WO2007004928A1

WO2007004928A1 - A repeater device for increasing coverage in a mimo-system

Info

Publication number: WO2007004928A1
Application number: PCT/SE2005/001079
Authority: WO
Inventors: Sven Petersson; Martin Johansson; Ulrika ENGSTRÖM; Mats Andersson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2005-07-04
Filing date: 2005-07-04
Publication date: 2007-01-11
Anticipated expiration: 2008-01-04

Abstract

The invention discloses a repeater device (151, 152, 153) equipped with means for receiving transmissions from a first radio unit (110) at a first site within a system for wireless telecommunications and means for retransmitting those transmissions to a second radio unit at a second site within said system. The means for receiving transmissions are means for receiving MIMO transmissions which have been transmitted from the first radio unit with a certain degree of decorrelation between them, and the means for retransmitting those transmissions are means for MIMO transmissions. The repeater is further equipped with means for increasing the coverage of the system given by the MIMO transmissions before or upon retransmission of the received MIMO transmissions .

Description

TITLE

A REPEATER DEVICE FOR INCREASING COVERAGE IN A MIMO-SYSTEM

TECHNICAL FIELD The present invention discloses a repeater device equipped with means for receiving transmissions from a first radio unit at a first site within a system for wireless telecommunications and with means for retransmitting those transmissions to a second radio unit at a second site within said system.

BACKGROUND ART

Operators of wireless telecommunications systems such as, for example, 2G, 3G and WiMax, to mention a few such systems, need to increase the transmission capacity of the radio base stations in their systems, the radio base stations being stations that handle all transmissions to and from users within a defined area referred to as a cell.

One way of achieving the desired increase in capacity is to introduce so called MIMO-technology into the systems. MIMO, Multiple Input, Multiple Output, is a technology which is based on the simultaneous transmission of multiple streams of information, usually on one and the same frequency.

Since there are multiple transmissions on one and the same frequency, the different data streams need to be transmitted with some form of decorrelation between them. One example of such decorrelation is transmission of the data streams in different antenna beams which have a sufficiently high degree of angular separation between them. The separation between the antenna beams can also be a separation caused by different polarizations in the different antenna beams, the different polarizations suitably being orthogonal with respect to one another.

In order to be used efficiently, MIMO transmissions need a minimum Signal to Noise Ratio, SNR. MIMO is usually employed in Non Line of Sight applications, NLOS, in other words applications in which a number of objects such as buildings, hills, mountains etc block the radio base station's Line Of Sight, LOS, to the users of the system.

In an NLOS environment, the SNR drops rapidly with increased distance from the transmission source, the radio base station, which thus limits the coverage area in which MIMO can be used.

DISCLOSURE OF THE INVENTION As shown above, there is a need for a method or a device which would increase the coverage given by a MIMO-system in an NLOS-area.

This need is addressed by the present invention in that it discloses a repeater device equipped with means for receiving transmissions from a first radio unit at a first site within a system for wireless telecommunications and means for retransmitting those transmissions to a second radio unit at a second site within said system.

In the repeater device, the means for receiving transmissions are means for receiving MIMO transmissions which have been transmitted from the first radio unit with a certain degree of decorrelation between them, and the means for retransmitting these transmissions are means for MIMO transmissions.

The repeater device is further equipped with means for increasing the coverage of the system given by means of the MIMO transmissions before or upon retransmission of the MIMO transmissions.

The means for increasing the system coverage can in one embodiment of the repeater device comprise passive amplification of the signals. In other embodiments, the means for increasing the system coverage can comprise active amplification of the signals or recreation of the original data streams comprised in the MIMO transmissions.

Naturally, at least the latter two methods can be combined in one and the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawing, Fig 1 , which shows a schematic overview of an example of a system with a number of repeater devices.

EMBODIMENTS OF THE INVENTION

Fig 1 shows a schematic overview of a MIMO system 100. It is assumed in this example that the system is a cellular telephony system such as second or third (2G/3G) generation systems, but the invention can also be applied in other wireless communications systems where MIMO technology is used, such as, for example, wireless LAN systems in office environments.

The system 100 comprises a base station 110, which handles the external control of all of the user equipments within the system, and which also handles all the traffic to and from said user equipments. In fig 1 , the area 120 which it is desired to cover with the system 100 is indicated by means of a dashed line.

The transmissions from the base station 110 to the users in the system are so called MIMO-transmissions, i.e. the base station simultaneously transmits multiple streams of information, usually on one and the same frequency. Since there are multiple transmissions on one and the same frequency, the different data streams need to be transmitted with some form of decorrelation between them so that they do not interfere with each other. The desired decorrelation can be achieved by means of the base station transmitting in a plurality of antenna beams, one beam being used for each data stream, with a sufficient angle between the beams to ensure that the transmissions do not interfere with each other. As an alternative, the beams can transmit in different polarizations, which suitably are orthogonal to each other.

Another method of achieving a high degree of decorrelation between the transmitted data streams is to use spatially separated antennas at the base station.

As is also indicated in fig 1 , there are a number of objects 131 ,132; 141 , 142, 143, within the area which block transmissions to and from certain sub-areas as seen from the base station 110. Examples of such objects are hills, mountains, high-rise buildings etc.

Thus, much of the area 120 which it is desired to cover by means of the base station will be a so called NLOS (Non Line of Sight) area. Within an NLOS- area, the Signal to Noise Ratio (SNR) of the signal drops rapidly, which limits the coverage of the system.

The decrease in coverage can be due to reduced signal strength (low SNR) caused by the distance from the base station, or it can be due to the MIMO transmission having lost all or part of their mutual decorrelation, or it can be caused by a combination of these two factors.

In order to increase the coverage given by the MIMO transmissions in the area 120, there are in the system 100 arranged a plurality 151 , 152, 153, of repeater devices of the invention, which are suitably arranged as shown in fig 1 , i.e. in locations where the repeaters can obtain a good coverage of sub- areas where there will be a low SNR and/or low decorrelation in signals from the base station, as well as having a good connection to the RBS 110. An example of suitable locations for the repeaters is on top of the high-rise buildings 141 , 142, 143. The number of repeater devices 151 , 152, 153, in this case three, is merely an example, one or more can be used according to the system in question.

A repeater device 151 will be equipped with means for receiving MIMO transmissions, i.e. means for receiving the transmissions from the base station 110. In order to achieve the purpose of covering a sub-area within the area 120 with MIMO-transmissions, the repeater station 151 is also equipped with means for MIMO-retransmission of the transmissions received from the base station into one or more of the obscured areas.

Since the received signals may have lost SNR and/or some of their decorrelation before they are received in the repeater device, the system will thus have a degraded coverage at and beyond the points of the repeater devices.

In order to increase the coverage of the system which is given by the MIMO- signals, the repeater device comprises means for increasing the coverage of the system given by the MIMO transmissions before or upon retransmission of the received MIMO transmissions.

The means for increasing the coverage can be either passive or active: in the passive case, the means for retransmission may simply carry out passive amplification of the received signals, suitably by means of a plurality of transmit antennas, suitably but not necessarily one for each MIMO data stream.

In this passive case, the MIMO transmissions are received in the receive means, suitably one antenna or antenna beam per MIMO transmission, and each received MIMO transmission (data stream) is simply connected to a transmit antenna or antenna beam with a sufficient degree of decorrelation with respect to the other antenna beams which are used for retransmission.

Naturally, the transmit antenna or antenna beams of the repeater device is/are directed so that they give the MIMO transmissions the desired coverage.

Said decorrelation between the antennas/antenna beams may be achieved by means of different polarizations in the different antennas or antenna beams, or spatial separation between different antennas, or angular separation between antenna elements in one and the same antenna. Naturally, these three methods may be combined.

In this case, i.e. passive retransmission, the SNR of the transmissions will be increased, due to the gain of the retransmit antennas.

The retransmit antennas mentioned in the passive case are used in the case of active retransmission means as well. However, in one embodiment of active retransmission, the received signals are amplified before they are retransmitted. This may, for example, be achieved by an amplifier which is connected between the receive and transmit antennas. In this case, the SNR of the transmissions will also be increased, due to the amplifier as well as the gain of the retransmit antennas.

In the case of active retransmission, the decorrelation of the signals can also be increased on retransmission by means of recreating the original data streams contained in the MIMO-transmissions before retransmission. In this case, the received signals are converted to baseband level in a receiver in or connected to the repeater antenna, and data streams that correspond to the original data streams in the MIMO-transmissions are recreated and transmitted, decorrelated in one or more of the ways described above. Thus, there exist a number of ways of making the data streams decorrelated upon or before transmission from the base station 110 or from the repeater device 151 , 152, 153. Naturally, these methods can be combined with each other. Regardless of which method or methods that is employed, the users of the system, i.e. the telephones in the case of cellular telephony systems, can be equipped with means for signalling in their transmissions to the repeater device or to the base station the "status" of each of the data streams. Such status signalling can include information about the SNR in each stream, or its decorrelation with respect to the other data streams.

Upon receiving said status signalling, the repeater device can, if equipped with means for responding to said signalling, increase the decorrelation of the data streams in accordance with the status which was reported for the individual data streams, so that the original level of decorrelation may be reached, i.e. the level of decorrelation which the signals had when transmitted from the base station.

It can be pointed out that although the transmissions from the base station to the users have been described as being MIMO-transmissions, MIMO can also be used when transmitting from the users to the base station. However, the need for MIMO transmissions in the so called up-link (user to base station) may not be as high as the need for MIMO in the downlink, since more data will usually need to be transferred on the downlink than the uplink. However, the repeater device of the invention can be designed to handle MIMO in either direction.

Claims

1. A repeater device (151 , 152, 153) equipped with means for receiving transmissions from a first radio unit (110) at a first site within a system for wireless telecommunications and means for retransmitting those transmissions to a second radio unit at a second site within said system, the repeater device being characterized in that the means for receiving transmissions are means for receiving MIMO transmissions which have been transmitted from the first radio unit with a certain degree of decorrelation between them, and in that the means for retransmitting those transmissions are means for MIMO transmissions, the repeater device also being characterized in that it further is equipped with means for increasing the coverage of the system given by the MIMO transmissions before or upon retransmission of the received MIMO transmissions .

2. The repeater device (151 , 152, 153) of claim 1 , in which the means for increasing the system coverage comprises passive amplification of the signals .

3. The repeater device (151 , 152, 153) of claim 1 , in which the means for increasing the system coverage comprises active amplification of the signals.

4. The repeater device (151 , 152, 153) of claim 1 , in which the means for increasing the system coverage comprises recreating the original data streams comprised in the MIMO transmissions.

5. The repeater device (151 , 152, 153) of any of the previous claims, in which the means for increasing the system coverage are arranged to function in accordance with information received from the second radio unit.