KR20010031969A - Access channel slot sharing - Google Patents

Abstract

The present invention is directed to a system and method for increasing user capacity on a slot random access channel using a multi-part access probe (502) in a spread spread communication system (100). The first and second portions 508, 510 of the access probe preamble 604 are modulated using a short PN code sequence 620 and the second portion and the remainder of the access probe 502 use a long PN code sequence. Is modulated. The information to be transmitted by the access probe 502 is modulated through the second portion 606 of the access probe 502 and the access probe is transmitted such that the first portion 508 of the probe preamble 604 receives an access channel slot ( Within the boundary of 402). In one embodiment, the time slot 402 of the access channel used to receive the access signal consists of the length of the first portion 508. In another embodiment, the time slots 402 of the plurality of adjacent access channels used for receiving access signals are longer than the first portion 508 but offset from each other by the length or period of the first portion.

Description

Share access channel slots {ACCESS CHANNEL SLOT SHARING}

Several multiple access communication systems and technologies have been developed for transmitting information to numerous system users. But. Spread spectrum modulation technology systems, such as code division multiple access (CDMA) communication systems, have great advantages over other modulation technologies, particularly when serving a large number of communication system users. Such technologies include U.S. Patent 4,901,307 entitled ″ Spectrum Spread Access Communication Systems Using Satellite or Terrestrial Repeaters ″, issued February 13, 1990, and U.S. Patent 5,691,974, titled November 25, 1997, ″ Individual Receive Phases. A method and apparatus for using the maximum spectrum transmitted to a spread spectrum communication system that tracks time and energy are disclosed herein.

The patents disclose a multiple access communication system in which a large number of mobile system users or remote system users use at least one transceiver to communicate with system users or users of other connected systems, such as public switched telephone networks. The transceiver communicates through a gateway and a satellite or terrestrial base station (also called a cell-site or cell).

The base station covers the cell, and the satellite has a coverage area (also called a "spot") on the earth's surface. In a system, the capacity gain is achieved by sectoring or dividing the covered terrain. The cell may be divided into ″ sectors ″ by using a directional antenna at the base station. Similarly, the satellite's coverage area can be divided into ″ beams ″ using a beamforming antenna system. These techniques of subdividing corner areas can be thought of as being separated using relative antenna directivity or spatial division multiplexing. Also, if there is available bandwidth, each of these partitions, sectors or beams can be allocated to multiple CDMA channels using frequency division multiplexing (FDM). In satellite systems, each CDMA channel is called a ″ sub-beam ″ because there are several for the ″ beam ″.

In a communication system using CDMA, a separate link is used to transmit a communication signal to a gateway or base station. The forward link issues a communication link from the base station or gateway to the user terminal, and the communication signal is generated at the gateway or base station and transmitted to the system user or users. The reverse link refers to a communication link from a user terminal to a gateway or base station, and a communication signal is generated at the user terminal and transmitted to the gateway or base station.

The reverse link consists of at least two separate channels (access channel and reverse traffic channel). In general, there are several access channels and reverse link traffic channels in a communication system. The access channels are separated in time and used for one or more user terminals to initiate or respond to communication from a gateway or base station. Each such communication process is called an access signal transmission or ″ access probe ″. The reverse traffic channel is used to transmit user and signaling information or data from a user terminal to one or more gateways or base stations during a ″ call ″ or communication link setup. One of the structures or protocols for access channels, messages and calls is described in detail in the Telecommunications Industry Association IS-96 standard entitled ″ Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular Systems ″, which is incorporated herein by reference.

In a typical spread spectrum communication system, one or more preselected pseudonoise (PN) code sequences are used to modulate or " spread " the user information signal over a predetermined spectrum band before being modulated onto a carrier for transmission as a communication signal. Known PN spreading and spread spectrum transmission methods form a transmission signal having a larger bandwidth than the data signal. In a communication link from a base station or gateway to a user terminal, a PN spreading code or binary sequence is used to distinguish between signals transmitted by different base stations or over different beams and between multipath signals. These codes are generally shared by all communication signals in a given cell or sub-beam. In some communication systems, the same set of PN spreading codes are used for the reverse link for the reverse traffic channel and the access channel. In another proposed communication system, the forward link and reverse link use different sets of PN spreading codes.

In general, PN spreading is achieved using a pair of pseudonoise (PN) code sequences to modulate or " spread " the information signal. In a technique generally called quadrature phase shift modulation, one PN code sequence is used to modulate the in-phase (I) channel, and the other PN code sequence is used to modulate the quadrature phase (Q) channel. PN spreading occurs before an information signal is modulated by a carrier signal and transmitted over a forward link from a gateway or base station to a user terminal as a communication signal. PN spreading codes are also called short PN codes because they are ″ short ″ relative to other PN codes used in communication systems. In general, the same set of PN spreading codes are shared by the forward link and reverse link traffic channels and another set of PN spreading codes is used for the access channel as described above.

Certain communication systems may use some short PN code length depending on whether the forward link or reverse link is used. In forward links, such as satellite systems, short PN codes are typically 2 ¹⁰ to 2 ¹⁵ chips long. These short PN codes are used to distinguish different signal sources such as gateways, satellites and base stations. In addition, timing offsets within certain short PN codes are used to distinguish beams of specific satellite or cell and sector of terrestrial systems.

In the proposed satellite communication system, the short PN code used for the reverse link has a length of about 2 ⁸ chips. These short PN codes are used to allow the gateway or base station receiver to quickly search for user terminals attempting to access the communication system without the complexity associated with the ″ long ″ short PN codes used in the forward link. In the description, ″ short PN code ″ refers to these short PN code sequences 2 ⁸ used in the reverse link.

Another PN code sequence, called a channelization code, is used to distinguish communication signals transmitted to different user terminals via reverse links in cells or sub-beams. PN channelization codes are also called long codes because they are ″ long ″ relative to other PN codes used in communication systems. Long PN codes are typically about 2 ⁴² chips long but can be short or obscured as needed. In general, an access message is modulated by a short PN code and by a long PN code before being sent to the gateway or base station as an access probe or signal. However, short PN codes and long PN codes may be combined before modulating or spreading the access message.

When the receiver of the gateway or base station receives the access probe, the receiver must despread the access probe to obtain the access message. This is accomplished by hypothesizing or guessing which long PN code and which short PN code pair were used to modulate the access message. The correlation between a given hypothesis and an access probe is generated to determine which hypothesis is the best estimate for the access probe. The hypothesis that forms the most optimal correlation generally associated with a given threshold is chosen as the hypothesis for the most similar code and timing match. Once the selected hypothesis is determined, the access probe is despread using the selected hypothesis to obtain an access message.

In a communication system with many users, more than one access probe can reach the gateway or base station at the same time or within a preselected time period during which a signal is detected. When this situation occurs, the access probes may collide or interfere with each other, making them unrecognized at the gateway or base station. One way to avoid such a collision is to use a centralized access technique, where the communication system manages the time of user terminal access probe transmission. One disadvantage of such techniques is that a large amount of access channel bandwidth is required for such time management mechanisms.

Another way to avoid such collisions is the slotted random access technique, such as the ″ slot ALOHA ″ technique. In slotted random access techniques, the general system-wide timing structure forms an acceptable transmission or reception time. An access channel is generally divided into a series of fixed length frames or time ″ slots ″ or windows, each having the same fixed period. Slots are used to receive signals. The access signal is generally configured as a ″ packet ″, which consists of a preamble and a message part, which must reach the beginning of the time slot being acquired. The user terminal transmits at its own choice but is limited to transmitting only within the range of a single slot for having received messages. The use of this technique on an access channel reduces the likelihood that access probes from multiple users will collide at the gateway or base station.

Unfortunately, slotted random access techniques also result in a significant amount of unused time on the access channel. Since the access probe must be transmitted within a single slot, the slot period should be chosen to exceed the period of the longest access probe possible. Since all slots have the same period, the slot is partially empty for all but the longest access probe. This wastes a significant amount of bandwidth on the access channel and reduces the user capacity of the access channel.

If it fails to capture the access probe during a particular frame period, the transmitter wishing to access should retransmit the access probe so that the receiver detects the probe again during the next frame. Multiple access signals that arrive together are ″ conflicting ″ and are not captured and require transmission again. In this case, the timing of the next access transmission when the initial attempt fails is based on the length of the time slot, and generally at least equal delay time for a random number of time slots or frames. Thus, a significant amount of time passes before the access probe is sent and received again. The delay length in probe acquisition is increased by the resetting acquisition circuitry of the receiver for scanning several hypotheses and the delays in other probes that were first captured. As a result, the access probe is never captured, at least within the actual time range, unless timing instability is resolved.

Accordingly, what is needed is a system and method for increasing user capacity on a slotted random access channel in a spread spectrum communication system. It is desirable to allow access probes to be received with minimal delay and efficiently.

The present invention relates to a multiple access spread spectrum communication system and network. In particular, the present invention relates to increasing user access capacity in a spread spectrum communication system.

1 illustrates an example of a wireless communication system constructed and operative in accordance with one embodiment of the present invention.

2 shows an example of a communication link used between a gateway and a user terminal in the communication system of FIG.

3 shows the structure of an access channel in detail.

4 is a timing diagram illustrating the general timing structure of an access probe in a typical slotted random access channel.

5 is a timing diagram for an access probe in a slotted random access technique according to a preferred embodiment of the present invention.

6 illustrates a protocol for generating an access probe in accordance with one embodiment of the present invention.

7 is a block diagram of an example of an access channel transmitter used to transmit an access probe in accordance with an embodiment of the present invention.

8 is a flowchart of an access channel transmitter according to an embodiment of the present invention.

9 is a block diagram of an example of an access channel receiver for receiving an access probe in accordance with an embodiment of the present invention.

The present invention provides a system and method for increasing user capacity on slotted random access technology in a spread spectrum communication system using multiple partial access probes. The present invention also has the advantage of reducing the delay in successful access after the initial access fails.

The present invention is implemented by a method and apparatus for transmitting multiple access signals over at least one access channel, the access signal comprising a preamble and a message portion, the preamble having a first and a second stage. . The access probe preamble does not contain message information but consists of null data.

The access signal is generated by modulating the first and second stages of the preamble into a first signal, by modulating the second stage of the preamble into a second signal and by modulating the message into the first and second signals. The access signal is transmitted in the form of a first stage, a second stage and a message. The access signal thus formed may be transmitted and received over an access channel divided into time slots such that the preamble falls within one of a plurality of preselected time slots. Thus, the signal can be captured even when one or more access signals are transmitted at one time so that the second stage or message overlaps the first stage of the one or more transmitted access signals.

In a preferred embodiment, the access signal may be transmitted and received over an access channel divided into signal receiving time slots having the same length as the first stage. Optionally, the access signal may be received via a plurality of access channels divided into signal receiving time slots having time offset from each other by a period of length equal to the first stage.

The first portion of the access probe is preferably formed by first modulating or spreading the access signal using a short PN code, wherein the short PN code is used to spread the second portion. In a preferred embodiment, the short PN codes are a pair of right angle PN codes. This spreading is generally accomplished using an apparatus that transmits a multi-part access probe, having first and second PN code modulators, data modulators, and transmitters.

The first PN code modulator spreads the first and second portions of an access probe with a given short PN sequence, and the second PN code spreads the first portion of an access probe with a long PN code. The transmitter sends an access probe so that the first portion belongs to one of the access channel slots.

An apparatus for receiving a multipart access probe includes a plurality of demodulators and a search receiver. The search receiver captures the first portion of the access probe and sends it to one of the demodulators for further processing of the probe (ie, the second portion). The search receiver may capture a first portion of another access probe, and the demodulator demodulates a second portion of the first access probe. This process can be repeated for a period of time and captures and hands off as many access probes as can be received, demodulated and captured.

Hereinafter, the present invention will be described with reference to the accompanying drawings.

The present invention provides a system and method for increasing user capacity on a slotted random access channel in a spread spectrum communication system by using multiple access probes. The present invention also reduces the delay in retransmission of unsuccessful access probes or signals. In one embodiment of the invention, the access probe is sent from the user terminal to a gateway or base station.

Although the present invention has been described based on specific embodiments, various modifications may be made without departing from the scope of the present invention. For example, the present invention is also suitable for transmissions other than access channel transmissions spread over multiple PN code sequences. In addition, the communication channel of the present invention is not limited to the wireless link but may also be used in wired, optical fiber cables, and the like.

In a typical CDMA communication system, a base station in a predetermined terrain or cell uses several spread spectrum modems or transmitter and receiver modules to provide a communication signal to system users in the service area. Each receiver module generally utilizes a digital spread spectrum data receiver, at least one search receiver and associated demodulator, and the like. During normal operation, a particular transmitter module and receiver module or single modem in a base station is assigned to a user terminal to accommodate transmission of communication signals between the base station and the user terminal. In some cases, multiple receiver modules or modems may be used to accommodate diversity signal processing.

In a communication system using a satellite, a transmitter module and a receiver module are generally located in a base station called a gateway, which communicates with a system user by transmitting a communication signal through the satellite. In addition, other related control centers in communication with the satellite or gateway exist to maintain system-wide traffic control and signal synchronization.

I. System overview

An example of a wireless communication system constructed and operating in accordance with the present invention is shown in FIG. Communication system 100 employs a spread spectrum modulation technique in communication with a user terminal (shown as user terminals 126 and 128). In a terrestrial system, communication system 100 communicates with a mobile station or user terminal 126, 128 using a base station (shown as base station 114, 116). Cellular phone type systems in large urban areas may have hundreds of base stations 114, 116 serving thousands of user terminals 126, 128.

In a satellite system, communication system 100 is configured to communicate with user terminals 126 and 128 using satellite repeaters (shown as satellites 118 and 120) and system gateways (shown as gateways 122 and 124). do. Gateways 122 and 124 transmit communication signals to user terminals 126 and 128 via satellites 118 and 120. Satellite systems generally use fewer satellite repeaters to serve more users on larger terrain in terrestrial systems.

The mobile station or user terminal 126, 128 has a wireless communication device, such as a cellular phone, data transceiver, or switching device (e.g., computer, personal digital assistant, facsimile), but may have other communication devices. . In general, each unit is portable or mounted in a vehicle. Although these user terminals are described as moving, the present invention can be applied to fixed units or other types of terminals requiring remote wireless service. The latter form of service is particularly suitable for using satellite repeaters to establish communication links in many remote areas of the world. A user terminal is also called a subscriber unit, mobile unit, mobile station or simply ″ user ″, ″ mobile ″ or ″ subscriber ″ in some communication systems depending on preference.

Examples of user terminals are described in U.S. Patent 5,691,974, U.S. Patent Application 08 / 673,830, ″ Pilot Signal Strength Control for Low Orbit Satellite Communications Systems ″ and U.S. Patent Application 08 / 723,725, ″ Accurate Positioning Using Two Low Orbit Satellites ″. Which are hereby incorporated by reference.

It may be contemplated to provide multiple beams within a ″ spot ″ in which satellites 118 and 120 are to cover separate terrain that is not overlapping. In general, multiple beams of different frequencies are also referred to as CDMA channels, ″ sub-beams ″ or FDM signals, frequency slots, or channels, which may allow overlapping the same area. However, beam coverage or service areas for antenna patterns for different satellite or terrestrial cell sites may overlap completely or partially in certain areas, depending on the communication system design and the type of service provided, and spatial diversity or Can be achieved between devices. For example, each may serve a different set of users of different terrain at different frequencies, and certain mobile units may utilize multiple frequencies and / or multiple service providers, each overlapping a geographic service area.

As shown in FIG. 1, the communication system 100 utilizes a system controller and a switch network 112, which is typically a (ground) command for a satellite system that communicates with a mobile switching center (MTSO) and a satellite at a ground station. Also called a control center (GOCC). Such controllers generally include an interface and processing circuitry that provides system-wide control to base stations 114 and 116 or gateways 122 and 124 through operations including PN code generation, allocation and timing. The controller 112 also controls the routing of communication links or telephone calls between the public switched telephone network (PSTN) and the base stations 114, 116 or gateways 122, 124 and the user terminals 126, 128. However, the PSTN interface generally forms part of each gateway for direct connection to such a communication network or link.

The communication link for connecting the controller 112 and various system base stations 114, 116 or gateways 122, 124 uses known techniques such as, for example, dedicated telephone lines, fiber optic links, and microwave or dedicated satellite communication links. Can be formed.

Although only two satellites are shown in FIG. 1, a communication system generally utilizes multiple satellites 118, 120 traveling in orbit. Several multi-satellite communication systems have been proposed, including the use of an array of low orbit (LEO) satellites serving a large number of user terminals. However, those skilled in the art will appreciate that the concepts of the present invention can be applied to both terrestrial and satellite systems.

In FIG. 1, some possible signal paths for the communication link between base stations 114, 116 and user terminals 126, 128 are shown by lines 130, 132, 134, 136. Arrows on these lines indicate the link signal direction, such as forward link or reverse link, and are provided for illustration only and do not limit the actual signal pattern.

In a similar manner, the signal path for the communication link between the gateways 122, 124, the satellite repeaters 118, 120, and the user terminals 126, 128 is the line 146, 148, 150, 152 from the gateway to the satellite link. And lines 140, 142, 144 from satellite to user links. In some configurations, it is possible to form a satellite link at the satellite directly as illustrated by line 154.

As will be apparent to those skilled in the art, the present invention is suitable for both terrestrial or satellite systems. Thus, gateways 122 and 124 and base stations 114 and 116 are collectively represented by gateway 122 for simplicity. The terms base station and gateway are also used interchangeably, where a gateway is recognized as a special base station that communicates directly via a satellite. Similarly, satellites 118 and 120 are collectively represented as satellite 118 and user terminals 126 and 128 are collectively represented as user terminal 126.

II. Communication link

2 illustrates an implementation of a communication link used between gateway 122 and user terminal 126 in communication system 100. Two links are used in the communication system 100 to facilitate the transmission of communication signals between the gateway 122 and the user terminal 126. These links are referred to as forward link 210 and reverse link 220. The forward link 210 processes the transmission signal 215 transmitted from the gateway 122 to the user terminal 126. The reverse link 220 processes the transmission signal 225 transmitted from the user terminal 126 to the gateway 122.

The forward link 210 includes a forward link transmitter 212 and a forward link receiver 218. In one embodiment, the forward link transmitter 212 is implemented at the gateway 122 in accordance with the known CDMA communication technology disclosed in the patent. In one embodiment, forward link receiver 218 is implemented in user terminal 126 in accordance with known CDMA communication techniques as disclosed in the patent.

Reverse link 220 includes reverse link transmitter 222 and reverse link receiver 228. In one embodiment, the reverse link transmitter 222 is implemented in the user terminal 126. In one embodiment, the reverse link receiver 228 is implemented in the gateway 122.

As noted above, reverse link 220 utilizes at least two channels, including one or more access channels and one or more reverse traffic channels. These channels may be performed by separate receivers or the same receiver operating in distinct mode. As mentioned above, the access channel is used by the user terminal 126 to initiate or respond to communication with the gateway 122. A separate access channel is required at some time for each active user. In particular, the access channel is shared in time by several user terminals 126 such that transmissions from each active user are separated in time from each other. The structure of the access channel and signal is described in detail below.

The system may utilize one or more access channels depending on known factors such as moderate levels of gateway complexity and access timing. In a preferred embodiment, one to eight access channels are used per frequency. In a preferred embodiment, different sets of PN spreading codes are used between the reverse traffic channel and the access channel. In addition, the access channel may use a very short PN code selected from a dedicated code set (or code generator) assigned only for use of the access channel through the communication system 100. This technique provides a very effective mechanism for quickly capturing access signals at gateways where signal delays and Doppler and other known phenomena exist.

III. Access channel

3 illustrates the access channel 300 in detail. The access channel 300 includes an access channel transmitter 310, an access channel receiver 320 and an access signal or probe 330. The access channel transmitter 310 may be included in the reverse link transmitter 322. Access channel receiver 320 may be included in reverse link receiver 328.

Access channel 300 is used for short signaling message exchanges, including call initiation, page response, and registration from user terminal 126 to gateway 122. In order for the user terminal 126 to initiate or respond to communication with the gateway 122 via the access channel 300, a signal called an access probe 330 is transmitted.

The access channel is also associated with one or more specific paging channels used in the communication system. This allows for a more efficient response to the paging message in terms of the system knowing where to request the user terminal access transmission in response to the page. Correlations or assignments are known based on a fixed system design and can be presented to the user terminal within the structure of the paging message.

Ⅳ. Timing Uncertainty in Access Probes

The timing uncertainty of the access probe 330 is caused by a change in the distance or propagation path length between the user terminal 126 and the satellite 118 by the satellite 118 orbit around the station. This timing uncertainty is bounded by the minimum propagation delay and the maximum propagation delay. The minimum propagation delay is the amount of time required for signals transmitted from the user terminal 126 to the satellite 118 (and gateway) when the satellite 118 is directly above the user terminal 126. The maximum propagation path is the amount of time required for a signal from user terminal 126 to satellite 118 when satellite 118 is located at a predetermined available limit of user terminal 126. The overall delay is affected by the gateway's position with respect to the satellite and can change the satellite location where the maximum or minimum occurs. Similarly, some time uncertainty may arise with respect to relative movement between the user terminal and the base station 114, 116 or other signal source, but this is small.

This timing uncertainty must be addressed to capture the access probe 330. In particular, the PN code phase and timing, ie the start time of the PN code sequence, must be known to despread the long and short PN codes used in forming the access probe 330. This is done by correlating the various timing (appropriate code) hypotheses with the access probe 330 to determine which timing hypothesis is the best estimate for capturing the access probe 330. The timing hypothesis is a time (and frequency for the Doppler effect) offset and represents various estimates of the PN code used to generate the timing or access signal of the access probe 330. Hypotheses that show the highest correlation with access probes 330, generally those that exceed certain correlation thresholds, are the most likely estimates of timing (" accurate " or appropriate) for use with the particular access probe 330. It is a hypothesis with If timing uncertainty is resolved in this manner, the access probe 330 may be despread using timing and long PN codes and short PN codes obtained according to known techniques.

Ⅴ. System Timing for Access Probe Transmission

A common access technique for access signals is slot type random access known as ″ slot ALOHA ″. According to this technique, the communication system 100 forms a regular timing structure on the access channel to coordinate access probe transmissions. 4 is a timing diagram illustrating a general timing structure for an access signal or probe of a conventional slotted random access channel 400. Channel 400 includes an access slot 402, a boundary 404, a guard band 406, and an access probe 408. Channel 400 is divided into blocks of the same period of time known as access slot 402 with boundary 404. In a preferred embodiment, each access slot 402 includes a leading guard band 406A and a trailing guard band 406B to accommodate the timing uncertainties described above.

If the user terminal wants to access the communication system 100, ie to initiate or respond to communication, the user terminal sends an access signal or probe 408 to the gateway 122. A typical access probe 408 includes an access preamble and an access message and is sent by the access channel transmitter 310 of the user terminal 126 to the channel receiver 320 of the gateway 122. In a typical spread spectrum system, the preamble and access messages are spread at right angles to a pair of PN codes and channelized by long PN codes. The preamble generally contains null data, i.e. all ″ 1 ″ or mode ″ 0 ″ or preselected ″ 1 ″ and ″ 0 ″ patterns. The preamble is first sent to provide the access channel receiver with an opportunity to capture the access probe 408 before the access message is sent. When the access channel receiver 320 receives the preamble, the access channel receiver 320 must despread it using a short PN code pair and a long PN code pair. If the short PN code and the long PN code are determined by the access channel receiver 320, the access probe is considered captured. After the preamble has been sent for a predetermined time period, the access message is sent by the access channel transmitter 310. The access message is spread using the same short PN code pair and long PN code used to spread the preamble.

The preamble must be long enough that the access channel receiver 320 has time to process the hypothesis and capture the access probe before the access message is sent. Otherwise, the access channel receiver 320 will continue to attempt to capture the access probe while the access message is being sent. In this case, the access message is not properly received. The time required to acquire an access probe, called acquisition time, varies depending on how many receivers are used simultaneously to process the hypothesis, how many different code sequences exist in the timing uncertainty range in signal transmission, and so on. In addition, the repetition length and frequency of the preamble are selected to minimize collisions between access probes transmitted by different user terminals. Each of these factors should be considered in system design when determining the length of the preamble.

In conventional designs, access probes interfere with each other when transmitted simultaneously. Thus, only one conventional access probe can be successfully received in one access slot on a slotted random access channel. Because an access slot is not reserved for a particular user, the user can transmit in any access slot. Thus, the user waits for a response signal from the receiver before sending another message. If no response signal is received after a predetermined period, the user retransmits the access message, considering that the access probe has collided with another user's access probe or simply has not been received.

The access slot period (small guard band) of a typical slotted random access channel is chosen to exceed the length of the most likely access probe. A typical access probe is sent to be completely within one access slot 402. This arrangement reduces the collision to some extent. However, this arrangement also ensures that a significant amount of access channel 400 is not used. Since adding a communication channel is expensive, it is desirable to minimize the unused portion on the communication channel, especially the portion used for access to the system or setup communication link.

5 is a timing diagram for an access probe in a slotted random access channel according to a preferred embodiment of the present invention. In FIG. 5, a conventional access probe 408 is replaced with a multi-part access probe 502 in accordance with the present invention. Such multi-part access probes are co-pending and are disclosed in ″ Fast Signal Acquisition and Synchronization for Access Transmissions, filed June 16, 1998, which is incorporated herein by reference in US Patent Application 09 / 098,631. As described below, such multi-part access probes may be partially overlapped in certain circumstances. This technique not only significantly reduces the unused portion of the access channel 400, but simultaneously allows multiple access probes 502 to share the access channel 400 for at least a predetermined period. The basic difference between the present invention and the conventional protocol 400 is that the preamble is first spread with only short PN code pairs and then with short PN codes and long PN codes. This allows the access channel receiver 320 to resolve timing uncertainty using only a short PN code pair 440. In contrast, conventional protocol 400 requires the use of short PN code pair 440 and long PN code 450 to resolve timing uncertainty.

Ⅵ. Protocol for transmitting an access probe according to the present invention

6 illustrates a protocol or process structure 600 for generating an access probe 502 in accordance with one embodiment of the present invention. In protocol 600, access probe 502 includes an access probe preamble 604 and an access probe message 606. According to the present invention, the preamble 604 is transmitted in two stages (the first stage 508 and the second stage 510). The access message 606 is sent in a single message stage 512. Stages 508, 510, 512 are grouped into two parts (first part 504 and second part 506) for modulation. The first portion 504 includes a first stage 508 and is spread with a short PN code 602. The second portion 506 includes a second stage 510 and a message stage 512 and is spread with a long PN code 622. In the preferred embodiment, short PN codes 602 are orthogonal PN code pairs and are used to spread the signal using known techniques. In one embodiment, the PN code sequence for spreading the Q channel may be delayed than the PN code sequence for spreading the I channel, but a separate code is preferred.

In the first stage 508, the preamble 604 of the access probe 502 is short PN code 620 for a length of time sufficient for the access channel receiver 320 to determine the timing of the short PN code 620. By spreading. The preamble 604 can include a predetermined bit pattern that facilitates the acquisition of the access probe 502. In a preferred embodiment, the bit pattern for the preamble is null data, such as all 1's, all 0's bit patterns or preselected ″ 1 ″ and ″ 0 ″ patterns. For fast acquisition of the access probe 502 by the gateway 122, the long PN code 632 is not used to spread the first stage 508.

In the second stage 510, the preamble 604 of the access probe 502 is spread by a short PN code as in the first stage 508. The preamble 604 is spread by the long code 622 to facilitate synchronization of the long code by the gateway 122. When user terminal 126 attempts to access over a particular access channel, long code 622 generates a pseudo orthogonal PN code including the mask associated with that access channel. The gateway uses the same mask to demodulate the signal for that particular access channel. At the end of the second stage 510, the access channel receiver 320 must capture the access probe 502.

The access message may be encoded in a manner similar to data on a general traffic channel that is M-array modulated using an orthogonal code set such as a Walsh function. The data can also be modulated using a single Walsh function, but timing uncertainty generally works the opposite way.

In an alternative embodiment, during message stage 512, message data is modulated by one or more orthogonal codes selected from an orthogonal code set, spread by short code 620, and spread by long code 622. An example of an orthogonal PN code set is disclosed in co-pending US patent application 08 / 627,831 ″ Using Orthogonal Waveforms to Share a Single CDMA Channel (PA208) ″, which is incorporated herein by reference.

Two access probes 502 generated using protocol 600 may collide or interfere with each other in some environment. For example, two signals modulated with the same short PN code 620 interfere with each other if the difference in arrival time in the access channel receiver 320 is less than 256 chips in a 1/2 chip module. Thus, two access probes 520 may collide if their first stage 508 is transmitted and received within the same access slot 402.

In addition, two signals modulated with the same short PN code 620 and the same long code 622 may interfere with each other in certain circumstances. In particular, two signals modulated by the same short PN code 620 and the same long code 622 interfere with each other when the difference in arrival time in the access channel receiver 320 is less than 256 chips in a 1/2 chip module. Thus, the two access probes 520 may interfere with each other once the second stage 510 is transmitted and received within the same access slot 402.

However, only signals modulated with short PN code 620 do not collide with signals modulated with long PN code 622. Thus, the first stage 508 of one access probe may occupy the same access slot 402 as the second stage 510 and / or message stage 512 of the other access probe.

Also, a signal modulated with one orthogonal code (when used) does not collide with a signal modulated with another orthogonal code selected from another identical orthogonal PN code set. Thus, the message stage 512 of one access probe can occupy the same access slot 402 as the message stage 512 of another access probe.

Thus, in accordance with the present invention, the access probe 502 may share the access slot 402 or portions thereof. Thus, when slotted random access techniques are taken for the first stage 508 of each access probe 502 and the arrival times of the second stage of the access probe 502 do not match as described above, FIG. 6. Communication signals modulated in accordance with the protocol of P may overlap partially, which is shown in FIG. This causes wasted and unavailable slot time to be used. Thus, the present invention effectively uses communication channels.

Also, the length of each of the access slots is generally the sum of the lengths of the respective portions of the access signal (i.e., the sum of the guard band (if used) to the preamble and the message portion) (stage 508 + stage 508 + 512). Is defined as)). This gives the number of slots in the given time period available. The number of available access channels over a given frequency is limited by the number of short PN codes. This provides the number of time slots that a user may wish to access communication system 100. However, according to the present invention, the number of access channels is efficiently increased.

For example, the fact that parts or stages of an access probe can overlap can be used to create multiple access channels. That is, an access channel can be formed based on or using a short PN code whose timing structure is shifted by a preselected time period applied or used in the first portion of the preamble (only short PN codes). The channels use the same short PN codes that are shifted from one another so that multiple portions of adjacent access signals or probes that may be received do not collide. The access probe can be received on one channel, while the other channel uses the same short PN code but receives another access probe with a length or long time offset of the first preamble stage so that the two signals do not collide. Receipt of the second preamble stage and message portion does not cause a collision in this technique, and these portions need not be considered directly when forming the channel offset. The receiver may form a channel according to the time shifted PN code used for hypothesis in the signal acquisition and demodulation process. It is estimated that at least twice the channel can be formed in the same frequency space, depending on the length of time used for the time offset and the desired guard band to ensure preamble reception.

However, the preferred embodiment of the present invention can optionally be reduced to the total (fixed) length of each slot by adding the guard band or extra time to the short PN code period required for system performance. Since the access probe does not collide except over a short time period when the same short PN code is used, long time slots are not necessary to separate, capture and demodulate the access signal. This allows a large number of access slots (called channels in some systems) per channel to be efficiently created on the access channel or frequency. This technique increases access channel capacity and facilitates access without increasing the complexity of the hardware or control system used to create and monitor the access channel.

채널 .Access channel transmitter

7 is an example of a circuit block diagram for an access channel transmitter 310 transmitting an access probe 502 in accordance with the protocol or signal structure of FIG. 6. The access channel transmitter 310 includes a data modulator 702, a PN code modulator 704, a transmitter 706, and an antenna 708.

8 is a flowchart for explaining the operation of the circuit of FIG. In step 802, the data modulator 702 modulates a carrier signal (baseband) (not shown) of a conventional design into an access message so that the message stage 512 of the second portion 506 of the access probe 502. To form. In step 804, the PN code modulator 704A modulates a portion of the signal generated by the data modulator 702 using the long PN code 622 to open the second portion 504 of the access probe 502. To create it. In step 806, the PN code modulator 704B modulates the first portion 504 and the second portion 506 of the signal generated by the PN code modulator 704A using the short PN code 620. . In step 808, the transmitter 706 transmits the access probe 502 through the antenna 708 so that the first portion 504 of the access probe 502 completely belongs to one access slot 402. .

Iii. Access channel receiver

9 is an example of a circuit block diagram of an access channel receiver 320 for receiving an access probe 502 in accordance with the protocol of FIG. 6. Access channel receiver 320 includes searcher 902, demodulators 904A- 904N, and antenna 908. The two stage architecture of the access channel receiver 320 is a concept of processing the multipart access probe of the present invention in a pipelined manner as described below.

In operation, searcher 902 vomits antenna 908 to receive access probe 502 and to capture preamble 604. The preamble 604 is captured by capturing the short PN code 620 and the long PN code as described above and despreading the access probe 502. When searcher 902 captures preamble 604, searcher 902 sends the despread access probe to one of demodulator 904. Demodulator 904 demodulates the despread access probe to obtain access message 606.

Since the preamble 604 and the access message 606 are obtained by separate functional units, they occur simultaneously for different access probes. That is, in particular, demodulator 902 can demodulate the access message of one access probe and searcher 902 captures the preamble of another access probe. This arrangement is suitable for more efficient use of the overlap of multiple partial access probes in accordance with the present invention. As mentioned above, since an unsuccessfully received access signal can be transmitted again before the entire normal access period has elapsed, even an unacquired signal or a failed access signal can access the communication system more efficiently. Also, if there is an additional offset access channel in place or if a short time slot is used, the likelihood of not being acquired decreases with the time to retransmit and acquire the access signal.

Iii. conclusion

The above description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. While the invention has been shown and described with reference to particularly preferred embodiments, those skilled in the art can make various changes without departing from the spirit and scope of the invention. For example, the present invention is equally suitable for transmissions other than access channel communications that are spread over multiple code sequences.

Claims (29)

10. A system for transmitting multiple partial access probes with access messages each over a slotted random access communication channel having multiple access channel slots. A first modulator for modulating the first and second portions of the access probe into a short pseudo noise sequence; A second modulator for modulating the first and second portions of the access probe into a long pseudo noise sequence; A data modulator for modulating the second portion into the access message; And And a transmitter for transmitting the access probe such that the first portion belongs to one of the access channel slots. 10. The system of claim 1 wherein the short PN sequence is 2 ⁸ chips long. 2. The system of claim 1 wherein the length of the long PN sequence is 2 ⁴² chips. 2. The system of claim 1, wherein the short PN sequence is a right pair of short pseudonoise sequence pairs. The method of claim 1, wherein each access channel slot has a first guard band and a second guard band, And said transmitting means comprises means for transmitting said access probe such that said first portion falls within one of the access channel slots between said first and second guard bands. A system for receiving multiple partial access probes over a slotted random access channel having multiple access channel slots, the system comprising: Each access probe has a first portion modulated with a short pseudonoise sequence and a second portion modulated with the short pseudonoise sequence and a long pseudonoise sequence, the system comprising: A plurality of demodulators for demodulating the access probe; And And a search receiver for capturing and despreading the access probe and for delivering the despread access probe to one of the plurality of demodulators. 7. The system of claim 6 wherein the slotted random access communication is a slotted ALOHA channel. A method of transmitting a multi-part access probe with an access message on a slot-type random access communication channel having a plurality of access channel slots, the method comprising: Modulating a first portion and a second portion of the access probe into a short pseudo noise sequence; Modulating a first portion and a second portion of the access probe into a long pseudonoise sequence; Modulating the second portion into the access message; And Transmitting the access probe such that the first portion belongs to one of the access channel slots. 10. The method of claim 8 wherein the short PN sequence is 2 ⁸ chips long. 9. The method of claim 8 wherein the length of the long PN sequence is 2 ⁴² chips. 10. The method of claim 8, wherein the short PN sequence is a right pair of short pseudonoise sequence pairs. 9. The apparatus of claim 8, wherein each of the access channel slots has a first guard band and a second guard band. And the method further comprises transmitting the access probe such that the first portion is within one of the access channel slots between the first and second guard bands. A method for transmitting a plurality of access signals, each having a preamble and a message portion, each having a first stage and a second stage, over at least one access channel, wherein: Modulating a first stage and a second stage of the preamble with a first signal; Modulating a second stage of the preamble with a second signal; Modulating the message with the first and second signals; And Transmitting the access signal in the form of a first stage, a second stage and a message such that the preamble belongs to one of a plurality of preselected time slots having a length corresponding to the first stage. A plurality of access signal transmission method. 14. The method of claim 13, wherein the one or more access signals are transmitted in time such that the second stage or message portion overlaps with the first stage of the one or more other transmitted access signals. 14. The method of claim 13, comprising guard bands forming a boundary for the preselected time slots. 14. The method of claim 13, wherein said modulated first stage of said preamble is transmitted for a time sufficient for a receiver to capture timing of said first stage. 17. The method of claim 16, wherein the modulated second stage of the preamble is transmitted for a time sufficient for the receiver to capture timing of the second signal. 14. The method of claim 13, wherein the first signal is a right-angle spread pseudo noise sequence pair. 19. The method of claim 18, wherein the second signal is a channelized pseudo noise sequence. 20. The method of claim 19, wherein the access channel includes a message following the preamble, wherein the message is modulated by the first code sequence and the second code sequence. A method of using an access signal in a wireless communication system, Transmitting an access signal comprising a preamble and a message having a first length and a second stage modulated by the first signal and a second stage having data modulated by the first signal and having a first predetermined length; step; And And receiving the access signal through an access channel divided into a signal receiving time slot having the same length as the first stage. 22. The method of claim 21, wherein the first stage of the preamble consists of null data. 22. The method of claim 21, wherein the second stage of the preamble consists of null data. 22. The method of claim 21, wherein the first and second signals are PN sequences. A method of using an access signal in a wireless communication system, Transmitting an access signal comprising a preamble and a message having a first length and a second stage modulated by the first signal and a second stage having data modulated by the first signal and having a first predetermined length; step; And And receiving the access signal through a plurality of access channels divided into signal receiving time slots which are time offset from each other by a period of the same length as the first stage. 27. The method of claim 25, wherein the first stage of the preamble consists of null data. 27. The method of claim 25, wherein the second stage of the preamble consists of null data. 27. The method of claim 25, comprising a guard band forming a boundary for the signal reception time slot. A method for acquiring at a receiver a transmission having a preamble having a first stage and a second stage from a transmitter, the method comprising: Performing a coarse search on a transmission received by a receiver to determine a timing offset of the first signal during a first stage of the preamble modulated by a first signal; By the receiver to determine a timing offset of a second signal determined using the timing offset of the first signal and the first signal during a second stage of the preamble modulated by the first signal and the second signal Performing a fine search for the received transmission; And And demodulating the transmission using the first signal, the second signal, the timing offset of the first signal, and the timing offset of the second signal.