data are input to both multipliers and are subjected to combination with or modulation by the codes individual The multiplier 302 multiplies the input signal S (t) W (t) by a PN code in PNI phase (from the PN generator 216. The resulting signal is then filtered by the filter 310, a filter of conventional design, the which is typically used to provide pulse configuration, in order to contain the bandwidth of the transmitted signal.The filtered signal is then applied to multiplier 304, where it is multiplied by the carrier signal in phase cos (? t). similarly, the multiplier 306 multiplies the input signal S (t) W (t) by the PNQ quadrature PN code, from the PN 218 generator. The resulting signal is then filtered * by the filter 312 and is applied to the multiplier 308, where it is multiplied by the carrier signal 'of quadrature sin (? t). As will be apparent to one skilled in the relevant art, other waveforms may be used as carrier signals. The resulting in-phase and quadrature components are then added by the analog adder 314 to produce a broadcast signal of QPSK M (t), which can also be amplified and filtered before being added to other forward link signals and irradiated by an antenna , like before. IV. A Modality of the Present Invention Prior to the present invention, it was thought that multiple transmitters could not share a single CDM channel by multiple assignment of a set of orthogonal channelization codes. It was further thought that to carry out the multiple channel assignment, the respective carrier phases of the transmitted signals would have to be aligned in the receiver. Unfortunately, such coordinated precorrection of carrier phases from the multiple transmitters at the geographically distributed sites is not considered technically feasible at the carrier frequencies of interest. As described below, applicants have found that, contrary to conventional wisdom, multiple transmitters can share a single CDM channel using orthogonal channelization codes although the respective transmitter carrier phases do not align after reception. The signals of the transmitters remain mutually orthogonal, without taking into account the carrier phase, under certain circumstances. The reasons for the irrelevance of the carrier phase are best described by examples. Consider two transmitters, Transmitter X and Transmitter Y, as in base stations 112 and 114 or accesses 120 and 122, each generating basic carrier waveforms having phase 'x' and phase 'and', respectively. The X Transmitter channels an Sx data signal using the Walsh Wx (i) function and modulates the carrier to produce a transmitted signal Tx (i), where i represents the chip number in the Walsh sequence; in this example, i varies in value from 0 to 127. The Transmitter Y channels an Sy data signal using Walsh function Wy (i) and modulates its carrier to produce a transmitted signal Ty (i). Therefore, the transmitted signals can be represented as: Tx (i) = Sx Wx (i) ejfx (4) and Ty (i) = SyWy (i) ejfy (5) Both transmitted signals are received by a Receiver X (124, 126) and are discovered or de-channeled using the Walsh Wx (i) function. It is assumed that with the frequency precorrection, any relative difference in the signal phase for the arrival signals is substantially constant. That is, although the phases may differ, they remain relatively constant over the period of Walsh function that is being used. Because the product of a Walsh sequence with the same Walsh sequence is a unit sequence, the result for the Tx signal is given by the relation: ? Tx (i) W i) =? SxWx (i) Wx (i) ejf * = Sxejf *? (L) = 128Sxejf * (6) i = 0 i = 0 í = 0 which is the desired data signal . Because the product of a Walsh sequence with another Walsh sequence from the same set is zero, the result of the Ty signal is given by the relation: ? Ty (i) W i) =? SyW i) Wy (i) ejf = Sye ^? Wx (i wy (i) = Sye3f? * (0) = 0 <7> 1 = 0 1 = 0 1 = 0 resulting in no interference.Thus, the carrier phase is irrelevant when the conditions are met described above and the frequency alignment does not vary over the short period of the Walsh functions In accordance with a preferred embodiment of the invention, each transmitter employs the same pair of quadrature PN diffusion codes or sequences and offsets. of PN code is a predetermined delay between a reference time and the start time of the PN code sequence.) Additionally, no orthogonal channelization code is assigned to more than one transmitter during the time period in They share a CDMA channel, the displacements are pre-corrected in time to ensure time alignment in the receiver, the signal frequencies are pre-corrected to ensure frequency alignment in the receiver. Circuit diagram illustrating a preferred embodiment of the present invention is shown in Figure 4. Figure 4 presents a simple application of the invention, where only two transmitters, the transmitter 400A and the transmitter 400B, share a single CDMA channel. According to a preferred embodiment, a predefined set of Walsh codes is divided among the multiple assignment transmitters. This is illustrated in Figure 4, which shows Walsh codes W -_ (t) - Wn (t) assigned to transmitter 400A and Walsh codes Wn + 1 (t) - Ww (t) assigned to transmitter 400B, where "w "is the total number of Walsh codes in the set. It should be readily apparent to those skilled in the art that Walsh functions need not be assigned or grouped in a strictly serial order but can be assigned using other assignment patterns as desired. That is, the present invention does not require that said Walsh functions 1-16 are assigned to a transmitter while the Walsh functions 17-32 are assigned to another transmitter as continuous 'blocks' or sequences (1 to n and n + l to w). For example, the Walsh 1, 3, 5 ... 31 functions could be assigned to one transmitter while another receives the Walsh 2, 4, 6 ... 32 functions for its use. Functions can be assigned as small groups or alternate sequences or by using other known patterns. Any variety of groupings, combinations or orderings of Walsh functions can be used since the respective transmitters are not using common Walsh functions at the same time in the same CDM channel. An example of how such assignments work is shown for a preferred mode illustrated in Table I below. In the illustrated allocation scheme, two accesses, labeled as a first access (GW) and a second access (GW), share a common beam and frequency in a broadcast spectrum communication system of CDMA. The functions designated for a particular set of nine channels are listed together with their respective Walsh function assignments. TABLE I In this specification, the preferred embodiment is described as having two transmitters and one receiver.

It will be apparent to one skilled in the art that the principle of the present invention can be extended to allow multiple transmitters and multiple receivers to share a single CDMA channel. In addition, it will be apparent to one skilled in the art that the receivers can be replaced by repeaters (eg, satellite transponders, terrestrial repeaters, etc.) and that the time and frequency precorrection of the present invention can be carried out either by the transmitter or the repeater. For example, the pre-correction of time and frequency could be carried out for a group of users by multiple signaling of a single transponder on a satellite, or repeater, and by pre-correction of the signal to the point of transmission by the transponder. In this specification, the present invention is described with respect to signal transmission. As will be apparent to a person skilled in the relevant art, a variety of receptors can be employed with the present invention. A typical receiver is disclosed in U.S. Patent No. 5,103,459 entitled "System and Method for Forming Signal Wave Forms in a CDMA Cell Phone System", assigned to the assignee of the present invention, and incorporated herein by reference. Furthermore, according to a preferred embodiment, the same polynomial of PN and offset are assigned to each multiple assignment transmitter. This is illustrated in Figure 4, which shows a quadrature pair of PNQ and PNZ PN sequences assigned to both the transmitter 400A as well as the transmitter 400B. Referring to Figure 4, the transmitters comprise multipliers 402A, 402B, QPSK 300 separators, analog adders 404A, 404B, time precorrectors 406A, 406B, frequency pre-receivers 408A, 408B, and antennas 410A, 410B. In Figure 5 there is illustrated a flow chart depicting the operation of a preferred embodiment of the present invention. Now, a preferred embodiment of the present invention is described in detail with reference to Figures 4 and 5. Referring to Figure 5, in a step 502, a number of user signals exist in multiple transmitters that must share a single channel of CDMA. The user signals can be voice, data, etc. These signals are represented in Figure 4 as SA1-SAX in the transmitter 400A and SB1-SBY in the transmitter 400B. In a step 504, each user signal is multiplied by a different Walsh code sequence by multipliers 402A and 402B. Neither of the two user signal signals SA1-SM and SB1-SBY are multiplied by the same Walsh code sequence. The Walsh codes are illustrated in Figure 4 as Wx (t) - Wn (t) assigned to the transmitter 400A and Wn + 1 (t) - Ww (t) assigned to the transmitter 400B. Then, in a step 506, the output of each multiplier 402A, 402B is broadcast by QPSK by one or more QPSK 300 separators using the same pair of PN polynomials of quadrature and offsets. The operation of the QPSK 300 separator is described in Section III above. Then, in a step 508, the signals broadcasted by QPSK, encoded by Walsh, resulting, are summed in each transmitter by the analog adders 404A and 404B respectively. In a step 510, the composite signals are pre-corrected in time by time pre-correctors 406A, 406B, respectively, to ensure that PN shifts of the composite signals emanating from the transmitters align in time to the receiver or receivers for the signals. which reception is desired. As described above, the transmitters 400A, 400B are generally located in the base stations or accesses, and the approximate distances to the various receivers / transponders are known; in this way, the required synchronization precorrections can be easily calculated. In a step 512, the composite signals pre-corrected in time are pre-corrected in frequency by the frequency pre-receivers 408A, 408B to ensure that composite signals emanating from the transmitters align in frequency at the receiver or receivers. In a step 514, the composite signals are ready for transmission through antennas 410A, 410B. After reading the above description, it will become apparent to a person skilled in the relevant art how to implement the invention through the use of other alternative modalities. V. Frequency Estimation Through the Use of Multiple Pilot Signals In a CDMA receiver, the frequency of a transmitter carrier is generally estimated through the use of the pilot signal of a single transmitter occupying the CDMA channel. It is generally desirable to minimize the power of a transmitted signal. However, the difficulty of searching for frequency in CDMA systems can be exacerbated by the use of low power pilot signals. A feature of the present invention is that it allows the use of the multiple pilot signals of the plurality of transmitters that share the CDMA channel to estimate the carrier frequency of the transmitters. (As noted above, the carrier frequencies of the transmitters that share the CDMA channel are aligned.) In addition, because the carrier phases of the multiple assignment transmitters do not need to be aligned, each transmitter transmits a separate pilot signal to allow the coherent demodulation). A circuit block diagram of an automatic frequency control cycle 600 that employs multiple pilot signals to obtain an estimate of the carrier frequency of a received QPSK signal is shown in FIG. 6. The circuit in FIG. 6 comprises an antenna 602, a rotor 604, a PN 606 de-diffuser, a Walsh demultiplexer 608, coherent pilot filters 610 (610A-610N), frequency error signal generators 612 (612A-612N), an analog adder 614, a cycle filter 616, and a voltage controlled oscillator (VCO) 618. Figure 7 shows a flow diagram illustrating the operation of an automatic frequency control cycle 600. Now, the operation of the automatic frequency control cycle 600 is describes in detail with reference to Figures 6 and 7. Referring to Figure 7, in a step 702, the composite signal, comprising the signals transmitted from multiple transmitters that share a CDMA channel, e receive at antenna 602. In a step 704, rotor 604 subverts the received composite signal to the baseband. In a step 706, the baseband signal is de-diffused by the use of a PN code at an appropriate time offset by the PN 606 de-diffuser. In a step 708, the de-diffused baseband signal is demultiplexed into separate Walsh channels , A to N, using the Walsh demultiplexer 608. Among the resulting Walsh channels is a pilot channel for each transmitter that shares the CDMA channel. In a step 710, each pilot channel is filtered by coherent pilot filters 610A-N, which may include a full and empty function. In a step 712, each error signal generator 612A-N calculates a term proportional to the frequency error for each pilot signal. In a preferred or exemplary embodiment, the frequency error signal is calculated by taking the cross product between the vectors representing the current sample of the pilot signal and the previous sample of the pilot signal for both in-phase channels, I, and quadrature, Q. For a current pilot sample of Ik, Qk and a previous pilot sample lk-n Qk-i the resulting frequency error is given by Ik_! Q "Qk-i1] * - L &; Error signal can be positive or negative; an error signal of zero indicates a null frequency error. In a step 714, the frequency error signals for all the pilot signals are combined by an analog addition or summing element 614. In a step 716, the composite error signal is filtered by the cycle filter 616. In one step 718, the filtered error signal is converted to a phase estimate by the VCO 618. In a step 720, the phase estimate is applied to the rotor 604 to adjust the phase of the received composite signal. SAW. Conclusion Although various embodiments of the present invention have been described above, it should be understood that they have been presented only by way of example and not as limitation. In this way, the spirit and scope of the present invention should not be limited by any of the exemplary embodiments described above, but should be defined only in accordance with the following claims and their equivalents. What is claimed as our invention is:

Claims (37)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. A method for allowing a plurality of transmitters to share a single CDM channel in a CDM communication system having a plurality of transmitters, each transmitter having at least one communication channel for transmitting a data signal, comprising the steps: allocating a predefined set of orthogonal channelization codes to the plurality of transmitters in a predetermined manner; and channeling each of said data signals using one of said orthogonal channelization codes in order to produce a channelized data signal. The method according to claim 1, characterized in that said orthogonal channelization codes are Walsh functions. The method according to claim 2, characterized in that no Walsh function is assigned to more than one transmitter, at a time. The method according to claim 3, characterized in that it further comprises the step of broadcasting said channeled data signals by using at least one PN code to produce a broadcast user signal. The method according to claim 4, characterized in that a PN broadcast code is assigned to the plurality of transmitters. The method according to claim 5, characterized in that it further comprises the step of adding said broadcast data signals before their transmission to produce a composite signal. The method according to claim 6, characterized in that said composite signal of each transmitter is pre-corrected prior to transmission in such a manner that said PN diffusion codes of said composite signals are aligned at time after reception. The method according to claim 7, characterized in that the composite signal of each transmitter is pre-corrected before transmission in such a manner that said composite signals align in frequency after reception. The method according to claim 1, characterized in that said CDM communication system comprises a wireless broadcast spectrum CDMA communication system, each data signal being a user signal, and said channeling stage comprises the channeling of each of said user signals that use one of said orthogonal channelization codes to produce a channelized user signal. 10. A CDM communication system having multiple transmitters, each transmitting at least one data signal that shares a single CDM channel, each transmitter comprising: at least one signal processing path carrying one of said data signals , - and multiplying means for combining each data signal with a different orthogonal channelization code. The system according to claim 10, characterized in that said orthogonal channelization codes are Walsh functions. The system according to claim 11, characterized in that no Walsh function is assigned to more than one transmitter over a common operating time period. The system according to claim 12, characterized in that each of said signal processing paths further comprises a separator, coupled to said multiplying means, for spreading the signal produced by said multiplier means by the use of at least one diffusion code. of PN. The system according to claim 13, characterized in that a PN code is assigned to the plurality of transmitters. 15. The system according to claim 14, characterized in that it further comprises an analog adder, coupled to said signal processing paths, to sum the signals produced by said signal processing paths in each transmitter before transmission. 16. The system according to claim 15, characterized in that it also comprises a time precorrector, coupled to said analog adder, to pre-correct the signal produced by said analog adder so that the PN codes of the signals transmitted from the plurality of transmitters align at time after reception. The system according to claim 16, characterized in that it further comprises a frequency pre-corrector, coupled to said time pre-corrector, to pre-correct the signal produced by said time pre-corrector such that the carrier frequencies of the transmitted signals of the plurality of Transmitters are aligned after reception. The system according to claim 13, characterized in that said separator is a QPSK separator and said at least one PN code comprises a pair of quadrature PN diffusion codes. The system according to claim 10, characterized in that said CDM communication system comprises a CDMA of wireless broadcast spectrum, each of said data signals being a user signal, wherein said at least one signal processing path contains one of said user signals, and said multiplier means combines each user signal with a different orthogonal channelization code. 20. A CDM communication system having multiple transmitters, each transmitting at least one data signal that shares a single CDM channel, comprising a plurality of transmitters, each transmitter comprising: means for assigning a set of channelization codes orthogonal to the plurality of transmitters in a predetermined manner; and means for channeling each of said data signals by using one of said orothogonal channelization codes to produce a channelized data signal. The system according to claim 20, characterized in that said orthogonal channelization codes are Walsh functions. 22. The system according to claim 21, characterized in that no Walsh function is assigned to more than one transmitter at a time. The system according to claim 22, characterized in that it further comprises means for broadcasting the channelized signal by using at least one PN broadcast code to produce a broadcast signal. The system according to claim 23, characterized in that a PN code is assigned to the plurality of transmitters. The system according to claim 24, characterized in that it further comprises means for adding said broadcast signals before their transmission to produce a composite signal. 26. The system according to claim 25, characterized in that it further comprises means for pre-correcting said composite signal of each transmitter before its transmission in such a way that said PN codes of said composite signals align in time on reception. The system according to claim 26, characterized in that it further comprises means for pre-correcting said composite signal of each transmitter before transmission in such a manner that said composite signals align in frequency after reception. The system according to claim 20, characterized in that said communication system is a broadcast spectrum communication system of CDMA and said data signals are user signals sharing a single CDMA channel, wherein said channeling means comprise means to channel each of said user signals by using one of said orthogonal channelization codes to produce a channelized user signal. 29. The communication system according to claim 20, characterized in that it further comprises: means for receiving at least two user signals that share a single channel as a combined signal; means for de-diffusing said received signals with respect to at least one predetermined PN broadcast code; means for demultiplexing said composite signals into a plurality of individual data signals with respect to preselected orthogonal channelization codes, - means for coherently separating each of at least two pilot signals corresponding to said individual data signals by coherent filtering; means for generating an error signal from each of said pilot signals; and means for adding the resulting error signals. The communications system according to claim 29, characterized in that it further comprises: rotation means for downconverting the broadcast spectrum signals received at a baseband frequency before demultiplexing; means for filtering the resulting error signals summed; and means for adjusting the operation of said rotation means in response to filtered summed error signals. The communication system according to claim 29, characterized in that said means for generating an error signal comprises means for forming a cross product between the current samples of each pilot signal and the previous samples thereof. The method according to claim 1, characterized in that it further comprises the steps of: receiving at least two user signals that share a single channel as a combined signal; de-diffusing said received signals with respect to at least one predetermined PN broadcast code; demultiplexing said composite signals into a plurality of individual data signals with respect to preselected orthogonal channelization codes; coherently separate each of at least two pilot signals corresponding to said individual signals by coherent filtering; generating an error signal from each of said filtered pilot signals; and add the resulting error signals. The method according to claim 32, characterized in that it further comprises the steps of: subverting the broadcast spectrum signals received at a baseband frequency by rotation before demultiplexing; filter the resulting error signals added; and adjusting said downconversion in response to the summed error signals filtered. 34. The method according to claim 33, characterized in that said step for generating an error signal comprises the formation of a cross product between the current samples of each pilot signal and the previous samples thereof. 35. A method for automatically controlling the frequency in a CDM communication system with a plurality of transmitters that share a single CDM channel, each transmitter having at least one communication channel for transmitting a data signal, comprising the steps of: assigning a predefined set of orthogonal channelization codes to the plurality of transmitters in a predetermined manner; channeling each of said data signals using one of said orthogonal channelization codes to produce a channelized data signal; receive at least two user signals that share a single channel as a combined signal; de-diffusing said received signals with respect to at least one predetermined PN broadcast code; demultiplexing said composite signals into a plurality of individual data signals with respect to preselected user orthogonal codes; coherently separating each of at least two pilot signals corresponding to said individual signals by coherent filtering; generating an error signal from each of said filter pilot signals; and add the resulting error signals. 36. The method according to claim 35, characterized in that it further comprises the steps of: subverting the broadcast spectrum signals received at a baseband frequency by rotation before demultiplexing; filter the resulting error signals added; and adjusting said downconversion in response to filtered summed error signals. 37. An apparatus for automatic frequency control in a CDM communication system having multiple transmitters that share a single CDM channel which each transmits at least one data signal, comprising: means for assigning a set of codes channeling orthogonal to the plurality of transmitters in a predetermined manner, - means for channeling each of said data signals using one of said orthogonal channelization codes to produce a channelized user signal; means for receiving at least two user signals that share a single channel as a combined signal; means for de-diffusing said received signals with respect to at least one predetermined PN broadcast code, - means for demultiplexing said composite signals into a plurality of said individual data signals with respect to preselected orthogonal channelization codes; means for coherently separating each of at least two pilot signals corresponding to said individual data signals by coherent filtering; means for generating an error signal from each of said filter pilot signals; and means for adding the resulting error signals.