WO2005034382A1

WO2005034382A1 - Method for power ramping in a telecommunication system

Info

Publication number: WO2005034382A1
Application number: PCT/SE2004/001394
Authority: WO
Inventors: Johan Torsner; Stefan Parkvall
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2003-10-06
Filing date: 2004-09-28
Publication date: 2005-04-14
Also published as: SE0302655D0

Abstract

The present invention aims to solve the problem that the preamble ramping process on a random access channel (RACH) of a 3rd generation telecommunication network can take a considerable time and, as a consequence, cause significant delays in RRC signalling and user data transmission on RACH. The present invention suggests to base the calculation of the initial preamble power value on the last performed RACH transmission instead of calculating the initial preamble power from the information that is broadcasted in the system information. This results in fewer preamble rampings until the preamble is detected in the Node-B.

Description

METHOD FOR POWER RAMPING IN A TELECOMMUNICATION SYSTEM

FIELD OF THE INVENTION

The present invention relates to a method for power ramping on the random access channel (RACH) in a mobile 3^rd generation telecommunication system.

BACKGROUND OF THE INVENTION

Figure 1 shows the power ramping procedure on the random access channel (RACH) as disclosed in current 3GPP- specifications . The initial power value P_ιrut is calculated from information that is broadcasted by the UMTS Radio Access Network (UTRAN) in its system information and calculated from the measured received power. At each transmission on the RACH a new preamble ramping procedure is started, where the value of P_ιnιt is recalculated. This implies that, if all parameters in the system information as well as the measured received power remain constant, the value of P_ιnιt will be the same for each RACH access.

The information that is broadcasted in the system information and used to calculate the initial preamble power can only be broadcasted rather infrequently due to the limited resources on the broadcast channel. Typically the needed information can be broadcasted every 160-320ms. However, this also means that the received information may be quite old when the preamble ramping process actually starts. The specified procedure in 3GPP also includes a back-off mechanism if a maximum number of preambles have been reached without having received any acknowledgement from the Node-B. Also other parameters than mentioned above can be configured by UTRAN, namely the step size Pstep, and been reached without having received any acknowledgement from the Node-B. Also other parameters than mentioned above can be configured by UTRAN, namely the step size Pstep, and the time interval between preambles . The power ramping procedure and the calculated preamble initial power is specified in technical specification documents 3GPP TS25.214: "Physical layer procedures (FDD)", 3GPP TS25.321: "Medium Access Control (MAC) protocol specification", and 3GPP TS25.331: "Radio Resource Control (RRC) protocol specification" all issued by the 3^rd Generation Partnership Project.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an overview of the current RACH-preamble structure according to the state of the art.

DETAILED DESCRIPTION OF THE INVENTION

In a Wideband Code Division Multiple Access (WCDMA) communication system different types of transport channels are specified for various purposes. Typically a large amount of data is transmitted on dedicated channels whereas small amounts of data are transmitted on common/shared transport channels. In order to be able to communicate on dedicated channels, resources must first be negotiated between a user equipment (UE) and the UTRAN. Also this negotiation is performed on common channels. Thus, even if the user data is transmitted mainly on dedicated channels, an initial signalling must first be performed on common channels before a dedicated channel can be used.

Uplink transmissions on the random access channel do not use closed loop power control. Instead, a probing process is used where the user equipment transmits preambles with gradually increasing power values until the Node-B can detect the preamble and will send an acknowledgement on the AICH. This procedure is known as power ramping. When the user equipment receives the acknowledgement from the Node-B the power ramping procedure is aborted and the actual message is transmitted with the same power as the last transmitted preamble. Thus, a suitable transmission power for the RACH message can be found without closed loop power control .

In order not to cause too high interference the initial preamble power cannot be too high. In addition, due to inaccuracies of measurements in the user equipment and the fact that the latest received system information can be several hundred milliseconds old, the parameters in the calculation must be set to achieve a rather conservative, i.e. sufficiently low, initial power. Therefore, the method according to the present invention aims to solve the problem that the preamble ramping process on the random access channel can take a considerable time and, as a consequence, cause significant delays in RRC signalling and user data transmission on RACH. The problem is particularly evident when the transmitted data does not fit into one TTI . In this case, the data need to be segmented into several TTI : s and the preamble ramping process need to be applied for each TTI.

The solution according to the present invention suggests to base the calculation of the initial preamble power value on the last performed RACH transmission instead of calculating the initial preamble power from the information that is broadcasted in the system information. The transmission power of the latest RACH transmission is a better estimate of the needed power than provided by the current calculations according to the state of the art and, consequently, a smaller margin can be used when calculating the initial preamble power. This in turn results in fewer preamble rampings until the pre.amble is detected in the Node-B, i.e. a lower delay is required for the ramping process. Hence, it is beneficial to set the power for an attempt to access the random access channel as a function of the latest, i.e. the previous, RACH transmission. In a preferred embodiment of the present invention the back-off from the latest performed RACH transmission is increased over time in order to consider that the radio conditions may have changed since the latest RACH transmission occurred. This can be achieved, e.g., with a simple linear filter. The example below illustrates one conceivable approach to calculate the preamble power Pιnιt₍₁₎ that is used at a frame i:

Pimt₍ = τaax(P_w ,Pinit_(fΛIS) )

P(_θ) denotes the transmission power of the latest RACH transmission, P₍i) is the calculated preamble power if a RACH transmission takes places in frame i, and Pini (_Rei₅) is the preamble power that is calculated according to the current specifications. Finally, (O≤α≤l) denotes a filter coefficient.

This method will lead to a back-off that is increasing over time until the calculated preamble power equals the power according to the one defined in the current specifications. By configuring different values of the filter coefficient α, the UTRAN can control how aggressive this back-off shall be.

A simpler alternative of the method described above would be to apply a fixed back-off from the power that was used for the latest RACH transmission if the subsequent RACH transmission takes place within a given time. The back-off and the time during which the aggressive setting is allowed to be used is configured by the UTRAN. If no transmission is done within said configured time the current method according to the state of the art as defined in Release 5 of the 3GPP-specifications is used.

As an additional embodiment of the present invention, a more aggressive RACH setting could be achieved by changing the power step size Pstep in the ramping process, either alone or in conjunction with adjusting the initial power value. If a larger power step size is used, the ramping procedure will be faster; however, this is possibly at the cost of increased interference in the system. A larger power step size could be used during the aggressive period, and the user equipment returns to the normal setting once the backoff time has expired.

Another embodiment of the present invention takes the traffic priorities into account for the process. High priority traffic, for instance, could employ a more aggressive ramping scheme as outlined above, while low priority traffic always uses the "normal" set of ramping parameters .

Still another embodiment of the present invention is to use a similar approach also for the first RACH transmission. In this case the current solution would be extended with a time varying back-off, e.g. with a linear filter as described above. The result would be that the back-off is small when the information in the calculation is recent and gradually increasing when the information gets older. This would mean that UTRAN could apply a more aggressive setting of the initial power also for the first RACH transmission. The method allows a more aggressive power ramping scheme where the initial preamble power can be set higher than with the current solution and still avoiding excessive interference. This results in a faster power ramping cycle which can significantly reduce the delay for RACH transmissions. A reduced delay on RACH improves the performance both for control signaling and user data.

Claims

1. A method in a telecommunication system comprising a random access channel (RACH) , c h a r a c t e r i s e d b y determining the RACH initial preamble power of a subsequent RACH transmission based on the power value of the latest RACH transmission.

2. The method according to claim 1, whereby the RACH initial preamble power is based on a priority value for the message to be transmitted.

3. The method according to claim 1 or 2 , whereby the power step size of the initial preamble procedure is variable.

4. The method according to claim 1 or 2 , whereby a fixed back-off value compared to the one of the latest RACH transmission is used during a given time.

5. The method according to claim 1 or 2 , whereby the backoff value is increased over time.

6. The method according to claim 5, whereby the back-off is increased by using a linear filter.

7. The method according to one of the preceding claims, whereby the initial preamble power for the first RACH transmission uses an offset that increases over time.