WO2001019117A1

WO2001019117A1 - Improved utilization of uplink random access radio channels

Info

Publication number: WO2001019117A1
Application number: PCT/SE2000/001658
Authority: WO
Inventors: Rolf Hansson; Anders Herlitz
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1999-09-08
Filing date: 2000-08-29
Publication date: 2001-03-15
Also published as: AU7326200A; JP2003509923A

Abstract

In a mobile radio communication system, a mobile radio transceiver station (11, 15) can attempt to obtain a replacement uplink random access channel (13, 17) in response to detected unavailability of an uplink random access channel (13, 17) currently assigned to the mobile radio transceiver station.

Description

IMPROVED UTILIZATION OF UPLINK RANDOM ACCESS RADIO

CHANNELS

CROSS REFERENCE TO RELATED APPLICATION

The subject matter disclosed herein is related to subject matter disclosed in copending U.S. Serial No. 09/132,058 (Attorney Docket No. 34646-311), filed on August 11, 1998, the disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to radio communications between mobile and fixed-site radio transceivers and, more particularly, to the use of random access channels for communication from mobile transceivers to fixed-site transceivers.

BACKGROUND OF THE INVENTION

In mobile radio communications systems, the connection between the mobile stations (MS) and the fixed-site transceivers, or base stations (BS), is generally referred to as the air interface. One conventional technique for implementing the air interface is referred to as time division multiple access (TDMA), wherein each frame of a repetitive frame structure is divided into a plurality of time slots (TS), during which time slots the radio transceivers in the mobile stations and the radio transceiver in the base station transmit and receive communications to and from one another. Communications from the mobile station to the base station are referred to as uplink communications, and communications from the base station to the mobile station are referred to as downlink communications.

Higher speed mobile stations (often referred to as full-rate mobile stations) may require two or three of the time slots available in the frame, while lower speed mobile stations typically require only a single time slot. One time slot of a frame on one frequency carrier is referred to as a physical channel.

In a typical conventional random access channel arrangement, downlink communications are transmitted in a particular time slot of a downlink frequency carrier, and uplink communications are transmitted in a particular time slot of an uplink frequency carrier. The uplink and downlink time slots are typically offset from one another in time so that the corresponding uplink and downlink communications do not timewise overlap one another. Thus, as seen from the foregoing discussion, the downlink channel can be conventionally assigned to a particular time slot on a particular downlink frequency carrier, and the uplink channel can be conventionally assigned to a particular time slot on a particular uplink frequency carrier.

In one conventional example of random access channel operations, a mobile station monitors the downlink channel to determine whether the corresponding uplink random access channel is busy or idle. If the uplink random access channel is determined to be idle, then the mobile station can begin transmitting its information on the uplink random access channel. If the information transmitted by the mobile station is received accurately at the base station, then the base station informs the mobile station, via the downlink channel, that the uplink random access channel has been assigned to the mobile station. In this case, the mobile station can continue using the uplink random access channel until it has completed its desired communication.

On the other hand, if the downlink channel indicates that the uplink random access channel is busy, then the mobile station conventionally selects a time delay of random duration (for example, a random number of time slots) and thereafter monitors the downlink channel again to determine the busy/idle status of the uplink random access channel. Similarly, if the message that the mobile station transmitted was not received at the base station, then the base station conventionally indicates this on the downlink channel, and the mobile station selects a time delay of random duration and thereafter checks the status of the uplink random access channel again. One reason that the message of a mobile station might not be accurately received at the base station is that more than one mobile station may have been attempting to transmit on the idle uplink random access channel at the same time, thus causing the base station to receive unintelligible noise on the uplink random access channel.

In cellular radio communications systems, such as, for example, the Personal Digital Cellular (PDC) System of Japan, there may be cells with one or more of a particular type of random access channel. For example, a given cell might have one or more of a given type of random access channel such as the user packet channel (UPCH), depending on the traffic load within the cell. Continuing with this example, when a mobile station enters a given cell, it selects a UPCH based on its own conventional IMSI (International Mobile Subscriber Identity). This implies that the traffic on any given UPCH of the cell may be regarded as independent from the traffic on any other UPCH of the cell. The UPCH handles, for example, general Internet Protocol (IP) traffic, and therefore communications thereon tend to have a bursty nature. More specifically, communications on UPCH are typically characterized by periods of intensive communication use, with lengthy idle periods interposed therebetween.

In general, should a channel such as UPCH be unable, during intensive bursts of traffic, to handle all traffic that is offered to it, it will typically use some of the idle time between intensive bursts to "repair" itself, that is, deliver packets that it was unable to deliver during the traffic peak. If the backlog of undelivered packets represents more packets than the channel is able to deliver during a given idle period or periods, then the backlog of undelivered packets can continue to grow, and the channel will choke. Once choked, the channel will not recover to proper operation until the load offered to it becomes very low.

It is therefore desirable to accommodate bursty traffic on uplink random access channels while also avoiding the aforementioned channel choking phenomenon associated with bursty traffic on conventional uplink random access channels.

According to the present invention, a mobile station is permitted to search for an idle uplink random access channel when it detects that the uplink random access channel to which it is presently assigned is unavailable, for example, due to heavy traffic. If an idle channel is found, then the mobile station can switch to the idle channel and attempt to transmit thereon its desired uplink communication. Thus, heavy, bursty traffic can be accommodated with reduced likelihood of the conventional channel choking phenomenon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 illustrates diagrammatically conventional utilization of random access channels between mobile stations and a base station. FIGURE 2 illustrates diagrammatically the utilization of random access channels between mobile stations and a base station according to the invention.

FIGURE 3 illustrates diagrammatically pertinent portions of an example mobile station according to the invention.

FIGURE 4 illustrates a pertinent portion of a downlink channel utilized by the present invention.

FIGURE 5 illustrates in flow diagram format exemplary operations of the mobile station of FIGURE 3.

FIGURE 6 illustrates further exemplary operations of the mobile station of FIGURE 3, including examples of the search criteria of FIGURE 5.

FIGURE 7 illustrates the search operation of FIGURE 5 in greater detail.

DETAILED DESCRIPTION

FIGURE 1 is a block diagram which illustrates the aforementioned conventional allocation of communication channels (for example, UPCHs) between mobile stations and a base station 19 in a cellular communication network such as PDC. As shown in FIGURE 1, one or more mobile stations can be assigned to a single communication channel that includes an uplink channel and an associated downlink channel, the uplink channel being a random access channel (RAC). However, any given mobile station can use only the random access uplink channel of its currently assigned communication channel. For example, each of plural mobile stations designated at 11 is currently assigned to the random access uplink channel of communication channel 13, while each of plural mobile stations designated at 15 is currently assigned to the random access uplink channel of communication channel 17.

FIGURE 2 is a block diagram which illustrates exemplary random access channel allocation according to the present invention. In particular, any of the individual mobile stations illustrated at 21, 23 and 25 can decide to utilize any of the plurality of uplink random access channels associated with the respective communication channels of base station 19. For example, mobile station 21 can decide to utilize the uplink random access channels of either of communication channels 13 and 17, as can mobile stations 23 and 25. FIGURE 3 is a block diagram which illustrates pertinent portions of an example mobile station according to the present invention. In the exemplary mobile station portion of FIGURE 3, the random access channel communications processing section 31 is coupled to a radio transceiver (XCVR) 33 which is in turn coupled to an antenna 35 for communication via radio signaling with the base station 19 of FIGURE 2. The implementation and execution of random access radio channel communications between a mobile station transceiver and a base station transceiver are of course well known in the art.

The mobile station of FIGURE 3 further includes a search controller 37 which implements a random access channel searching capability wherein the mobile station of FIGURE 3 can switch from a busy uplink random access channel (such as a busy UPCH) to another, hopefully idle, uplink random access channel (such as an idle UPCH). The search controller 37 includes a status detector 32, search logic 34, a timer 36 and a memory 38. The status detector 32 is coupled for communication with the random access channel communications processing section 31 , and is also coupled for communication with the search logic 34. The search logic 34 is further coupled for communication with the timer 36 and the memory 38, as well as with the random access channel communications processing section 31. The status detector 32 obtains status information about the random access channel that is currently assigned to the mobile station, as well as information about other candidate random access channels available to the mobile station. The status detector 32 also obtains other network information as described in more detail below. The operation of the exemplary mobile station portions of FIGURE 3 will be most clearly understood with reference to FIGURES 5-7.

FIGURE 5 illustrates one example of a process executed by the mobile station of FIGURE 3 in order to decide whether or not to search for another random access channel. In particular, if the uplink random access channel currently assigned to the mobile station is either busy too often at 51 or will be busy too long at 53, or if the mobile station has lost too many contests for the idle uplink channel at 55, it is then determined at 57 whether or not a search for another uplink random access channel is possible. If none of the conditions at 51 , 53 or 55 are satisfied, then the mobile station continues to use its currently assigned random access channel.

With respect to the decision criterion at 51, whenever the random access channel communications processing section 31 needs to access the uplink random access channel but the downlink channel indicates that the uplink channel is busy, this busy condition is detected by the status detector 32 and is reported to the search logic 34. The search logic 34 maintains in memory 38 a record of how many times the uplink channel has been busy when needed. If the search logic 34 determines at 51 that the uplink channel has been busy too often, then the search logic 34 determines at 57 whether a search for another random access channel is possible.

Regarding the decision criterion at 53 of FIGURE 5, the status detector 32 also detects from the downlink channel how much longer the uplink random access channel will be busy. The search logic 34 receives this information from the status detector 32 and then determines at 53 whether or not the channel will be busy too long. If so, then the search logic 34 determines at 57 whether a search for another channel is possible.

Regarding the decision criterion at 55, the status detector 32 detects from the section 31 each instance when the mobile station loses a contest for the idle uplink channel. This information is passed to the search logic 34, which maintains in memory 38 a record of how many times the mobile station has lost a contest for the channel. If the search logic 34 determines at 55 that the number of losses is too large, then the search logic 34 determines at 57 whether a search for another channel is possible.

If it is possible at 57 to search for another channel, then the search logic 34 obtains from memory 38 information indicative of another available random access channel which is a candidate to replace the current channel, and provides this information to the random access channel processing section 31 via communication path 39 to begin the search for another channel. On the other hand, if the search logic 34 determines at 57 that a search is not possible, then the search logic 34 takes no further action, so that the mobile station continues at 59 to use its currently assigned random access channel. FIGURE 4 illustrates a portion 40 of the information conventionally communicated from the base station to the mobile stations on the downlink channel in, for example, PDC systems. The portion 40 illustrated in FIGURE 4 includes 22 bits which are commonly referred to as the E field of a downlink channel in PDC. The I/B section 41 (3 bits) in FIGURE 4 indicates whether or not the associated uplink channel is idle (I) or busy (B), and the R/N section 43 (3 bits) in FIGURE 4 indicates whether a message has been received (R) or not (N). The PE (partial echo) section 45 (16 bits) of FIGURE 4 is conventionally used to communicate a CRC (cyclic redundancy code) checksum. The CRC checksum is transmitted when a message on the uplink random access channel has been received at the base station.

The CRC checksum indicates, to each individual mobile station that may have been attempting to obtain the uplink channel, whether or not the message that was received by the base station was in fact the message that the particular mobile station transmitted. The CRC checksum in the PE section 45 indicates to only one mobile station that its message was received. Any other mobile station which attempted to transmit on the uplink channel will also know, from the CRC checksum in the PE section 45, that its message was not received by the base station and therefore that it has not obtained (i.e., has lost the contest for) the uplink channel. The above described organization and use of the E field portion 40 of the downlink channel in PDC is well known in the art.

Once the base station of FIGURE 2 has received the initial message from a mobile station on the uplink channel, and has indicated in R/N section 43 and PE section 45 that a message has been received and from which mobile station the message has been received, the PE section 45 is not conventionally used in the downlink channel for the remaining duration of the mobile station's use of the uplink channel. Accordingly, and as disclosed in detail in aforementioned copending U.S. Serial No. 09/132,058 (Attorney Docket No. 34646-311), the PE section 45 can be used to transmit from the base station to all mobile stations information indicative of how much longer the mobile station which has the uplink channel will continue to communicate on the uplink channel, in other words, how much longer the uplink channel will be busy. This information is obtained by the status detector 32 (FIGURE 3) and provided to search logic 34, which can then make the determination illustrated at 53 in FIGURE 5, namely whether or not the uplink channel will be busy too long.

The base station can easily provide the information indicative of how much longer the uplink channel will be busy, because the mobile station conventionally informs the base station, in the first time slot of the mobile station's uplink transmission, how long the uplink communication will last (e.g., how many time slots). Accordingly, after the CRC checksum has been transmitted by the base station using the PE section of the E field, the base station can then use the PE section of each downlink time slot to provide information indicative of how many more time slots will be needed for the mobile station currently transmitting on the uplink channel to complete its transmission. As indicated above, if the search logic 34 of FIGURE 3 determines at 53 in FIGURE 5 that the uplink channel will be busy too long, then the search logic 34 proceeds to determine whether or not a search is possible at 57.

FIGURE 6 illustrates exemplary operations of the FIGURE 3 mobile station, including a detailed example of the decision process performed at 51, 53 and 55 in FIGURE 5. The procedure in FIGURE 6 is executed when the mobile station wants to access the random access channel that is currently assigned to the mobile station, designated as a home channel in FIGURE 6. Initially at 60 in FIGURE 6, the search logic 34 initializes variables BUSY and LOSS to zero. At 61, the search logic 34 determines from status detector 32 whether the uplink channel is busy. If the uplink is not busy (i.e., idle) at 61, then the search logic 34 takes no action, and the communications processing section 31 attempts to obtain the uplink channel at 62 as it would conventionally. It is thereafter determined from the downlink channel at 63 whether the mobile station won the channel, lost the channel to another mobile station, or whether the base station received no message at all, in which latter case no mobile station currently has the uplink channel, so the mobile station of FIGURE 3 has neither won nor lost the uplink channel.

If it is determined at 63 that the mobile station has won the channel, then the search logic 34 takes no action and communications processing section 31 proceeds at 64 with the desired uplink communication. If the mobile station has neither won nor lost the channel at 63, then search logic 34 takes no action and the communications processing section 31 tries again at 62 to obtain the channel. If it is determined at 63 that the mobile station has lost the contest for the uplink channel, the search logic 34 increments the variable LOSS (maintained in memory 38) at 65, and determines at 66 whether the variable LOSS is greater than a predetermined loss threshold TH_L (also stored in memory 38).

If the number of losses represented by the variable LOSS does not exceed the threshold TH_L at 66, then the search logic 34 increments the variable BUSY (stored in memory 38) at 67, because the uplink channel is known to be busy due to the mobile station's loss of the channel at 63. The search logic 34 also increments the variable BUSY at 67 if it was determined at 61 that the uplink channel is busy.

After incrementing the variable BUSY at 67, the search logic 34 determines at 68 whether or not the variable BUSY exceeds a threshold TH_B (also stored in memory 38). If the number of times that the uplink channel has been busy, as represented by the variable BUSY, does not exceed the threshold TH_B at 68, then the search logic 34 determines at 69 whether or not the uplink channel will be busy for too long, for example, by comparing the information in the PE section 45 of E field 40 to a predetermined threshold time TH_T stored in memory 38. If the uplink channel will not be busy longer than TH_T, then at 79 the search logic 34 directs the communications processing section 31 (via communication path 39) to wait for the remainder of the busy period (as known from the PE section 45 of FIGURE 4), and thereafter try to obtain the uplink channel again.

If either the loss threshold TH_L or the busy threshold TH_B are exceeded at 66 or 68, respectively, then at 70, the search logic 34 directs the status determiner 32 to check appropriate network conditions to determine whether or not there is time to search for another channel. If it is determined at 71 that there is enough time, then at 58, the search logic 34 selects a new candidate channel from a list of available channels stored in the memory 38, and sends information identifying the new candidate channel to the communications processing section 31 to begin the search. If it is determined at 71 that there is not enough time to search for a new uplink channel, then at 79 the search logic 34 directs the communications processing section 31 (via communication path 39) to wait for the known remaining duration of the current uplink communication (as specified in the PE section 45 of FIGURE 4), and then try to obtain the channel again at 62.

If it is determined at 69 that the uplink channel will be busy longer than the threshold time TH_T, then at 73 the search logic 34 directs the status detector 32 to check appropriate network conditions to determine whether or not there is enough time to search for another uplink channel. If there is enough time at 74, then at 58 the search logic 34 selects a new candidate channel and initiates a search as described above. If there is not enough time at 74, then the search logic 34 directs the communications processing section 31 to wait at 79 for the known remaining duration of the current uplink communication, and then try again at 62 to obtain the channel.

Regarding blocks 70 and 73 of FIGURE 6, wherein the status detector 32 checks appropriate conditions of the network to determine whether or not there is enough time to search for another uplink channel, this can be accomplished in the following exemplary manner. Taking the user packet channel UPCH as an example random access channel, the conventional OSI Layer 1 header of the downlink UPCH includes information indicative of the remaining duration of the current downlink communication from the base station. If the current downlink communication from the base station is not directed to the mobile station of FIGURE 3, then for as long as the current downlink communication continues, the search logic 34 can be certain that no new information will be transmitted to the mobile station on the downlink UPCH. Thus, the search logic can determine, for example by comparing the known remaining duration of the downlink communication to a predetermined value stored in memory 38, whether or not there is enough time left in the current downlink communication to permit a search for a new random access channel (in this example, a new UPCH).

The predetermined value in memory 38 can be, for example, an empirically determined value indicative of how much time typically elapses during a minimal (shortest reasonable) search for another channel. If the search logic 34 determines at 74 (or 71) that there is enough time to conduct at least a minimal search, then the search logic 34 initializes the timer 36 to a time limit value which is preferably no more than the remaining duration of the downlink communication. Thus, if the search for a new channel is not successful within the time limit, then the mobile station can return to its home (currently assigned) channel in time for any new downlink communication that might possibly be directed to it.

The aforementioned use of the conventional OSI Layer 1 header information to determine how much time is available before the base station could possibly transmit information to the mobile station is only one example of ensuring that the mobile station can safely search for another uplink random access channel.

The broken line in FIGURE 6 represents another embodiment wherein, if the mobile station has neither won nor lost the channel at 63 , this result is treated the same as a loss of the channel.

FIGURE 7 illustrates exemplary operations which can be carried out by the example mobile station of FIGURE 3 when it has been decided to search for another channel to replace the currently assigned channel. In FIGURE 7, it is first determined at 75 whether or not the uplink channel of the new candidate channel is busy. If not, then at 76 the communications processing section 31 attempts to obtain the channel. If the mobile station wins the channel at 77, then at 78 the search logic 34 designates the candidate channel as the new home channel, stores this information in the memory 38, and informs the communications processing section 31 (via path 39) that the candidate channel is now the home channel. At 88, the communications processing section 31 continues uplink communications with the base station using the new home uplink channel. When a conventional base station next wishes to send data to the mobile station, it will conventionally use the downlink channel corresponding to the uplink channel on which the latest uplink communication was received from the mobile station, that is, the downlink channel corresponding to the new home uplink channel.

If it is determined at 77 that the mobile station has neither won nor lost the channel, it is thereafter determined at 80, from timer 36, whether or not there is enough time to try again to obtain the candidate channel. This can be implemented, for example, by the search logic 34 comparing the remaining search time indicated by timer 36 with an empirically determined value stored in memory 38, which empirically determined value is indicative of how much time typically elapses during an attempt to obtain an uplink random access channel. If the search logic 34 determines at 80 that there is enough time, it then directs the communications processing section 31 to try- again at 76 to obtain the channel. Otherwise, at 81, the search logic 34 directs section 31 to return to the home channel.

If the candidate channel is busy at 75, or if the mobile station lost a contest for the uplink channel at 77, the search logic 34 determines at 82 whether or not the uplink channel will be busy longer than the threshold time TH_T. If so, then at 84 the search logic 34 consults the timer 36, to determine whether or not there is enough time left to try another candidate channel. This determination can be made by comparing the timer value with a suitable empirically determined value stored in memory 38, in a similar manner to that described above relative to decision 80. If there is enough time left at 84, then at 85 the search logic 34 designates the next candidate channel, and the procedure of FIGURE 7 is repeated for the new candidate channel. If there is not enough time left at 84, then at 89 the search logic 34 directs a return to the home channel.

If it is determined at 82 that the candidate uplink channel will not be busy too long, it is then determined at 86 whether or not there is enough time to wait until the current uplink communication is complete and then try again to obtain the candidate uplink channel at 76. This determination can be made by, for example, comparing the current value of timer 36 with the sum of the remaining uplink communication time (from PE section 45 of FIGURE 4) and the empirically determined time value discussed above relative to decision 80. If there is enough time left at 86, then at 87 the search logic 34 directs the communications processing section 31 to wait for the remaining duration of the current uplink communication, and then try again at 76 to obtain the channel. However, if there is not enough time left at 86, then at 89 the search logic 34 directs a return to the home channel.

The broken line in FIGURE 7 represents another embodiment wherein, if the mobile station has neither won nor lost the channel at 77, this result is treated the same as a loss of the channel.

Workers in the art will recognize that the inventive techniques described above will increase the data throughput over the air interface in cellular systems such as PDC. Instead of having certain random access channels choked while others remain idle, advantageous load sharing can be accomplished. Therefore, the user will experience a higher data rate, and the network operator will be able to utilize resources more effectively and economically.

It will be apparent to workers in the art that a mobile station having the exemplary inventive portions of FIGURES 3 and 5-7 can be readily implemented by appropriately modifying software, hardware or both in the data processing portions of conventional mobile stations, for example a mobile station for operation according to the requirements of the PDC standard.

Although exemplary embodiments of the present invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.

Claims

WHAT IS CLAIMED IS:

1. A method for a mobile radio transceiver station operating in a mobile radio communication system to utilize uplink random access channels by which the mobile radio transceiver station can be coupled to a fixed-site radio transceiver, comprising: the mobile radio transceiver station detecting that an uplink random access channel currently assigned to the mobile radio transceiver station is unavailable for uplink radio communications from the mobile radio transceiver station to the fixed-site radio transceiver; and the mobile radio transceiver station attempting to obtain another uplink random access channel in response to said detected unavailability of the uplink random access channel that is currently assigned to the mobile radio transceiver station.

2. The method of Claim 1 , wherein said detecting step includes the mobile radio transceiver station determining that another mobile radio transceiver station will be using the currently assigned uplink random access channel for a length of time, and comparing the length of time to a predetermined threshold value.

3. The method of Claim 1 , wherein said detecting step includes the mobile radio transceiver station determining that the currently assigned uplink random access channel has been busy during a number of attempts by the mobile radio transceiver station to access the currently assigned uplink random access channel, and comparing the number of attempts to a predetermined threshold value.

4. The method of Claim 1 , wherein said detecting step includes the mobile radio transceiver station determining that the mobile radio transceiver station has lost a number of contests for the currently assigned uplink random access channel, and comparing the number of lost contests to a predetermined threshold value.

5. The method of Claim 1, wherein said attempting step includes the mobile radio transceiver station selectively attempting to obtain another uplink random access channel in response to said detected unavailability of the uplink random access channel that is currently assigned to the mobile radio transceiver station.

6. The method of Claim 5, including the mobile radio transceiver station selectively attempting to obtain the currently assigned uplink random access channel subsequent to said detection of the unavailability of the currently assigned uplink random access channel.

7. The method of Claim 5, including determining, in response to said detecting step, whether the mobile radio transceiver station can ignore a downlink channel corresponding to the currently assigned uplink random access channel long enough to perform said attempting step, and performing said attempting step only if the mobile radio transceiver station can ignore the downlink channel long enough to perform said attempting step.

8. The method of Claim 1, including time- limiting said attempting step to a predetermined duration of time, and returning to the currently assigned uplink random access channel if said predetermined time expires without the mobile radio transceiver station having obtained said another uplink random access channel in said attempting step.

9. The method of Claim 1, wherein said attempting step includes the mobile radio transceiver station detecting that said another uplink random access channel is unavailable for uplink communications from the mobile radio transceiver station to the fixed-site radio transceiver, and further including the mobile radio transceiver station attempting to obtain a further uplink random access channel in response to said detected unavailability of said another uplink random access channel.

10. The method of Claim 9, including determining, in response to said last- mentioned detecting step, whether the mobile radio transceiver station can ignore a downlink channel corresponding to the currently assigned uplink random a c c e s s channel long enough to perform said last-mentioned attempting step, and performing said last-mentioned attempting step only if the mobile radio transceiver station can ignore the downlink channel long enough to perform said last-mentioned attempting step.

11. The method of Claim 9, including time-limiting said last-mentioned attempting step to a predetermined time duration, and returning to the currently assigned uplink random access channel if said predetermined time expires without the mobile radio transceiver station having obtained a further uplink random access channel in said last-mentioned attempting step.

12. The method of Claim 1, including the mobile radio transceiver station obtaining the another uplink random access channel and replacing the currently assigned uplink random access channel with the another uplink random access channel.

13. A mobile radio transceiver station for transmitting uplink radio communications to a fixed-site radio transceiver in a mobile radio communication system via an uplink random access channel, comprising: a random access channel communications apparatus for controlling communications by the mobile radio transceiver station on an uplink random access channel that is currently assigned to the mobile radio transceiver station; and a search controller coupled to said random access channel communications apparatus, and responsive to an indication from said random access channel communications apparatus that the currently assigned uplink random access channel is unavailable for uplink radio communications from the mobile radio transceiver station to the fixed-site radio transceiver, for directing said random access channel communications apparatus to attempt to obtain another uplink random access channel for transmission of uplink radio communications from the mobile radio transceiver station to the fixed-site radio transceiver.

14. The mobile radio transceiver station of Claim 13, wherein said search controller includes a detector coupled to said random access channel communications apparatus for determining when the currently assigned uplink random access channel will be unavailable for a length of time, and said search controller further including search logic coupled to said detector, and a memory coupled to said search logic, said search logic for comparing said length of time to a predetermined threshold value stored in said memory.

15. The mobile radio transceiver station of Claim 13 , wherein said search controller includes a detector coupled to said random access channel communications apparatus for determining when the currently assigned uplink random access channel has been busy during a number of attempts to access the currently assigned uplink random access channel, said search controller further including search logic coupled to said detector, and a memory coupled to said search logic, said search logic for comparing said number of attempts to a predetermined threshold value stored in said memory.

16. The mobile radio transceiver station of Claim 13 , wherein said search controller includes a detector coupled to said random access channel communications apparatus for determining when the mobile radio transceiver station has lost a number of contests for the currently assigned uplink random access channel, said search controller further including search logic coupled to said detector, and a memory coupled to said search logic, said search logic for comparing said number of lost contests to a predetermined threshold value stored in said memory.

17. The mobile radio transceiver station of Claim 13, wherein said search controller is selectively operable in response to said indicated unavailability of the currently assigned uplink random access channel for directing said random access channel communications apparatus to attempt to obtain another uplink random access channel.

18. The mobile radio transceiver station of Claim 13, wherein said search controller includes a timer which provides a time reference for time-limiting said attempt to obtain another uplink random access channel.

19. The mobile radio transceiver station of Claim 13 , wherein the currently assigned uplink random access channel is a user packet channel (UPCH).

20. The mobile radio transceiver station of Claim 13, wherein the mobile radio communications system is the Personal Digital Cellular (PDC) system.