WO2001091322A1

WO2001091322A1 - Link adaptation method and quality estimation in a cellular radio system

Info

Publication number: WO2001091322A1
Application number: PCT/SE2001/000812
Authority: WO
Inventors: Erik Dahlman; Stefan Parkvall
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 2000-05-23
Filing date: 2001-04-11
Publication date: 2001-11-29
Also published as: AU2001247037A1; SE0001918L; SE0001918D0

Abstract

The invention is based on the recognition that conventional pilot-based channel quality estimation does not take into account the future transmission activity of neighboring access points, and that this may lead to inaccurate channel quality estimation as well as inefficient link adaptation. By controlling the transmission power of the pilots based on future data transmission activity, the transmission conditions during the pilot-based estimation of channel quality (e.g. frame n) can be correlated to the transmission conditions of the future frame (e.g. frame n+1) in which the corresponding modulation and coding scheme is to be used. In this way, more accurate predictions of the channel quality can be made so that optimal or near-optimal modulation and coding schemes can be selected during link adaptation. This in turn results in increased link data rates.

Description

Link adaptation method and quality estimation in a cellular radio system

TECHNICAL FIELD OF THE INVENTION

The present invention generally concerns a cellular radio system, and more specifically link adaptation, channel quality estimation as well as transmission of pilots in such a system.

BACKGROUND

The rapid growth of digital wireless or cellular telephony has also given rise to an increasing demand for wireless data services as well. Currently, service providers offer data rates equivalent to those of wireline modems ten or fifteen years ago, but the gap is closing. Circuits are now becoming available for data rates above 64 Kbits/s, and service providers are already planning for wireless data rates up to and above 2Mbits/s. Such wireless data services are mainly driven by the demand for rapid, low latency access to the Internet.

One way of increasing the data rate for a radio link involves link adaptation. In link adaptation, the transmission parameters used for communication on the link are adapted to the channel conditions of the link in order to provide as high a data rate as possible.

For example, in a cellular system employing link adaptation for the downlink, the modulation and coding scheme used by the access point for communication with the user equipment can be varied and is typically selected in dependence on the downlink channel quality. This means that the modulation and coding scheme can be optimized to the channel conditions, leading to a considerable improvement of the downlink channel throughput (normally expressed in terms of bits/s). This is also known as an Adaptive

Modulation and Coding Scheme (AMCS). With AMCS, the modulation and coding is typically fixed over a certain time interval, here denoted frame, but may be changed between consecutive frames. To select an appropriate modulation and coding scheme (MCS) for communication with the user equipment, it is necessary to estimate or predict the downlink channel quality during the frame for which the MCS is to be used. The downlink channel quality is typically expressed as the downlink signal-to-interference ratio or some other related measure. The channel-quality estimate is typically derived by the user equipment some frames in advance, and reported to the network using uplink signaling. The network then selects a suitable MCS to be used for downlink transmission to this user equipment in a given future frame. Alternatively, the user equipment itself decides on a suitable MCS and reports the selected MCS to the network for subsequent use in the downlink transmission to the user equipment.

In direct-sequence spread-spectrum systems, a common way of providing the user equipment with a means to estimate the channel quality is to have each access point periodically transmit a predetermined chip sequence, also known as a pilot, for reception by the user equipment. During pilot transmission, there is no other transmission from the access point. Neighboring access points generally use different chip sequences for the pilot or different shifts of the same chip sequence, and the pilot can either be transmitted once per frame or several times per frame.

An example of a known cellular radio system that employs link adaptation is the HDR (High Data Rate) system, which typically uses separate RF carriers for high rate data services. For example, it is envisaged that HDR will be used as a separate add-on technique to upgrade existing CDMA architectures.

Fig. 1 is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and possible data from a plurality of access points in a cellular system during a number of consecutive frames. Each transmitted frame comprises a pilot, indicated in black, followed by possible payload data. The example of Fig. 1 is representative of a cellular system with tight inter-cell synchronization, and therefore all access points AP-1 to AP-N are able to transmit the frames including the pilots so that they are substantially time aligned to each other. Ignoring propagation delays, it can be assumed that the overall signal received by a given user equipment will be the sum of the pilot signals from all the access points AP-1 to AP-N attenuated by the respective propagation losses.

According to the prior art, by measuring the signal strength of the pilot from a specific access point in frame n, as well as the total interference from all the other access points during the pilot part of frame n, the user equipment can estimate the signal-to- interference ratio (SIR) during the pilot part of frame n for the link between the user equipment and the access point under consideration. This estimate can then be used for selecting the modulation and coding scheme (MCS) for the transmission of data by the access point in a future frame such as frame n+1 or frame n+2.

A problem with the prior art technique outlined above is that the estimated interference will be the worst-case interference corresponding to the case when all access points are actively transmitting at full power. This may lead to the selection of a non-optimal MCS, and hence inefficient link adaptation.

SUMMARY OF THE INVENTION

The present invention overcomes these and other drawbacks of the prior art arrangements.

It is an object of the present invention to provide an efficient link adaptation method and system.

It is another object of the invention to provide an improved channel quality estimation method and system. In particular, it is desirable to provide an accurate prediction of channel quality for link adaptation purposes, where the prediction is consistent with the transmission conditions during the predetermined future frame in which the corresponding modulation and/or coding scheme is to be used. In this respect, it is particularly important that the future transmission activity of the neighboring access points, also generally referred to as transmitters, is taken into account.

It is also an object of the invention to provide a method and system for pilot transmission that facilitate accurate channel quality estimation and efficient link adaptation.

These and other objects are met by the invention as defined by the accompanying patent claims.

The present invention is based on the recognition that conventional pilot-based channel quality estimation does not take into account the future transmission activity of neighboring access points or transmitters, and that this may lead to inaccurate channel quality estimation as well as inefficient link adaptation.

Channel quality estimation is based both on what is transmitted on the channel for subsequent measurement as well as the actual measurement, and the present invention focuses on the transmission part of the overall channel-quality estimation mechanism. By controlling the transmission power of the pilots based on future transmission activity, the transmission conditions during the pilot-based estimation or prediction of channel quality can be correlated to the transmission conditions of the future frame, or other time period, in which the corresponding modulation and coding scheme is to be used. In this way, more accurate predictions of the channel quality can be made so that optimal or near-optimal modulation and coding schemes can be selected during link adaptation. This in turn results in increased data rates and higher data throughput. Preferably, the pilot transmission power is based on the amount of data to be transmitted in the future frame. The transmission power is normally zero when the data queues are empty and no data is to be transmitted, and non-zero only when data is to be transmitted.

The channel quality is conveniently represented by the signal-to-interference ratio of the link, or some other related measure.

With regard to link adaptation, the channel quality is typically estimated by the user equipment a predetermined number of frames in advance based on the received pilots, and a representation of the channel quality estimate is reported by uplink signaling so that the access point transmitter can select a suitable modulation and coding scheme for downlink transmission in a given future frame.

The invention offers at least the following advantages: More accurate prediction of channel quality; Efficient link adaptation; and Increased data rates.

Other advantages offered by the present invention will be appreciated upon reading of the below description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, will be best understood by reference to the following description taken together with the accompanying drawings, in which: Fig. 1 is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and possible data from a plurality of access points in a conventional cellular system during a number of consecutive frames;

Fig. 2 is a schematic overview of an exemplary communication system in which the present invention may be used;

Fig. 3 is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and data from a plurality of access points in a conventional cellular system;

Fig. 4 is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and data from a plurality of access points according to a first embodiment of the invention;

Fig. 5 is a schematic timing diagram illustrating an example of the transmission of time-multiplexed pilots and data from a given access point according to a second preferred embodiment of the invention;

Fig. 6 is a schematic block diagram of a receiver which can be used by the present invention; and

Fig. 7 is a schematic block diagram of a transmitter according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Throughout the drawings, the same reference characters will be used for corresponding or similar elements. Fig. 2 is a schematic overview of an exemplary communication system in which the present invention may be used. The communication system 10 comprises a number of remote mobile terminals 1-1 to 1-3 which are connected by radio links to a number of access points 2-1 to 2-3. The access points 2-1 to 2-3 are normally representative of sectors or cells served by base stations or equivalents. The access points of these base stations are connected to a communication network 3, which connects different access points to each other, and connects access points to the core network for communication with other parts of the communication network.

For a better understanding of the invention we will begin with a more detailed analysis of the problem associated with the conventional technique for channel quality estimation and the adverse effects this may have on the subsequent link adaptation. Reference is made to Fig. 3, which is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and data from a plurality of access points in a conventional cellular system during a number of consecutive frames.

The user equipment can estimate the channel quality of a link between the user equipment and a specific access point as the signal-to-interference ratio (SIR) of the link during the pilot part of a given frame n by measuring the signal strength of the pilot from the access point as well as the total interference from all the other access points during the pilot part of frame n. This estimate is then used for selecting the modulation and/or coding scheme (MCS) for the transmission of payload data by the access point in a future frame; for example frame n+1, assuming a one-frame delay between the SIR estimation and the use of the selected MCS. This is indicated in Fig. 3 for access point AP-N by the line extending from the pilot part of frame n to the data part of frame n+1.

However, in conventional systems the estimated interference will always be the worst- case interference corresponding to the case when all access points are actively transmitting at full power, and no account is taken to the possible inactivity (no transmission of data) of neighboring access points during the future frame in which the selected MCS is to be used. The interference measurement takes the interference from all pilots, including also the pilot of access point AP-2, into account. However, during frame n+1, the access point AP-2 will not transmit any data, e.g. due to empty data queues. This means that AP-2 will not contribute to the interference during the payload data part of frame n+1, which is the frame in which the MCS that is selected based on the interference measurements in frame n will actually be used. Consequently, the SIR in frame n+1 has been underestimated since the interference level has been overestimated, and this may lead to the selection of a non-optimal MCS for frame n+1. In. the same way, measurements during the pilot part of frame n+1 will give an overestimate of the interference level during frame n+2, and a corresponding underestimate of the SIR, as no data is transmitted from access points AP-3 and AP-N in frame n+2. For completeness, the case when all access points are transmitting data is illustrated in frame n+3. In this case, the measurements during the pilot part of frame n+2 actually corresponds to the actual data transmissions in frame n+3, and consequently there will be no underestimate of the SIR value for this particular frame.

Fig. 4 is a schematic timing diagram illustrating an example of the transmissions of time-multiplexed pilots and data from a plurality of access points according to a first embodiment of the invention.

In the same manner as for conventional systems, measurements during the pilot part of a frame, such as frame n, are used for estimating the SIR or some other measure representative of the channel quality of a link, and for subsequently selecting a suitable MCS for frame n+1, assuming a one-frame delay between SIR estimation to MCS selection.

However, with regard to the transmission of pilots by the access points, the invention proposes an active correlation of the transmission conditions during the pilot-based estimation of channel quality to the transmission conditions during the future frame in which the corresponding modulation and coding scheme is to be used. This is achieved by controlling the transmission of pilots based on the future transmission activity of the access points so that the pilots, indicated by black in Fig. 4, are transmitted in a given frame n only when there will actually be transmission of data by the corresponding access points in the future frame n+1. This means that the user equipment measuring interference during the pilot part of frame n, will only receive interference from those access points that will be actively transmitting data during frame n+1. An efficient way of determining whether or not there will be any transmission of data in frame n+1 is to investigate the corresponding data queue.

With reference to Fig. 4, it can be seen that since no data is to be transmitted by access point AP-2 during frame n+1, this access point AP-2 does not transmit any pilot in frame n. It can also be seen that this means that the transmission conditions during the pilot part of frame n (in which the channel quality estimation is performed) are now consistent with the transmission conditions during the data part of frame n+1 (in which the MCS that has been selected based on the channel quality estimation is actually used). In this way, it is possible to more accurately estimate or predict the actual channel quality for the future frame in which the corresponding MCS is to be used.

It can also be seen from Fig. 4 that interference measurements during frame n+1 will not include any pilots from access points AP-3 and AP-N since these access points will not be active during the data part of frame n+2 and thus not cause any interference in that frame. In the same way, interference measurements during frame n+2 will not include any pilots from access point AP-2, since that access point will not transmit any data in frame n+ 3.

In a more general scheme, schematically illustrated in Fig. 5, the transmission power of a pilot is controlled based on the expected transmission activity of the corresponding access point AP-K during a given future frame. This means that a pilot need not be transmitted with the maximum output power of the corresponding access point, but rather with a power equal to the expected power for transmission of data in the given future frame. An efficient way of determining the expected power for transmission of payload data during a future frame is to investigate the data queue of the corresponding access point, as will be explained later on.

Considering access point AP-K in Fig. 5, it can be appreciated that since data is to be transmitted at full power in frame n+1, the pilot of frame n is also transmitted at full power. In frame n+2 on the other hand, data is to be transmitted at approximately half power, and consequently, the pilot of frame n+1 is transmitted at half power. In frame n+3, data is to be transmitted at approximately 2/3 of full power, and therefore, the pilot of frame n+2 is transmitted at 2/3 of full power.

In some sense, the pilots can be regarded as activity-indicating pilots since the pilot transmission power is based on the future transmission activity of the corresponding access point.

Fig. 6 is a schematic block diagram of a receiver which can be used by the present invention. The receiver 20 is normally provided in user equipment such as a mobile terminal or equivalent, and comprises a signal receiver element 21, a switch element 22, an interference measuring unit 23, a pilot signal strength measuring unit 24, a channel quality estimator 25 and a MCS selector 26. The receiver also comprises a demodulator 27 and a decoder 28 for demodulating and decoding a data signal modulated and coded according to a previously selected MCS and transmitted to the receiver by a base station access point.

The switch element 22 is closed during the pilot periods, and based on the pilots received by the signal receiver element 21 from the different access points, the user equipment determines an estimate of the channel quality of the link between the user equipment and a given access point. The channel quality is typically estimated as the signal-to-interference ratio (SIR) of the link. The SIR estimate is formed by measuring the pilot signal strength "S" in the pilot strength measuring unit 24, and the interference signal strength "I" in the interference measuring unit 23.

There are several ways of measuring the interference of a link, as discussed in standardization work groups for the 3GPP standard. A promising way of measuring the interference is to reserve a channelization code as an "interference-measurement code", which is never used for information transfer. The user equipment can then obtain an interference estimate by means of a de-spreading process using the reserved channelization code. The reserved code used in the de-spreading process can be considered as a virtual code that is orthogonal to the pilot transmitted by the given downlink access point.

The signal strength estimate "S" from the pilot signal strength measuring unit 24 and the interference estimate "I" from the interference measuring unit 23 are combined in the channel quality estimator 25 to generate a SIR estimate, which is transferred to the MCS selector 26. The MCS selector 26 decides on a suitable MCS based on the generated SIR estimate, and transmits the selected MCS to the relevant access point on the uplink.

Of course it is equally possible to transmit the SIR estimate itself or any suitable representation thereof to the access point, and have the network decide which MCS to use for downlink transmission. For example, it is possible to map the estimated SIR into one of a number of data rate modes, and feed this channel state information back to the access point via the uplink. However, in the present example, the user equipment itself decides on a suitable MCS and transmits the selected MCS to the relevant access point on the uplink. In this respect, the present example generally corresponds to the existing HDR system. A possible set of modulations schemes to choose from could for example be {QPSK, 8PSK, 16QAM}, and a possible set of coding schemes could for example be turbo codes with coding rates {1/4, 3/8, 1/2}. A specific modulation and/or coding scheme is selected depending on estimated channel quality, typically the estimated signal-to- interference ratio (SIR). If the channel quality is poor, a lower order modulation, such as QPSK, may be selected together with a coding scheme of relatively low coding rate. With improved channel quality comes the possibility to use a higher order modulation, such as 8PSK or even 16QAM, and higher coding rates. In general, higher order modulation and higher coding rates give higher data transfer rates than lower order modulation and lower coding rates. For example, the data rate of link employing a modulation and coding scheme based on 16QAM and a coding rate of 1/2 is normally 4 times higher than that of a link employing a modulation and coding scheme based on QPSK and a coding rate of 1/4, provided all other relevant link parameters are the same for the two cases.

Of course, it should be understood that the above modulation and coding schemes are only illustrative examples of schemes that can be used by the invention. In fact, any suitable MCS known in the art can be selected for transmission of data.

Fig. 7 is a schematic block diagram of a transmitter according to a preferred embodiment of the invention. The transmitter 30 is normally provided in a base station or equivalent, and comprises a data buffer 31, a channel coder 32, a modulator 33, a pilot generator 34, a pilot output power amplifier 35, a multiplexor 36, a transmission element 37 as well as a control unit 38. Based generally on the amount of data in the data buffer 31, and more particularly on the amount of data to be transmitted in a predetermined future frame, the control unit 38 determines the output power of the pilot in a current frame by controlling the gain of the pilot output power amplifier 35. The gain factor is preferably normalized to the interval of 0-1 so that the pilot can be transmitted at any power between zero power and full power. Furthermore, the control unit 38 is responsive to the MCS selection made by user equipment communicating with the access point, and controls the modulation and coding scheme used by the channel coder 32 and the modulator 33 accordingly. The control unit 38 also controls the multiplexor 36 that switches between the pilot input and the data input. During the pilot part of a frame, the multiplexor 36 forwards the pilot sequence, which is appropriately amplified by the amplifier 35, to the transmission element 37 for transmission to the user equipment. During the data part of a frame, the multiplexor 36 forwards channel coded and modulated pay load data as input to the transmission element 37 for transmission to the user equipment.

Preferably, each and every access point in the radio system is configured so that it is capable of controlling the pilot transmission power based on the transmission activity expected in a predetermined future time period. However, it should be understood that the prediction of channel quality is generally improved compared to the prior art even though the inventive pilot transmission power control is exercised in only a single access point.

Although the invention mainly has been described under the assumption of a one-frame delay between channel quality estimation and MCS selection, it is apparent that other timing relations between pilot measurements and channel quality estimation on hand and MCS selection and utilization on the other hand are possible. For example, pilot measurements made during frame n could be used to select the MCS in frame n+2, corresponding to a delay of two frames between SIR estimation and MCS selection.

It is also possible to use the channel quality measurements during several consecutive pilots as a base for the MCS selection, for example by using different averaging techniques.

Furthermore, although the examples shown may have indicated that the activity- indicating pilot has replaced the conventional pilot, it is equally feasible to have a system in which the activity-indicating pilot co-exists with a second pilot. In this case, the second pilot may be used for other purposes such as cell search and phase estimation.

The embodiments described above are merely given as examples, and it should be understood that the present invention is not limited thereto. Further modifications, changes and improvements which retain the basic underlying principles disclosed and claimed herein are wit n the scope and spirit of the invention.

Claims

1. A link adaptation method for a radio link in a radio system, comprising the steps of estimating channel quality of the radio link based on transmission and measurement of pilots and selecting a link adaptation scheme to be used in a predetermined future time period based on the estimated channel quality, characterized by controlling, for at least one transmitter in said radio system, the transmission power of a corresponding pilot based on transmission activity of the transmitter in said future time period to enable correlation of the transmission conditions during the pilot-based channel quality estimation to the transmission conditions of said future time period.

2. A method of estimating channel quality of a radio link in a radio system based on transmission and measurement of pilots, characterized by: controlling, for at least one transmitter in said radio system, the transmission power of a corresponding pilot based on transmission activity of the transmitter in a predetermined future time period; receiving said transmission-power controlled pilots; and estimating said channel quality based on measurement of said received pilots, thereby correlating the transmission conditions for the pilot-based channel quality estimation to the transmission conditions of the future time period.

3. A method of pilot transmission in a radio system, characterized by controlling, for at least one transmitter in said radio system, the transmission power of a pilot based on transmission activity of the transmitter in a predetermined future time period to enable correlation of the transmission conditions for pilot-based channel quality estimation to the transmission conditions of the future time period.

4. The method according to claim 1, 2 or 3, characterized in that said transmission power of said pilot is based on the amount of data to be transmitted in said predetermined future time period.

5. The method according to claim 4, characterized in that said data is payload data.

6. The method according to claim 1, 2 or 3, characterized in that the transmission power of said pilot is non-zero only when data is to be transmitted in said predetermined future time period.

7. The method according to claim 1, 2 or 3, characterized in that said predetermined future time period is a future frame.

8. The method according to claim 1, 2 or 3, characterized in that said future transmission activity is determined by investigating the amount of data in a data buffer in said transmitter.

9. The method according to claim 1 or 2, characterized in that said channel quality is represented by the signal-to-interference ratio of the link.

10. The method according to claim 1, characterized by selecting a modulation and/or coding scheme for transmission of data in said predetermined future time period based on the estimated channel quality of said radio link.

11. The method according to claim 1 , characterized in that said channel quality is estimated by a receiver a predetermined number of frames in advance based on received pilots, and a representation of said channel quality estimate is reported by uplink signaling from said receiver to said transmitter, which effectuates a suitable modulation and/or coding scheme for downlink transmission to said receiver in a given future frame.

12. A link adaptation system comprising means for estimating channel quality of a radio link in a radio system based on reception and measurement of transmitted pilots, and means for selecting a link adaptation scheme to be used in a predetermined future time period based on the estimated channel quality, characterized by means for controlling, for at least one transmitter in said radio system, the transmission power of a corresponding pilot based on transmission activity of the transmitter in said future time period to enable correlation of the transmission conditions during the pilot-based channel quality estimation to the transmission conditions of said future time period.

13. A system for estimating channel quality of a radio link in a radio system based on transmission and measurement of pilots, characterized by: means for controlling, for at least one transmitter in said radio system, the transmission power of a corresponding pilot based on transmission activity of the transmitter in a predetermined future time period means for receiving said transmission-power controlled pilots; and means for estimating said channel quality based on measurement of said received pilots, thereby enabling correlation of the transmission conditions for the pilot- based channel quality estimation to the transmission conditions of the future time period.

14. A system for pilot transmission in a radio system, characterized by means for controlling, for at least one transmitter in said radio system, the transmission power of a pilot based on transmission activity of the transmitter in a predetermined future time period to enable correlation of the transmission conditions for pilot-based channel quality estimation to the transmission conditions of the future time period.

15. The system according to claim 12, 13 or 14, characterized in that said controlling means is operable for determining the transmission power of said pilot based on the amount of data to be transmitted in said predetermined future time period.

16. The system according to claim 15, characterized in that said data is payload data.

17. The system according to claim 12, 13 or 14, characterized in that the transmission power of said pilot is non-zero only when data is to be transmitted in said predetermined future time period.

18. The system according to claim 12, 13 or 14, characterized in that said predetermined future time period is a future frame.

19. The system according to claim 12, 13 or 14, characterized in that said system further comprises means for determining said future transmission activity by investigating the amount of data in a data buffer in said transmitter.

20. The system according to claim 12 or 13, characterized in that said channel quality is represented by the signal-to-interference ratio of the link.

21. The system according to claim 12, characterized in that said system further comprises means for selecting a modulation and/or coding scheme for transmission of data in said predeteπnined future time period based on the estimated channel quality of said radio link.

22. The system according to claim 12, characterized in that said channel quality is estimated by a receiver a predetermined number of frames in advance based on received pilots, and a representation of said channel quality estimate is reported by uplink signaling from said receiver to said transmitter, which effectuates a suitable modulation and/or coding scheme for downlink transmission to said receiver in a given future frame.