HK1033052B - System and method for rapid signal acquisition and synchronization for access transmissions - Google Patents

Description

System and method for fast signal acquisition and synchronization for access transmissions

Background

I. Field of the invention

The present invention relates to multiple access, spread spectrum communication systems and networks. The present invention enables, among other things, the elimination of timing ambiguities in received access channel transmissions in a spread spectrum communication system.

Description of the related Art

Various multiple access communication systems and techniques have been developed for communicating information among a large number of system users. However, spread spectrum modulation techniques, such as those used in Code Division Multiple Access (CDMA) communication systems, have significant advantages over other modulation techniques, particularly when serving a large number of communication system users. The principles of these technologies are described In U.S. Pat. No. 4,901,307 entitled "Spread Spectrum multiple Access Communication System Using Satellite or Terrestrial Repeaters" entitled at 13/1990, And U.S. Pat. No. 5,691,974 entitled "Method And Apparatus For Tracking Phase time And Energy of receivers Using Full spectral Transmission Power In A Spread Spectrum Communication System" entitled "Method And Apparatus For Tracking Phase time And Energy of receivers In an extended Spectrum Communication System at 25/1997, entitled" extended Spectrum Transmission System For use In Spread Spectrum Communication System "entitled" extended Spectrum receiver And antenna engineering ". Both patents are assigned to the assignee of the present invention and are incorporated herein by reference.

In the multiple access communication system disclosed in the above-identified patent, a large number of generally mobile or remote system users each employ at least one transceiver to communicate with other system users or other connected systems, such as users of the public switched telephone network. These transceivers communicate through a gateway and a satellite, or terrestrial base station (also sometimes referred to as a cell site or cell).

The base stations cover cells, while the satellites cover footprints or points on the surface of the earth. In either system, capacity gains may be achieved by sectoring or subdividing the geographic area covered. A cell may be divided into "sectors" with directional antennas located at the base stations. Similarly, the footprint of the satellite is geographically divided into "beams" using the beams that make up the antenna system. These techniques for subdividing the coverage area can be viewed as creating isolation with relative antenna directivity or spatial division multiplexing. In addition, if there is available bandwidth, each of these subdivided portions, either sectors or beams, may be allocated multiple CDMA channels using Frequency Division Multiplexing (FDM). In a satellite system, each CDMA channel is referred to as a "sub-beam" because there may be several such "sub-beams" per "beam".

In communication systems employing CDMA, communication signals are sent back and forth between gateways or base stations using separate links. The forward link refers to the base-to-user terminal or tandem-to-user terminal communication link where signals originate at the tandem or base station and are transmitted to system users or subscribers. The reverse link refers to a communication link from a user terminal to a gateway or to a base station, where signals originate and are transmitted to the gateway or base station.

The reverse link is composed of at least two separate channels: an access channel and a reverse traffic channel. The access channel is initiated by one or more user terminals at separate times or in response to communications from a gateway or base station. The process of communication is referred to as an access transmission, or an "access probe". The reverse traffic channel is used for user transmissions and to send signaling information from the user terminal to one or more gateways or base stations during a "call" or call setup. A more detailed description of the structure or protocol of the access channels, messages and calls IS given in the IS-95 Standard of the telecommunications industry association, entitled Mobile Station-Base Station compatibility Standard For Dual-Mode broadband Spread Spectrum cellular systems, which IS hereby incorporated by reference.

In a typical spread spectrum antenna system, one or more preselected Pseudorandom Noise (PN) code sequences are employed to modulate or "spread" a user information signal over a preselected frequency band prior to modulating the user information signal onto a carrier for transmission as a communication signal. PN spreading is a spread spectrum transmission method well known in the art that produces a signal for transmission that has a much wider bandwidth than the data signal. In the forward link, PN spreading codes or binary sequences are used to distinguish between signals transmitted by different cell sites or on different beams, and to distinguish between different multipath signals. These codes are typically shared by all communication signals within a given cell, beam, or sub-beam.

In some communication systems, the same set of forward link PN spreading codes is used in the reverse link for reverse link traffic and access channels. In other proposed communication systems, different sets of PN spreading codes are used between the forward and reverse links. In other communication systems, it has been proposed to employ different sets of PN spreading codes between the reverse link traffic and the access channel.

PN spreading is performed using a pair of pseudo-noise (PN) code sequences or PN code pairs to modulate or "spread" an information signal. Typically, one PN code sequence is used to modulate an in-phase (I) channel, while another PN code sequence is used to modulate a quadrature-phase (Q) channel. The PN modulation or coding occurs prior to the information signal being modulated by a carrier signal and transmitted by a gateway or base station as a communication signal on the forward link. PN spreading codes are also sometimes referred to as short PN codes or sequences because of their relative short length compared to other PN codes or code sequences used by the communication system.

Depending on whether a forward link channel or a reverse link channel is used, a particular communication system may employ short PN code sequences of several lengths. For the forward link, the short PN code is typically 2 in length¹⁰To 2¹⁵One chip. These short PN codes are used to distinguish between signals transmitted by various satellites or gateways and base stations. In addition, the timing offset of a given short PN code is used to distinguish the beams of a particular satellite or cell.

For the reverse link in a satellite system, the short PN sequence has a length of 2⁸A sequence of the order of a chip. These short PN sequences are used to enable a gateway receiver to quickly search for user terminals attempting to access the communication system without the complexity associated with the "longer" short PN codes used in the forward link. For ease of discussion, "short PN code" refers to a short PN code sequence (2) used in the reverse link⁸One chip).

Another PN code sequence is called a channelization code and is used to distinguish communication signals transmitted by different user terminals in a cell or sub-beam. PN channelization codes are also known as long codes because they are relatively "long" compared to other PN codes used by the communication system. The long PN code is typically 2 in length⁴²On the order of a chip. Typically, an access message is modulated by a long PN code or a particular "masked" version thereof before being modulated by a short PN code and then transmitted to a gateway or base station as an access probe. However, the short PN code and the long PN code may also be combined before the access message is modulated.

When a receiver at a tandem office or base station receives an access probe, the receiver must despread the access probe to obtain the access message. This is accomplished by forming hypotheses or guesses as to which long PN codes and which short PN codes to modulate the received access message. Correlations between a given hypothesis and the access probes are generated to determine which hypotheses are the best estimates of the access probes. The hypothesis that yields the greatest correlation (typically with respect to a predetermined threshold) is the selected hypothesis. Once the appropriate hypotheses are determined, the access probe is despread with the selected hypotheses to obtain the access message.

This timing uncertainty creates a problem for spread spectrum communication systems. This timing uncertainty corresponds to the uncertainty of the beginning of the PN code sequence, which is the starting point or timing of the encoding. As timing uncertainty increases, more assumptions are made to determine the start of a PN code sequence. Proper demodulation of signals in these communication systems depends on whether or not to "know" where to start each PN code sequence in the received signal. Not knowing the starting point of the PN code sequence, or not being properly synchronized with their respective timing, would result in the inability to demodulate the received signal.

However, in satellite communication systems, access probes are particularly difficult to acquire due to the varying distance between the user terminal and the satellite transponder. Because the satellite orbit is around the earth, the distance between the user terminal and the satellite varies greatly. The maximum distance occurs when the satellite is located at a level relative to the user terminal. The minimum distance occurs when the satellite is directly on top of the user terminal. This difference in distance makes the timing of the access probe one way (i.e., from the user terminal to the gateway) uncertain by as much as 20 milliseconds (ms). This uncertainty can also be greater depending on the system.

To solve the timing uncertainty problem, the gateway receiver may have to search through tens of thousands of hypotheses. This search may take several seconds to complete, resulting in a delay in establishing the communication link that is unacceptable to the user. In addition, due to the limited number of channels in a communication system, a particular user may actually lose one access to the communication system in a few minutes because one or more users first set up a link or call.

Similar problems arise in communication systems employing slotted ALOHA access signal protocols or techniques. In this technique, the access channel is divided into a series of fixed length frames or time slots for receiving signals. The structure of the access signal is usually some "packets", which consist of a preamble part and a message part, which have to arrive at the beginning of the time slot from which they are captured. The lack of acquisition of an access probe in a particular frame period may result in the transmitter wishing to access having to retransmit the access probe so that the receiver can detect it again in a subsequent frame. Multiple access signals arriving together can "collide" and are not captured, requiring both parties to retransmit. In either case, the timing of the subsequent access transmission when the initial attempt fails is based on a delay equal to the random number of the slot or frame. The length of the delay in acquisition of a probe is increased by any delay in resetting the acquisition circuit in the receiver to scan for various hypotheses and, as noted above, is acquired first in other probes. In the extreme case, if the timing uncertainty problem is not resolved, the access probe is never captured, at least for a practical time period.

There is a need for a system and method for rapidly acquiring access probes in a spread spectrum communication system in the presence of expected timing ambiguities.

Summary of The Invention

The present invention is a new and improved system and method for rapidly acquiring and synchronizing an access probe transmitted by a user terminal in a spread spectrum communication system. Rather than initially spreading the access probe with a short pseudo-noise (PN) code pair and a long PN code, the access probe is spread in stages. In the first phase, the preamble of the access probe, which consists of null data, is first extended with only short PN code pairs. In the second stage, the preamble of the access probe is spread with a short PN code pair and a long PN code pair. The purpose of extending the access probes in both phases is to reduce the total number of hypotheses required by the receiver to resolve the timing uncertainty problem in the access probes. During the first phase of access detection, the receiver employs a coarse search function or operation to determine short PN code pairs that modulate the null data in the preamble. The determination of the short PN code pair accounts in part for timing uncertainty as a function of the short PN code pair length.

During the second phase of access acquisition, and after the receiver has determined the short PN code pairs to use, the receiver employs a fine search function or operation to determine the long PN code that modulates the zero data in the preamble that is also spread with the short PN code pairs and the long PN code. The determination of the long PN code completely resolves the timing uncertainty of the access probe.

It is a feature of the present invention to reduce the total number of hypotheses required by a receiver in acquiring an access signal or probe. The reduction in the number of hypotheses results in a reduction in the amount of time necessary to capture an access probe. Therefore, the delay experienced by the user terminal in accessing the communication system is shorter than in systems employing conventional techniques. The reduction of the number of hypotheses also increases the probability of establishing contact between the user terminal and the gateway.

Brief Description of Drawings

The features, objects, and advantages of the present invention will become more apparent to the reader after a detailed description of the invention with reference to the accompanying drawings. In the drawings, the same reference numerals are used for the same purposes.

Fig. 1 is an exemplary wireless communication system constructed and operative in accordance with an embodiment of the present invention;

fig. 2 is an exemplary architecture of a communication link used between a tandem office and a user terminal in a communication system;

FIG. 3 is a further detail of an access channel;

fig. 4 is a conventional protocol for transmitting access probes in a typical CDMA communication system;

FIG. 5 is a protocol for sending access probes according to one embodiment of the invention;

FIG. 6 is a block diagram depicting an access channel transmitter in accordance with one embodiment of the present invention;

FIG. 7 is a block diagram of a preamble phase switch (switch) of the access channel transmitter of FIG. 6 in further detail;

FIG. 8 is a block diagram of further details of another embodiment of the switching of the preamble phase of the access channel transmitter shown in FIG. 6;

FIG. 9 is a block diagram of an access channel receiver according to one embodiment of the present invention; and

fig. 10 is a state diagram of the operation of an access channel receiver in accordance with one embodiment of the present invention.

Detailed description of the preferred embodiments

The present invention relates to a system and method for fast acquisition of an access probe in a spread spectrum communication system. In one embodiment of the invention, the acquired access probe is sent by the user terminal or mobile station to the gateway or base station.

In a typical CDMA communication system, a base station within a predetermined geographic area or cell employs several spread spectrum modems or transmitter and receiver modules to process communication signals for system users within the base station's access area. Each receiver module typically employs a digital spread spectrum data receiver and at least one searcher receiver and associated demodulator, etc. During typical operation, a particular transmitter module and a particular receiver module or modem in a base station is assigned to a subscriber terminal to regulate the communication of communication signals between the base station and the subscriber terminal. In some cases, multiple receiver modules may be used to accommodate diversity signal processing.

For communication systems employing satellites, the transmitter and receiver modules are typically located in base stations called gateways or hubs that communicate with system users via satellite-transmitted communication signals. In addition, there may be other associated control centers that communicate with the satellite or gateway to maintain system wide traffic control and signal synchronization.

I. Overview of the System

An example wireless communication system constructed and operative in accordance with the present invention is shown in fig. 1. Communication system 100 employs spread spectrum modulation techniques to communicate with user terminals, illustratively user terminals 126 and 128, having wireless data terminals or telephones. In terrestrial systems, the communication system 100 communicates with the user terminals 126 and 128 through system base stations (shown as base stations 114 and 116). A cellular telephone type system in a large city may have hundreds of base stations 114 and 116, serving thousands of user terminals 126 and 128 with ground base station repeaters.

Each of the mobile stations or user terminals 126 and 128 has or includes a wireless communication device such as, but not limited to, a cellular telephone, a data transceiver or forwarding device (e.g., a computer, personal data assistant, facsimile machine), or a paging or position location receiver. Typically, these devices may be hand-held, or vehicle-mounted, as desired. Although these user terminals are discussed as being mobile, it should be understood that the principles of the present invention are equally applicable to fixed devices, or other types of terminals requiring remote wireless service. The latter type of service is particularly useful for establishing communication links with satellites in many remote areas of the world.

Typical user terminals are found in the above-mentioned U.S. patent 5,691,974, and U.S. patent application 08/627,830 entitled "Pilot Signal Strength control for a Low Earth orbit satellite communication System" and U.S. patent application 08/723,725 entitled "clear positioning with two Low Earth orbit satellites". These documents are incorporated herein by reference.

In a satellite-based system, communication system 100 employs satellites (shown as satellites 118 and 120) and system gateways (shown as gateways 122 and 124) to communicate with user terminals 126 and 128. Gateways 122 and 124 transmit communication signals to user terminals 126 and 128 via satellites 118 and 120. Satellite-based systems typically employ a small number of satellites to provide service to a large number of users over a larger geographic area.

It should be noted that in this example, the satellites provide multiple beams to cover separate geographic areas that typically do not overlap. Multiple beams at different frequencies, referred to as CDMA channels, 'sub-beams' or FDM signals, bins or channels, may cover the same area. It should be understood, however, that the beam footprints or service areas of different satellites, or the antenna patterns of ground-based stations, may overlap, in whole or in part, in a given area, depending on the design of the communication system and the type of service being provided. Diversity or handover may also be implemented between all of these communication areas or devices. For example, each may provide services to a different group of users having different characteristics at different frequencies, or each given mobile unit may employ multiple frequencies and/or multiple service providers, each having overlapping geographic areas.

As shown in fig. 1, the communication system 100 employs a system controller and switching network 112, also referred to as a Mobile Telephone Switching Office (MTSO) in a terrestrial system and a (terrestrial) command and control center for a satellite system. These controllers typically include interface and processing circuitry that provides wide system control of the base stations 114 and 116 or the gateways 122 and 124. Controller 112 also typically hosts the routing of telephone calls among the Public Switched Telephone Network (PSTN), base stations 114 and 116 or gateways 122 and 124, and mobile units 126 and 128. However, a PSTN interface typically forms part of each tandem that is directly connected to these communication networks or communication links. The communication links coupling the controller 112 to the various system base stations 114 and 116 gateways 122 and 124 may be established using known techniques such as, but not limited to, dedicated telephone lines, fiber optic links, or microwave or dedicated satellite communication links.

In fig. 1, the possible signal paths of the communication links between the base stations 114 and 116 and the user terminals 126 and 128 are represented by straight lines 130, 132, 134 and 136. The arrows in the line segments indicate the signal direction of the typical link (either the forward or reverse link), and such depiction is merely for clarity and does not limit the actual signal manner.

In a similar manner, the signal paths of the communication links between the gateways 122 and 124, the satellites 118 and 120, and the user terminals 126 and 128 are represented by segments 146, 148, 150, and 152 for the gateway-to-satellite links, while segments 140, 142, and 144 are used for the satellite-to-user links. In some configurations, it is also possible and desirable to establish a direct satellite-to-satellite link, represented typically by line segment 154.

As is known in the art, the present invention is applicable to either ground-based systems or satellite-based systems. Therefore, for clarity, the gateways 122 and 124 and the base stations 114 and 116 will be collectively referred to as the gateway 122. Similarly, satellites 118 and 120 will be collectively referred to as satellites 118, and user terminals 126 and 128 will be collectively referred to as user terminals 126. Additionally, although the user terminal 126 is discussed as being 'mobile', it should be understood that the principles of the present invention may also be applied to fixed units requiring remote wireless service.

Although only two satellites are shown in fig. 1, communication systems typically employ a plurality of satellites that intersect different orbital planes. It has been proposed, for typical systems employing on the order of 48 or more satellites, to employ a plurality of satellite communication systems, the satellites operating in Low Earth Orbit (LEO) along 8 different orbital planes, serving a large number of user terminals. However, those skilled in the art will understand how the principles of the present invention apply to a variety of satellite systems and gateway architectures, including to other orbital distances and constellations.

The terms base station and tandem are sometimes used interchangeably in the art, with a tandem being considered a specialized base station that directs communications through satellites and has more 'functionality', with associated equipment to maintain a communication link through a moving transponder, while a base station directs communications in a surrounding geographic area with a terrestrial antenna. The central control center also typically has a number of functions that are performed when interacting with the gateways and satellites. A subscriber terminal is also sometimes referred to as a subscriber unit, mobile station, or simply as a "subscriber", "mobile subscriber", or "user" in some communication systems, depending on preference.

Communication link

Fig. 2 illustrates an exemplary configuration of a communication link used between a gateway 122 and a subscriber terminal 126 in communication system 100. At a minimum, and typically also, two links are employed in communication system 100 for communication signals between gateway 122 and subscriber terminal 126. These links are referred to as forward link 210 and reverse link 220. Forward link 210 processes a transmitted signal 215 that is transmitted from gateway 122 (or a base station) to a user terminal 126. The reverse link 220 processes a transmit signal 225 transmitted from the user terminal 126 to the tandem office 122 (or base station).

The forward link 210 includes a forward link transmitter 212 and a forward link receiver 218. In one embodiment, forward link transmitter 212 is located within tandem office 122 (base station), in accordance with well known CDMA communication techniques disclosed in the above-mentioned patents. In one embodiment, forward link receiver 218 is located in user terminal 126 in accordance with well-known CDMA communication techniques disclosed in the above-identified patents.

Reverse link 220 includes a reverse link transmitter 222 and a reverse link receiver 228. In one embodiment, the reverse link transmitter 222 is located in the user terminal 126. In one embodiment, the reverse link receiver 228 is located in the gateway 126 (base station).

Reverse link 220 includes at least two channels: one or more access channels, and one or more reverse traffic channels. These channels may be implemented with different receivers or the same receiver operating in different ways. As discussed above, the access channel is used by the user terminal 126 to originate or respond to communications with the tandem office 122. A separate access channel is used for each active user at any given time. In particular, the access channel is time-shared by several user channels 126, with transmissions from each active user being separated in time from each other. The system may employ one or more access channels, depending on known factors such as the required level of tandem complexity and access timing. The proposed embodiment employs 1 to 8 access channels per frequency. The access channel will be discussed in detail below.

Access channel

Fig. 3 depicts further details of access channel 300. The access channel 300 includes an access channel transmitter 310, an access channel receiver 320, and an access probe 330. The access channel transmitter 310 is included in the reverse link transmitter 222 described above. The access channel receiver 320 is included in the reverse link receiver 228 described above.

The access channel 300 is used for short signaling message exchanges including call origination, page response, and registration (registration) originating from the subscriber terminal 126 and targeted to the tandem office 122. To enable the user terminal 126 to issue or respond to communications with the gateway 122 (or base station) on the access channel 300, a signal referred to as an access signal or access probe is transmitted.

The access channel is also typically associated with one or more particular paging channels used in the communication system. The response to the paging message is made more efficient since the system knows where to look for user terminal access transmissions in response to the page. This correlation or assignment may be known based on a fixed system design or prompt to the user terminal in the paging message structure. As is known, with the slotted access channel approach, the access channel is divided into a series of fixed length frames or slots during which access transmissions or probes can be received from user terminals.

Timing uncertainty in access probes

Uncertainty in access probe timing can arise due to varying distances or propagation path lengths between the user terminal 126 and the satellite transponder 118 as the satellite 118 orbits the earth. This timing uncertainty is limited by the minimum and maximum propagation delays. Minimum propagation delay D_minIs the propagation time of the signal from the user terminal 126 to the satellite 118 when the satellite 118 is directly above the user terminal 126. Maximum propagation delay D_maxIs the time that a signal travels from user terminal 126 to satellite 118 when satellite 118 is at a predetermined useful horizontal position of user terminal 126. Adopt the classSimilarly, some degree of timing uncertainty can arise for relative motion between the user terminal and the base station 114, although typically of a small magnitude.

Resolving this timing uncertainty is necessary in order to properly capture the access probe 330. Specifically, the timing (i.e., the starting time of the PN code) must be known in order to despread the access probe 330 or its message content with the long PN code and the short PN code. This is done by correlating the access signals forming the access probes 330 with various timing hypotheses to decide which timing hypothesis is the best estimate for solving the access probes 330. The timing hypotheses are time offsets from each other and represent various estimates of the timing of the access probe 330 or PN code used to generate the probe. The assumption that the highest correlation (typically one that exceeds a predetermined threshold) with the access probe 330 is generated is the most likely estimate (assuming "correct") of the timing of a particular access probe 330. Once the timing uncertainty is resolved in this manner, the access probe 330 can be despread with the timing estimate and the long and short PN codes in accordance with well-known techniques.

V. legacy protocol for transmitting access probes

Fig. 4 depicts a conventional signal structure or protocol 400 for transmitting a conventional access signal 410 (also referred to as an access probe) on an access channel used in a conventional CDMA communication system. When a user terminal 126 desires access to the communication system 100, i.e., initiates or responds to a communication, the user terminal 126 sends a conventional access signal or probe 410 to the gateway 122 in accordance with the conventional protocol 400. The conventional access probe 410 includes an access probe preamble (preamble)420 and an access probe message (access message) 430. The conventional access probe 410 is transmitted by the access channel transmitter 310 in the subscriber terminal 126 to the access channel receiver 320 in the gateway 122.

In conventional spread spectrum systems, each of preamble 420 and access message 430 is orthogonally spread with a pair of short pseudo-noise code sequences (short PN code pairs) 440 and channelized with a long pseudo-noise code sequence (long PN code) 450. The preamble 420, which is typically composed of zero data (i.e., all "1" s or all "0" s or a preselected pattern of "1" s and "0" s), is transmitted first, providing the channel receiver 320 with an opportunity to acquire the access probe 410 before sending the access message 430.

The short PN code pair 440 is used to modulate or "spread" the information signal. Pseudo-noise modulation or coding occurs before the information signal is modulated with a carrier signal and transmitted to the gateway 122. The short PN code pair 440 is used to distinguish communication signals transmitted on a particular CDMA channel. In one embodiment of the present invention, the short PN code pair 440 is used to distinguish access channel signals used in the reverse link 220 from other communication signals. According to one embodiment of the invention, each tandem office 122 employs its own short PN code pair 440. In other embodiments of the present invention, different short PN code pairs 440 are employed for each frequency band in the gateway 122, depending on the amount of communication traffic to be accommodated. In these embodiments, there may be up to 8 short PN code pairs 440 per tandem office. However, other numbers (more or less) of PN code pairs may be employed for this function.

The long PN code 450 is used to distinguish communication signals transmitted by different user terminals 126 in a cell or beam. Typically, in conventional systems, the preamble 420 and access message 430 are modulated or encoded with a long PN code 450 before being spread 440 with a short PN code. However, in other conventional systems, the short PN code 440 and the long PN code 450 may be combined and subsequently used to modulate the preamble 420 and access the message 430.

When the access channel receiver 320 accepts the preamble 420, the access channel receiver 320 must despread the preamble 420 with the short PN code pair 440 and the long PN code 450. This is done by forming which long PN code 450 and which short PN code pair 440 are hypothesized or guessed to modulate the null data included in the preamble 420. The given assumption is related to the preamble 420. The correlation results of preamble 420 with each hypothesis are compared. The particular hypothesis that yields the highest correlation in terms of magnitude or energy is the selected hypothesis. The specific long PN code 450 and the specific short PN code 440 that comprise the hypothesis are used to demodulate the access probe 410. The transmission of the access probe 410 may have to be repeated to ensure acquisition.

Once the short PN code pair 440 and the long PN code 450 are determined by the access channel receiver 320, the conventional access probe 410 is considered acquired. After the preamble 420 is transmitted within a predetermined time period, an access message 430 is sent by the access channel transmitter 310. As discussed above, the access message 430 is spread according to a conventional protocol or access signal structure 400 using the same short PN code pair 440 and long PN code 450 used to spread the preamble 420.

The preamble 420 must be of sufficient length so that the access channel receiver 320 has time to process the hypothesis and capture the conventional access probe 40 before transmitting the access message 430. Otherwise, the access channel receiver 320 will still attempt to acquire the conventional access probe 410 when transmitting the access message 430. In this case, the access message 430 will not be received correctly. The time required to acquire access probe 410, referred to as the acquisition time, depends on how many receivers are used to process the hypotheses in parallel, how long the individual code sequences are, the extent of timing uncertainty in the signaling, and so on. Each of these factors will affect the number of hypotheses that must be formed, as well as the time required to capture the conventional access probe 410. In addition to factors that affect acquisition time, the length and repetition frequency of the preamble 420 are selected to minimize collisions between access probes 410 transmitted by different user terminals 126. It will be apparent that each of these factors is considered in light of system design considerations in determining the length of the preamble 420.

The present invention employs an access signal structure or protocol to transmit access probes that are intended to form far fewer hypotheses than required by conventional access probes 410. This access probe is discussed in further detail below.

Protocol for transmitting access probes according to the invention

Fig. 5 illustrates a signal structure or protocol 500 for transmitting an access probe 510 according to one embodiment of the invention. The access probe 510 includes an access probe preamble (preamble)520 and an access probe message (access message) 530. One basic difference between the protocol 500 and the conventional protocol 400 is that the preamble 510 is initially spread or modulated with only the short PN code pair 440, and then the short PN code is used to modulate 440 and the long PN code 450. This enables the access channel receiver 320 to resolve timing uncertainty problems with only the short PN code pair 440. In contrast, the conventional protocol 400 wants to solve the timing uncertainty problem with both the short PN code pair 440 and the long PN code 450.

Modulating the preamble 520 in stages (i.e., first with only the short PN code pair 440, followed by the short PN code pair 440 and the long PN code 450) significantly reduces the number of hypotheses required to obtain the access probe 510 by the access channel receiver 320. By reducing the number of hypotheses, the time required for access channel receiver 320 to acquire access probe 510 (i.e., the acquisition time) is correspondingly reduced.

According to the invention, the preamble 520 is transferred in two stages: a first stage preamble 560 and a second stage preamble 570. In the first stage preamble 560, the preamble 520 is modulated with the short PN code pair 440 for a sufficient amount of time so that the access channel receiver 320 can determine the timing of the short PN code pair 440.

In the second stage preamble 570, the preamble 520 is modulated with a short PN code pair 440 and a long PN code 450. The second stage preamble 570 is transmitted by the access channel transmitter 310 for a sufficient time to enable the access channel receiver 320 to determine the timing of the long PN code 450. At the end of the second stage preamble 570, the access channel receiver 320 should have acquired the access probe 510.

After the second stage preamble 570, the message stage 580 is transmitted by the access channel transmitter 310. During the message phase 580, the message 530 is modulated with a short PN code pair 440 and a long PN code 450.

By transmitting the preamble 520 in stages, the ambiguity in the decision making and number of hypotheses required to acquire the access probe 510 is reduced. In systems employing the conventional protocol 400, the number of hypotheses required is determined by multiplying the timing uncertainty by the chip rate, since one hypothesis is required for each potential code start time (start of frame) of the conventional access probe 410 for the duration of the timing uncertainty. In other words, each potential PN code timing (i.e., the time at which the access probe begins) must be evaluated for the duration of the uncertainty.

In a preferred embodiment of the present invention, the access channel receiver 320 solves the timing uncertainty problem in part by first despreading the first-stage preamble 560 with a short PN code pair 440 derived from a prior check. Because we want the short PN code pair 440 to be much shorter than the timing uncertainty, the number of hypotheses needed to acquire the short PN code pair 440 is the number or number of possible code start points for the short PN code pair 440. Therefore, for a short PN code pair 440 having a length of 256, the number of hypotheses required to acquire the short PN code pair 440 is 256.

In a preferred embodiment of the present invention, the access channel receiver 320 completely solves the timing uncertainty problem by despreading the second stage preamble 570 with the a priori short PN code pair 440 and the a priori long PN code 450. After acquisition of the short PN code pair 440, an integer order of magnitude (order) ambiguity in the length of the short PN code pair 440 exists in the timing of the access probe 510. In other words, the short PN code pair 440 repeats an integer number of times for the duration of the timing uncertainty. The number of repetitions is the number of hypotheses that need to be formed during the transfer of the second stage preamble 570. The value is determined by dividing the timing uncertainty by the period of the short PN code pair 440.

The total number of hypotheses required by the present invention to resolve the timing uncertainty is determined by the sum of the hypotheses required by each of the first-stage preamble 560 and the second-stage preamble 570. A comparison of the number of hypotheses required to resolve the timing uncertainty is shown in table I. Table I compares the number of hypotheses required for a system employing conventional access probes 410 with a system employing access probes 510 having respective short PN code lengths (L) in accordance with the present invention. Table I is generated for a typical CDMA communication system having a chip rate of 1.2288 megachips per second (megachips) and a timing uncertainty of 10 milliseconds. In this typical comparison, the assumption of one-half chip is ignored.

TABLE I

Timing uncertainty comparison
				System for controlling a power supply	Number of hypotheses required
First stage	Second stage	Total number of
			Conventional		N/A	N/A	≈12,500
L＝128	128	96		224
			L＝256		256	4	304
L＝512	512	24		536
			L＝1024		1024	12	1036

The reduction of the number of hypotheses becomes more important when frequency uncertainty is taken into account. According to one embodiment of the present invention, frequency uncertainty is resolved during the transmission of the first-stage preamble 560, while timing uncertainty is also fully resolved during the transmission of the second-stage preamble 570. In this embodiment, the number of hypotheses required during the first-stage preamble 560 is increased by a factor of the number of frequency hypotheses (e.g., N) measured while the number of hypotheses required for the second-stage preamble 570 remains unchanged. The number of frequency hypotheses, N, depends on factors well known in the art, such as the expected amount of Doppler and other frequency shifting effects, and the size and number of bins (bins) employed to partition the total frequency space to be searched. A comparison of the number of hypotheses required to solve the timing and frequency problems with the same system shown in table I above is shown in table II.

TABLE II

Frequency and timing uncertainty comparison
				System for controlling a power supply	Number of hypotheses required
First stage	Second stage	Total number of
			Conventional		N/A	N/A	≈12,500*N
L＝128	128*N	96		128*N+96
			L＝256		256*N	48	256*N+48
L＝512	512*N	24		512*N+24
			L＝1024		1024*N	12	1024*N+12

Access channel transmitter

Fig. 6 is a block diagram of an example access channel transmitter 310, in accordance with one embodiment of the present invention. The access channel transmitter 310 includes a transmit data pre-processor 610, a long code generator 635, a preamble stage switch 640, and a transmit data post-processor 690.

The transmit data pre-processor 610 pre-processes information to be transmitted in accordance with various signal processing techniques employed in CDMA communications. In an exemplary embodiment according to the present invention, the transmit data pre-processor 610 includes an encoder 615, a symbol repeater 620, an interleaver 625, and an M-array of quadrature modulators 630. Transmit data pre-processor 610 may include these elements as well as other pre-processing elements without departing from the scope of the present invention. Those skilled in the art are familiar with the various types of signal processing and associated components used to prepare information signals.

An exemplary embodiment of transmit data pre-processor 610 is described below. In this embodiment, encoder 615 is a conventional encoder that encodes data using generator functions well known in the art. Encoder 615 receives data input as binary bits and outputs as encoded symbols. Symbol repeater 620 repeats the code symbols received from encoder 615 to maintain a total number of code symbols per frame at the respective data rates. Interleaver 625, which is typically a block interleaver, interleaves the code symbols according to well-known techniques. The M-array quadrature modulator 630 modulates the interleaved code symbols with an M-array quadrature code modulation process. These M-matrix orthogonal codes may be walsh functions or codes, which are commonly used in CDMA communication systems, as is well known.

When Walsh codes are used as the orthogonal codes, eachGroup log₂The M code symbols are mapped to one of M mutually exclusive orthogonal modulation symbols, which may be referred to as walsh symbols. In this embodiment of the invention, a 64 array of quadrature modulators is employed. Therefore, in this embodiment, each walsh symbol is composed of 64 walsh chips, and 6 code symbols are mapped to one walsh symbol or orthogonal function. Other code lengths, having different sets or numbers of code symbols, may be used, as is known in the art.

Preamble stage switch 640 receives data from transmit data pre-processor 610 and long PN code 450 from long code generator 635. Preamble stage switch 640 outputs data to transmit data post processor 690. Preamble stage switch 640 will be described in detail below.

Transmit data post processor 690 post-processes the information output from the front end stage switch 640 before it is transmitted. In an exemplary embodiment of the invention, the transmit data post processor 690 includes an I channel modulator 645, an I channel short code generator 648, a Q channel modulator 650, a Q channel short code generator 649, a delay or delay element 655, an I channel baseband filter 660, a Q channel baseband filter 665, an I channel carrier signal modulator 670, a Q channel carrier signal modulator 675, and a signal combiner 680. Transmit data post processor 690 may include these elements as well as other post processing elements without departing from the scope of the present invention. For example, the transmit signal may not be composed of in-phase or quadrature elements as described above. In other words, communication system 100 may not use phase shift keying. In this example, only one signal path in transmit data post processor 690 may be employed. It is therefore apparent that in this example only one of the short code generators 648, 649, one of the baseband filters 660, 665 and one of the carrier signal modulators 670, 675 is employed. At this point, transmit data post-processor 690 performs various filtering and modulation operations in accordance with techniques well known in CDMA communications.

In a preferred embodiment of the present invention, the modulator 645 and 650 are used to self-shortThe short PN code pairs 440 of the code generators 648, 649 quadrature spread the output of the preamble stage switch 640. Short PN code pair 440 contains sequences sometimes referred to as Q pilot PN sequences and I pilot PN sequences. This term is used in embodiments where the short code pair 440 is selected to match the forward link short PN code, as in terrestrial cellular and some satellite communication systems. Conversely, the term "pilot" is not necessarily used with reference to coding, but is used only on the reverse link, where no pilot is employed, or only on the access channel. Short code generator 648 generates IPN (PN)₁) And (4) sequencing. Short code generator 649 generates a Q PN sequence (PNQ). The I and Q sequences may be completely different sequences or the same sequence, with a delay of one sequence offset from the other sequence.

In another embodiment (not shown), the short code generators 648, 649 are replaced with a short code generator 618 and a delay. In this embodiment, the output of the short code generator is applied directly to modulator 645 and to modulator 650 which is then delayed. The modulators 645,650 may be implemented as combiners, multipliers or modulo-2 adders or other techniques as will be apparent.

In one embodiment of the invention, after modulation with short code generator 649, PN_QSequence vs. PN₁The sequence is delayed by one-half PN chip time by delay 655. In this embodiment of the invention, the one-half chip delay provides an offset for quadrature phase shift keying and improves the power envelope for subsequent baseband filtering.

The output of the spreading operation is applied to baseband filters 660, 665 and modulated with a carrier signal by modulators 670, 675, respectively. The combined modulated signals are combined by combiner 680 and transmitted in accordance with well-known communication techniques.

Preamble stage switch

Fig. 7 depicts in further detail an exemplary architecture of the preamble baseband switch 640. The preamble stage switch 640 includes a first switch 710, a second switch 720, two zero code generators 730, and a modulator (or spreading element) 740. The first switch 710 includes two terminal positions, a first terminal position being labeled "a, B" and a second terminal position being labeled "C". The second switch 720 includes two terminal positions, a first terminal position being labeled "a" and a second terminal position being labeled "B, C". "a" identifies the end positions of the first switch 710 and the second switch 720 during the occurrence or transmission of the first stage preamble 560. "B" identifies the terminal position of the first switch 710 and the second switch 720 during the generation and transmission of the second stage preamble 570. "C" identifies the terminal positions of the first switch 710 and the second switch 720 during the occurrence of the message phase 580.

The operation of the preamble stage switch is described below with reference to fig. 5 and 7. During the first stage preamble 560 of the access probe 510, each of the first switch 710 and the second switch 720 are in their respective terminal positions labeled "a". In this position, the first switch 710 passes zero data to the modulator 740, and the second switch 720 also passes zero data to the modulator 740. During the first stage preamble 560, the output consists of zero data. The null data is modulated with the short PN code pair 440 discussed above. Thus, in the first stage preamble 560 section, the null data is modulated with the short PN code pair 440, rather than the long PN code 450.

Zero data refers to data having a constant or known value, or all "0" s or all "1" s, or data of a known pattern such as alternating "1" s or "0" s, etc. The null data represents a fixed pattern that helps capture the access probe 510 known to the receiver. The zero data does not contain any message information. In this embodiment of the present invention, zero data means all "1".

The second stage preamble 570 is transmitted after a receiver, such as the access channel receiver 320, has sufficient time to determine the short PN code pairs 440 from the first stage preamble 560. After the second stage preamble 570 occurs or is transmitted, the first switch 710 and the second switch 720 are in their respective terminal positions labeled "B". In this position, the first switch 720 continues to pass zero data to the modulator 740, while the second switch 720 passes the long PN code 450 to the modulator 740. During generation or transmission of the second stage preamble 570, the output 642 is zero data modulated by the long PN code 570. Output 642 is then modulated with a short PN code pair 440 as discussed above. Thus, during the second stage preamble 570, the null data is modulated by the long PN code 450 and the short PN code 440.

The message phase 580 is transmitted after the receiver (access receiver 320) has sufficient time to determine the short PN code pair 440 from the first-stage long-sequence portion, as there is sufficient time to determine the long PN code 450 from the second-stage preamble 570. During the generate or transmit message phase 580, the first switch 710 and the second switch 720 are in their respective terminal positions labeled "C". In this position, the first switch 710 passes the access channel information 638 to the modulator 740, while the second switch 720 continues to pass the long PN code 450 to the modulator 740. During the message phase 580, the output 642 is comprised of message data modulated by the long PN code 570. Output 642 is then modulated with a short PN code pair 440 as discussed above. Therefore, during message phase 580, the message data is modulated with long PN code 450 and short PN code pair 440.

Fig. 8 depicts further details of another example structure of preamble stage switch 640. In this embodiment, preamble stage switch 640 includes a switch 810, zero code generator 820, and a modulator (or spreading element) 830. The switch 810 includes two terminal positions, a first terminal position labeled "a" and a second terminal position labeled "B, C". "a" indicates the terminal position of the switch 810 during the first stage preamble 560. "B" denotes the terminal position of switch 810 during the second stage preamble 570. "C" represents the terminal position of switch 810 during the occurrence or transmission of message phase 580.

The operation of preamble stage switch 640 in this embodiment is described below with reference to fig. 5 and 8. During the first stage preamble 560 of the access probe 510, the switch 810 is located in the terminal position labeled "a". In this position, switch 810 passes all "0" s from zero data generator 820 to modulator 830. Meanwhile, the access channel information applied to the access channel transmitter 310 is composed of null data (i.e., either "0" or "1"). The data is provided by and generated within known user terminal transmit elements under the control of the user terminal controller using techniques known in the art. For example, the input of the encoder 615 may be controlled to provide a particular desired output, or the output of the modulator 630 or the pre-processor 610 may be interrupted and the input of the preamble switch 640 connected to another source that produces zero data. Therefore, access channel information 638 is comprised of null data for processing by transmit data pre-processor 610. Access channel information 638 is applied directly to modulator 830.

The particular combination of the spreading element 830 and the null data generator 820 shown in fig. 8 ensures that when the access channel information 638 is modulated by the output of the null data generator 820, the result is the same as the access channel information 638, which, as discussed above, is comprised of null data. It will be apparent that other combinations of these elements will similarly ensure that output 642 is comprised of access channel information 638. The output is then modulated by the short PN code pair 440 as discussed above. As with the previously discussed embodiments, during the first stage preamble 560, the zero data of the output 642 is modulated by the short PN code pair 440, rather than the long PN code 450.

The second stage preamble 570 is transmitted after a receiver, such as the access channel receiver 320, has had sufficient time to determine the short PN code pairs 440 from the first stage preamble 560. During the transfer of the second stage preamble 570, the switch 810 is in the end position labeled "B". In this position, switch 810 passes long PN code 450 to modulator 830. Meanwhile, the access channel information applied to the access channel transmitter continues to consist of null data. During the second stage preamble 570, the output 642 consists of zero data modulated by the long PN code 570. The output 642 is then modulated by the short PN code pair 440 as discussed above. Thus, during the second stage preamble 570, the null data is modulated by the long PN code 450 and the short PN code pair 440.

The message stage 580 is transmitted after the receiver (accessing the receiver 320) has had sufficient time to determine the long PN code 450 from the second stage preamble 570. During the transmission of message phase 580, switch 810 is in the position labeled "C". In this position, switch 810 continues to pass long PN code 450 to modulator 830. Meanwhile, the access channel information applied to the access channel transmitter becomes message data opposite to the null data. Therefore, access channel information 638 is message data as processed by transmit data pre-processor 610. Thus, during the message phase 580, the output 642 is comprised of message data modulated by the long PN code 570. The output 642 is then modulated by the short PN code pair 440 as discussed above. Therefore, during message phase 580, the message data is modulated by long PN code 450 and short PN code pair 440.

IX. Access channel receiver

Fig. 9 is a block diagram of an exemplary structure of an access channel receiver 320 according to an embodiment of the present invention. The access channel receiver 320 includes an analog-to-digital (a/D) converter 910, a rotator 920, a first memory 925, a Fast Hadamard Transformer (FHT)930, a second memory 935, a delay 940, adders 945 and 950, a coherent integrator 960, a squarer 965, a channel adder 970, and a non-coherent integrator 980.

The a/D converter 910 receives I, Q the channel signal from an antenna (not shown) and quantizes the received signal. Rotator 920 adjusts the frequency of the received signal to remove frequency uncertainty in the received signal due to the Doppler effect or other known effects.

The output of the rotator 920 is stored in the memory 925. FHT930 performs a Fast Hadamard Transform (FHT) according to well-known techniques. The output of the FHT930 is stored in memory 935. The memory 925 and memory 935 operate according to well-known procedures to permute data before and after FHT operation. This process quickly and efficiently determines the number of possible offsets for the short PN code pair 440 in view of possible timing uncertainties. The outputs of the memory 925, FHT930, and memory 925 are the periodic autocorrelation of the short PN code pairs 440.

The remainder of the access channel receiver 320 calculates the energy of the received signal in accordance with well-known communication techniques. Delay 940 and adders 945 and 950 calculate estimates of the in-phase and quadrature components of the received signal. The coherent integrator 960 accumulates each of the in-phase and quadrature components over a preselected period. Typically, the period corresponds to one symbol period. A squarer 965 determines the magnitude of each of the accumulated components. These amplitudes are called coherent sums. Channel summer 970 combines the coherent sums from the in-phase and quadrature channels. The non-coherent integrator 980 accumulates the combined coherent sums over an interval beginning and ending at the walsh code boundary to provide a non-coherent sum combination of sums 990. The incoherent sum 990 correlates with the net energy of the communication signal that is correlated or despread with the short PN code specific timing offset 440. The value of the incoherent sum 990 with the timing offset of the short PN code pair 440 corresponds to the timing or timing offset of the communication signal being acquired.

The incoherent sum 990 is compared to one or more thresholds (not shown) to establish a minimum energy level for determining proper signal correlation and timing alignment (alignment). When the incoherent sum 990 exceeds one or more thresholds, the timing offset of the short PN code pair 440 is a selected timing offset that is then used to track and demodulate the communication signal. If the incoherent sum 990 does not exceed the threshold, a new timing offset (i.e., another hypothesis) is tested and the above-described accumulation and threshold comparison operations are repeated.

Fig. 10 is a state diagram of the operation of one embodiment of access channel receiver 320. The state diagram includes a coarse search state 1010, a fine search state 1020, and a demodulate message state 1030.

The access channel receiver 320 begins operating in the coarse search state 1010 searching for the access probe 510. In the coarse search state 1010, the visited channel receiver 320 performs a coarse search. According to a preferred embodiment of the present invention, the coarse search includes a time search and a frequency search. The time search attempts to lock onto the short PN code pair 440 used in the access probe 510. In particular, the search attempts to determine the timing offset of the short PN code pair 440. Frequency searching attempts to solve the frequency uncertainty problem in the access probe 510.

The time search and the frequency search may be performed in series or in parallel. Since the expected timing uncertainty is greater than the frequency uncertainty, one embodiment of the present invention performs a parallel time search and a serial frequency search. This embodiment is particularly useful when the FHT930 is in access to the channel receiver 320. In this embodiment, rotator 920 increments the frequency by a predetermined amount based on the expected frequency uncertainty range. The FHT930 performs a parallel timing search of the short PN code pairs 440 at each frequency increment. The particular frequency increment and the particular timing of the short PN code pair 440 maximizes the output 990 of the non-coherent integrator 980. If the maximum output 980 exceeds a predetermined threshold, then the coarse search has detected an access probe 510. When this occurs, the particular frequency increment solves the frequency uncertainty problem, while the timing of the short PN code pair 440 partially solves the timing uncertainty problem.

If the maximum output 990 does not exceed a predetermined threshold, the coarse search does not detect the access probe 510. At this point, the access channel receiver 320 remains in the coarse search state 1010.

Upon detecting an access probe 510, the access channel receiver 320 changes from the coarse search state 1010 to the fine search state 1020. After changing from the coarse search state 1010 to the fine search state 1020, the access channel receiver 320 changes characteristics to acquire the long PN code 450. In particular, as is known, the operation of the memories 925, 935 and FHT930 is different for the long PN code 450 than for the short PN code pair 440. According to one embodiment of the invention, the memory 925, 935 is reconfigured to search for the long PN code 450. In another embodiment, a separate dedicated access channel receiver 320 is employed. Short code access channel receiver 320 is used to acquire short PN code pair 440 and long code access channel receiver 320 is used to acquire long PN code 450. In this embodiment, the memories 925, 935, and FHT930 are designed to capture the short PN code pair 440 or long PN code 450, respectively. In this embodiment, the short code access channel receiver 320 switches the timing of the short PN code pair 440 to the long code access channel receiver 320 during the transition from the coarse search state 1010 to the fine search state 1020.

In the fine search state 1020, the access channel receiver 320 performs a fine search. According to a preferred embodiment of the invention, the fine search comprises only a temporal search. The fine search attempts to lock onto the long PN code 450 used in the access probe 510. During the fine search, the particular frequency increment and timing of the short PN code pair 440 obtained during the coarse search state 1010 are used to fully resolve the timing uncertainty in the access probe 510.

A process similar to the coarse search described above is used to acquire or lock onto the long PN code 450. The particular timing of the long PN code 450 maximizes the output 990 of the non-coherent integrator 980. If the maximum output 990 exceeds a predetermined threshold, the fine search has captured an access probe 510. When this occurs, the particular timing of the long PN code 450 completely resolves the timing uncertainty.

If the maximum output 990 does not exceed a predetermined threshold, the fine search may not capture access probes. At this point, the access channel receiver 320 changes from the fine search state 1020 to the coarse search state 1010 in an attempt to detect the access probe 510.

After acquiring the access probe 510, the access channel receiver 320 changes from the fine search state 1020 to the demodulate message state 1030. During the demodulate message state 1030, the access channel receiver 320 demodulates the message 530 included in the access probe 510 with a particular frequency increment and timing obtained during the fine search state 1020.

If the output 990 falls below a predetermined threshold during the demodulate message state 1030, the access channel receiver 320 loses acquisition of the access probe 510. This will occur in a variety of situations, including completion of the access probe 510 transfer or some failure. Regardless of the cause, the access channel receiver 320 changes from the demodulate message state 1030 to the coarse search state 1010, attempting to detect the access probe 510.

X. conclusion

Although the invention has been described in detail with respect to specific embodiments thereof, various modifications can be made without departing from the scope of the invention. For example, the present invention is equally applicable to transmissions other than access channel transmissions spread with multiple code sequences.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims (21)

1. A system for wireless communication, comprising:

a transmitter for transmitting an access probe signal including a preamble and a message, the preamble having a first phase and a second phase, the first phase having data modulated by a first signal, the second phase having data modulated by a second signal and the first signal; and

a receiver for receiving said access probe, said receiver comprising a coarse searcher and a fine searcher, said coarse searcher being configured to determine a first timing offset of said first signal to said first stage of said preamble portion, and said fine searcher being configured to determine a second timing offset of said second signal to said second stage, and being based on said first timing offset.

2. The system of claim 1, wherein the first signal and the second signal are pseudo-noise sequences.

3. The system of claim 1, wherein the first signal and the second signal are coded sequences.

4. The system of claim 1, wherein the first signal is a pair of orthogonally spread pseudo-noise sequences.

5. The system of claim 1 wherein said second signal is a channelized pseudonoise sequence.

6. The system of claim 1, wherein the data of the first phase is zero data.

7. The system of claim 6, wherein the data of the second stage is zero data.

8. The system of claim 1, wherein the access probe signal enables a receiver to quickly determine a timing associated with the access probe signal, and wherein the access probe signal comprises:

a preamble having a first stage and a second stage, the first stage being modulated with a first code sequence and the second stage being modulated with the first code sequence and a second code sequence,

wherein the first stage is transmitted before the second stage such that a receiver can determine the timing of the first code sequence modulated on the first stage of the preamble portion before determining the timing of the second code sequence modulated on the preamble portion of the second stage.

9. The system of claim 8, wherein said access probe further comprises a message following said preamble, said message being modulated with said first code sequence and said second code sequence.

10. The system of claim 8 wherein said first code sequence is a pair of orthogonally spread pseudonoise sequences and said second code sequence is a channelized pseudonoise sequence.

11. A method of transmitting an access probe, the access probe including a preamble and a message, the preamble having a first stage and a second stage, the method comprising the steps of:

modulating said first stage of said preamble with a first signal;

transmitting the modulated preamble of the first stage;

modulating the second stage preamble with the first signal and a second signal;

after transmitting the modulated first phase preamble, transmitting the modulated second phase preamble;

modulating the message with the first signal and the second signal; and

after transmitting the modulated second stage preamble, transmitting the modulated message.

12. The method of claim 11, wherein the preamble of the modulated first stage is transmitted for a sufficient time for a receiver to acquire a first timing offset of the first signal.

13. The method of claim 12, wherein the modulated second stage preamble is transmitted for a sufficient time for a receiver to acquire a second timing offset of the second signal.

14. The method of claim 11, wherein the first signal is a pair of orthogonally spread pseudo-noise sequences.

15. The method of claim 11, wherein said second signal is a channelized pseudonoise sequence.

16. A method of acquiring a transmission from a transmitter at a receiver, the transmission having a preamble with a first stage and a second stage, the method comprising the steps of:

performing a coarse search of a transmitted signal received by the receiver during a first stage preamble, wherein the first stage preamble is modulated with a first signal, the coarse search determining a timing offset of the first signal;

performing a fine search of a transmitted signal received by the receiver during a second stage preamble, wherein the second stage preamble is modulated with the first signal and a second signal, the fine search to determine a timing offset of the second signal, wherein the timing offset of the second signal is determined with the first signal and the timing offset of the first signal; and

demodulating the transmission signal with the first signal, the second signal, the timing offset of the first signal, and the timing offset of the second signal.

17. The method of claim 16, wherein the first signal and the second signal are pseudo-noise sequences.

18. The method of claim 16, wherein said first signal is a pair of orthogonally spread pseudonoise sequences and said second signal is a channelized pseudonoise sequence.

19. The method of claim 16, wherein the first stage preamble is comprised of null data.

20. The method of claim 16, wherein the second stage preamble is comprised of null data.

21. A method for utilizing an access signal in a wireless communication system, comprising:

transmitting an access probe signal including a preamble and a message, the preamble having a first phase and a second phase, the first phase having data modulated with a first signal, the second phase having data modulated with a second signal and the first signal;

receiving the access probe signal;

determining a first timing offset of the first signal to the first stage of the preamble; and

a second timing offset of the second signal to the second stage is determined and is based on the first timing offset.