A digital radio link system and a method of adjusting the transmission power in a digital radio link system

Digital radio link system and method for adjusting transmission power in a digital radio link system

The invention relates to a method for adjusting the transmission power in a digital radio link system by monitoring the received signal level and the error rate estimate at the receiving end.

When determining the transmission power of a transmitter in a digital radio link system, the so-called in addition to standard losses, be prepared for random signal fading caused by, for example, multipath propagation. Therefore, the transmission power of the transmitter is often set significantly higher than the minimum transmission power that would normally be sufficient for error-free reception at the other end of the link15 tendon. The extra transmission power creates a power margin for the link voltage, which ensures error-free reception even in difficult conditions. On the other hand, however, high transmission powers increase interference to adjacent channels or systems. This makes it difficult to implement dense radio link networks, efficient use of frequency bands and re-use of the same frequency in the same area.

U.S. Patent No. 4,004,224 discloses a method in which upon detecting fading on a particular link25 voltage, the transmission power on that link voltage is automatically increased so as to maintain a constant received power at the receiver. The receiver generates a signal indicating the received signal level, which is transmitted along the return channel to the transmitter for transmission power control.

U.S. Pat. No. 4,309,771 monitors the bit error rate of a received digital signal instead of a received signal level and adjusts the transmission power through the return channel so that the bit error rate of the received signal is below a predetermined threshold.

In FR 8518919, the control range is basically divided into two parts: at low reception levels, the transmitter is instructed via the return channel to increase the power to compensate for the fading. At high levels, the error rate is also monitored. If the error rate is high despite a sufficient level, the transmission power is temporarily increased beyond the normal 6 dB backoff (e.g., 3dB backoff).

The aim of the above methods has been to reduce the transmission power required under normal conditions and thereby interference to other link spans or channels by means of the transmission power control, but at the same time still enables sufficient data transmission quality even during fading periods. However, they do not succeed in their task well enough in all situations.

It is an object of the present invention to provide an improved method for adjusting the transmission power of a digital radio link.

This is achieved by a method of the type described in the introduction, which according to the invention is characterized in that the transmission power is temporarily increased to a value close to the maximum transmission power if the received signal level change rate or error rate estimate exceeds a predetermined threshold value.

By monitoring the rate of change of the received signal level, rapid, multipath fading can be effectively detected. In a multipath situation, fading can be very fast (even greater than 100 dB / s), so compensating for changes in reception level by adjusting the transmission power through the return channel in real time can be difficult due to, for example, the return channel delay. Therefore, in the method according to the invention, the transmission power is set to normal full transmission power for some time when a fast fading is detected. This solution makes it easier to manage rapidly changing situations and makes the adjustment stable. In addition, during multipath fading, there are situations where the received signal level is sufficient, but errors occur in reception due to the interaction. Therefore, in a preferred embodiment of the method according to the invention, the transmission power is increased from a value close to the maximum transmission power to the maximum transmission power if, in addition to the detected multipath situation, the error rate estimate exceeds a predetermined limit value. This causes some distortion in the signal, so that the long-term error rate can be of the order of 1E-7 ... 1E-8. But in a multipath situation, the error rates 1E-3 ... 1E-6 must be controlled, in which case it makes sense to use the maximum transmission power for a limited time.

If the observed error rate estimate in a normal situation (not a multipath situation) exceeds a predetermined threshold, the transmission power is temporarily raised close to the maximum transmission power. This provides a quick response to errors. Such a situation may arise due to temporary propagation conditions, in which case some distant system may interfere for some time.

In a preferred embodiment of the invention, the slow variation in the reception level due to, for example, variations in the k-value is compensated by monitoring the reception level. If the reception level falls below a predetermined minimum level, the transmission power is adjusted through the return channel by increasing it by a small increment. The quality of the signal thus received remains good enough, but on the other hand, too much transmission power is not used to avoid interference with nearby systems.

The method according to the present invention provides a transmission power control which keeps the transmission power optimal in normal situations, but which reacts quickly to interference situations, ensuring error-free reception.

The invention also relates to a digital radio link system according to claim 8, which applies the method according to the invention.

The invention will now be described with reference to exemplary embodiments with reference to the accompanying drawings, in which Fig. 1 shows a block diagram of a digital radio link system according to the invention, Fig. 2 shows a in the reception situation.

Figure 1 illustrates a radio link system according to the invention comprising a transmitter unit 1 and a receiver unit 2. In the transmitter unit 1, the transmitter or modulator part 6 supplies a modulated high frequency signal to a high frequency amplifier stage 7, e.g. a power stage 3, which amplifies the signal. At the other end of the link string 15, the receiver antenna 12 receives the signal transmitted by the transmitter unit 1 and supplies it to the receiver 8, where the signal is detected.

Block 9 of the receiver unit 2 monitors the rate of change of the received signal level, in particular the rapid lowering of the level, to indicate a multipath situation, and generates a 1-bit signal S_mp, whose status is 1 if a multipath situation exists, and 0 if there is no multipath situation. In the case of Figure 1, block 9 compares the AGC voltage P of the receiver 8_rec the current value to the short-term average of the voltage or to the previous value only. If the AGC voltage P_recthe current value differs by at least a short-term average or the previous value of a predetermined amount, the change is interpreted as caused by a rapid fading, and the output signal S of block 1_mp the mode is set to one.

Block 10 of the receiver unit monitors the received signal level, and generates a 1-bit signal S_Bln, if the received signal level is less than the set limit value L_mln. In the preferred embodiment, block 10 also monitors the AGC voltage P of the receiver 8_rec value. Limit value L_bin is set so that the residual error rate in the system becomes sufficiently good. Suitable size class for limit value L_min is about 10-12 dB above the 8 threshold levels of the receiver.

In block 11 of the receiver unit, a 1-bit signal S is generated_err, whose status is 1 if there is an error condition in reception, and 0 if there is no error condition. The error situation can be, for example, N errors per unit time (eg 1 minute). In a preferred embodiment of the invention, the signal S_err gets state 1 as soon as N errors are full, even if the time has not yet elapsed. Errors can be calculated, for example, from the parity of the frame. The error signal can also be generated from the pseudo-error signal connected to the receiver 8. When the estimated error rate is greater than a predetermined threshold, the signal S_err the mode is set to one.

The binary signals generated by blocks 9, 10 and 11 are multiplexed by a multiplexer 13 and transmitted along a low-capacity return channel 14 to the transmitter unit 1. In the transmitter unit 1, the demultiplexer 5 demultiplexes the received signal and supplies the signals S_bp, S_Bln and S_errto the transmission power control block 6. In the transmission power control block, a control voltage c (t) is generated on the basis of these signals, which controls the amplifier stage 7 of the transmitter unit 1. Thanks to the control voltage c (t), the high frequency power P_OUT can be adjusted according to the quality of the signal received by the receiver unit 2.

The tasks performed by the transmission power control unit 4 are illustrated in the operation diagram of Fig. 2.

First, it is examined whether this is a multipath situation by checking whether the signal S indicating an excessive signal level change rate is S._mp in mode 1. If a multipath situation is detected (S_ep = 1), it is checked in step 22 whether the error condition is further examined by examining whether the signal S indicating the error condition is present_err status 1. If S_arr = 1, selects both the multipath situation and the error situation at the same time, as a result of which the maximum transmission power P is set in step 23._Bax and the transmission is resumed at this power time T1 from the last detected common multipath and error condition. The maximum transmission power may be, for example, 2-4 dB in the case of a 16QAM signal and about 0-2 dB below the level of the 1 dB compression point in the case of a 4QAM signal. The time T1 may be, for example, of the order of 510 minutes. This use of maximum transmission power is intended to minimize the time during which the error rate in reception is worse than 1O '.³... 1O⁶.

If step 22 yields S_err = 0 (no error condition), the transmission power in step 25 is set to P_x, which is slightly lower than the maximum transmission power and, for example, with a 16QAM signal, about 6-8 dB below a compression level of 1 dB. At this transmission power P_x the distortions are small and it is possible to achieve a good residual error rate (RBER). Transmission continues at power Pi for time T2 from the last detected multipath situation. The time T2 is, for example, of the order of 15-20 minutes.

If the result is obtained in step 21_mp = 0, i.e. there is no multipath situation, the signal S is checked in step 24_err with the help of a possible error situation. If in step 24 S_arr - 1, proceeds again to step 25, where the transmission power is set to the above-mentioned power Pj for the time T2. Errors that occur without multipath propagation may be due to interference caused by exceptional propagation conditions. In this case, the situation can continue for a long time and the extreme maximum transmission power P_vmax there is no reason to send.

If step 24 yields S_err = 0, at 26 the received signal level of the signal S is checked_mln through. If s_mln = 1, i.e. the minimum reception level L is exceeded, is added to the previous value of the transmission power Ρ_η__χ increment k «d, where k is the proportionality factor (> 1, typically

2-4) and d is a basic addition. After increasing the maximum transmission power, a maximum of the above-mentioned power Pj is allowed. Selecting k> l makes the increase in transmission power faster than the decrease.

If in step 26 the result is S_min = 0, i.e. the reception level is above the minimum reception level L, the transmission power is subtracted from the previous value P_of__x incrementin d verran. These changes are typically less than 1 dB because the update is done, for example, once per second or more often. The rate of change of transmission power should be less than 1 dB / s, so that the change is not interpreted as a multipath situation at the receiving end. The transmission power is not reduced to less than a predetermined value P_o, which is the normal transmission power. Your links are dimensioned so that normally the transmission power P_o results in a reception level higher than the minimum reception level L. Thus, most of the time, the transmission power is constantly P_o and the system operates at about 12-14 dB above the receiver threshold level. Only in exceptional situations, multipath and error, higher power levels are transmitted.

Figure 3 shows a schematic diagram of a possible implementation of the transmission power control unit 4.

Signals S_np and S_err is conducted to the AND gate 41, the output of which is connected to the flip-flop 42. If both signals S_mp and S_err are in state 1, the gate 41 and the flip-flop 42 reset and start the counter 43, which counts the time T1. The output of flip-flop 42 directs the output L0AD1 of the second flip-flop 44 to state 1 during the count. When the counting of the counter 43 after the stop T1 stops, the counter 43 sets the output L0AD1 of the flip-flop 44 to 0. The signal L0AD1 controls the memory circuit 45. The maximum power P is charged from the memory circuit 45 with each period of the clock signal CL._max the corresponding number in the counter 55 as long as the signal LOAD1 is in state 1.

Signals S_bp and S_err is also fed to an exclusive OR gate 46 which, via flip-flop 47, controls a counter 48 which counts the time T2. The counter 48 is reset and started whenever one of the signals S_bp and S_err is in state 1, i.e. when either a multipath situation or an error situation exists. The output signal of the flip-flop 49 is set to state 1 during the counting of the counter 48 and is fed to the second input of the AND gate 50. A signal L0AD1 is applied to the second, inverting input of the AND gate, whereby the output signal LOAD2 of the gate 50 is in state 1 only if the signal L0AD1 is in state 0 and the output signal of flip-flop 49 is in state 1. Transmission power P is read from memory circuit 51_x the corresponding number on the counter 55 for each clock signal CL in the period as long as the signal LOAD2 is in state 1. The gate 50 ensures that in a possible competition situation the maximum power P is primarily charged_m._.

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Signal S_min fed to AND gates 53, 54 directly and signals L0AD1 and LOAD2 through EITAI gate 52. This ensures that the transmission power is adjusted by the signal S_Bln only if there is neither a multipath situation nor an error situation, in which case the signals LOAD1 and L0AD2 are in state 0 and the output of gate 52 is 1. In this case, the signal S_Blncan act on the counter 55 via gates 53 and 54. If the signal S_Bln = 1, i.e. the minimum reception level L is exceeded, an increase k · d is given to the input of the counter 55. If the signal S_Bln = 0, i.e. above the minimum reception level, the contents of counter 55 are decremented increment d via input D. The counter 55 internally monitors that the transmission power controlled via this does not exceed the power P_x or underpower P_o.

The above operation can, of course, also be implemented as a microprocessor program.

The digital output of counter 55 is applied to a D / A converter 56 which produces an analog voltage. If necessary, a non-linear conversion can be performed in block 57 if it is desired that the power increase steps be approximately evenly spaced on the dB scale. The result is a control voltage c (t) which controls some amplifier in the transmitter gain circuit, for example a high frequency power amplifier.

The calculation part 6 presented above can also be implemented in simpler versions. One implementation possibility is to use T1 and the maximum power P at the same time_max that the power of P_x sending. In this case, only one circuit is needed for timing. Also in a microprocessor implementation, a few program steps could be saved in this way.

Another implementation that may be useful in the context of constant amplitude modulation methods is to use the P_x and P_max instead of only one higher power value P_x and correspondingly only one time value T2. This greatly simplifies the implementation.

The control principle could also be changed so that the exceedance of a predetermined maximum reception level U is also monitored. If the received level were between L and U, the transmission of the former power level would be continued. Only if the level U was exceeded would the transmission power be reduced. This implementation would have the advantage of less frequent changes in transmission power and more reliable identification of multipath change situations. The disadvantage is the need for one additional bit to transmit information to the transmitter.

Figure 4 illustrates the operation of the control method according to the invention in an imaginary reception situation. The signal level received at A is below the set minimum reception level L, whereby a small control is given to the transmitter (increment k «d is added to the counter 55) to increase the power. Point B states that the reception level exceeds the minimum reception level L and the increment d is subtracted from the counter 55. The same happens in the next clock cycle at C.

At point D, a rapid change in the received signal level compared to the short-term average is observed, e.g., a 3 ... 5 dB difference or a rate of change greater than 3 ... 5 dB / s. In this case, the transmitter is connected to a power close to the maximum power Ρ_χ. In E, in addition to the multipath situation, an error situation is also detected and the maximum transmission power P is connected to the transmitter._max for Tl. There will be no new error findings, but the multipath situation will continue. At F, the transmission power is calculated from the maximum power P_max to power P_lz when time T1 has elapsed since the last error detection E. In point G, time T2 has elapsed since the last detection of the multipath situation, at which time the power P_x sending and switching to signal S_min adjustment. In this case, it is stated in section G that the minimum reception level L has been exceeded and the output power P_OUT reduced by increment d. The same is repeated at point H for the next clock cycle and the transmission power gradually decreases until the normal transmission power P is reached at point I._o.

At point K, an error condition is detected without multipath fading. In this case, the transmission power is increased to P_x. This power is transmitted at time T2 after the last detected error condition. At point M, the signal S is switched again_mln which gradually or monotonically reduces the transmission power towards P_o.

The accompanying figures and the related description are intended to illustrate the present invention only. The details of the method and system of the invention may vary within the scope of the appended claims.