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

(74) Agent

Nokia OY, Pl 780, SF-00101 Helsinki, Fl

Jukka Henriksson, Espoo, Fl Tandbergs Patent Office AS, Oslo (54) Designation Digital radio line system, and method for setting the transmit power in a digital radio line system (56) Published publications EP Al 217042, US 4777653, US 4309771 (57) Summary A digital radio line system and a method for setting the transmit power in a digital radio line system. At a receiving end (2), the system comprises a first device (11) for monitoring the error rate calculation and for generating a first control signal (S_{> rr}) if the error rate calculation exceeds a predetermined threshold value. At a transmission end (1), the system comprises a device (4) for adjusting the transmission power, the device responding to the occurrence of the first control signal (S, ") by increasing the transmission power. In the system, a second device (9) is further provided at the receiving end for monitoring the rate of change of the received signal level and for generating a second control signal (S._p) if the rate of change exceeds a predetermined threshold value. The transmitting power setting device (4) responds to the occurrence of the first (S_r) or the second (S ^) control signal by temporarily increasing the transmit power to near the maximum transmit power.

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The invention relates to a method of setting the transmit power in a digital radio line system having a transmitting end and a receiving end, comprising the steps of transmitting such a digital signal from the transmitting end of the line system with a normal transmit power value less than a maximum transmit power value, to receive the digital signal at the receiving end of the line system, the received digital signal sometimes having errors and signal level changes, to detect and monitor the received signal level io and the error rate calculation at the receiving end, and to set the transmit power value at the transmit end in accordance with the detected signal level and error rate calculation.

The invention also relates to a digital radio line system, which at a receiving end comprises a first device ice for monitoring the error rate calculation and for generating a first control signal if the error rate calculation exceeds a predetermined threshold value, and at a transmitting end comprises a device for setting the transmit power, the setting device responds on the occurrence of the first control signal by increasing the transmit power.

In determining the transmit power of a digital radio line system, not only the so-called constant power losses caused by the line or line voltage must be taken into account, but also random signal fades or signal variations caused by e.g. multipath propagation. The transmit power of the transmitter is therefore often set to a value considerably above the minimum transmit power which would, under normal conditions, be sufficient for interference-free reception at the other end of the line. Excessive transmit power gives the line 30 line a power margin which ensures interference-free reception even in difficult conditions. On the other hand, however, high transmission power increases the interference in adjacent channels or systems. This prevents the realization of dense radio-line networks, efficient utilization of frequency bands and repeated use of the same frequency within the same range.

U.S. Patent No. 4,004,224 discloses a method by which transmit power within a certain line distance is automatically increased if fading is observed within that particular line so that a constant reception power is maintained at the receiver.

The receiver produces a signal representing the level of the received signal, and this signal is sent through a return channel to the transmitter for setting the transmit power.

In US Patent 4,309,771, the bit error rate or bit error rate is monitored by the received digital signal instead of the received signal level, and the transmit power is set via the return channel in such a way that the bit error rate of the received signal does not exceed a predetermined threshold value.

In US Patent 4,777,653 (corresponds to FR Patent 8518919) the setting 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 fading. At high levels, the error rate is also monitored. If the error rate is high regardless of the sufficient level, the transmit power is temporarily increased to exceed the normal reserve of 6 dB (eg a reserve of 3 dB).

EP 0 217 042 discloses a digital radio line system incorporating a fading detector for rapid recognition of fading processes, for the derivation of predetermined threshold values ​​for setting the transmit power. In this system, the fading state is detected on the basis of the absolute level and / or distortion of the received signal level, i.e. the power level control criterion is the depth of the fading.

The above-mentioned methods aim to adjust the transmit power to reduce the transmit power required under normal conditions, to reduce the interference caused by other line voltages or channels while enabling adequate quality of data transmission, even during fading periods. However, these methods are not sufficiently effective in all situations.

The object of the present invention is to provide an improved method for adjusting the transmit power to a digital radio line.

In order to achieve the above object, a method of the type initially introduced which is characterized by the further steps of monitoring the rate of change of the received signal level and temporarily increasing the transmit power value at a first value between the normal transmit power value and the maximum value is provided. transmit power value at times when the rate of change of the received signal level exceeds a predetermined threshold value.

According to the invention there is also provided a digital radio line system of the type initially provided, characterized in that a device is also provided for monitoring the rate of change of the received signal level and for generating a second control signal if the rate of change exceeds a predetermined threshold value. and said device for setting the transmit power responds to the occurrence of the second control signal by temporarily increasing the transmit power to a first value between a normal transmit power and the maximum transmit power.

By monitoring the rate of change of the received signal level, rapid variations or fading occurring at multipath propagation can be detected efficiently. In multipath propagation, the fading can be very fast (as fast as 100 dB / s and more), so it can be difficult to compensate for changes in the reception level by real-time setting of the transmit power via the return channel, for example due to the return channel delay. In the method of the invention, therefore, the transmit power is set to its highest normal value for a certain time when rapid fading is detected. This makes it easier to control rapidly changing situations and stabilizes the setting. Furthermore, during multipath fading, the received signal level may be sufficient, but errors occur in the reception due to interaction between channels. Therefore, in the preferred embodiment of the method of the invention, the transmit power is increased from a value near the maximum transmit power to the maximum transmit power if the error rate calculation exceeds the predetermined threshold value in addition to the detected multipath condition. This degrades the signal to some extent, so that the long-term error rate may be of the order of 1E7 ... 1E-8. However, in a multi-way mode, it is necessary to bring the error rates 1E-3 ... 1E-6 under control so that the application of the maximum transmit power for a limited period of time is advisable.

If the monitored error rate calculation exceeds the predetermined threshold value under normal conditions (no multipath mode), the transmit power is temporarily increased close to the maximum transmit power. In this way, quick response to errors is achieved. This type of situation can occur as a result of temporary propagation conditions, whereby some remote system may interfere with a certain time.

In the preferred embodiment of the invention, it is compensated for slow variation in the reception level, which may be due to e.g. variation in the k value, by monitoring the level of reception. If the reception level falls below a predetermined minimum, the transmit power is set via the return channel by increasing it by a small increment. In this way, the quality of the received signal remains sufficiently good while avoiding the use of excessive transmit power and interference with adjacent systems.

By the method of the present invention, the transmit power can be set in such a way that it is maintained at an optimum value under normal conditions while providing a fast response to interference so as to ensure interference-free reception.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail by way of example with reference to the drawings, in which FIG. 1 is a block diagram of a digital radio line system according to the invention; FIG. 2 is a flow chart of the transmit power setting unit 4 of FIG. 1, FIG. 3 is a circuit diagram of the transmit power setting unit of FIG. 1, and FIG. 4 illustrates graphically the setting of transmit power according to the invention in a conceived reception situation.

Fig. 1 shows a radio line system according to the invention comprising a transmitter unit 1 and a receiver unit 2. In the transmitter unit 1, a transmitter or modulator part 6 supplies a modulated high frequency signal to a high frequency amplifier stage 7, such as an output stage, which amplifies the signal and supplies it to a transmitter antenna 3. At the other end of a line 15, a receiving antenna 12 receives the signal transmitted by the transmitter unit 1 and feeds it to a receiver 8 in which the signal is detected.

A block 9 in the receiver unit 2 monitors the rate of change of the received signal level, especially fast level reductions, to detect a multipath state, and produces a 1 bit signal S_rap if condition is 1 if a multipath state occurs and 0 if no multipath state is present.

In FIG. 1, block 9 compares the present value of receiver 8 AGC voltage P_rec with a short-term average of the voltage, or simply with a previous value. If the present value of the AGC voltage P_ree deviates at least 5 to a predetermined degree from the short-term average of the previous value, the change is interpreted to have been caused by a fast fading, and the state of the output signal S_mp from block 9 the value 1 is given.

A block 10 in the receiver unit monitors the received signal level and produces a 1-bit signal S_my if the received signal level is below a predetermined threshold value L_my. In the preferred embodiment, block 10 also monitors the value of receiver 8 AGC voltage P_rec. Threshold value L_myis set so that the system's residual error rate will be sufficiently good. Limit value L_nin is preferably of the order of approx.

to 12 dB above the receiver's 8 threshold.

A block 11 in the receiver unit produces a 1-bit signal S_{is R} if condition is 1 if an error condition occurs at reception, and 0 if no error condition occurs. The error condition may be e.g. N error pr. unit of time (eg per minute).

In the preferred embodiment of the invention, the signal S is obtained_{is R}condition 1 immediately after N error has occurred, even if the time should not have expired. The errors can be counted, e.g. based on the parity of the byte unit or frame.

The error signal can also be generated from the pseudo-error signal that occurs in connection with the receiver 8. When the calculated error rate is greater than a predetermined threshold value, the state of the signal S is given._{is R} earn 1.

The binary signals generated by blocks 9, 10 and 30 are multiplexed by a multiplexer 13 and transmitted through a low capacity return control channel 14 to the transmitter unit 1.

In the transmitter unit 1, a demultiplexer 5 demultiplexes the received signal and supplies the signals S_np, S_nin and S_{is R} to the transmit power setting block 4. On the basis of these signals, the transmit power setting block generates a setting voltage c (t) which controls the amplifier stage of transmitter unit 1. By means of the setting voltage c (t), a high frequency power P_out supplied by the amplifier stage 7 to the antenna 3 is set or adjusted in response to the quality of the signal received by the receiver unit 2.

The functions of the transmit power setting unit 4 are illustrated by means of the operation diagram of FIG. 2.

To begin with, the presence of a multipath mode is monitored by detecting the state of the signal S_rap which represents an excessive rate of change of signal level.

If a multi-path mode is detected (S_mp = 1), the state of the signal S is monitored or investigated_{is R} representing an error state, at a point 22 to determine if an error state is present. If S_{is R} = 1, a multipath state and an error state are simultaneously present, and as a result, a maximum transmit power P is set_max at a point 23, and the transmission is continued at this value for a time period T1 from the last detection of simultaneous multipath and error states. With an ice 16QAM signal, the maximum transmit power can be e.g. 2 to dB, and with a 4QAM signal approximately 0 to 2 dB below the level of a 1 dB compression point. The time period T1 can e.g. be of the order of 5 to 10 min. This application of a maximum transmit power strives to minimize the time during which the reception error rate is less than 10 '³...10‘⁶.

If point 22 results in S_{is R} = 0 (no fault condition), the transmit power is set at a point 25 to a value P_x which is slightly below the maximum transmit power, and with e.g. a 16QAM signal approx. 6 to 8 dB below the 1 dB compression level.

With the transmit power P_x distortions are small and it is thus possible to obtain a good residual error rate (BBER). The transmission is continued on the effect P_x for a time period T2 from the last detection of a multipath state. The time period T2 is e.g. of the order of 15 to 20 min.

If point 21 results in S_mp = 0, i.e. no multipath state is present, the possible presence of an error state at a point 24 is checked by the signal S_{is R}. If S_{is R} = 1 at point 24, go to point 25 at which the transmit power is set to the above value P_x for a time period T2. Errors that occur without any multipath condition may be due to disturbance caused by exceptional propagation or propagation conditions. In such a case, the situation may continue for a long period of time and it is not advisable to transmit the maximum transmit power P_raax.

If point 24 gives the result that S_{is R} = 0, the received signal level is monitored or examined at a point 26 by the signal S_nin. If S_my = 1, i.e. the received signal level is below a minimum reception level L, the previous value P is increased_n__x of the transmit power with an increment k'd, where k is a proportional coefficient (&gt; 1, typically between 2 and 4) and d is a fundamental increase or base increase. After the increase, the maximum value of the transmitting power must not exceed the aforementioned power P_x. By choosing k&gt; 1, the increase in transmit power is faster than its reduction.

If point 26 results in S_oin = 0, i.e. the reception level is above the minimum reception level L, the transmit power from the previous value P is reduced_n._x with the increment d. These changes are typically below 1 dB, because the update is performed e.g. once per minute or more frequently. The rate of change of the transmit power must be less than 1 dB / s to prevent interpretation of the change as a multipath state at the receiving end. The transmit power will not fall below a predetermined value P_o which is the transmit power under normal conditions. The line distance is dimensioned so that the transmit power P_o under normal conditions results in a reception level that is above the minimum reception level L. The transmission power is thus almost permanently equal to P_o, and the system works approx. 12 to 14 dB above the receiver's threshold level. Higher power levels are used only in unusual cases, ie in the case of multi-way and fault conditions.

Fig. 3 shows schematically the principles for one possible embodiment of the unit 4 for setting the transmit power.

Signals S_sp and S_{is R} is applied to an AND gate 41 whose output is coupled to a flip-flop 42. If both signals S_mp and S_{is R} has state 1, the gate 41 and the flip-flop 42 are reset and a counter 43 starts counting the time period T1. The output of flip-flop 42 causes an output LOAD1 of another flip-flop 44 to have condition 1 during the count. When the counter 43 count is completed after the time period T1, the counter 43 sets the output L0AD1 of flip-flop 44 to state 0. The signal L0AD1 controls a memory or memory circuit 45. During each cycle of a clock or clock signal CL, a count corresponding to the maximum power P is loaded_max from the storage circuit 45 to a counter 55 as long as the signal LOAD1 has state 1.

Signals S_AEP and S_{is R} is also supplied to an exclusive OR gate 46 which via a flip-flop 47 controls a counter 48 which counts the time period T2. Counter 48 is reset and started each time one or the other of the signals S_mp and S_{is R} has the state 1, 5, ie when either a multi-way or a fault state is present. An output of a flip-flop 49 is set to state 1 during the count of the counter 48 and it is applied to one input of an AND gate 50. The signal LOAD1 is applied to another input, i.e., an inverting input, to the AND gate, so that the output signal io L0AD2 from the port 50 has the state 1 only if the signal LOAD1 has the state 0 and the output signal from the flip-flop 49 has the state

1. A count similar to the transmit power P_x is supplied from a storage circuit 51 to the counter 55 during each cycle of the clock signal CL as long as the signal L0AD2 is in the state 1. The gate 50 ensures that the maximum power P_raax first and foremost loaded in case of competition between P_ma3I and P_x.

The signal S_my is applied directly to AND gates 53, 54, and the signals LOAD1 and LOAD2 are applied to these ports via a NORport 52. In this way, the setting of the transmit power is effected by 2o by the signal S_my only if neither a multipath state nor an error state is present, the signals L0AD1 and LOAD2 both having the state 0 and the output of the port 52 having the state 1. The signal S_my are thereby capable of affecting the counter 55 via ports 53 and 54. If the signal S_nin = 1, i.e., the received level is below the minimum receiving level L, an increase kd is added to the counter 55's input. If the signal S_nin = 0, i.e., the minimum reception level is exceeded, the increment d is extracted from the contents of the counter 55 via an input D. The counter 55 internally checks that the transmit power set via this does not exceed the power P_x or is under the effect P_o.

Of course, the operation or operation mentioned above can also be realized as a microprocessor program.

The digital output of the counter 55 is applied to a D / A converter 56 which produces an analog voltage. If required, this voltage can be subjected to a non-linear conversion in block 57 if the power increment steps on the dB scale are to be substantially the same size. As a result, a setting voltage c (t) is obtained by which one amplifier in the transmitter amplifier sequence is set, such as the high frequency end amplifier.

The above-mentioned counting block 4 can be realized in a simpler form. One alternative is to use the same time period T1 for transmitting both the maximum power P_food and the effect P₁.

Thus, one circuit is sufficient for the count of time. In the microprocessor realization, it may be possible to save a few program steps in this way.

Another alternative, which is probably applicable in conjunction with constant-amplitude modulation methods, is to use a single higher power value P_x instead of the effects P_xand P_max, and correspondingly a single time period T2. This greatly simplifies realization.

The setting principle could also be modified by monitoring any exceedance of a predetermined maximum reception level U. If the received level is between L and U, there is no need to change the power level. If the level U is exceeded, the transmit power is reduced. An advantage of this alternative would be the less frequent changes in transmit power and more reliable multipath mode detection. A disadvantage is the need for a further bit to transmit information to the transmitter.

Fig. 4 illustrates the operation of the setting method according to the invention in a conceived reception situation. At a point A, the received signal level is below the predetermined minimum reception level L, so that the transmitter is adjusted to a certain degree (an increment k'd is added to the counter 55) to increase the power. At a point B, it appears that the reception level exceeds the minimum reception level L and increment d is extracted from the counter 55. The same happens during the subsequent clock cycle 30 at a point C.

At a point D, a rapid change in the received signal level compared to the short-term average is detected, such as a difference of 3 to 5 dB or a rate of change greater than 3 to 5 dB / s. Effects P_x which is close to the maximum power, 35 is thereby adjusted in the transmitter. At a point E, both multipath mode and error state are detected, and the maximum transmit power P_Naxconnected to the transmitter for a time period T1. No further errors are detected, but the multipath mode continues. At a point F, the transmit power from the maximum power P is reduced_max to the value

P_x after the time period T1 has elapsed from the last detection E of errors. At a point G, the time period T2 has elapsed from the last detection of multipath mode, whereby the transmission on the effect P_x is canceled and the setting starts with signal S_nin. It is thereby observed at a point G that the minimum reception level L is exceeded, and the output power P_outdecreases with the increment d. The same happens at a point H within the subsequent clock cycle, and the transmit power gradually drops to the normal transmit power P_o is obtained in point I.

At a point K, an error condition occurs without multi-way fading. The transmission power is thereby increased to the value P_x. The power is transmitted in the time period T2 after the last detection of a fault condition. At a point L, the setting is resumed with the signal S_my which reduces the transmit power gradually or monotonously in the direction of the value P_o.