WO1996009697A1

WO1996009697A1 - Device at radio base communication system operating with repeaters

Info

Publication number: WO1996009697A1
Application number: PCT/SE1995/000998
Authority: WO
Inventors: Johan Wickman
Original assignee: Telia Ab
Priority date: 1994-09-21
Filing date: 1995-09-06
Publication date: 1996-03-28
Also published as: EP0783805A1; SE9403161D0; SE9403161L; SE513448C2; AU3580095A

Abstract

In a radio based communication system in the form of a mobile and/or stationary digital radio based radio system is used time shared channel access. Furthermore there is as an integral part one or more repeater functions or repeaters arranged between concerned base stations and the terminals or the mobile units. The mentioned repeaters are arranged to achieve macro diversity with the aim of reducing influence from fading and time dispersion.

Description

TTTLEOFTHEINVENTION

Device at radio based communication system operating with repeaters.

TECHNICAL FIELD

The present invention can be classified among such communication systems that are indicated in the introduction to the subsequent patent claim 1.

STATE OF TECNOLOGY

The use of macro diversity is in itself previously known in communication systems. By the American patent document 5 260 943 it is previously known to decide of hand-off in a TDM-system. Time measuring is made on repeaters which are considered to be candidates. Hand- off is made to that repeater which at the comparison of the point of time for incoming synchronization mark is regarded to be closest to the mobile unit. As candidates are those repeaters considered to be, which have sufficient signal strenght measured at the mobile unit. Above mentioned measuring is made in the mobile unit. The repeater function according to mentioned American patent document is not of the same kind as the repeater function in connection to the present invention.

By the American patent document 5 233 626 it is in itself known to make use of repeater diversity. A signal which is transmitted by a transmitter is repeated by a number of repeaters. The accordingly repeated signal is repeated over a number of signal routes and received in a receiver. In the receiver the different signals are combined to a utility signal which is comparatively insensitive to fading. In the system, according to the document, no decision is taken about which repeater which is to repeat, but all adjacent repeaters repeate.

DESCRIPTION OF THE INVENTION

TECHNICAL PROBLEM

There is a need for offering macro diversity with repeaters where the signalling between the repeaters occurs in one for the invention unique way. The intention with the invention is among other things to solve this problem.

There is a need for the possibility to integrate the present invention with in itself known systems, for instance the DECT-system. The invention also solves this problem.

Repeaters as such will get a very big economic and functional potential for systems which use time shared channel access. It is of importance to improve function and economy in such systems. The invention is intended to solve also this problem.

In the present case there may arise need for the invention to function without digital encoding/decoding. The invention is intended to solve also this problem. THE SOLUTION

What can in the main be regarded as significant for the new device is that the repeaters or repeater functions in the system are arranged to achieve macro diversity and by that reduce fading and time dispersion.

Embodiments of the main ideas of the invention concept will be clear from the subsequent sub claims.

ADVANTAGES

By the invention is achieved bigger advantages with repeaters in radio based communication systems where the repeaters as such are of very great importance to the economy and functionality of the system. The repeater application can at that be used at radio based local network applications comprising area restricted mobile access. The application implies that the terminating wire connection to the subscriber is replaced by or completed by surface covering radio. The subscriber is offered terminal mobility within one or more restricted areas. In this application can by use of repeaters the number of base stations be reduced by a factor 10-100, with the result that among other things wire connection of base units can be dramatically reduced. The potential importance of repeaters for DECT is confirmed by heavy accelerating interest within the field. Repeaters can consequently be used for creating radio covering also at high signal attenuation in relation to the dynamics of the system. By using the invention the repeater functions are obtaining even higher advantages in radio based communication systems than what was previously possible. By the invention can also be achieved improved functions regarding repeating principles. The repeaters can at that in their turn solve problems of surface covering for new powerful local network applications such as, for instance, area restricted mobile access.

DESCRIPTIONS OF DRAWINGS

One at present suggested embodiment of a device which shows those for the invention characteristic features will be described below, while referring to enclosed drawings, where

figure 1 in the form of a principal diagram shows example of use of DECT with repeaters for local net replacement with area restricted mobility,

figure 2 shows example of two adjacent repeaters with overlapping coverage area, and

figuer 3 shows in schematic form macro diversity.

DETAILED EMBODIMENT

Repeaters can, consequently, be used to create radio coverage also at high signal attenuation in relation to the dynamics of the system. The invention, however, entails that repeaters can also be used for creating macro diversity and by that reduce the influence of fading and time dispersion, for mobile and stationary digital radio systems with time shared channel access.

Fading consists of variations of the signal strength in time or room due to interference between dfferent paths of propagation and (varying) shadowing. The different propagations paths have different transit times which causes time dispersion. High time dispersion implies risk for intersymbolic interference, unless counteracting measures are taken (channel equalizer, directional antenna, antenna diversity, multi carrier modulation etc) . The time dispersion is often well correlated to the fading.

Macro diversity implies that influence of both short (rapid) and long (slow) fading is reduced by continuously selecting the best one of two separated antennas. In this way necessary fading margin can be considerably reduced (often more than 10 dB) . It can be interesting to make a comparison with macro diversity, a technology which is expected to be used generally for DECT. Micro diversity, however, results in that only the short fading can be reduced, consequently not the long fading which is also called shadow fading. Furthermore the micro diversity antennas must be efficiently separated in many outdoor applications. A thing that can bring about more expensive installations.

Reduced fading margin implies that the system can offer surface covering with higher transmission quality and capacity at lower costs. Reduced influence from time dispersion implies increased transmission quality i surroundings with extensive multipath propagation.

In the following the following abbreviations are used;

REP signifies repeater in a wireless system with time shared channel access.

PP signifies mobile or stationary user unit (portable part in the DECT-standard) , in wireless system with time shared channel access.

BAS signifies base station, (radio fixed part in the DECT-standard) , in wireless system with time shared channel access.

The following significant features can be distinguished at the invention:

Adjacent REP with common or partly common coverage area measure signal strength in uplink from one and the same PP.

Repeating to BAS is performed by that REP which has measured strongest PP-signal. But for such REP which can perform bit error checking on received PP-signal, also the bit error level can constitue a basis for selection of REP.

Which REP that has measured strongest signal strength (or lowest bit error rate) is decided in a comparison, by adjacent REP communicating mutually, for instance via wireless communication (radio, IR etc) or by wire connection. The invention comprises a new, simple protocol for such comparing communication (NB; the protocol can also be used for representing bit error level): The comparing communication can be performed by measured signal level being represented by a message where the length of the message corresponds to the signal level.

The message must be long enough to make possible necessary resolution, short enough to make possible a comparison before repeating must be done, and short enough to make it possible for a sufficiently number of PP to be managed by REP. Further, the solution of the message must not be so high that the slight time dispersion which can occur between adjacent PP in wireless communication threatens the correct interpretation of the message.

By way of example can for DECT the following suppositions be made if communication is performed via radio within or at the DECT-band:

Time dispersion and time delay between REP depends among other things on the distance. For area restricted mobile access is the max. distance between adjacent REP probably 200 m. This makes possible a maximal solution of about 1 μs. If we choose to represent the signal strength with 20 different intervals, this implies that the length of the message is 20 μs. Further is needed in all about 10 μs for start- and finish operations of the message, which results in a total of 30 μs. This can be compared with the length of a time frame 10 ms, time slot 420 μs, and guarding period between time slots 56 μs. The technology can consequently be used for very rapid repeating within a time frame or even between consecutive time frames. At one for both REPs known point of time/channel in relation to PP-channel the message is sent. The message can for instance be sent in a guard-period immediately after the PP time slot which the REP has received.

REP starts listening as soon as it is no longer transmitting the message. That REP which finished first will "hear" the other REP. This implies that the REP with least received PP-signal strength will be eliminated and consequently shall not repeat to the base station. That REP which does not "hear" any signal strength message shall, on the contrary, repeat to the base station.

There is a risk that both REP have measured the same signal strength. The risk is furthermore increased if only a small number of discrete signal strength intervals represent the signal strength. For avoiding double transmission a random or repeater unique (need only be unique within a limitet range) sequence can be added to the signal strength message. The one who first "hears" the other's transmission "loses" the struggle for the portable.

Since the signal strength message is sent at a point of time/channel with a known relation to the portable's channel, there is no risk that different PP-signal strengths are mixed up at the comparison.

Repeating of traffic from BAS to PP is performed by that repeater which made the latest uplink repeating from the portable in question, if nothing else is announced Regarding figure 3 is referred to the following description of sequences:

Sequence Description

I: REPla and REPIB receive signal from PP.

II: REPla and REPIB compare received signal strength.

Ill: REP with max PP-signal repeats to BAS.

IV: REP with max PP-signal repeats from BAS to PP.

The signalling procedure for communication between repeaters according to above is not previously known.

The invention is not restricted to the above as an example shown embodiment, but may be subject to modifications within the scope of the following patent claims and the idea of invention.

Claims

PATENT CLAIMS

1. Device at radio based communication system in the form of a mobile and/or stationary digital radio based radio system operating with time shared channel access and comprising repeaters/repeater functions arranged between base stations and terminals/mobile units, c h a r a c t e r i z e d in that the repeaters/repeating functions are arranged to achieve macro diversity and by that reduce the influence of fading and time dispersion.

2. Device acccording to claim 1, c h a r a c t e r i z e d in that the macro diversity implies that the influence of both short/rapid and long/slow fading is possible to reduce by the best one of two separated anntennas being continuously selected.

3. Device acccording to claim 1 or 2, c h a r a c t e r i z e d in that adjacent repeaters/repeater functions with completely or partly common coverage areas measures signal strength in uplink from one and the same mobile or stationary user unit.

4. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that repeating to respective base station is performed by that repeater or repeater function which measures or have measured the strongest user unit signal.

5. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that, at repeater or repeater function which is capable to perform bit error checking on received signal from respective user unit, the bit error level will constitute a basis for selection of the repeater or repeater function.

6. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the repeater or repeater function which has measured the strongest signal level and/or lowest bit error level is possible to appoint at a performed comparison function by adjacent repeaters or repeater functions being mutually communicable, for instance via wireless commumication (radio, IR, etc), or by wire communication.

7. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that, comparing devices for selection of repeater or repeater function which has measured the strongest signal level and/or the lowest bit error level operates with a protocol for comparing communication.

8. Device acccording to claim 7, c h a r a c t e r i z e d in that the comparing devices operate with comparing communication allowing measured signal level to be represented in the form of a message where the length of the message preferably corresponds to the signal level.

9. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the mentioned message has a length which makes possible necessary solution and is sufficiently short to make comparison possible before repeating is performed, and also is sufficiently short for a sufficient number of user units shall have possibility to be managed by the repeater or repeater function.

10. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the solution in the message is so high that the slight time dispersion that may occur between adjacent user units at wireless communication does not correctly interpretes the message.

11. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that, at two involved repeaters or repeater functions, a current signal strength message is sent at a known point of time or in a known channel in relation to the user channel.

12. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that respective message is sent in the guard period immediately after the user unit time slot which the repeater has received.

13. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the repeater or repeater function starts listening as soon as it no longer transmits the message, at which the repeater which first finished will "hear" the other repeater, implying that the repeater with the lowest received user unit signal strength will be eliminated and does not perform the repeating function to the concerned base station, whereby, on the contrary, that repeater of those involved, which does not "hear" any signal strength message, shall repeat to the mentioned base station.

14. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that, at measuring the same signal strength, a random or repeater unique sequence is added to the signal strength message, at which that repeater which first "hears" the other repeater's transmission loses the struggle for the user unit/portable.

15. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the signal strength message is sent at a point of time/channel with a known relation to the user units/portables channel with the aim to eliminate risks for different user units signal strengths being mixed up at the comparison.

16. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that repeating of traffic from concerned base station to concerned user unit/portable is being performed by that repeater which performed the latest uplink repeating from current user unit/portable, if no other messsage is sent out in the system.

17. Device acccording to any of the previous claims, c h a r a c t e r i z e d in that the radio system consists of a system for wireless PABXes (Private Automatic Branch Exchanges) and/or system which operates with public access, for instance the DECT system.