WO2010107354A1

WO2010107354A1 - High priority random access

Info

Publication number: WO2010107354A1
Application number: PCT/SE2009/050841
Authority: WO
Inventors: Vera Vukajlovic; Tobias Tynderfeldt
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2009-03-17
Filing date: 2009-06-30
Publication date: 2010-09-23

Abstract

Higher priority for a mobile terminal (100) making an emergency call or other high priority access attempt is provided by allowing the mobile terminal (100) to transmit a preamble at a higher initial power than a normal access, or by allowing the mobile terminal (100) to use a larger power ramping step for emergency or high priority access attempts.

Description

HIGH PRIORITY RANDOM ACCESS TECHNICAL FIELD

The present invention relates generally to random access procedures in mobile communication networks and, more particularly to random access for emergency calls and other high priority access attempts.

BACKGROUND

The random access channel (RACH) in LTE networks and other mobile communication networks provide contention-based access to mobile terminals to request connection setup when no traffic channel has been allocated to the mobile terminal. When a mobile terminal needs to establish a connection, the mobile terminal transmits a random access preamble on the RACH to the serving base station. The serving base station sends the Random Access Response message on a downlink common channel in response to the preamble received from the mobile terminal. The response message contains an uplink grant for layer 3 Radio Resource Control (RRC) signaling. The mobile terminal then transmits a random access request message (e.g., connection request or re-establishment request) that contains a unique mobile terminal identity and an "establishment cause" indicating the purpose of the access to the serving base station. The mobile terminal can indicate in the establishment cause that the access is an emergency access. The RRC Request message is the first point in the random access procedure in which requests for emergency calls can be distinguished from the other calls. The cause value can thus be used to prioritize the random access request in the further handling by the serving base station.

Contention between mobile terminals can occur during random access attempts because each mobile terminal randomly selects which preamble to use, and when to transmit the preamble. If multiple mobile terminals transmit the same preamble on the RACH, there will be contention between those mobile terminals that needs to be resolved through contention resolution. There is no guarantee that the contention will be resolved in favor of mobile terminals that need to access the network for an emergency call or other high priority user. There should be a mechanism that gives emergency and other high priority calls higher priority in random access procedures than non-emergency or other low priority calls. Therefore, random access procedures are needed to ensure that emergency calls and other high priority calls receive priority over normal calls in the event of contention.

The TS 36.331 Radio Resource Control Protocol Specification includes Access Class Barring functionality. This functionality allows the network to limit the access attempts due primarily to network overload for normal users not making emergency calls. Access is limited by broadcasting a barring value between 0 and 1, where 1 corresponds to "no barring" and 0 corresponds to "all users are barred." Once this barring value is broadcast, the mobile terminal, before accessing the cell, generates a random number and compares it to the barring value. If the random value is lower than the barring value, the mobile terminal considers the cell available (not barred) and random access followed by connection establishment is allowed. On the other hand, if the random value is greater than the barring value, the mobile terminal considers the cell to be barred and the mobile terminal is not allowed to access the cell. For additional, prioritized access classes 11 - 15 (as stored on the USIM), a bit is broadcast for each class, wherein the broadcast bits indicate whether or not access for these specific classes is allowed (see TS 22.011 and 23.122). Finally, a separate bit, indicating whether or not emergency call is allowed is also included in broadcast information. This bit allows the network, in case of emergency situations, to prevent all other access to the network except emergency access. Access Class Barring, to some extent, controls the number of users accessing the cells and can therefore increase a probability for a successful emergency call. However Access Class Barring is a reactive function and is triggered only as a result of already existing overload, either on the network side or over the radio. This method does not guarantee access in case of RACH overload or latency in case of high load on a collision prone RACH. Of course, triggering of barring by the network can be offset in a way to ensure that users performing emergency calls are successful, but this would compromise cell capacity from the point of view of users using non-prioritized services.

Priority on the random access channel is currently not supported by the LTE standard but may be is essential for a mobile terminal in IDLE mode that needs to access the network both for emergency calls or, for other types of prioritized access with high requirements on latency without barring access of other users and without over allocating capacity for potential priority access. As it is specified today, the random access capacity, in terms of random access occasions, is configurable in the base station and can be provisioned to support high load scenarios. There are several methods of providing priority on the random access channel. One method is to reserve certain preambles for set-up of prioritized sessions, e.g., emergency sessions. In this case, the mobile terminal must recognize the user has dialed an emergency number and select a prioritized preamble when the random access burst is sent (requires additional protocol interactions). The base station receiving a reserved preamble could serve the request with priority. A second method to provide priority for emergency sessions is to reserve some occasions on the PRACH for set up of prioritized sessions only. The occasions reserved for prioritized sessions should be broadcast to the mobile terminals as system information. Neither of these two methods is part of the 3GPP standard Release-8.

Statically reserving resources for high priority access may be very costly from a system capacity point of view. In addition, it may be difficult to dimension resources that should be allocated for priority access. Also, in the scenario where many users are attempting emergency calls at the same time, using only a part of available preambles for emergency calls may increase collision probability on reserved resources and decrease success rate of random access, which seriously impairs system performance.

SUMMARY The present invention relates to the handling of high priority access attempts on a random access channel. In embodiments of the present invention, the power domain is used to prioritize emergency or high priority access attempts over normal access attempts. In one exemplary embodiment, a mobile terminal obtains high priority access by transmitting a preamble at a higher initial power than a normal access. An offset corresponding to the additional power the mobile terminal should add to the preamble received target power when performing emergency or high priority access can be included in broadcast system information. In other embodiments, the base station (e.g., base station 20) can provide a larger power ramping step for retransmissions of the preamble for emergency or high priority access attempts as compared to normal access attempts. Consequently, mobile terminals making an emergency access are more likely to win a contention against a mobile terminal making a normal access.

BRIEF DESCRIPTION OF THE DRAWINGS

Rg. 1 illustrates an exemplary mobile communication network. Fig. 2 illustrates an exemplary random access procedure for a mobile communication network.

Fig. 3 illustrates a probability density function for the received preamble power at the base station.

Fig. 4 illustrates a probability density function for the received preamble power at the base station where high priority accesses use a higher received target power.

Fig. 5 illustrates an exemplary method for random access implemented by a mobile terminal.

Fig. 6 illustrates an exemplary mobile terminal. Fig. 7 illustrates an exemplary base station.

DETAILED DESCRIPTION

Referring now to the drawings, Fig. 1 illustrates an exemplary mobile communication network 10 for providing wireless communication services to mobile terminals 100. Three mobile terminals 100 are shown in Fig. 1. The mobile terminals 100 may comprise, for example, cellular telephones, personal digital assistants, smart phones, laptop computers, handheld computers, or other devices with wireless communication capabilities. The mobile communication network 10 comprises a plurality of cells or sectors 12. Each cell or sectors 12 is served by a base station 20, which is referred to in LTE as a NodeB or Enhanced NodeB (eNodeB). A single base station 20 may provide service in multiple cells or sectors 12. The mobile terminals 100 receive signals from a serving base station 20 on one or more downlink (DL) channels, and transmit signals to the base station 20 on one or more uplink (UL) channels.

For illustrative purposes, an exemplary embodiment of the present invention will be described in the context of a Long-Term Evolution (LTE) system. Those skilled in the art will appreciate, however, that the present invention is more generally applicable to other wireless communication systems, including Wideband Code-Division Multiple Access (WCDMA) and WiMax (IEEE 802.16) systems.

The random access channel (RACH) in LTE systems provides contention-based access to mobile terminals 100 to request connection setup when no traffic channel has been allocated to the mobile terminal 100. Fig. 2 illustrates an exemplary random access procedure that is used at initial access from an IDLE state in order to establish a signaling connection. In step 1 , mobile terminal 100 sends a random access preamble in the uplink (UL). The transmission of the preamble enables the base station 20 to estimate the timing of the mobile terminal 100 and to calculate a timing correction for uplink synchronization. The information on allocated preambles is broadcast to the mobile terminals 100 via system information and includes time and frequency resources used for transmission of random access preambles as well as PRACH preamble format. In addition, information on the number of preambles and grouping of preambles is provided as a part of RACH configuration. In response to the detected random access attempt, the base station 20 transmits a random access response message on the downlink shared channel (DL-SCH) (step 2). The random access response message contains the index of the transmitted preamble, the timing correction calculated by the base station 20, a scheduling grant indicating the resources that the mobile terminal 100 should use for Radio Resource Control (RRC) signaling, and a temporary identifier to identify the mobile terminal 100. The mobile terminal 100 then transmits a RRC random access request message (e.g., connection request or re-establishment request) to the base station 20 (step 3) that contains a unique mobile terminal identity and an "establishment cause" indicating the purpose of the access to the serving base station (step 3). The mobile terminal 100 can indicate in the establishment cause that the access is an emergency access. Contention between mobile terminals 100 may occur when the mobile terminals 100 transmit the preamble because each mobile terminal 100 independently selects the preamble to use and decides when to transmit the preamble. If two or more mobile terminals 100 transmit the same preamble on the RACH at the same time, there will be contention between the mobile terminals 100. An example of contention is illustrated in Fig. 1. In Fig. 1 , two mobile terminals transmit the same preamble, p5, at the same time. A third mobile terminal also transmits on the same RACH₁ but with a different preamble, p1, so there is no contention between this mobile terminal and the other two mobile terminals. In the event of contention, the mobile terminals 100 contending for the channel will both receive the same response message from the base station 20 and thus both have the same temporary identifier. Therefore, in the final step (step 4) of the random access procedure, the base station 20 transmits a contention resolution message to the mobile terminals 100 that contains the mobile terminal identifier received from the mobile terminal 100 that won the contention. The mobile terminals 100 contending for the channel compare the mobile terminal identifier received in the contention resolution message with the one transmitted in the random access request message. If a match is observed, the mobile terminal 100 declares the random access attempt a success. The ability to provide some users with prioirty access on the random access channel is currently not supported by the LTE standard. It is desirable to include prioritization for random access in order to ensure provisioning of the service for emergency calls. Also, the network operator may want to provide high priority access for some classes of users or services types having stringent latency requirements without barring access of other users and over allocating capacity for potential priority access.

The present invention provides a random access procedure for providing priority access on the RACH to mobile terminals 100 needing to place an emergency call and to mobile terminals 100 associated with other high priority users. The inventive random access procedure uses the power domain to prioritize emergency access or high priority access over "normal accesses". In one exemplary embodiment, the mobile terminal 100 obtains priority access by transmitting a preamble at a higher initial power than a normal access. A "delta" value corresponding to the additional power the mobile terminal 100 should add to the preamble received target power when performing emergency or high priority access can be included in broadcast system information. In other embodiments, the base station 20 can provide a larger power ramping step for emergency or high priority access by mobile terminals 100.

For the random access preamble transmission, the mobile terminal 100 uses open loop power control to determine the transmission power. The open loop power control ensures that the mobile terminal 100 transmits with sufficiently high power to be heard by the base station 20, but not so high that it causes interference to other mobile terminals 100 in the same cell 12 or in neighboring cells 12. The preamble transmission power P_PRACH may be determined according to: ppκ_ΛCH = MlN {P_sm , PREAMBLE _RECEIVED JARGET _POWER + PL) , (1) where P_MλX represents the maximum allowed transmission power configured by higher layer signaling, PL represents the path loss, and PREAMBLE _ RECEIVED _TARGET _ POWER represents the targeted received power at the base station 20. The

PREAMBLE _ RECEIVED JTARGET _POWER may be computed according to: PREAMBLE _ INITIAL _ RECEIVED _ TARGET _ POWER +

DELTA _ PREAMBLE + (2)

(PREAMBLE _ TRANSMISSION _ COUNTER - 1) x POWER _ RAMP _ STEP where PREAMBLE JNlTIAL _ RECEIVED JTARGET _ POWER represents the initial preamble power, DELTA _PREAMBLE represents a format based preamble offset, PREAMBLE TRANSMISSION _COUNTER represents the transmission counter value indicating the number of preamble transmissions, and POWER _ RAMP STEP represents a power ramp factor indicating the power ramp step size between transmission attempts.

Due to fast fading variations and noisy path loss estimates, the received power is distributed around the targeted received power as shown in Fig. 3. To increase the success probability for mobile terminals 100 whose received power is too low, the transmission power is increased between transmission attempts. If, upon transmission, the mobile terminal 100 does not receive a random access response within a given time, the mobile terminal 100 repeats the preamble transmission until a maximum number of transmissions (configured by higher layers) is reached. For each successive transmission within one random access cycle, the mobile terminal 100 increases the transmission power by POWER _ RAMP _STEP , and therefore increases the probability of success. If the mobile terminal 100 loses the contention, which may occur, e.g., when the mobile terminal 100 does not receive the contention resolution message, the mobile terminal 100 ramps up the transmission power by increasing the preamble transmission counter.

The open loop power control ensures that all mobile terminals 100 target the same received power at the base station 20 for the initial transmission attempt. Thus, mobile terminals 100 selecting the same random access preamble contend on equal terms if they have made the same number of transmission attempts. On average, each mobile terminal 100 is as likely to win the contention as any other mobile terminal 100. Due to the power ramping, a mobile terminal 100 that has made a larger number of transmission attempts is more likely to win a contention.

The basic idea of the present invention is to use the power domain to prioritize between different users/random access causes. By allowing a mobile terminal 100 initiating random access due to a high priority cause, for example an emergency call, to transmit the preamble with higher power, it is more likely that the mobile terminal 100 with the higher priority cause will win contention over a mobile terminal 100 that initiates random access due to a cause with lower priority.

In one exemplary embodiment, a power offset is added is added to the preamble received target power, PREAMBLE _ RECEIVED _T ARGET _ POWER so that higher priority mobile terminals 100 will transmit with higher power on the first transmission of the preamble. This offset can either be signaled as part of system information transmitted to the mobile terminal 100 or be "hard-coded." In this embodiment, when the Medium Access Control (MAC) layer calculates the preamble received target power, it includes a received target power offset δ . Thus, the preamble received target power may be calculated by:

PREAMBLE _ INITIAL _ RECEIVED _ TARGET _ POWER + DELTA _PREAMBLE + δ + (3)

( PREAMBLE _ TRANSMISSION _ COUNTER - 1) x POWER _ RAMP_ STEP

Alternatively, a higher PREAMBLE JNITIAL _RECEIVED JTARGET _POWER value can be used in Equation (2) for mobile terminals 100 attempting a priority access as compared to mobile terminals 100 attempting a normal access.

Fig. 4 illustrates the probability density function for the received preamble power at the base station 20 where the high priority access uses a higher received target power. The received target power offset δ can be used to tune the probability that a "normal access" wins the contention over a high priority access. The probability that a "normal access" wins the contention over a high priority access should be low.

In another exemplary embodiment, mobile terminals 100 with a higher priority cause are given a larger value for the power ramp step, POWER _RAMP_ STEP , so that the higher priority mobile terminals 100 will increase the transmission power at a faster rate than mobile terminals 100 with a lower priority. That is, a mobile terminal 100 with a high priority cause will use the same received target power at the base station 20 for the initial transmission of the preamble. However, after a few repetitions, the mobile terminal 100 with higher priority cause will have higher received target power than mobile terminals 100 with lower priority and thus be more likely to win the contention. When the MAC layer calculates the preamble received target power, it adds a power ramping offset K to the power ramp step size denoted by POWER _ RAMP _STEP . The power ramping offset K may be specified in the standard for different establishment causes. Thus, the preamble receive target power may be calculated by: PREAMBLE _ INITIAL _ RECEIVED _ TARGET _ POWER + DELTA _ PREAMBLE + ( PREAMBLE _ TRANSMISSION _ COUNTER - 1 ) x ⁽⁴⁾

( POWER _ RAMP_ STEP + K)

In each of the embodiments, it is assumed that additional offsets can be broadcast within system information and if needed, multiple parameters could be defined for emergency access, for example, as well as for different additional high priority causes. Additionally, if the configured maximum allowed power P_MAX is lower than the maximum transmission power capability of the mobile terminal 100, the maximum allowed power could be increased for high priority accesses, either to the maximum transmission power of the mobile terminal 100 or by a maximum transmission power offset γ . For the latter the preamble transmission power may be calculated as:

Pp_HACH = MIN[P₁^_x + γ, PREAMBLE _ RECEIVED _ TARGET _ POWER + PL) . (5)

In another embodiment of the present invention, a mobile terminal 100 attempting a high priority access may transmit at maximum power, or some other specified power level, on the first transmission attempt.

As previously described, some mobile terminals 100 that transmit the same preamble will transmit the random access request (step 3 in Fig. 2) on the same PUSCH resources. If the same preamble received target power is used regardless of priority cause, there is a risk that the base station 20 decodes the random access request message of a contending low priority access even though the base station 20 detected the preamble of the high priority access. Therefore, the power offset between priority causes can also be used for the transmission of the random access request message.

In LTE, the power control of PUSCH, which is the physical channel used to transmit the random access request message, and PUCCH comprises an open loop component based, for example, on downlink path loss measurements and a closed loop component that is controlled through transmit power commands signaled by the base station 20. According to the current standard [3GPP TS 36.213], the closed loop power control functions for PUSCH, /(/) , and

PUCCH, g(i) , are initialized in the following way: /(0) = ΔP_roιnpap + ^_{2 l} and (6)

g(0) = AP_raPpup + δ_λisg2 , (7) where δ_mg2 represents the TPC command indicated in the random access response, and ΔP_mmpup is provided by higher layers (e.g., the MAC layer) and represents the total power ramp- up from the first to the last preamble.

One method of ensuring that the received power of the random access request for a high priority cause is higher than that of a lower priority cause is to include the received target power offset in the AP^^ parameter, AP^^ may then be calculated according to:

where δ represents the received target power offset, N represents the number of preamble transmission attempts, and Δ_ffl(npJlep is the POWER_RAMP_STEP. If a power ramping offset K is used instead, AP_nmpup may be calculated according to:

ΔP_rampup = (N-\)*(A_ro^_slep + tc) . (9) In LTE, it is possible for the base station 20 to indicate backoff to the mobile terminal 100 through the random access response message (step 2 in Fig. 1). There is one MAC subheader that indicates if backoff should be applied before transmitting a new preamble, or if the mobile terminal 100 should transmit a new preamble at the next available PRACH occasion. If backoff is signaled, the latency of each preamble retransmission will increase significantly. In case of too low initial transmission power (missed detection) or high probability of contention, the random access latency could thus increase significantly with backoff.

For high priority access causes, the high priority mobile terminal 100 may ignore the backoff indication and transmit a new random access preamble as soon as possible. As long as the high priority accesses are a small minority of all accesses, this is not expected to impact the overall load on the random access channel. However, if the intensity of high priority accesses exceeds a certain threshold, it could be relevant to apply backoff also for the high priority accesses. This could, for example, be signaled explicitly as part of system information. Alternatively, the backoff for the high priority accesses could be coupled to access class barring. Fig. 5 illustrates an exemplary method 150 implemented by a mobile terminal 100 for making a random access attempt. A triggering event (block 152) initiates the random access attempt. In response to the triggering event, the mobile terminal 100 determines the event/cause that triggered the access attempt (block 154) and determines a priority level (block 156). If the event/cause is a normal or low priority cause, the mobile terminal 100 calculates the preamble transmission power normally according to the standard (block 158). On the other hand, if the event/cause is a high priority cause, e.g., an emergency call, the mobile terminal 100 implements a high priority power control scheme for the random access attempt (block 160). More particularly, the mobile terminal 100 may apply a higher initial receive target power to increase the transmission power on the initial access attempt (block 162). Alternatively, or in addition, the mobile terminal 100 may use a power ramping offset to increase the rate at which the mobile terminal 100 ramps up the transmission power on successive access attempts (block 164). The mobile terminal 100 may also apply the received target power offset or power ramping offset to the random access request (block 166).

Fig. 6 illustrates an exemplary mobile terminal 100 having a transceiver 102 connected to one or more antennas 108, a control processor 104, and a memory 106. Transceiver 102 comprises a transmitter and receiver that operates according to the LTE standard, or other standard now known or later developed. Control processor 104 controls the operation of the transceiver 102 as hereinabove described. The control processor 104 may comprise one or more processors, microcontrollers, hardware, or a combination thereof. Memory 106 stores program code or instructions for causing the control processor 104 to operate as described, as well as data needed for operation.

Fig. 7 illustrates an exemplary base station 20 having a transceiver 22 connected to one or more antennas 28, a control processor 24, and a memory 26. Transceiver 22 comprises a transmitter and receiver that operate according to the LTE standard, or other standard now known or later developed. Control processor 24 controls the operation of the transceiver 22 as hereinabove described. The control processor 24 may comprise one or more processors, microcontrollers, hardware, or a combination thereof. Memory 26 stores program code or instructions for causing the control processor 24 to operate as described, as well as data needed for operation.

The present invention provides advantages over the prior art in that no additional over- provisioning of resources to be used by emergency or high priority access is needed by the system thus optimizing system capacity. Reserving random access may not help in case many users try to do emergency or high priority access, i.e., it may increase risk of collisions thus impairing access to the system. With the described invention, emergency or high priority users may utilize full capacity on random access with the higher probability to succeed (higher probability to "win" the contention) due to increased power at which they access the system. Furthermore, since higher transmission power is used, the probability of missed detection will decrease, which reduces the latency also in case of low random access load, e.g., low probability of contention.

Claims

CLAIMSWhat is claimed is:

1. A method of random access implemented in a mobile terminal (100) in a mobile communication network (10), said method comprising: determining a priority level for an access attempt; determining a transmit power level for transmitting a preamble on an uplink channel based on the priority level of the access attempt; and transmitting the preamble on the uplink channel at the determined transmit power level.

2. The method of claim 1 wherein determining a transmit power level for transmitting a preamble on an uplink channel based on the priority level of the access attempt comprises determining an initial transmit power for an initial transmission of the preamble on the uplink channel based on the priority level of the access attempt.

3. The method of claim 2 wherein determining the initial transmit power for an initial transmission of the preamble on the uplink channel based on the priority level of the access attempt comprises applying a transmit power offset to the initial transmit power to increase the initial transmit power for a high priority access attempt.

4. The method of claim 3 further comprising receiving the transmit power offset from the network (10) over a downlink control channel.

5. The method of claim 1 wherein determining a transmit power level for transmitting a preamble on an uplink channel based on the priority level of the access attempt comprises determining the transmit power for a retransmission of the preamble on the uplink channel based on the priority level of the access attempt.

6. The method of claim 5 wherein determining the transmit power for a retransmission of the preamble on the uplink channel based on the priority level of the access attempt comprises applying a power ramp offset to compute a transmit power for a retransmission to increase the power step size between retransmissions for high priority access attempts.

7. The method of claim 6 further comprising receiving the power ramp offset from the network (10) over a downlink broadcast channel.

8. The method of claim 1 wherein determining a transmit power level for transmitting a preamble on an uplink channel based on the priority level of the access attempt comprises determining the maximum allowed transmit power for a transmission of the preamble on the uplink channel based on the priority level of the access attempt.

9. The method of claim 1 further comprising retransmitting said preamble without regard to back-off indications received from said base station for high priority access attempts.

10. The method of claim 1 further comprising: receiving a random access response; determining a transmit power level for a random access message transmitted on an uplink channel responsive to the random access response based on the priority level of the access attempt; and transmitting the random access message at the determined transmit power level.

11. A mobile terminal (100) for a mobile communication network (10) comprising: a transceiver (102) for transmitting a preamble on an uplink channel during a random access attempt; and a control processor (104) configured to: determine a priority level for the random access attempt; and determine the transmit power level for transmitting the preamble on the uplink channel based on the priority level of the random access attempt.

12. The mobile terminal (100) of claim 11 wherein the control processor (104) is configured to determine an initial transmit power for an initial transmission of the preamble on the uplink channel based on the priority level of the access attempt.

13. The mobile terminal (100) of claim 12 wherein the control processor (104) is configured to applying a transmit power offset to the initial transmit power to increase the initial transmit power for a high priority access attempt.

14. The mobile terminal (100) of claim 13 wherein the control processor (104) receives the transmit power offset from the network (10) over a downlink control channel.

15. The mobile terminal (100) of claim 11 wherein the control processor (104) is configured to determine the transmit power for a retransmission of the preamble on the uplink channel based on the priority level of the access attempt.

16. The mobile terminal (100) of claim 15 wherein the control processor (104) is configured to apply a power ramp offset to compute a transmit power for a retransmission to increase the power step size between retransmissions for high priority access attempts.

17. The mobile terminal (100) of claim 16 wherein the control processor (104) is configured to receive the power ramp offset from the network over a downlink control channel.

18. The mobile terminal (100) of claim 11 wherein the control processor (104) is configured to: receive a random access response; determine a transmit power level for a random access message transmitted on an uplink channel responsive to the random access response based on the priority level of the access attempt; and control the transceiver (102) to transmit the random access message at the determined transmit power level.

19. The mobile terminal (100) of claim 11 wherein the control processor (104) is further configured to determine a maximum allowed transmit power for a transmission of the preamble on the uplink channel based on the priority level of the access attempt.

20. The mobile terminal (100) of claim 11 wherein the control processor (104) is further configured to retransmit said preamble without regard to back-off indications received from a base station (20) for high priority access attempts

21. A method implemented by a base station (20) in a mobile communication network (10) of providing priority access on a random access channel, said method comprising: transmitting a power parameter for high priority access from said base station (20) to one or more mobile terminals (100) over a downlink control channel, wherein said power parameter is used by a mobile terminal (100) attempting a high priority access.

22. The method of claim 21 wherein the power parameter comprises a power offset to be added to a received target power by a mobile terminal (100) attempting a high priority access.

23. The method of claim 21 wherein the power parameter comprises a power ramp step for a mobile terminal (100) attempting a high priority access.

24. A base station (20) in a mobile communication network (10) comprising a transceiver (22) and a control processor (24), said control processor (24) configured to: transmit a power parameter for high priority access via said transceiver (22) to one or more mobile terminals (100) over a downlink control channel, wherein said power parameter is used by a mobile terminal (100) attempting a high priority access.

25. The base station (20) of claim 24 wherein the power parameter comprises a power offset to be added to a received target power by a mobile terminal (100) attempting a high priority access.

26. The base station (20) of claim 24 wherein the power parameter comprises a power ramp step for a mobile terminal (100) attempting a high priority access.