HK1102070A - Access channel with constrained arrival times - Google Patents

Description

Access channel with constrained arrival time

Requesting priority according to 35U.S.C.119

This patent application claims priority from provisional application No.60/551,689, entitled CDMA-ALOHA RANDOM ACCESS CHANNEL with connected ARRIVAL TIMES, filed 3, 9, 2004, which is assigned to the assignee of the present application and is hereby expressly incorporated by reference.

Technical Field

The present invention relates generally to the field of electronic communications. In particular, the present invention relates to the field of configuring and interacting with access channels in a communication system.

Technical Field

In the last decade, many cellular communication standards have chosen between orthogonal and non-orthogonal Code Division Multiple Access (CDMA) physical layer interfaces.

In the case of many-to-one communication, where multiple users attempt to send information to a central receiver, non-orthogonal CDMA is an alternative. The reverse links of cdma2000 and WCDMA are good examples thereof.

A basic feature of non-orthogonal CDMA channels is that they are self-interference limited. The degradation of the communication between the user and the central receiver is mainly due to the simultaneous access of the channel by other users in the system on the same frequency band. Each parallel transmitter can only be distinguished by the code it uses. Furthermore, in order for the system to operate, the energy present on the medium due to the transmission of other users needs to have substantially the same statistical properties as white noise. It is because of this randomness that multiple users can successfully transmit information at the same time and in the same frequency band as long as the number of concurrent users does not exceed some maximum value N. Typically, each user is assigned a different transmission code by the central entity. The nature of these codes ensures the desired characteristics of the interference.

In a circuit-switched CDMA channel, such as the reverse link of CDMA2000, the number of users U actually present in the system is of the same order of magnitude as the maximum number of concurrent users N allowed for successful transmission. This connection-oriented configuration is well suited for applications with steady traffic requirements, such as voice. For example, a typical speech coder produces 192 bits every 20 milliseconds. Further, the transmission of frames is arranged as follows: once the receiver has acquired a particular user, it knows exactly the expected time of the next information frame. Conceptually, the receiver consists of U parallel receivers, each operating according to one of a plurality of codes. For a typical cdma2000 configuration, U is approximately 60, which can be implemented in a relatively low complexity receiver.

For different types of user services, such as browsing web pages, the channel usage of each user is more sporadic, so that the total number U of users that can be effectively supported by the system is much larger than the allowed number N of concurrent transmissions. Some systems have been proposed, where N is 30 and U15000. Furthermore, the sporadic nature of the traffic is reminiscent of connectionless Aloha-type access protocols. In an Aloha-based access channel, each user can access the channel as long as there is data to transmit. If multiple users attempt to access the same channel space at the same time, a collision may occur and the transmissions of both parties may be unsuccessful.

In an Aloha-based access channel, the arrival time of the information frame is unknown at the receiver and the probability distribution is flat in time. This adds an extra dimension (arrival time) to the demodulator complexity by requiring each possible transmission code to be detected continuously for the arrival of a packet. In fact, when the signal arrival time is unknown, it is much more complicated to demodulate a signal transmitted with a given code. The individual demodulators required for the CDMA-Aloha channel are orders of magnitude more complex than the connection-oriented protocols described above.

In principle one would not want to allocate 15000 different codes and have 15000 different parallel demodulators in terms of receiver complexity. One possible solution is to use a smaller set of codes C < U, from which the user can randomly choose one each time they want to start a transmission. Limiting the number of access codes increases the likelihood of collisions occurring.

Two different transmitters that use the same code and arrive at the receiver at the same time do not randomly interfere with each other when concurrent transmission is allowed. The mixing of information symbols on the same code may cause the loss of both packets at the same time and frequency band. This can be solved by using a set C of codes that is large enough to make collisions highly unlikely. However, the receiver complexity increases as the number of available codes C increases.

In a communication system, access to channel configurations and protocols is required, allowing a large number of active intermittent users, while reducing the likelihood of collisions between data transmissions from different users, and maintaining or reducing the complexity of the associated receivers.

Disclosure of Invention

Systems, methods, and apparatus for configuring and accessing a random access channel in a CDMA communication system are disclosed. The number of users supported by the random access channel may be optimized by assigning a different arrival time to each of the plurality of users. The different arrival times of different users can be as small as a single chip.

Each user may be time synchronized and transmit data at a time that is compensated for propagation delay so that the data arrives at the intended receiver at the assigned time. In a CDMA system, each user may transmit data spread with the same spreading code, provided that the cross-correlation of the codes is sufficient to identify a source that is time-offset with respect to another user. Alternatively, a user may be assigned a code from a table of predetermined code sequences. The time of arrival may be determined based on the number of active users and may be assigned each time each user makes each transmission.

A receiver receiving time constrained transmissions from a plurality of users is able to change the search space for each of a plurality of active users to a predetermined spreading code and a predetermined time window corresponding to that user. Assigning arrival times reduces receiver complexity while allowing the system to support more users than can be supported by a random access channel (e.g., Aloha) using unrestricted channel access.

The present application includes a method of allocating channel access. The method comprises the following steps: determining a transmission cycle timing sequence; determining an arrival time allocated to one of a plurality of active user terminals during the transmission period; transmitting the arrival time to the user terminal at the beginning of the arrival time, thereby allocating the channel to the user terminal.

The present application also includes a method of allocating channel access. The method comprises the following steps: receiving a request from a user terminal to access the channel; synchronizing a time reference with the user terminal; determining a transmission period having a duration proportional to a CDMA chip duration; determining an arrival time occurring at a chip boundary within the transmission period; transmitting the arrival time to the user terminal at the beginning of the arrival time, thereby allocating the channel to the user terminal.

The present application also encompasses a method for transmitting data in a channel. The method comprises the following steps: requesting access to the channel; receiving a time-of-arrival allocation result in response to the request; transmitting a data block at a time offset from the arrival time such that an initial portion of the data block arrives at a receiver at the assigned arrival time.

The present application also encompasses a method for receiving data in a channel. The method comprises the following steps: determining an arrival time allocated to a user terminal during a transmission period; receiving transmissions from a plurality of active user terminals; searching for transmissions from the user terminal within a time window that includes the arrival time; a data block is received from the user terminal.

The present application also includes an apparatus operating on a constrained arrival time channel. The device comprises: a data buffer storing a data block; a data modulator coupled to the data buffer. A data modulator direct sequence spreads the data within the data block with a code to produce modulated data. The device further comprises: a transmitter which receives the modulated data from the data modulator and selectively transmits the modulated data; a transmit timing module coupled to the transmitter that receives the time of arrival assignment and controls the transmitter to transmit the modulated data at a time offset from the time of arrival assignment such that the transmitted data initially arrives at the receiver substantially at the time of arrival assignment.

The present application also includes an apparatus operating on a constrained arrival time channel. The device comprises: a transmission cycle module that determines a transmission cycle timing; a time boundary module that determines an arrival time allocated to a user terminal during the transmission period; a receiver that receives a plurality of transmissions from a plurality of active user terminals and searches for the plurality of transmissions within a time window that includes the arrival times of transmissions from the user terminals.

Drawings

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a functional block diagram of one embodiment of a wireless communication system implementing a time-constrained access channel of the present invention;

FIGS. 2A-2B are timing diagrams of an embodiment of an Aloha random access channel and a time-bounded access channel in accordance with an embodiment of the invention;

FIG. 3 is a functional block diagram of one embodiment of a base station that manages the time-constrained access channel of the present invention;

FIG. 4 is a functional block diagram of one embodiment of a user terminal interacting with the time-constrained access channel of the present invention;

FIG. 5 is a flow diagram of one embodiment of a process for allocating channels;

FIG. 6 is a flow diagram of one embodiment of a process of operating in a constrained arrival time channel; and

figure 7 is a flow diagram of one embodiment of a process for channel-by-channel signaling from a constrained arrival time.

Detailed Description

A wireless communication system with constrained time of arrival, which is a device operating on an access channel, and a method of interacting with the access channel are disclosed. A wireless communication system can implement an access channel in which a transmission from a particular user terminal is constrained to a predetermined time of arrival.

The arrival time may be selected from a plurality of predetermined arrival time boundaries and may be determined based in part on the number of active users on the channel. For example, the communication system assigns arrival times to particular user terminals modulo the number of active users on the channel. In another embodiment, the communication system assigns the arrival time modulo a predetermined value to a particular user terminal. In other embodiments, the communication system may also randomize the arrival time assigned to each of the plurality of users. This randomization may occur at each transmission or may occur over multiple transmissions or a period of time. The communication system may transmit the time of arrival for a particular user before each time interval or at other time intervals, which may be based on the manner in which the communication system determines the time of arrival.

Initially, a user terminal may contact a base station to establish an active session on an overhead channel that may be configured as a random access channel by communicating on the channel. For each active communication session, the user terminal may access the overhead channel a limited number of times, e.g., for initial establishment and termination of the communication session. The random access channel may comprise the same frequency band as the restricted arrival time channel. However, the user terminal is typically not time synchronized with the base station before establishing communication with the base station. Alternatively, the random access channel may be located on a frequency band that partially overlaps or is distinct from the restricted arrival time channel. Because the user terminal communicates on the overhead channel a relatively small number of times, there is a low probability of collision with a transmission from another user terminal. The user terminal may synchronize a time reference with the communication system, establish an active communication session on an overhead channel, and assign an arrival time thereto for transmission on a time constrained channel.

The time of arrival of the user transmission is constrained, simplifying the configuration of the receiver. At each point in time of arrival (time epoch), the receiver already knows which user terminal of a limited number of active user terminals is assigned to that point in time of arrival. The receiver may search a predetermined time window and correlation code for the CDMA system. The number of these codes is significantly reduced relative to the number of codes required for a non-orthogonal CDMA random access channel and can be reduced to the use of only one code by all users.

Fig. 1 is a functional block diagram of one embodiment of a wireless communication system 100 implementing a time-constrained access channel. System 100 includes one or more fixed elements that may communicate with one or more user terminals 110a-110 n. User terminals, such as 110a, may communicate using different communication protocols on the forward link and the reverse link. The forward link refers to the communication link from base station 120b to user terminal 110 a. The reverse link refers to the communication link from the user terminals, e.g., 110a, to the base station 120 b. The user terminal 110 may be a portable unit, a mobile unit or a stationary unit. User terminal 110 is also referred to as a mobile station, mobile unit, mobile terminal, user equipment, portable device, telephone, or the like.

Although only two user terminals 110a-110N are shown in the wireless communication system 100, the wireless communication system 100 is capable of supporting a first number N of concurrent transmissions and a second number U of active users transmitting sporadically to the base station 120 b. For simplicity, the following description is for a particular user terminal 110 a. It should be understood that the description applies equally to all user terminals 110a-110n within the coverage area of the wireless communication system 100.

In one embodiment, user terminal 110a communicates directly with one or more base stations 120b, although only one base station is shown in fig. 1. In the present embodiment, base station 120b is shown as a sectorized cellular tower. User terminal 110a typically communicates with base station 120b, which provides the strongest signal strength at one of the receivers in user terminal 110 a.

In another embodiment, the user terminal 110a communicates with the satellite 120a via an earth station (earth station). The earth station may be internal to the user terminal 110a or may be external to the user terminal (not shown). Satellite 120a communicates with a base station 120b, commonly referred to as a ground station or gateway. User terminal 110a transmits a reverse link signal to satellite 120a via the earth station, and satellite 120a relays the reverse link signal to base station 120 b. Base station 120b may transmit the forward link signal to satellite 120a, and satellite 120a may relay the forward link signal to user terminal 110 a.

The base stations 120b, whether in direct communication with the user terminals 110a-110n or indirectly via the satellites 120a, can be coupled to a Base Station Controller (BSC)140 that passes communication signals back and forth to and from the appropriate base stations 120 b. The BSC 140 is coupled to a Mobile Switching Center (MSC)150, which may be configured as an interface between the user terminal 110a and a Public Switched Telephone Network (PSTN)160 or some other network, which may be a packet network 170. In one embodiment, packet network 170 may be a Wide Area Network (WAN) such as the Internet. Thus, the MSC 150 may also be coupled to the PSTN 160 and the packet network 170. MSC 150 may also coordinate inter-system handovers with other communication systems.

Because of the structure of the reverse link, a large number of user terminals 110a-110n may each have an active communication session with the same base station 120b, the wireless communication system 100 may implement a channel on the reverse link with a constrained arrival time.

The user terminal 110a initially communicates with the wireless communication system 100 and requires access to a channel with a restricted access time. User terminal 110a may initially communicate with base station 120b over a random access overhead channel. The random access overhead channel may be located on the same or different frequency band as the restricted arrival time channel. The wireless communication system 100 may implement, for example, an Aloha protocol for random access overhead channels. Fig. 2A is a timing diagram 200 of an Aloha random access channel showing transmissions from three different user terminals attempting to communicate with a base station. In the example of fig. 2A, the first user terminal experiences two collisions 202A and 202b before a successful transmission occurs. Similarly, the second user terminal experiences two collisions 204a and 204b before a successful transmission occurs. In addition, the second user terminal experiences two collisions 206a and 206b before a successful transmission occurs. Of course, the number of collisions experienced by any one user terminal is not limited to 2.

Although fig. 2A shows each terminal experiencing collisions and unsuccessful attempts to access the channel, the sporadic nature of communication on the random access channel can greatly reduce the likelihood of collisions occurring. The initial setup may require a random access channel since the user terminals 110a-110n may be asynchronous with the wireless communication system 100 and may not have the capability to send a request at a predetermined arrival time.

The user terminal 110a may also synchronize with the wireless communication system 100 after requesting that an active session be established over a channel having a restricted access time. The user terminal 110a may synchronize with the wireless communication system 100 using any of a variety of synchronization techniques. For example, user terminal 110a may be in accordance with the technology of U.S. patent application No.10/428,953 entitled "ORTHOGONAL CODIVISION MULTIPLE ACCESS ON RETURN LINK OFSATELLITE LINKS", filed ON 5/1/2003, assigned to the assignee of the present application and hereby incorporated herein in its entirety.

Once the user terminal 110a is synchronized with the wireless communication system 100, the wireless communication system can determine the time of arrival of the data transmitted by the user terminal 110a and can assign the time of arrival to the user terminal 110 a. The wireless communication system 100 can communicate the time of arrival assignments to the user terminal 110a using the forward link channel.

The wireless communication system 100 may assign different times of arrival to active user terminals, e.g., 110a and 110n, instead of different codes. Thus, the wireless communication system 100 may assign U different times of arrival to each of U different user terminals. The wireless communication system 100 may select points in time of arrival times from a set of evenly spaced time boundaries for allocation. Alternatively, the time points of arrival times may be irregularly spaced or randomly determined.

In one embodiment, a transmission from a particular user terminal 110a in a CDMA based system can arrive at base station 120b starting at the remainder modulo U with any chip in the ith position. In other words, each user terminal 110a-110n (u) may have transmissions at any chip boundary b of the set_uAt the arrival receiver:

b_u∈u+kU k∈{0，1，2...} (1)

many variations of the present embodiments are possible and/or desirable, and the actual implementation may depend on design balances within the system. For example, the wireless communication system 100 may assign a remainder modulo the arrival time by the number of active user terminals 110a-110 n. Alternatively, the wireless communication system 100 may assign a remainder modulo the arrival time by a predetermined constant. If the number of active users exceeds the predetermined constant modulus value, the wireless communication system may implement a priority scheme to ensure that all user terminals are eventually assigned an arrival time.

In one embodiment, the wireless communication system 100 may determine and assign times of arrival so that transmissions from any two users do not arrive at the receiver at the same time. In such an embodiment, all user terminals 110a-110n may use a single code, provided that the code has a pseudo-random nature when cross-correlated with a shifted version of itself. Codes of this nature can be obtained by using a linear feedback left shift register (LFSR). With this transmission strategy, the probability of collision occurrence can be effectively reduced to zero.

In embodiments where all user terminals 110a-110n use one code, the receiver in the base station 120b becomes simpler since the code is known. Furthermore, the point in time at which the receiver needs to look for the transmission of a particular user is now a discrete set of hypotheses, thus also reducing the complexity of this dimension.

The constrained arrival time embodiment introduces a delay on the channel that may not be present in a pure CDMA-Aloha scheme where the terminal decides to transmit by itself. This delay depends on the transmission period, which may be the chip interval between the arrival times of two consecutive transmissions of a user. In one of the above embodiments, each user terminal gets a transmission opportunity in each period of U chips, so the delay experienced by a single packet varies uniformly and randomly with a parameter U.

It is noted that even for very large values U-15000, the delay time introduced is in the order of a few milliseconds when the chip rate is in the order of a few mega chips per second. Some digital communication systems, such as those using geostationary satellites, have inherent propagation delays on the order of hundreds of milliseconds, even without taking into account the additional delays that may be introduced by the end-to-end communication link, such as the delays introduced via an internet router. In such systems, the delay percentage increase for constrained arrival time access implementations is very small.

Fig. 2B is a timing diagram 210 of one example of a constrained arrival time channel. The timing diagram 210 of fig. 2B shows three active user terminals, each transmitting data block arriving at a constrained arrival time. The first user terminal transmits data blocks 222a-222c that arrive at the assigned arrival time assigned to the first user terminal. Although only three data block transmissions 222a-222c are shown, it should be understood that the user terminal may continue to transmit data blocks arriving at the assigned time until the user terminal relinquishes the channel. Time t between successive transmissions_cIs the transmission period. In the example of fig. 2B, the point in time allocated to the first user terminal is the same in each transmission cycle. The transmission period 230 shown in the example of fig. 2B has a duration that is greater than the duration required to cycle through all user terminal transmissions. Wherein the transmission period 230 is a multiple of the minimum time increment, i.e., t_c＝D×t_bThe distribution arrival time is determined as the remainder of the modulo operation performed on the time distribution result with D as the modulo. Furthermore, although fig. 2B shows that the duration of a data block, such as 222a, is less than the duration of the transmission period 230, the duration of the data block 222a may also exceed the duration of the transmission period. In this case, the receiver does not need to search for a transmission from the user terminal at the assigned point in time because it has already received the transmission from the user terminal. Furthermore, if the duration of the data block exceeds the duration of the transmission period, the system may not need to transmit a new point in time allocation result to the user terminal.

Similarly, the second user terminal transmits data blocks 224a-224c, which arrive at the assigned arrival time assigned to the second user terminal, each data block may be shorter or longer in duration than the transmission period. Similarly, the point in time assigned to the second user terminal is the same at each transmission period.

The third user terminal transmits data blocks 226a-226b that arrive at the assigned arrival time assigned to the third user terminal. The point of time allocated to the third user terminal is the same at each transmission period. However, the third user terminal has no data to transmit during the second transmission period and thus no data to receive at the base station.

Time increment 240, t between successive time point allocation results_bAnd may be fixed or variable. The minimum time increment 240 may be determined based on the synchronization level and the configuration of the user terminal.

For example, in a wireless communication system where the user terminal is stationary and there are no significant multipath signal components at the base station, the minimum time increment can be made relatively small. For example, the duration of the minimum time increment 240 may be one CDMA chip, 2 chips, 3 chips, 4 chips, 5 chips, 10 chips, and so on, or some other time increment.

In other embodiments, the user terminal may be mobile or portable, or a large number of multipath signal components may arrive at the base station. In such an embodiment, the minimum time increment is made relatively large so that significant multipath components from the first user terminal arrive before the assigned arrival time of the second user terminal.

In the embodiments discussed above, the arrival time of each user is always the same chip number modulo D, and there is a possibility of an unexpected phenomenon. Analysis shows that data blocks starting at different points in time may have consistently different levels of interference. For example, in the timing diagram example of fig. 2B, one physical portion of the data block transmission (e.g., 226a) from the third user terminal experiences no other interference sources from other user terminals. The overall consequence is a decrease in system capacity. One solution to this effect is for the wireless communication system to assign each user terminal a point in time that varies with each transmission period, i.e., D chips. The continuous transition makes the interference more evenly distributed over the time points. The process of assigning points in time may be random, pseudo-random or may follow a predetermined sequence or algorithm.

In CDMA-Aloha channels, the receiver at the base station does not know which user terminal is transmitting. Typically, the identity of the sender is only known after the information has been correctly decoded. In one embodiment where the user terminal broadcasts only sporadically, the base station cannot determine who the sender is when a decoding error occurs. With the constrained time of arrival configuration, the receiver at the base station knows which user terminal is transmitting the data block. Such information can be used, for example, to update the power control loop of each user or to inform a particular user that a packet loss has occurred, if a decoding error has occurred. CDMA networks typically rely on closed loop control of the transmit power of the user terminal. If the data transmitted by the user terminal transmitter is not received correctly, the wireless communication system may use a power control loop to inform the user terminal transmitter to increase its transmit power.

Fig. 3 is a functional block diagram of one embodiment of a user terminal 110 operating in a restricted arrival time channel. The user terminal 110 may be, for example, one of the user terminals 110a or 110n shown in the embodiment of fig. 1. For simplicity, only the portions of user terminal 110 relevant to the present invention are shown and described.

User terminal 110 includes a receiver 302 that receives forward link transmissions from one or more base stations. As discussed above with respect to fig. 1, receiver 302 may receive forward link transmissions transmitted by a base station or relayed via an intermediate element (e.g., a satellite). The receiver 302 may receive data and instructions from a wireless communication system. The instructions and associated data may be transmitted using an overhead channel and may include parameters related to point-in-time assignments that constrain the time-of-arrival channel. Other user data may be transmitted on the traffic channel. Alternatively, some or all of the control data and instructions may be transmitted on the forward link traffic channel.

The receiver 302 may direct instructions and data received on the overhead channels to the appropriate modules. For example, the output of the receiver 302 may be connected to a synchronization module 310, a transmit timing module 320, and a power control module 330.

The synchronization module 310 synchronizes a timing reference of the user terminal 110 with a time base of the wireless communication system. The synchronization module 310 may be configured with the remaining modules of the user terminal 110, for example, to implement the synchronization techniques described in U.S. patent application No.10/428,953. The synchronization module 310 may provide a predetermined synchronization accuracy, which may be on the order of or better than one CDMA chip.

The transmit timing module 320 may receive the point in time assignment and control the transmit path within the user terminal to transmit the data block at a time that allows the data block to be received at the base station at the specified point in time. In one embodiment, the transmit timing module 320 receives the time allocation results before each transmission period. In another embodiment, the transmit timing module 320 receives an initial point in time assignment and determines a future point in time assignment based in part on a predetermined algorithm. The predetermined algorithm may include pseudo-randomizing the point in time assignment. In such an embodiment, the base station may similarly determine the point-in-time assignment using a complementary algorithm. In other embodiments, the transmit timing module 320 may receive the time point assignments at intervals of lower frequency. The frequency may be periodic, such as once every predetermined transmission period, or may be event based. An example of event-based point-in-time allocation is point-in-time reallocation consistent with a change in the number of active user terminals accessing the channel.

The randomized or transformed permutation of the point in time allocations may be determined at the base station and transmitted to the user terminals 110, or may be determined by the transmit timing module 320, particularly if the point in time allocations are pseudo-random or deterministic.

The transmit timing unit 320 may receive the chip allocation result and the modulus and may determine a time point allocation result together with the synchronization module 310. In other embodiments, the transmit timing module may receive the point in time assignment and use the time offset determined by the synchronization module, where the data block needs to be transmitted in order for it to arrive at the base station at the assigned point in time. In other embodiments, the transmit timing module 320 may receive other types of timing information.

Power control module 330 may control transmitter 350, and in particular power amplifier 352 in transmitter 350, to increase or decrease transmit power based in part on a power control signal received in forward link communications.

The transmit data path of user terminal 110 may include a data buffer 340 that stores data to be transmitted to the base station. The data may include control and overhead signaling and traffic to be transmitted on the reverse link, and may originate from one or more sources (not shown). The user terminal 110 retrieves a data block from the data buffer 340 and passes the data block to the data modulator 342. The data blocks may be selected from a predetermined set of data block sizes, or may vary in size based on the amount of data the user terminal 110 wishes to transmit, or may be a combination of predetermined block sizes based on the amount of data to be transmitted.

The data modulator 342 may modulate data included in the acquired data block. For example, the data modulator 342 may utilize a predetermined code sequence for direct sequence spreading of the data bits. The data modulator 342 may use a code generated by the LFSR in the data modulator 342 or select a code from a predetermined number of codes stored or generated in the user terminal 110. Based on instructions or control signals received from the base station via the receiver 302, the data modulator 342 may be controlled to use a particular code.

The modulated data is provided to a transmitter 350, which can transmit a signal at a time that includes a time offset that compensates for the propagation delay. Thus, the modulated data block is configured to arrive at the base station at the assigned point in time.

Processor 360, in conjunction with processor-useable instructions stored in an associated memory 362, may execute some or all of one or more modules of a user terminal. For example, some or all of the functions of the transmit timing module 320 may be stored in the memory 362 as software and executed by the processor 360.

Fig. 4 is a functional block diagram of one embodiment of a base station 120, which may be a base station of the wireless communication system shown in fig. 1. For simplicity, only those portions of the base station 120 relevant to the present invention are shown and described.

The base station 120 can include an analog receiver module 402 that can receive a signal broadcast on a random access channel to establish an active session on a constrained time-of-arrival channel. The analog receiver module 402 may also receive signals broadcast on the constrained arrival time channel. The output of the analog receiver module 402 may be converted to a digital signal for subsequent processing.

The base station may include a RAKE receiver coupled to the output of the analog receiver module 402. The RAKE receiver may include a searcher 410, for example, that searches for the strongest one of the potential number of multipath signals arriving from a particular user terminal. The searcher 410 may assign a first multipath signal to a first finger (finger)412 and may assign a second multipath signal to a second finger (finger) 414. Although only two fingers 412 and 414 are shown, any number of fingers may be implemented in a RAKE receiver. Searcher 410 may search for transmissions from a particular user terminal based on timing. Since each user terminal in the restricted arrival time channel is assigned a point in time of arrival, searcher 410 may search for transmissions from the relevant user terminals within a time window that includes the assigned point in time. Thus, for each point in time, the searcher 410 knows the user terminals assigned to that point in time.

Each branch 412 and 414 demodulates the multipath signal assigned to it, e.g., by spreading the signal with a corresponding code. The signal outputs of the various branches 412 and 414 may be connected to a combiner 420 where the multipath signals are time aligned and coherently summed. In embodiments where multipath signals are substantially absent, such as where a fixed user terminal transmits signals to a satellite relay station, a RAKE receiver comprising a plurality of fingers 412 and 414 and an associated combiner 420 may be omitted. Instead, a single receive path equivalent to a single branch that performs searching and demodulation may be used.

The output of the combiner 420 may be connected to a baseband processor 430. The base station processor 430 may feed the relevant portion of the data to the BSC (not shown). In addition, the baseband processor 430 may feed control and overhead signals to associated control modules.

The control module may include a transmission period module 440 that determines a duration of a transmission period. Transmission period module 440 may determine a transmission period, e.g., based on a number of active user terminals communicating with base station 120.

The control module may also include a time boundary module 450 that may determine a point in time representing an arrival time assigned to a particular user terminal. The time boundary module 450 may also perform a time point randomization or a transform permutation, thereby distributing the interference effects more evenly among all user terminals. The time border module 450 may transmit the time point assignment result to the processor 470, the baseband processor 430, and the searcher 410.

The control module may include a power control module 460 forming part of a power control loop. Power control module 460 may determine whether the transmit power of a particular user terminal should be increased or decreased. For example, the base station processor 430 may determine whether received data corresponding to an arrival time allocated for a particular user terminal is corrupted. The base station 120 may then transmit a message requesting retransmission of the data. In addition, baseband processor 430 may communicate information that the data cannot be recovered to power control module 460, so that power control module 460 may generate control information to the user terminal instructing the user terminal to increase its transmit power. Such a power control loop is not possible in random access channels, such as the Aloha channel, since the receiver does not know which user terminal is trying to transmit data, and thus cannot determine which user terminal is the initiator if a collision causes loss or corruption of data. Conversely, the baseband processor 430 may determine that the reception data corresponding to a particular user terminal has been received without error. Baseband processor 430 may communicate the error-free reception to power control module 460, and power control module 40 may then generate control information for the user terminal to instruct the user terminal to reduce its transmit power. Power control module 460 may determine power control information based in part on a received signal quality metric, such as a data error rate, a bit error rate, or a symbol error rate. The output of the power control module 460 and the outputs of the transmit period module 440 and the time boundary module 450 may be connected to a modulator 482.

The modulator 482 may also be connected to a data buffer 480 that stores data to be transmitted on the forward link channel to each user terminal. Modulator 482 can modulate each forward link signal with an appropriate code and can generate overhead signals from one or more control module outputs.

The modulated signals are fed to a transmitter 490 that provides the forward link signals to the various user terminals. Processor 470, in combination with processor-useable instructions stored in an associated memory 472, implements some or all of one or more modules of base station 120.

Fig. 5 is a flow diagram of a method 500 of assigning a constrained arrival time channel. For example, the base station shown in fig. 1 or fig. 4 may implement method 500.

The method 500 begins at block 502 when a base station receives a channel access request from a user terminal. The base station may receive a request from a user terminal, for example, over a CDMA Aloha random access channel provided for overhead signaling and communication. The request initiates an active session that is constrained to arrive on the channel.

The base station proceeds to block 510 and synchronizes the user terminal so that the user terminal and the base station are synchronized to the same time reference. In one embodiment, the user terminal is synchronized to the base station time reference to achieve an accuracy better than one CDMA chip.

The base station then proceeds to block 520 and determines the transmission period of the constrained arrival time channel. As previously mentioned, a transmission period represents the duration between the arrival times of two consecutive transmissions of a particular user terminal. As previously mentioned, the transmission period may be determined based on the number of active user terminals or may be independent of the number of active user terminals. In one embodiment, the number of time points, i.e. arrival time boundaries, is equal to the number of active user terminals, and thus the transmission period is equal to the minimum time increment multiplied by the number of active users. In other embodiments, the transmission period may be a fixed duration. Other embodiments may also employ a combination of different techniques. For example, the duration may be based on the number of active users, but may be further constrained to be at least some predetermined minimum transmission period duration.

The base station then proceeds to block 522 and determines the arrival time assigned to the user terminal. The arrival time assigned to a particular user terminal may be determined based in part on the arrival times assigned to other user terminals. These times of arrival may differ by as little as one CDMA chip or multiple chips. In one embodiment, the base station may assign the earliest available arrival time to the user terminal.

After determining the arrival time assigned to the user terminal, the base station proceeds to decision block 530 to determine whether the previously determined arrival time is the initial assignment for the user terminal. If the assigned arrival times are periodic, there may be uneven interference for different users assigned to different arrival times. Thus, if the time of arrival does not represent the initial time of arrival decision, the base station proceeds to block 532 and randomizes the time of arrival assignment. The base station may randomize the arrival time assignment and transmit the randomized value to the user terminal. In another embodiment, after the base station transmits the initial time allocation result to the user terminal, the base station and the user terminal may determine the arrival time based on a predetermined function, respectively. The base station then proceeds to block 540.

Returning to decision block 530, if the time of arrival assignment results in the first time of arrival assigned to the user terminal, then no randomization of the time of arrival is needed and the base station can proceed directly to block 540.

In block 540, the base station determines the code channel assigned to the user terminal. The base station may assign a different code channel to the user terminal in each transmission period so that multiple user terminals may use the same arrival time. Typically, the number of codes is limited, thereby reducing the complexity of the receiver in the base station. In other embodiments, all ues use the same code, so step 540 may be omitted.

The base station proceeds to block 550 and transmits the time of arrival assignment to the user terminal. For example, the base station may communicate the time of arrival to the user terminal via signaling on the forward link.

After transmitting the time of arrival assignment result, the base station proceeds to block 552 and transmits the code channel assignment result. If all user terminals use the same code allocation result, the base station does not need to transmit a code allocation to the user terminals.

The base station proceeds to decision block 560 to determine whether the base station receiver has received a termination message from the user terminal. The user terminal may transmit a termination message to indicate termination of the active session.

If the base station receives the termination message, the base station proceeds to block 570 whereupon method 500 ends for the subscriber station. Returning to decision block 560, if the base station receiver does not receive a termination message, the base station may conclude that the session is still active. The base station may then return to block 510 to maintain synchronization with the user terminal and determine the next arrival time assigned to the user terminal. The base station may determine the time of arrival at each transmission period or may determine the time of arrival less frequently. For example, if the number of active users changes, the base station may re-determine the time of arrival. In other embodiments, the base station may re-determine the time of arrival after a predetermined number of transmission cycles. Other embodiments may use other methods.

Figure 6 is a flow diagram of one embodiment of a method 600 of operating on a constrained arrival time channel. The method 600 may be implemented, for example, within one or more of the user terminals of fig. 1 or fig. 3.

The method 600 begins at block 602 when a user terminal transmits a channel access request to a base station. The user terminal may transmit the request, for example, over a random access channel of the base station.

The user terminal proceeds to block 610 and synchronizes with the base station to establish a common time reference. During synchronization, the user terminal may determine a time offset to compensate for propagation delay.

The user terminal proceeds to block 620 and receives or determines the time allocation of arrival. Typically, the user terminal receives an initial arrival time allocation result from the base station. However, the subsequent arrival time may be independently determined by the user terminal. For example, the ue may receive a time allocation that is represented by the remainder of the modulo operation on the chip duration modulo the number of active ues. The user terminal then continues to determine its arrival time assignment unless there is a change in the assignment or a change in the number of active users. In another embodiment, the user terminal may receive the arrival time assignment and determine a subsequent arrival time based on a predetermined function.

After receiving or determining the time of arrival assignment, the user terminal proceeds to block 630 and receives or determines a code assignment. In systems where the user terminal uses more than one code, the base station may determine the code channel, for example, based on the time of arrival allocation. In other embodiments, multiple ues may all use the same code, and therefore need not be assigned a code.

After determining the code, the user terminal proceeds to block 640 and transmits data, which is expected to arrive at the base station at the assigned arrival time. The user terminal transmits data at a time before the assigned arrival time to compensate for propagation delay from the user terminal to the base station.

The user terminal may buffer data to be transmitted while waiting for its assigned transmission opportunity. The user terminal may then take some or all of the buffered data and transmit the data so that it arrives at the base station at the beginning of the assigned arrival time. The user terminal may generate data having one of a plurality of predetermined data block sizes, or may generate a variable data block size. The data may be encoded with an assigned code, which may be generated using a Linear Feedback Shift Register (LFSR), for example.

After transmitting the data, the user terminal proceeds to decision block 650 and determines whether it should relinquish its partially constrained arrival time channel and terminate the active session. If not, the user terminal returns to block 610 and continues to operate on the channel.

Returning to decision block 650, if the user terminal determines that the active session is to be terminated and relinquishes access to the constrained arrival time channel, the user terminal proceeds to block 652 and transmits a termination message to the base station. In one embodiment, the user terminal transmits the termination message on a random access overhead channel used by the user terminal for the initial channel access request. In another embodiment, a termination message is included in the data transmitted on the restricted arrival time channel. After transmitting the termination message, the user terminal proceeds to block 660 whereupon method 600 ends.

Fig. 7 is a flow diagram of one embodiment of a method 700 of receiving a signal from a time constrained channel. The method 700 may be implemented, for example, within the base station of fig. 4. The method 700 begins at block 710 when a base station receives a transmission from at least one active user terminal, and typically from multiple active user terminals, on a constrained arrival time channel. The base station proceeds to block 720 and determines an arrival time assignment for a particular one of the plurality of active user terminals. The base station then proceeds to block 730 and searches for transmissions from the user within a time window that overlaps the arrival time assigned to the user terminal. The base station may receive multiple transmissions modulated with the same code. However, each transmission typically has a different arrival time assignment. Thus, different signals begin to be modulated at different times. If the code has sufficient cross-correlation over the increment of the assigned arrival time, the base station may recover the transmission from the particular user terminal in the presence of other signals.

The present invention describes a constrained arrival time channel that does not require a large number of codes C, thus simplifying the receiver, while at the same time significantly reducing the likelihood of collisions occurring. A wireless communication system may implement a channel as part of the reverse link between multiple user terminals and a single base station. The receiver in the base station is greatly simplified because the number of codes searched per time of arrival is reduced.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. Various steps or operations in a method or process may be performed in the order shown or in another order.

A software module may reside in RAM memory, flash memory, non-volatile memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. Further, the various methods may be performed in the order shown in the embodiments or with a modified order of steps. In addition, one or more process or method steps may be omitted or one or more process or method steps may be added to the method or process. Additional steps, blocks or actions may be added at the beginning, end or intervening elements of a method or process.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the scope or spirit of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (39)

1. A method for allocating channel access, the method comprising:

determining a timing of a transmission cycle;

determining an arrival time allocated to one of a plurality of active user terminals within the transmission period; and

transmitting the arrival time to the user terminal, thereby allocating the channel to the user terminal at the beginning of the arrival time.

2. The method of claim 1, wherein the time of arrival occurs within a period of time during which at least one other of the plurality of user terminals is transmitting.

3. The method of claim 1, wherein the time of arrival occurs within about one CDMA chip of the time of arrival assigned to the other of the plurality of user terminals that is transmitting.

4. The method of claim 1, wherein the arrival time occurs at least one CDMA chip away from a nearest arrival time assigned to one other user terminal.

5. The method of claim 1, wherein the arrival time occurs at substantially the same location as a start of the transmission period.

6. The method of claim 1, wherein the duration of the transmission period is proportional to the duration of a CDMA chip.

7. The method of claim 6, wherein determining the arrival time comprises:

determining a remainder obtained by performing modulo operation on the CDMA chip serial number by taking the number of chips in the transmission period as a modulus.

8. The method of claim 1, wherein the transmission period comprises a duration proportional to a number of active user terminals.

9. The method of claim 1, wherein the transmission period comprises a fixed duration.

10. The method of claim 1, further comprising:

randomizing the arrival time within the transmission period.

11. The method of claim 1, further comprising:

determining a code allocation result for the user terminal; and

and transmitting the code allocation result to the user terminal.

12. A method for allocating channel access, the method comprising:

receiving an access request for the channel from a user terminal;

synchronizing a time reference with the user terminal;

determining a transmission period having a duration proportional to the CDMA chip duration;

determining an arrival time occurring at a chip boundary within the transmission period; and

transmitting the arrival time to the user terminal, thereby allocating the channel to the user terminal at the beginning of the arrival time.

13. A method for transmitting data in a channel, the method comprising:

requesting access to the channel;

receiving a time-of-arrival allocation result in response to the request; and

transmitting a data block at a time offset from said arrival time such that an initial portion of said data block arrives at a receiver at said assigned arrival time.

14. The method of claim 13, wherein requesting access to the channel comprises:

access to the channel is requested through a random access channel.

15. The method of claim 14, wherein the random channel comprises a cdma aloha channel.

16. The method of claim 13, wherein receiving the arrival time comprises:

reception occurs at a CDMA chip boundary within a transmission period.

17. The method of claim 13, wherein receiving the arrival time comprises:

the time of arrival occurring approximately one CDMA chip duration from the time allocation result for another user terminal is received.

18. The method of claim 13, wherein receiving the arrival time comprises:

and receiving a remainder obtained by performing modulo operation on the time point distribution result by taking the number of the active user terminals as a modulus.

19. The method of claim 13, further comprising:

encoding the data block with a code used by at least one other user terminal transmitting over the channel for a period of time that at least partially overlaps with a time required to transmit the data block.

20. The method of claim 13, further comprising:

the data block is encoded with a code used by a plurality of active user terminals.

21. The method of claim 13, further comprising:

a future arrival time allocation occurring in a subsequent transmission cycle is determined based in part on the arrival time allocation.

22. The method of claim 21, wherein determining the future time of arrival allocation result comprises:

the remainder of modulo operation of the CDMA chip boundary modulo the number of active user terminals is determined.

23. The method of claim 21, wherein determining the future time of arrival allocation result comprises:

the CDMA chip boundaries are determined based on a predetermined algorithm.

24. A method for receiving data in a channel, the method comprising:

determining an arrival time allocated to a user terminal during a transmission period;

receiving transmissions from a plurality of active user terminals;

searching for a transmission from the user terminal within a time window that includes the arrival time; and

a data block is received from the user terminal.

25. The method of claim 24, further comprising:

determining a received signal quality metric corresponding to at least a portion of the data block;

determining a power control message based in part on the received signal quality metric; and

transmitting the power control message to the user terminal.

26. The method of claim 25, wherein the received signal quality metric comprises a symbol error rate.

27. The method of claim 25, wherein the received signal quality metric comprises a bit error rate.

28. An apparatus operating on a constrained arrival time channel, the apparatus comprising:

a data buffer storing data blocks;

a data modulator coupled to said data buffer for direct sequence spreading data within said data block with a code to produce modulated data;

a transmitter which receives the modulated data from the data modulator and selectively transmits the modulated data; and

a transmit timing module coupled to the transmitter and receiving the time of arrival assignment and controlling the transmitter to transmit the modulated data at a time offset from the time of arrival assignment such that the transmitted data arrives at the receiver substantially initially at the time of arrival assignment.

29. The apparatus of claim 28, wherein the data modulator comprises a Linear Feedback Shift Register (LFSR) that generates the code.

30. The apparatus of claim 28, wherein the time of arrival assignment result comprises CDMA chip boundaries occurring within one transmission period.

31. The apparatus of claim 28, further comprising a local receiver that receives the arrival time assignment and communicates the arrival time assignment to the transmit timing module.

32. The apparatus of claim 28, further comprising a synchronization module that synchronizes the transmit timing module to a system time reference.

33. An apparatus operating on a constrained arrival time channel, the apparatus comprising:

a transmission period module for determining a transmission period time sequence;

a time boundary module for determining the arrival time allocated to the user terminal in the transmission period; and

a receiver that receives a plurality of transmissions from a plurality of active user terminals, and searches for the plurality of transmissions within a time window that includes the arrival times of the transmissions from the user terminals.

34. The apparatus of claim 33, wherein the transmission period module determines the transmission period to have a duration substantially equal to a duration of a plurality (D) of CDMA chips.

35. The apparatus of claim 34, wherein said time boundary module determines said time of arrival to include a CDMA chip boundary.

36. The apparatus of claim 34, wherein said time boundary module determines said time of arrival comprising a remainder modulo D with respect to a CDMA chip boundary.

37. The apparatus of claim 33, wherein the plurality of transmissions comprise a plurality of time-overlapping transmissions encoded with a same code.

38. The apparatus of claim 37, wherein the plurality of time-overlapping transmissions comprises a plurality of overlapping transmissions, each transmission having a different time-of-arrival allocation result.

39. One or more processor readable storage devices storing one or more processor usable instructions that, when executed by the processor, perform a method comprising:

receiving a request from a user terminal to access the channel;

synchronizing a time reference with the user terminal;

determining a transmission period having a duration proportional to a duration of one CDMA chip;

determining an arrival time occurring at a chip boundary within the transmission period; and

transmitting the arrival time to the user terminal, thereby allocating the channel to the user terminal at the beginning of the arrival time.