Transmission system for transmitting digital signals in a radio terminal connecting network

A radio access network is a - typically nationwide - System of radio cells, each having a - Possible central generally - fixed base station contain around which in a radius of 1 km for example, be distributed more or less homogeneous, the network terminations (NTs) of the radio stations are located. Such radio cell is outlined in the drawing FIG 1, in which the base station with BS and the mobile subscriber (or its Network Termination) are denoted by NT. Since in a radio system the path loss increases quadratically with the distance, is the radio field strength at the cell edge is substantially smaller than inside the cell; in FIG 1 is this attenuation with concentric Illustrates circles around the base station around. For example caused by rain, also distance-dependent Additional attenuation can at the network terminations available at the cell border receive power addition greatly reduce.

In such a radio system which, in principle, a point-to-multipoint system represents (the transmitter of the base station can the recipient of many participants reach), the signal transmission downward from the base station to the radio subscribers back in time division multiplex (TDM - Time Division Multiplex) in go a 155-Mbit / s bitstream over and the signal transmission upstream of the radio subscribers towards the basestation in a TDMA (Time Division Multiple Access) -Zugriffsverfahren.

In such a system, the Shannon channel capacity not optimally exploited in two ways:

All network terminations (NT in FIG 1) to process the full Total bit rate of 155 Mbit / s of the down-time-division multiplex signal, although only a small part of this bitrate usually the respective network termination is determined. The high bandwidth of the time division multiplexed signal leads to a correspondingly poor signal / noise ratio at the subscriber-performance Receiver. The transmission power in particular the base station (BS in FIG 1) is from telecommunications law and / or technical limited reasons, resulting in as given parameters RF transmission frequency, total bit rate, antenna gain, signal / noise ratio the greatest possible cell radius results; in those Network termination (NT in FIG 1) that is closer to the Base station are, the reception performance - and thus the signal / noise ratio - this case greater than This is necessary for a given bit error rate, so that far 'away' transmission power.

The - usual in TDM - uniform distribution of available stationary overall performance over the time t and the frequency f illustrates the so-called illustrated in Figure 2. 'water-filling' Chart for a channel having a bandwidth B, a time duration T and a transmission power P_s, Such uniform power distribution, as in the in FIG 2 hatched planar surface of the diagram expressed comes, is ideal in a radio system, in which all Network Termination the same constant distance from the base station have. Taking the time T to be constant, so that they in Diagram may not be represented, and leads to the Distance r from the base station as a third variable, then is obtained for the constant for a bit error rate required Transmission power P 3 shaded scale in FIG curved Surface, which is obtained by one for each point of the Level the required power P for a given bit error rate (Eg 10^-9) Plotting. With negligible Multipatheffekten P of the frequency hardly dependent. In the 2 shows power distribution represented by the 'water-filling' algorithm ie independent of the distance r power P_s, In turn, is drawn in FIG 3 hatched. The distance between the curved and the straight surface is then a measure of the above so primed, at a given distance r 'gave away' performance.

To optimize the power distribution in the upward direction (Of the radio subscribers (NT) for base station BS back) can one within a radio cell in each case the received power at measure the individual network terminations NT and as a measure of use the respective pathloss, according to its then the transmission power of the respective Network Termination is adjusted accordingly. To optimize the power distribution in a downward direction (From the base station BS to the radio subscribers (NT) down) can with adaptive antennas each network terminations NT a radio cell are driven in succession, the higher profit such antennas towards omnidirectional Antennas with the same transmission power bridging accordingly greater distances allowed. Adaptive antennas are However, only at the beginning of development. One can also for more distant network terminations one hand and network terminations in the vicinity of the base station on the other hand use different modulation schemes, For example, 16QAM and QPSK for the inner for the outer Area of ​​the radio cell. The required signal / noise ratio (S / N) is smaller by 7 dB for QPSK than 16QAM. at 16QAM increase, however, especially in the amplifiers, the requirements for the linearity. Such a method is especially suitable for OFDM.

The invention now shows another way to optimize of power distribution within a radio cell.

The invention relates to a system for transmitting digital signals in a radio access network, particularly in a broadband RLL (Radio in the Local Loop) -Teilnehmeranschlussnetz; This transmission system according to the invention characterized in that for the digital signal transmission from the base station of a radio cell to the in the radio cell located radio subscribers towards the total transmission power the base station on a plurality of Frequency sub-bands and / or time periods with different Transmit power is divided and for in greater or lesser distance from the base station located mobile subscriber specific digital signals in frequency sub-bands and / or time intervals with correspondingly transferred higher or lower transmission power of the base station will. In this case, in a further embodiment of the invention, a solid Number of frequency sub-bands, each with a fixed predetermined be provided transmission power of the base station, of which each link from the base station to a wireless subscriber out in accordance with its distance from the base station one or more frequency subbands of the each required Transmission power can be allocated. In a further embodiment of the invention, the transmission power the base station having one or more integer Harmonics of a sine wave of a predetermined frequency be modulated and each link from the base station to a mobile subscriber towards pursuant to its removal from the base station a time portion of the required transmit power be assigned.

With such a power scaling in the frequency and / or Time domain, according to which the respective straight existing connections between the base station and radio subscribers each a frequency band portion or period only with the respectively assigned required base station transmission power is, the invention enables advantageously an optimal Power distribution within the radio cell, wherein either the total transmission power of the base station corresponding to can be reduced, or - at constant Total transmission power - the regenerated power to increase of the cell radius can be used.

The modulation of the transmission power with one or more Sinusoids, which are integer harmonics of a sine wave are a predetermined frequency, allowing the specific allocation of phases of large or small transmission power to the individual links between the base station and the more or less remote radio subscribers by Basis of the distance. For sufficiently small modulation frequencies (Eg 10 kHz) with a modulation of the transmit power only an insignificant broadening of the RF spectrum connected.

In a breakdown of the total available frequency band in frequency subbands of different base station transmission power varies for example 8-frequency sub-bands and a Derating for example, every 3 dB per frequency subband the transmission power by 24 dB. The benefits of Frequency sub-bands add up geometrically, ie Overall performance is substantially in the frequency sub-bands with the highest performance. The range extension is correspondingly large.

The signal transmission from the base station down to the Radio subscribers can turn out, if necessary in each frequency subband To go for in the time division before him; as the modulation method can be used for the digital signals to be transmitted be provided uniformly QPSK. The use of one another orthogonal carriers for the individual frequency subbands avoids the otherwise in FDMA (Frequency Division Multiple Access) unavoidable filter losses.

Other features of the invention will become apparent from the following a more detailed explanation with reference to the drawings. show case <sl><li>occurring 1 shows the typical picture of a radio cell with it losses and</li><li>2 shows the uniform distribution of data available Total transmission power with respect to time and frequency;</li><li>FIG 3 illustrates the course of the constant for a bit error rate required transmission power as a function of Distance of the mobile subscriber from the transmitting base station.</li><li>4 shows a division of the total transmission power in frequency subbands different transmit power, and</li><li>FIG 5 illustrates a breakdown of the total transmission power in time segments of different transmission power.</li></sl>

The Drawings Figure 1, FIG 2 and FIG 3 were already above explained, so that further explanations on this Place unnecessary.

In Figure 4, schematically an embodiment of a division the total transmission power of the base station (BS in FIG 1) of a radio cell (FIG 1) in frequency subbands FK0, FK1, ... Shown nudist different transmit power of where the digital signal transmission from the base station to the located in the radio cell of radio subscribers (NT in FIG 1) through each link from the base station to a mobile subscriber the way in accordance with its distance from A base station or a plurality of frequency subbands of each be assigned required transmission power (s) so that the for located at a greater distance from the base station Radio subscriber specific digital signals in frequency sub-bands with a correspondingly higher transmission power of the base station are transmitted and the for a small distance away from Base station located radio subscriber specific digital signals in frequency sub-bands with correspondingly lower Transmission power.

Of the base station BS (in FIG 1) with the individual emitted frequency-subband individual transmission powers Signals form an OFDM (Orthogonal Frequency Division Multiplex) signal with a few carriers, the actual multiple access time-division multiplexing (TDM) is going on. By Using only a few (eg, 4 to 16) support is the effort however low for the OFDM. On the other hand, the Symbol duration so greatly extended that an otherwise in TDM / TDMA because of so-called. Multipathverzerrungen required elaborate can be dispensed with equalizer.

In OFDM systems, the crest factor is very unfavorable in itself, which has a disadvantageous especially in highly nonlinear amplifiers noticeable. The breakdown of the total available different frequency band in frequency subbands Base station transmission power add up the benefits the frequency sub-bands, however, geometrically, that the overall performance is substantially of the frequency sub-band with the greatest performance determined. The crest factor is therefore much cheaper than OFDM with constant carrier amplitude.

The breakdown of the total available frequency band in Frequency sub-bands of different base station transmission power also allows an improvement in the frequency re-use. The co-channel interference from frequency sub-bands with smaller Transmission power is innately very low. The co-channel interference the frequency sub-bands with high power are indeed as great as in the previous systems, limited Now, however, a smaller to a substantial frequency range. In principle, the transmission system keeps the noise power as small as possible.

It can not even frequency sub-bands determined with each predetermined transmission power of the base station, but with a continuously controllable transmission power can be provided. A Such a configuration of the transmission system allows adaptively to each given in the radio cell Conditions (eg rain-induced increase of pathloss during the existence of a connection, Number of active Radio subscriber).

The for a Network Termination (NT in FIG 1) specific Different signals can also on frequency subbands Power are divided. The received bits are then each by frequency sub-band more or less reliable, with using a corresponding encoding method the less reliable bits are correct effectively.

The frequency band FK0 with the highest power can be used as Emergency channel for all connections from the base station (BS in FIG 1) to the network terminations of the mobile subscriber (NT in FIG 1) can be used on the then in - for example, by strong Rain conditional - greatly increased pathloss at least still telephony traffic is possible.

In Figure 5 is schematically an allocation of the total transmit power the base station (BS in FIG 1) of a radio cell (FIG 1) in periods ZK0, ZK1, ... zkk different transmit power shows, one of which for digital signal transmission from the base station to the located in the radio cell Radio subscribers (NT in FIG 1) out of each compound of the Base station to a wireless subscriber out in accordance with whose distance from the base station, one or more time periods posted to the required transmission power (s) be so that the for a greater distance from Base station located radio subscriber specific digital signals in periods with a correspondingly higher transmission power the base station is transmitted and the smaller for in Distance from the base station located radio subscriber specific digital signals in time periods with correspondingly lower transmission power. The sketched in FIG 5 Power modulation results from the superimposition the first and third harmonics of a (for example, 10 kHz) sinusoidal oscillation. In the receivers of radio subscriber can the power modulation by means of an opposite Modulation be rescinded.

The for a Network Termination (NT in FIG 1) specific Signals on different time periods Power are divided. The received bits are then each more or less on time slot reliably, which with Aid of an appropriate coding the less reliable bits are correct effectively.

The period Z0 with the highest power can be used as Emergency channel for all connections from the base station (BS in FIG 1) to the network terminations of the mobile subscriber (NT in FIG 1) can be used on the then in - for example, by strong Rain conditional - greatly increased pathloss at least still telephony traffic is possible.

It should be particularly noted that a power scaling in the frequency domain as erläuert above with reference to Fig 4 was charged with a power scaling in time domain, as was eräutert above with reference to FIG 5, also be combined may, without this still needs further explanation.