WO2003019851A1

WO2003019851A1 - Method for improving quality of transmitted data

Info

Publication number: WO2003019851A1
Application number: PCT/EP2001/009804
Authority: WO
Inventors: Thomas Stichelbout
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-08-24
Filing date: 2001-08-24
Publication date: 2003-03-06

Abstract

The present invention relates to a method for transmitting data between a mobile user equipment and a base station, a receiving unit and a RF-system for data transmission between a mobile user equipment and a base station. The object of the present invention is to provide a method, a receiving unit and a RF-system having an improved efficiency of data transmission and enhanced quality of the signals. It is a further object of the present invention to reduce the number of external measurement facilities avoiding manual operation of skilled operators. A even further object of the present invention is to achieve the aforesaid objects at a minimised amount of energy consumption at least at the side of the mobile equipment. As a solution a method according to the present invention proposes that during a communication with a given mobile station MS of a user, the base-station BS and the mobile station MS build a closed control loop L adapting the base-band characteristics to optimise the communication with this particular mobile station MS.

Description

METHOD FOR IMPROVING QUALITY OF TRANSMITTED DATA

The present invention relates to a method for transmitting data between a mobile user equipment and a base station, a receiving unit and a RF-system for data transmission between a mobile user equipment and a base station.

Within the field of radio communication, such as satellite communication or mobile communication, there is an increasing need of capacity, channels and bandwidth for data transmission caused by the still growing number of users and data services .

However, more complex methods of coding, modulation and strict standards are used to supply sufficient numbers of channels of a determined quality. So the Global Standard for Mobile communication GSM became extended to Enhanced Data rate for GSM Evolution, GSM-EDGE, using a quadrature modulation technique to meet the aforesaid needs.

Usually a direct conversion receiver is used in a radio frequency receiving unit. This is a receiver with a single mixer in the receive path, where the RF modulated frequency carrier is converted to a base-band directly. That means that the RF modulated frequency is transformed in a single step with the help of a frequency mixer and an appropriate local oscillator. However, in complex modulation scheme such as e.g. GSM- EDGE, the quality of the modulation transmitted at high frequency is highly sensitive to the linearity of the transmission chain and impairments such as amplitude, quadrature skew and phase noise, which are added during up and down frequency conversion. Each impairment added during the transmission of the signal degrades the received signal, including the impairment in the receiver chain. At the present time, if taken into account the amplitude and the phase imbalance of an in-phase component I and in- quadrature phase component Q of a quadrature modulated signal, these signals usually are tuned manually to optimise the modulator in the transmitter side of the transceiver. Such a method and a system for calibrating a quadrature phase modulator are known from US5847619. According to the disclosure of this document a spectrum analyser is placed at the transmitter TX output to calibrate the transmitter. This method is well suited for a single time calibration. In a mass production process this method is much too time consuming as manual operation is included. However, the process calibration requires skilled operators. Further more this method does not insure that the tuning will remain valid if there is a drift in impairments e.g. due to ageing of the device. Furthermore this calibration only optimises the transmitter side of the transceiver. Any impairment in the mixer placed after a low noise amplifier within the receiver is usually not compensated by hardware tuning.

US5930286 discloses a gain imbalance compensation for a quadrature receiver in a cordless direct sequence spread spectrum telephone. The method disclosed uses a base station to monitor the quality of I/Q amplitude impairment, with a communication between the mobile user equipment and the base station. However it is assumed that only the side of a transmitter of the mobile can be optimised.

The object of the present invention is to provide a method, a receiving unit and a RF-system having an improved efficiency of data transmission and enhanced quality of the signals. It is a further object of the present invention to reduce the number of external measurement facilities avoiding manual operation of skilled operators. A even further object of the present invention is to achieve the aforesaid objects at a minimised amount of energy consumption at least at the side of the mobile station. These objects are achieved using a method having the features of claim 1. Further these objects are reached using a receiving unit and a RF-system having the features of claims 7 and 12 respectively. The depending claims contain the features of preferred embodiments of the invention respectively.

Therefore, a method according to the present invention is characterised in that during a communication with a given mobile station of a user, the base-station and the mobile user station build a closed control loop adapting the base-band characteristics to optimise the communication with this particular mobile user equipment. Thus, according to the present invention an enhanced quality of the signals and a better communication are achieved by a closed loop using two compo- nents only where both of them make part of a communication system already. However no further components are required. Further a closed control loop is working automatically. That means that the loop is able to work and to optimise parameters characteristic for the quality of a communication with- out manual operation of skilled operators. This optimisation is carried out during a communication of the base station with a given mobile station of a user. However, no extra communication between the base-station and the mobile user station is necessary. Thus, a method according to claim 1 saves electric energy which still is a rare resource at the side of the mobile user equipment. Reducing the bit error rate by enhancing the quality of data transmission improves the efficiency of the whole system, too.

In a preferred embodiment of the present invention a control unit of the base station produces user specific settings for amplitude impairment between the I and Q components and for the quadrature phase error to compensate for impairment in the receiver mixer of the direct conversion receiver.

Further it is advantageous to keep impairments within the base station in mind when producing the aforesaid settings for impairments of the mobile station respectively, that means that the settings are produced in relation to impairments of the base station. However, in the best case the impairment between I and Q and the quadrature phase impairment between the mixer of the receiver and the total equivalent impairment transmitted by the base-station transmitted signal can partly cancel each other and thereby improve the performance of the Error vector magnitude EVM. This leaves an increased margin for non-linearities in other blocks in the re- ceiver, i.e. filter and amplifier chain of the receiver. Further, this feature reduces the effort to be taken as a matching point between base station and mobile station may be found laying slightly apart optimal standard characteristics.

The mobile unit requires the base station's response to modify it's transmitted signal to optimise the transmission quality and to obtain the lowest value of EVM. So, even if the transmitter presents impairments, it is possible to find an optimal settings of the digital I and Q amplitude and phase in TX to obtain an minimal value of EVM rms. The optimisation of the tuning process is done on a regular basis to compensate for the hardware static error and potential drift due to ageing, specially as these setting are user specific. However a method according to the present invention is car- ried out within fixed time intervals of requests sent by the mobile unit.

In an embodiment of the invention previous optimal values of settings are saved to be used as starting conditions for op- timisation in the next update of the tuning. Preferably the saving is done in a special memory of the mobile unit.

However the method disclosed is described with respect to GSM EDGE only it may also apply in any kind of homodyne appara- tus . An embodiment of the invention will now be described with reference to the appended drawing:

The drawing of the only figure shows a loop L comprising a base station BS and a mobile station MS, where the mobile station MS is connected to the base station BS via a up link channel UL and the base station is connected to the BS mobile station MS via a down link channel DL. The mobile station MS is a user equipment in the form of a mobile GSM EDGE tele- phone.

The mobile station MS comprises an antenna ANT for receiving data in the form of a RF signal from the base station BS via the down link channel DL. A low noise amplifier LNA amplifies the RF signal transmitting it to a frequency mixer M, where the amplified RF signal is multiplied with the output of a local oscillator LO. The output of the mixer M is given as I and Q signals, containing the data or information itself. The data or information are further processed in a way that a user can see or listen them depending on the type of data.

In a complex modulation scheme such as GSM-EDGE the quality of the modulation transmitted at high frequency is highly sensitive to the linearity of the transmission chain. Impair- ents such as amplitude, quadrature skew and phase noise, which are added during up and down frequency conversion degrade the received signal. This results in an enhanced bit error rate and/or a decreasing quality of speech signals. Each impairment added during the transmission of the signal degrades the received signal, including the impairment in the receiver chain. Thus optimising the characteristic parameters of the receiver within the mobile station MS is a suitable way.

The need of optimisation is meet by the method described here after: To start a process of optimisation according to the present invention the mobile station MS sends a tuning re- quest via the up link channel UL to the base station BS. However in the layer 1 of the GSM there is a possibility to handle an adjustment request form the base band just the way it is done for frequency control in present systems. In the state of the art the mobile station should remain locked in frequency with the base station and the frequency lock is controlled by the base station.

The base station BS closes the loop L by sending a response via the down link channel DL back to the mobile station MS. As GSM EDGE is a time domain multiple access TDMA process each channel is divided up into time slots. Among the eight time slots of GSM EDGE, there is one available for monitoring purposes that is used for this task. As impairment in the mo- bile mainly change due to ageing it is preferred not to measure often. However, as the data contained in the request signal has not been changed by the base station BS any kind of change is affected by impairments. However, the signal received from the base station BS is evaluated in a block C within the mobile station MS. The block C contains an error vector magnitude EVM measurement and adjustment means thus reducing the impairment within the mobile station MS.

In an alternative embodiment the base station BS executes measurements detection impairments extending beyond a level defined by the mobile communication standard respectively. The base station BS contains means for measuring the following kinds of impairment: amplitude impairment between I and Q component and quadrature phase error. Based upon a communica- tion standard used sets of user specific values are produced for compensation of the impairments respectively. This very special kind of information is sent from the base station BS to the mobile station MS via the down link channel DL as some kind of tuning data signal.

This signal is processed in the way described above, where this information is evaluated in said block C within the o- bile station MS. The block C thus contains adjustment means for reducing the impairment within the mobile station MS only. Any kind of analysing work is shifted to the base station saving energy at the mobile station MS.

However, the base station BS modifies the aforesaid impairments of the mobile station MS. As a result the system specification requirement are met, for example the EVM requirements for GSM-EDGE in the transmitter. Thus, there is a mar- gin or interval of tolerance reducing the efforts to be taken because of standard definitions. As a result any kind of data transfer coming from or going to any remote network N via the base station BS will benefit from the enhanced quality reached by the method described above.

Within a further embodiment according to the present invention it is kept in mind that any base station BS may show aforesaid kinds of impairment, too. However an enhancement of signal quality is achieved optimising the parameters of the mobile user station MS in relation to the characteristics of the base station BS. In the best case the impairment between I and Q and the quadrature phase impairment between the mixer of the receiver of the mobile station MS and the total equivalent impairment transmitted by the base station BS transmitted signal can partly cancel each other and thereby improve the performance of the Error vector magnitude EVM. This leaves an even increased margin for non-linearities in other blocks in the receiver, i.e. filter and amplifier chain of the receiver. Within in text and the drawings the following Abbreviations are used:

GSM Global system for Mobile communication EDGE Enhanced Data rates for GSM Evolution RF Radio frequency Direct conversion receiver receiver with a single mixer in the receive path, where the RF modulated frequency carrier is directly converted to a base-band, in a single step with the help of a frequency mixer and an appropriate local oscillator (LO) . I/Q in-phase and in-quadrature phase components of a quadrature modulated signal (whose envelope is given as I + j*Q)

ANT Antenna LO Local oscillator TX transmitter RX receiver M frequency mixer

EVM Error Vector Magnitude, see GSM 05.05 for the definition of EVM rms rms Root means square LNA Low noise amplifier DL Channel in a downlink direction UL Channel in a uplink direction MS mobile station UE User equipment BS Base station

Claims

1. Method for transmitting data between a mobile station

(MS) and a base station (BS) , c h a r a c t e r i s e d i n t h a t during a communication with the mobile station (US) of a user the base-station (BS) and the station (MS) build a closed control loop (L) adapting the base-band character- istics to optimise the communication with this particular mobile station (US) .

2. Method according to claim 1, c h a r a c t e r i s e d i n t h a t a control unit of the mobile station (MS) evaluates a signal received from the base station BS in a block (C) containing an error vector magnitude EVM measurement and adjustment means thus reducing the impairment within the mobile station (MS) .

3. Method according to claim 1, c h a r a c t e r i s e d i n t h a t a control unit of the base station (BS) produces user specific settings for amplitude impairment between the I and Q components and for the quadrature phase error to compensate for impairment in the receiver mixer of the direct conversion receiver of the mobile station (MS) .

4. Method according to at least one of the proceeding claims, c h a r a c t e r i s e d i n t h a t the settings are produced in relation to impairments of the base station.

5. Method according to at least one of the proceeding claims, c h a r a c t e r i s e d i n t h a t the method is carried out within fixed time intervals on requests sent by the mobile station (MS) .

6. Method according to at least one of the proceeding claims, c h a r a c t e r i s e d i n t h a t previous optimal values or settings are saved to be used as starting conditions for optimisation in the next update of the tuning.

7. Receiving unit c h a r a c t e r i s e d i n t h a t it is made to carry out a method according to one or more of the proceeding claims.

8. Receiving unit according to claim 7, c h a r a c t e r i s e d i n t h a t it comprises a block (C) evaluating information sent form the base station (BS) , where the block (C) containing an error vector magnitude EVM measurement and adjustment is connected to QAM-demodulation means.

9. Receiving unit according to claims 7 and/or 8, c h a r a c t e r i s e d i n t h a t it comprises a memory to store previous optimal values of settings.

10. Receiving unit according to at least one of the claims 7 to 9, c h a r a c t e r i s e d i n t h a t it makes part of a mobile station (US) that is a telephone .

11. Receiving unit according to at least one of the claims 7 to 10, c h a r a c t e r i s e d i n t h a t the unit is made to carry out data transmission according to the GSM EDGE standard.

12. RF-system for data transmission between a mobile user equipment and a base station c h a r a c t e r i s e d i n t h a t it is made to carry out a method according to one or more of the proceeding claims and/or it comprises a Receiving unit according to at least one of the claims 7 to 11.