WO2002054623A1

WO2002054623A1 - Allocation of amplification stages to particular transmissions to obtain favourable thermal load distribution

Info

Publication number: WO2002054623A1
Application number: PCT/SE2001/002447
Authority: WO
Inventors: Kristian Lindskog; Jacob ÖSTERLING
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2000-12-29
Filing date: 2001-11-06
Publication date: 2002-07-11
Also published as: SE0004921D0; SE0004921A0

Abstract

An Energy Managing routine, EM, for a cellular telecommunication system. Information relating to power demands for messages to be transmitted from the system, BSS, by base station transceivers, BTS 1-3, are collected from the systm together with information related to heat generated in power amplification stages. The EM allocates messages to be transmitted to amplification stages in transmitting units with due regard to power distribution among the amplification stages, power dissipation and temperature cycling.

Description

Allocation of amplification stages to particular transmissions to obtain favourable thermal load distribution

TECHNICAL FIELD OF THE INVENTION

The present invention relates to power amplification in radio base stations for cellular mobile communication systems. In particular the invention concerns power control and a way to increase the mean time between failure, MTBF, in the power amplification equipment of such systems.

BACKGROUND

In cellular telecommunication systems such as systems working according to the TDMA-principle, the transmission is sent out in the form of TDMA-frames, each divided into several time slots. In the Global System for Mobile communication, GSM, the frames are built up by eight time slots each with the duration of approximately 0,5 milliseconds. The short burst of information sent out during a time slot could be aimed for just one particular receiver. Other pieces of information sent out are speech frames of 20 ms thus occupying a number of time slots.

A base station transceiver, a BTS, is responsible for communicating information over the air interface to a remote mobile station MS. The BTS is composed of various parts such as the actual transceiver unit TRX with its RF- modulator, power amplifier and antenna at the radio end, and a base band unit with channel control and burst formatting equipment in the other end. This last end is connected to the base station controller, BSC, via the Abis interface. A BSC with a number of served BTSs is normally referred to as a base station system, BSS. At typical base station sites, there are several transceiver equipments arranged with antennas transmitting and receiving information on different frequencies and also covering different sectors of the area the sites are serving .

Traditionally there is a one to one mapping between the radio part and the control unit of each TRX and the output power from the radio part is dependant on the demand for a certain transceiver seen as a whole unit. The output power for one connection BTS-MS is not constant over time. There are several functions, which could alter the output power such as :

• TRX power control . A closed loop power control is used to minimise the output power of the transmitter. The aim is to keep down the interference level in the network. • Discontinuous transfer (DTX) . If the user on the base station side of an established connection between calling parties is quiet for more than a certain time period, only a control message is transmitted to the MS stating so. This control message contains much less data than the voice-coded speech does, resulting in many timeslots being unused.

As a conclusion and because traffic demand changes, the TRXs seldom, or never, uses full output power on all time slots. The output power of each connection varies on a short-term basis; even from burst to burst in single time slots of just 0.5 milliseconds duration.

The amplifiers of a BTS could be categorised as SCPA or MCPA. A Single Carrier Power Amplifier (SCPA) amplifies only one signal (carrier) at the time. Each amplifier has an upper limit regarding the highest desired output power it can deliver. This limit may differ over temperature. Different amplifiers in a BTS may also support different modulations .

A Multi Carrier Power Amplifier (MCPA) amplifies a number of signals (carriers) simultaneously. The MCPA has an upper output power limit, which is related to the sum of desired output power of the carriers it shall amplify, the number of carriers amplified simultaneously and also to the characteristics of each carrier like its modulation.

Because of the varying demands of the power amplifiers the temperature of the amplifiers and their surroundings varies. Normally cooling and overtemperature guards take care of the excess heat produced and other problems caused.

Another drawback with the varying temperature is the temperature cycling. The mean time between failure (MTBF) of a power amplifier depends mainly on this temperature cycling the power transistor has experienced. To maintain a high MTBF it is thus essential to keep the temperature of the power amplifiers as constant as possible or at least keep variations slow.

Especially the soldering together of the various components in the amplifier stage is sensitive to temperature cycling, but also e.g. bond wires within the transistor chips. The damaging effect is that the soldering, and bond wires, repeatedly is expanded and compressed due to the temperature variation.

Most BTS products have an overtemperature protection of . their power amplifiers. The working temperature range of the power transistor can vary from room temperature up to about 100 °C without being damaged. However, if the temperature rises too much for the design, the power amplifier must be taken out of use. The dissipated power from the power transistor depends directly on the input power. If the power amplifier has an efficiency of 33% at 10 , a desired output power of 10 would result in 30 being consumed and 20W dissipated. The dissipated power will heat the transistor. On the other hand, if only 5 W output power is required from the same power amplifier, about 21 will be consumed and 16 dissipated (efficiency decreases if less output power than biased for is requested) The relation between efficiency and output power depends on the amplifier design and is deterministic .

A way to control the temperature of the power amplifier is therefore to control the output power from the amplifier. One exemplary temperature protection mechanism for a power amplifier is to decrease the accepted desired output power level when the temperature is higher than a threshold. As a result of such an algorithm, a BTS which is designed for six 50 W power amplifier may, when operating out of temperature range, be configured to only accept 50 W output power on three of the power amplifiers, and the rest only supporting e.g. 30 .

The dissipated power is lead to cooling fins, which could be air-cooled e.g. by forced ventilation from a fan.

Prior art examples concerning protection of power amplifiers in radio base stations are found e.g. in the patent publications JP 11 312 989 and JP 11 243 320 together with WO 00/19602. A common feature concerning all three documents is protection against overheating. The last document also describes a cold start procedure with enhanced power consumption. The state-of-the art solutions work on a local basis - turning on and off the power amplifier based on the temperature of that particular power amplifier.

Other prior art solutions relate to cooling control and the use of air condition equipment. Fans can be regulated on the basis of temperature measured in the vicinity of power amplifiers and other power consuming components.

SUMMARY OF THE INVENTION

Although the prior art has coped very intense with the overtemperature problem, no solution has been presented also to overcome the problems of temperature cycling. Another drawback is the local control of the power amplifiers, which limits flexibility and makes an overall general power control of the power consumption in a base station system difficult.

It is an object of the present invention to introduce a flexible solution to the prior art problems. The object is to distribute the transmissions of messages intended for remote users within a radio base station system depending on the power needed for each particular transmission. Still another object is to distribute the transmissions with the goal to obtain a favourable thermal load on the amplification stages with due regard to power dissipation and temperature cycling.

These objects are accomplished by a method and an arrangement for Energy Managing of Power Amplifiers in a Telecommunication Interface, EMPATI . The solution is to manage the energy or power needed for the amplifiers of a base station transceiver based on the particular tasks for the station to handle. As mentioned above these tasks vary considerably between different TRXs, sometimes on a very short-term basis. The energy managing comprises assigning to each transmitter different tasks, on short- or long- term basis, based on the power needed for the tasks with the aim to keep the amplifiers busy on a more or less steady state basis.

According to a preferred embodiment the managing is performed by an Energy Manager, a unit collecting all information about the transmissions being asked for by the system for a particular base station in a certain moment, including the state of each power amplifier. The information involves a lot of data e.g. the kind of signalling over control or traffic channels, burst formatting, carrier allocation, modulating schemes, power required and the temperature and other conditions of the amplifiers like their performance and degree of efficiency for different power demands.

The invention greatly improves the flexibility of power control in a radio base station system by making it possible to more or less momentary change the allocation of radio transmitters depending on the respective power demand for messages to be broadcasted by the system. A further merit of the invention caused by the greater flexibility is the possibility to give the individual users the power they really want or actually need. This is not an obvious feature of today's systems.

The invention is further defined in the appended claims concerning a method, an arrangement and a device for power control in a radio base station system.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic overview of a traditional GSM- system with an Energy Manager according to the invention introduced in the chain from the BSC to the antenna in a base station system. Figure 2 illustrates a preferred embodiment of the present invention.

Figure 3 is a diagram explaining the underlying idea of the invention. Figure 4 is a flow chart showing an example according to the invention. Figure 5 shows the Energy Manager in a block diagram.

DETAILED DESCRIPTION OF THE FIGURES AND OF PREFERRED EMBODIMENT

Figure 1 is a general overview of the present invention. The radio base station system, BSS, preferably according to the GSM-standard is shown as a cloud. The inside of the system is occupied by all the usual items of the BSS, that is the BSC, a number of base station transceivers, BTS 1- BTS 3 and Abis interfaces connecting the BSC with the various base stations. The transmission input to the BSS is delivered by the Mobile Switching Centre, MSC and the downlink output is transmitted from the antennas A1-A3 to remote receivers.

The Energy Manager, EM, according to the invention collects information from the BSS and its inherent components, the BSC, the Abis interfaces and the base stations, wherever information about the transmission can be collected. Some information about bursts to be sent may be collected even at the input of the BSC, other information at its connection over the Abis interface to a particular BTS or at the base band part of the BTS itself all depending on the circumstances in the individual case.

The Energy Manager further collects information about the amplification stages of the transmission parts of the base stations BTS 1-3. The most important information concerns the temperature at and in the vicinity of the amplifier. The type of amplifier is also registered together with information about its actual performance, degree of degradation, e.g. due to ageing or experienced overtemperature .

Based on collected data the Energy Manager optimises the allocation of transmitted messages to amplification stages with due regard to temperature and temperature fluctuations of the stages in order to keep the temperature cycling down thereby keeping the MTBF up.

Another goal is to prioritise amplifiers with the highest degree of efficiency for a particular task. As mentioned above in the Background section, the efficiency of an amplifier is dependent on the power level it is adjusted to for the moment . Lower power levels is normally less efficient than higher. To keep dissipated power low the "best" amplifier should be preferred. The efficiency and its power dependency are inherent qualities of the amplifier, but it changes as the amplifier ages. The Energy Manager is updated with such information as well.

The two goals, low temperature cycling and low power consumption, are not always compatible with each other. Other reasons like economy, age of the equipment, need for maintenance will therefore also influence the choice.

An example of how the invention could be realised more in detail is illustrated in figure 2. In this figure three transceivers, TRX 1-3, belonging to one base station in a radio base station system are shown. The ordinary one to one mapping of the parts of each TRX are split up and to the left there are three base band parts la-lc and to the right three radio parts 2a-2c. The invented EM 4 is situated in-between and connected to all six parts of the base station via links 5a-5c and 6a-6c respectively. Each base band unit has a channel control unit and a burst formatting unit. The radio parts 2a-2c are composed of a modulator and a power amplifier 22. In close proximity to the amplifier a temperature sensor 23 is arranged. The outputs from the power amplifiers are directed via a power combiner 7 to the antenna. Instead of using a combiner, the outputs could be directed to separate antennas. The EM in the figure is used as a switching circuit which is able to connect any base band part la-lc with any radio part 2a-2c. The EM receives signals from temperature sensors 23 representing the heat developed at the site of the power amplifiers 22 and power demands from the base band parts la-lc together with additional information relating to the transmissio .

The purpose of connecting the EM between the burst formatting and channel control units of the base band part and the modulator of the radio unit, is because this is normally the last point where the information is still in digital form.

The EM has knowledge of each power amplification stage about :

• What output power it can transmit at and its degree of efficiency.

• The temperature at the power transistor of the amplifier.

• What modulation it supports (8PSK, GMSK and others) .

• What combinations of carriers it can amplify according to a multiple carrier power amplifier system (MCPA) . • What temperature threshold the maximum output power must be accounted for for each power amplifier ("alarm limit") .

• Cooling characteristics of the amplifier. Optionally the EM also has knowledge about the BTS mechanical design to optimise the allocation, such as:

• Cooling capacity of the cabinet . • The desired temperature distribution within the cabinet depending on cooling system design.

• Inlet and outlet air temperature values collected from further temperature sensors 24

One output from the EM is also used for control, via control circuit 25, of cooling fans 26 and other cooling and air condition equipment of the BTS. In cold environments the BTS could also be equipped with controlled heating arrangements.

An illustrative example of a possible switching scheme managed by the EM is shown in figure 3. Assume that an upcoming power demand for the TRXs of the base station is low, moderate and high for the respective transmissions to be delivered by the base band units la-lc. The EM has recorded low, moderate and high temperature at the respective amplifiers 23 in the radio parts 2a-2c. A suggestion for the EM is to assign the low demand in base band unit lc for the radio unit 2c with the low temperature and the high demand in la for the moderately heated radio unit 2b, which in this simple example, leaves the moderate demand in lb to the most heated unit 2a. With this solution the temperature changes will be kept low and at the same time there will be a less risk of overheating of the amplifiers.

To choose a scheme with a low^;- low, moderate - moderate and high - high combination, which leads to the least temperature change is not that suitable because it will keep an already hot amplifier at an high temperature level, which eventually could lead to breakage. A sudden high - low, low - high, moderate - moderate scheme tending to even up the temperature level is neither preferred. It involves too much and rapid temperature cycling. To even up the temperature is left to later power demands making temperature changes slow.

This example is not exhaustive. Assume that the hottest amplifier has the best efficiency, then this amplifier could be chosen anyway for the highest demand at least if there is no risk of overheating. Another variant is that the hottest amplifier could be exposed to extra external cooling. Also when designing the base station, an amplifier expected to be exposed to high power demands could be housed in the base station cabinet in a place with optimal cooling.

Another approach is to assign the same mean output power to all power amplifiers. This must however be done with some caution since there otherwise is a risk for temperature cycling due to varying load. Seen over a long time, this will cause all power amplifiers to run the same amount of traffic.

In a multi carrier amplifier, MCPA, the allocation combinations increase since each MCPA transmits a set of bursts. This allows for another degree of freedom when controlling the output power - or rather - the dissipated power of the PA. Since the dissipated power depends on the average power of the MCPA, but the combination is limited by the momentary top power of the MCPA, the bursts can be allocated both to maximise the efficiency and to control the temperature .

The dissipated power from an MCPA depends on the efficiency and the average output power. The efficiency depends on the wanted momentary peak power, which in turn depends on the average output power and the number of carriers. Controlling the dissipated output power by multiple MCPAs can therefore be done in different ways:

• Minimising the total dissipated power

• Having the same dissipated power

• Having any wanted relationship of dissipated power (as long as within the performance specification of the MCPA) .

The more carriers the greater the possibilities to achieve a wanted effect. To achieve maximum effect the bias shall be altered on time slot basis.

The algorithm can be enhanced by not only looking at the power of the bursts but also at their contents - to calculate the absolute momentary top power and to remove such by limiting the signal or adding signals in anti- phase.

For all allocation strategies, the EM must take into consideration the maximum output power that each power amplifier may output. This may differ due to over temperature protection or simply because two or more different power amplifier models are used in the same BTS. Also, the EM must take into consideration what other capabilities the power amplifier, and the radio associated with it, can handle (modulation, frequency range, etc) .

In figure 4 a flow chart is shown describing how the EM can work according to a further example. The temperature of the amplifiers PA has been read in step 41. In step 42 the target energy dissipation is estimated for a next burst to be handled by the respective PA. The estimation is based on the actual temperature registered in step 41 and the performance of the respective PA, such as its efficiency, degree of degradation, what modulation it supports etc. By this estimation the system will be aware of a suitable power level for the PA in the next burst. In the last step, 43, incoming new bursts are routed by the EM to appropriate PAs based on power and modulation demand for the new bursts. Reading the temperature again in step 41 closes the loop.

The flow in figure 4 makes one turn for each burst, that is every 0.5 ms . Of course it is possible to use the same routing for several burst if new bursts have about the same demands and as long as there is no considerable change at the PA in question.

With these principles in mind the skilled person in the art to which the invention pertains, realises how more comprehensive solutions based on well known estimating and prediction algorithms can be accomplished taking account of the relevant parameters of the transmissions and conditions of amplifiers aiming to keep low power consumption, minimise temperature variations or at least keep such variations slow and at the same time avoid overheating.

The Energy Manager is illustrated in figure 5. Data from the BSS relating to messages to be transmitted is input to a buffering memory 51a. The temperature values registered by the temperature sensors 23 and 24 in figure 2 is input to another memory 51 b. The data information in these memories 51a and 51b is thereafter transferred to the calculating logic 52. The calculating logic is also supplied with fixed information from a memory 53 relating to the radio amplifiers and the configuration of the cabinet housing the radio units and cooling facilities. This memory is updated as soon as there are changes in the fixed parameters, for example when an aged amplifier is replaced by a new one or when a new fan is installed in the cabinet .

The result of the calculation is controls switch 54 connecting the radio units to units delivering the bursts to be transmitted over the air. Calculations resulting in changes in the cooling of the cabinet is directed to a cooling control unit 56, which sends control information to fans or other air-conditioning equipment in the cabinet.

The invention has mainly been described with the GSM-system in mind. It is very well suited for circuit switched such systems with control on a time slot basis. However the invention is also applicable for other systems based on the TDMA-principle, for GPRS with longer radio block levels and on vocoder speech periods. The invention is relevant also for other cellular standards like, EDGE, IS95, WCDMA and UMTS.

Claims

1. A method in a cellular mobile telecommunication system for controlling power consumed in heat generating components, such as power amplification stages in a radio base station system (BSS) of said telecommunication system, the radio base station system (BSS) being responsible for the transmitting and receiving of communication signals over a radio interface to and from mobile station served by the telecommunication system c h a r a c t e r i s e d in an Energy Managing routine, EM, whereby a) data related to the transmission of communication signals - data such as required output power, duration, modulating scheme, carrier frequency and destination - is collected from the radio base station system (BSS) , b) data related to the individual power amplification stages in transmitting units (BTS 1-3) of the base station system (BSS) - data such as type of amplifier, its present performance, its efficiency and the actual temperature of the amplifier - is collected from the transmitting units (BTS1-3) of the system c) the allocation of ampli ication stages to particular transmissions asked for by the communication system is governed by the EM in relation to the collected data in order to control the choice of individual amplifiers thereby obtaining a favourable thermal load distribution on the amplification stages.

2. The method in claim 1 wherein the control is aimed to minimise the temperature variation in the amplification stages.

3. The method in claim 1 or 2 wherein the control is aimed to minimise the power consumed in the amplification stages.

4. The method in claims 1-3 wherein the data related to the transmission is collected from the base band units (la-lc) of transceivers belonging to base stations (BTS1-3) in the system.

5. The method in claims 1-4 wherein fixed data related to the power amplification stages is registered by the EM in advance and that the temperature in the vicinity of the ampli ication stages is registered by sensors (23) and regularly reported to the EM.

6. The method in claim 1-5 wherein the control loop (41-43) has a duration of one time slot in a TDMA- ystem and wherein a target power dissipation for an amplification stage in the next time slot is estimated (42) based on the registered temperature (41) and the performance of the amplification stage.

7. An arrangement in a telecommunication system including a radio base station system (BSS) with a base station controller (BSC) , and at least one base station transceiver (BTS 1-3) with a number of transceiver units (TRX 1-3) c h a r a c t e r i s e d in Energy Managing means, EM, collecting (5a-c) data related to the transmitting power needed for sending messages to remote users of the communication system together with data (23) related to heat generated in power amplification stages (22) in the radio parts (2a-2c) of the transceivers, wherein the collected data is used to allocate the messages to be sent to selected radio parts based on the power demand for the transmission.

8. The arrangement of claim 7 wherein the transceiver units are divided into two groups, the base band parts (la-lc) and the radio parts (2a-2c) with the EM arranged as an interface between the base band parts and the radio parts .

9. The arrangement of claims 7 or 8 comprising cooling control means (25) for controlling the temperature within the base station cabinet with the aid of climate control equipment (26) , the control being based on the registered temperature of the power amplification stages (22) .

10. The arrangement of claim 9 wherein the control is further based on temperature measured by sensors (24) within the cabinet but outside the transceiver units.

11. An Energy Manager in a telecommunication system for controlling the output power of transmitting units (2a-2c) of the system c h a r a c t e r i s e d in first means (51a) for collecting data related to the transmissions of messages to be sent by the system and second means (51b) for collecting data related to the heat generated in the transmitting units and in allocating means (54) for allocating messages to be broadcasted by the transmitting units (2a-2c) based the data collected by the first means (51) .

12. The Energy Manager of claim 11 further comprising a memory (53) storing fixed data relating to the performance of the amplifiers (22) in the transmitting units and a cooling control circuit (56) responsible for control of air- conditioning of the base station cabinet.

13. The Energy Manager of claims 11 or 12 further comprising a calculating logic (52) calculating the routing of messages to be transmitted to selected transmitting units .

14. The Energy Manager of claims 11-13 wherein the allocating means (54) controls a switching unit (55) connecting burst generating means (la-c) with transmitting means (2a-c) .