KR20040024371A - Method and apparatus to prevent spill-over signals that break into radio frequency repeater , using multi-path searcher - Google Patents

Abstract

PURPOSE: A device for preventing a re-inputted signal of a wireless repeater using a multi path searcher is provided to install a signal searcher for searching a pilot channel and synchronizing the searched pilot channel in an input part of a wireless repeater for receiving a signal of a base station, thereby efficiently reducing a re-inputted signal. CONSTITUTION: A receiving antenna(410) receives a base station transmission signal(213) and a forward re-input signal(214), and a portion of signals delivered to a transmission antenna. A signal portion of the receiving antenna(410) is branched, and is inputted to a multi path searcher(415). The multi path searcher(415) performs an autocorrelation integration process, searches a signal energy value and time difference information of a signal whose autocorrelation integration value exceeds a predetermined threshold value, and transmits the searched signal energy value and the time difference information to a controller(416). The controller(416) controls a repeater gain in order to minimize the forward re-input signal(214) by using the signal inputted from the multi path searcher(415).

Description

Method and apparatus for preventing re-input signal of wireless repeater using multipath searcher {method and apparatus to prevent spill-over signals that break into radio frequency repeater, using multi-path searcher}

The present invention relates to a method and an apparatus for preventing a repeater re-input signal for covering a partial frequency shadow area caused by extending a service area (also called coverage) or lacking electric field strength in a mobile communication system.

In general, a mobile communication system is composed of a mobile communication switch, a base station (including a repeater), and a mobile terminal. The mobile switching center connects each base station controller, and consists of information and authentication for subscribers and related additional equipment, and serves to connect with other networks. The base station is composed of a base station controller (BSC), a base station (BTS), a repeater and the like and performs a connection with the exchange so that the mobile terminal makes a call with a desired counterpart. The mobile communication exchange handles calls and calls from each base station and relays access to a public switched telephone network (PSTN) with a central control function so that all base stations can be operated efficiently. Of these, the location and number of base stations are most closely related to the quality of mobile communication service. However, since the installation cost of a base station is significant, the mobile service provider can install the base station as little as possible to secure the quality of communication service. We are working in various ways.

In this process, the mobile communication service provider uses a repeater that can be installed at a lower cost than the base station to expand the service area without installing a separate base station. The repeater does not increase the call channel, but it is sufficient to replace the base station in terms of coverage extension.

Mobile communication service providers prefer to place base stations in call dense areas when designing base station layouts, and optical repeaters or wireless repeaters that can be installed cheaper than base stations in areas outside the city, not in dense areas. Minimizes investment and operating costs compared to call quality

The repeaters used in mobile communication can be divided into optical repeaters that connect the base stations and repeaters with wired optical cables, etc., and wireless repeaters that connect wirelessly with the optical repeaters. Although it can be expensive compared to a wireless repeater, in particular, the cost of installing or renting an optical cable is considerable. On the other hand, the wireless repeater has the advantage that the call quality is lower than the wired repeater such as the optical repeater, but the installation or operation cost is low.

Wireless repeaters are mainly applied to communication services in buildings. In particular, in underground spaces such as underground parking lots of large buildings, the coverage of one wireless repeater must be small due to the effect of electromagnetic shielding by structures, etc. Therefore, the wireless repeater which uses less cost than the optical repeater that is expensive is used more frequently. .

However, when the wireless repeater is used in an open area such as outside the building, the transmission signal of the wireless repeater is received and input again to the receiving antenna of the wireless repeater for receiving the signal of the base station, thereby causing interference or oscillation of the signal.

1 is a block diagram of a general wireless communication repeater. In the case of the forward link from the base station to the terminal, the signal transmitted from the base station is received by the repeater receiving antenna 110 and then passed through the duplexer 111 to pass the low noise amplifier 112, the filter 113, and the power amplifier 114. After amplifying a signal having a bandwidth of a predetermined size, it is wirelessly transmitted through the transmission antenna 116 of the wireless repeater via the duplexer 115. In addition, in the reverse link from the mobile terminal to the base station, the transmit antenna 116 of the forward link serves as the receive antenna, and the receive antenna 110 of the forward link serves as the transmit antenna, and the signal received from the mobile terminal is a low noise amplifier 117. ), The filter 118, and the power amplifier 119 are transmitted to the support station.

FIG. 2 illustrates a problem that occurs when the wireless repeater described in FIG. 1 is applied to an open area, and a part of signals transmitted from a transmitting antenna of the wireless repeater becomes a forward re-input signal 214 as a receiving antenna of the wireless repeater. It shows the process being received. In the case of the forward link of the wireless repeater, which receives and amplifies the transmission signal of the base station and then retransmits it for reception by the mobile terminal, the signal transmitted from the base station 210 is received by the wireless repeater receiving antenna 110 and then the wireless repeater ( It is radiated to the mobile terminal 212 via the wireless repeater transmitting antenna 116 via 100. In this case, since the wireless repeater transmitting antenna 116 cannot have a perfect directivity pattern, some of the signals transmitted to the mobile terminal 212 are re-input to the receiving antenna 110 of the wireless repeater. The re-input signal (hereinafter referred to as 'forward re-input signal 214') is combined with the signal received from the base station, amplified through the wireless repeater, and then again radiated through the transmitting antenna 116 of the wireless repeater. . Therefore, the receiving antenna of the wireless repeater repeats the process of re-input of the signal transmitted from the base station and the signal transmitted by the relay wireless repeater, and the forward re-input signal 214 is delayed to primary, secondary, tertiary, etc. As a result, the strength of the received signal power increases, resulting in a saturated output of the wireless repeater having a fixed gain. In addition, the proportion of the forward re-input signal 214 becomes larger than the original signal received from the base station among the received signals of the wireless repeater, so that the original signal received from the base station cannot be amplified. In the mobile terminal, not only the call signal is received through various paths, but also the forward re-input signal 214 such as primary, secondary, and tertiary are received from the wireless repeater, so that the signal interference phenomenon increases and the reception characteristic is significantly reduced.

3 illustrates a signal and a re-input signal received from a base station. Each of A, B, C, and D of FIG. 3 represents information about a pilot signal which is a control channel for identifying a base station and acquiring synchronization among signals, and a vertical axis represents a signal-to-noise ratio, and their units are expressed in dB. Where A is a signal component directly received from the base station, B is a signal component received by the signal transmitted from the base station reflected by the building, etc., C and D are a part of the signal radiated by the transmitting antenna of the wireless repeater A re-input signal input to the receive antenna of, where C is a re-input signal for A and D is a re-input signal for B. The vertical axis represents the signal-to-noise level, and the horizontal axis represents the time axis as a chip. In digital mobile communication, in order to synchronize with a control channel (usually a pilot channel), the receiver has the same bit pattern as that of the control channel. The receiver determines that the synchronization is synchronized with the base station when the integral value is output over a certain intensity by performing a correlation integration with the received signal while moving the embedded bit pattern along the time axis. By synchronizing in this way, the receiving end completes preparation for communication with the mobile communication base station. At this time, the larger the integral value, the smaller the auxiliary error rate. When the receiver synchronizes with the base station, as shown in FIG. 3, the signal-to-noise ratio is maximized in the synchronized time zone, and the signal-to-noise ratio is the same in the unsynchronized time zone, and thus signal energy cannot be extracted.

A, B, C, and D of FIG. 3 show the point and magnitude at which the signal-to-noise ratio becomes more than a certain intensity by being synchronized with the control channel. B is a reflection signal delayed by one chip than A, C is a re-input signal delayed by 6 chips than A, and D is a re-input signal delayed by 6 chips than B. Although not shown in FIG. 3, C and D illustrate signals that are input again through the wireless repeater. Although not illustrated in the figure, in addition to the C and D signals caused by the system delay due to the A and B signals, other signals generated by the time delay due to the C and D signals are generated periodically. At this time, the difference between the signal strength between A and C is determined according to the value of the repeater output gain illustrated in FIG. 1. The larger the output gain, the smaller the gap between A and C signals. The smaller the output gain, the larger the gap between A and C signals. The same holds true between B and D signals. In addition, if the output gain is set large enough, the re-input signal component becomes large, resulting in a saturation of the repeater output value. In this case, due to the non-linearity of the repeater amplifier, the output signal exhibits a significant signal distortion, resulting in extremely poor service quality or even out of service.

In general, the wireless repeater takes 2 μs or more to output a received signal due to a signal delay effect such as a SAW filter, and thus C is delayed by 2 μs or more from A.

As shown in Fig. 3, in order to prevent re-input signals that can only occur by relaying the same frequency signal, the output gain of the wireless repeater is conventionally set sufficiently small to maintain the linearity of the repeater amplifier even in the worst case. There is a problem to use so as not to deteriorate. As a result, the capacity of the repeater amplifier is not effectively used, and thus the coverage area due to the repeater is reduced, thereby insufficiently obtaining the effect that the service provider wants to obtain.

In order to prevent oscillation of the wireless repeater due to the re-input signal, the separation distance of the transmitting / receiving antenna of the wireless repeater can be sufficiently secured or shielded to increase the signal isolation, but when the antenna is placed on the roof of a building, It is not easy to secure enough separation distance between and in the suburban area to increase the height of the pylon enough to secure the separation distance between the antennas, but there are many problems, such as increased cost and difficulty in maintenance due to the installation of the tower.

The present invention has been made to solve the above-mentioned problems, and is intended to prevent the radio repeater from oscillating by re-input of a signal transmitted through the transmit antenna of the radio repeater.

In addition, the present invention is to effectively reduce the re-input signal appearing that the signal transmitted to the transmitting antenna of the wireless repeater is re-input to the receiving antenna.

In addition, the present invention is to effectively reduce the re-input signal by installing a signal searcher (Searcher) for searching and synchronizing the pilot channel on the input side of the wireless repeater receiving the signal of the base station.

The present invention extends the coverage of a wireless repeater to improve the quality of wireless communication service.

In addition, the present invention is not only to reduce the installation cost, operating cost for installing the antenna for the wireless repeater, but also to solve the inconvenience in operation.

1 is a block diagram showing the configuration of a wireless repeater for a general mobile communication.

2 illustrates a process in which some of the signals transmitted from the transmitting antenna of the wireless repeater are received as re-input signals to the receiving antenna of the wireless repeater.

3 illustrates an example in which a signal-to-noise ratio and a time difference between a signal received from a base station and a re-input signal appear.

Figure 4 is a block diagram applying the re-input signal prevention apparatus of the wireless repeater according to the present invention in the forward communication.

FIG. 5 is a block diagram in which a re-input signal preventing device of a wireless repeater according to the present invention is applied to reverse communication in addition to FIG. 4.

<Description of Symbols for Main Parts of Drawings>

100,400,500: Wireless repeater 110,116,211,410,414,510,516: Antenna

111,115,511,520: Duplex 112,117,411,512: Low noise amplifier

113,118,412,513: filter 114,119,413,514: power amplifier

210: base station 212: mobile terminal

213: base station transmission signal 214: forward re-input signal

215,521: Wireless repeater transmission signal

217: mobile terminal transmission signal 415: multi-path searcher

416: controller 420: coupler

522: reverse re-input signal

A: transmission signal B: multipath signal

C, D: re-input signal

The present invention for solving the above technical problem is to add a multi-path searcher on the path of the forward link in the wireless repeater system chip delay information and signal-to-noise ratio (Ec / Io: Ec of the pilot channel of the re-input signal Power density, Io is the power density of the interfering signal), and provides the information to the repeater controller, which repeater controller maintains the signal-to-noise ratio of the pilot signal returned from the information below an appropriate value. It can be configured as a system for controlling gain.

4 is a block diagram of the apparatus for preventing the re-input signal of the wireless repeater according to the present invention to the forward link. In the case of Fig. 4, the multi-path searcher is configured to detect a signal at the coupler end of the base station side receiving antenna portion on the forward link transmitted from the base station toward the repeater. As described above, the location of the multipath searcher may exist anywhere on the forward link path in the repeater system. In fact, as shown in FIG. 3, in the case of a signal from the base station, multiple signals due to various paths are detected, and the re-input signal is also divided into a multipath signal. Therefore, in general, it is necessary to distinguish whether the signal is from a base station due to repeater re-input. In the present invention, there may be various methods for distinguishing the re-input signal of the wireless repeater and the multipath signal component received from the base station. When the repeater is installed, it is possible to measure the delay time including the repeater and antenna path using a network analyzer, etc., and to distinguish the re-input signal by comparing with the chip delay information of the multipath searcher. Another method is to implement an installation-related algorithm in the repeater controller to reduce or increase the gain sequentially, and then to repeat the multipath signal with a signal-to-noise ratio of the pilot signal that varies linearly with the repeater gain. It is also possible to determine by the input signal.

The receiving antenna 410, the low noise amplifier 411, the filter 412, the power amplifier 413, and the transmitting antenna 414 are basic components that constitute a general wireless repeater. The coupler 420 is for branching a signal and branching some of the signals received through the receiving antenna 410 and inputting them to the multipath searcher 415.

The multipath searcher 415 has the same bit pattern as the pilot channel which is the base station control channel in CDMA mobile communication. The multipath searcher 415 has a baseband signal demodulated from the signal received from the base station while moving the bit pattern along the time axis. Perform auto-correlation integration. The autocorrelation function outputs the largest value when two bit patterns overlap, that is, when synchronized, so that the bit pattern of the pilot channel, which is the control channel received from the base station, and the bits stored in the multipath searcher 415 If the pattern works correctly, the autocorrelation integrator of the multipath searcher 415 outputs a large energy value. The autocorrelation integrator in the multipath searcher 415 constantly moves the bit pattern, performs the bit pattern synchronization with the base station control channel, and finds a signal whose output value of the autocorrelation integrator exceeds a certain threshold. .

Typically, in a wireless communication environment, a plurality of signals exceeding the threshold exist due to the influence of multipath.

When the output value of the autocorrelation integrator exceeds the threshold, the multipath searcher 415 provides the controller 417 with a signal energy value at that point, that is, the largest signal among the autocorrelation integral value and the signal exceeding the threshold. Time difference information, that is, chip information. The controller 417 adjusts the repeater gain such that the signal-to-noise ratio value of the repeater re-input signal is equal to or less than a predetermined value (or set value) using the information. Alternatively, the controller 417 may control the repeater gain so that the difference value is equal to or greater than a predetermined magnitude by using a difference value between the signal-to-noise ratio of the dominant base station transmission signal and the noise ratio of the repeater re-input signal.

The repeater gain is adjusted by reducing the repeater gain if the signal-to-noise ratio of the re-input signal or the difference between the re-input signal and the signal-to-noise ratio of the largest base station pilot signal is greater than or equal to a certain intensity. This ensures maximum cell coverage that the repeater can maintain.

Referring to FIG. 4 in which the present invention is applied to the forward link, an operation relationship is shown. In the reception antenna 410 of the wireless repeater, the forward re-input signal 214 which is a part of a signal transmitted to the base station transmission signal 213 and the transmission antenna of the wireless repeater ) Is received together. A part of the received signal of the receiving antenna of the wireless repeater is branched and input to the multipath searcher 415. The multipath searcher 415 performs an autocorrelation integration, searches for signal energy values and time difference information for a signal whose autocorrelation integration value exceeds a predetermined threshold value, and transmits them to the controller 417. The controller 417 controls the repeater gain to minimize the forward re-input signal 214 using the signal input from the multipath searcher 415.

Since the coupler 420 is for branching a signal, a splitter or the like may be used. In addition, the multi-path searcher 415 is not necessarily located at the front end of the repeater as shown in FIG. 4, and may be installed close to the receiving antenna 410 of the wireless repeater to search for a wireless signal, if necessary, and is an essential component of the wireless repeater. Branching in any of the forward link paths transmitted from the repeater to the terminal, such as amplifiers (where amplifiers typically include low-noise amplifiers installed at the front of the filter or power amplifiers installed at the rear of the filter) To explore.

FIG. 5 is a block diagram of another embodiment of the present invention in which the apparatus for preventing re-input signal of the wireless repeater of the present invention is applied to a reverse link in a transmission direction from a mobile terminal to a base station.

The signal transmitted from the base station is relayed to the mobile terminal wirelessly through the transmitting antenna via the duplexer 511, the low noise amplifier 411, the filter 412, the power amplifier 413, the duplexer 520 via the receiving antenna. On the other hand, since the reverse link is opposite to the forward link, the receive antenna of the forward link acts as the transmit antenna 51 on the reverse link and the transmit antenna of the forward link acts as the receive antenna 516 on the reverse link.

In the case of the reverse link, the inflow of the reverse re-input signal 522 from the transmitting side of the wireless repeater to the receiving side is a problem as in the forward link, but in general, since the reception sensitivity of the base station is much better than that of the mobile terminal, the reverse direction of the wireless repeater The transmit power is much smaller than the forward direction. Therefore, when the repeater gain is properly set on the reverse link, the oscillation phenomenon caused by the reentry of the wireless repeater may not be a problem. In this way, the repeater gain of the reverse link can be managed efficiently.

5 is for preventing oscillation caused by the reverse re-input signal 522 in the reverse link as described above. The control gain value input to the repeater power amplifier in the forward link re-input signal prevention device of FIG. The same applies to the power amplifier, and both the forward and reverse links are configured to prevent re-input signals as much as possible. Therefore, as the forward repeater output increases, the reverse output increases, and when the forward repeater output decreases, the repeater operates to decrease the reverse output.

The present invention is not limited to the above-described embodiments, and may be variously implemented within the range permitted by the technical idea of the present invention.

The present invention prevents the radio repeater from oscillating by preventing a re-input signal appearing by re-input of the signal transmitted through the transmitting antenna of the wireless repeater back to the receiving antenna.

In addition, the present invention effectively prevents the re-input signal appearing by re-input of the signal transmitted to the transmitting antenna of the wireless repeater to the receiving antenna, while maintaining the repeater gain to the maximum.

In addition, the present invention effectively prevents the re-input signal by installing a multi-path searcher for searching and synchronizing the pilot channel on the input side of the wireless repeater receiving the signal of the base station.

The present invention can increase the quality of the wireless communication service by extending the coverage of the wireless repeater by ensuring the maximum gain of the wireless repeater.

Since the present invention can prevent the repeater oscillation in real time as possible, it is possible to reduce the height of the steel tower for installing the antenna, etc., thus reducing facility investment cost and operating cost, and eliminating operational inconvenience.

Claims (36)

In the wireless repeater consisting of a receiving antenna, amplifying means, a transmitting antenna, Signal search means for receiving a signal received by the reception antenna and searching for multiple paths of a pilot signal to detect signal magnitude and time difference information between signals; Control means for receiving the output of the signal searching means as a control command and controlling the transmission output gain of the wireless repeater; Device for preventing re-input signal of wireless repeater comprising repeater output means controlled by the control means 2. The apparatus of claim 1, wherein the signal searching means searches for a branched signal on a forward link path in a repeater connected from a base station to a mobile terminal. 2. The apparatus of claim 1, wherein the signal searching means comprises a receiving signal branching means for branching a signal received by the receiving antenna to receive the receiving side signal. . 2. The apparatus of claim 1, wherein the signal searching means searches for signal-to-noise ratio strength and time difference information between paths of CDMA pilot signals of various paths. The apparatus of claim 1, wherein the signal searching means separates a signal re-input from the transmitting antenna of the wireless repeater to the receiving antenna and a signal from the base station. The apparatus of claim 5, wherein system delay time measurement information based on a distance between a repeater and an antenna using a measuring device is used to separate the re-input signal and the signal from the base station. The method according to claim 5, wherein when the re-input signal and the signal from the base station are separated, the input value of the signal searched by the search means changes in proportion to the output value while sequentially changing the output value of the wireless repeater. Re-input signal preventing device of a wireless repeater, characterized in that the signal defined 2. The repeater of claim 1, wherein the control means for controlling the transmission output gain of the wireless repeater separates a signal re-input from the transmitting antenna of the wireless repeater to the receiving antenna and a signal from the base station. Input signal protection device The apparatus of claim 8, wherein system delay time measurement information based on a distance between a repeater and an antenna using a measuring device is used to separate the re-input signal from the base station. 10. 9. The method of claim 8, wherein when the re-input signal and the signal from the base station are separated, the input value of the signal searched by the search means changes in proportion to the output value while sequentially changing the output value of the wireless repeater. Re-input signal preventing device of a wireless repeater, characterized in that the signal defined The apparatus of claim 1, wherein the control means controls the repeater output means such that the signal-to-noise ratio of the re-input signal measured by the signal search means is equal to or less than a predetermined value. The radio control apparatus according to claim 1, wherein the control means controls the repeater output means to maintain a difference value between the signal-to-noise ratio of the re-input signal measured by the signal search means and the signal-to-noise ratio of the largest signal at a predetermined magnitude or more. Repeater signal prevention device of repeater 2. The apparatus of claim 1, wherein the control means controls the repeater output means so that the noise ratio of the largest signal does not fall below a constant level from a value maintained on average. 2. The apparatus of claim 1, wherein the control means controls the repeater output means by using an instantaneous value of information received from the signal search means. 2. The apparatus of claim 1, wherein the control means controls the repeater output means by using an average value of information received from the signal search means. 16. An apparatus according to any one of claims 1 to 15, wherein said wireless repeater is a forward link. 17. The method of claim 16, wherein the wireless repeater further comprises a reverse link transmitted from the terminal toward the base station, wherein the repeater output gain on the reverse link uses the change information of the forward link repeater output gain. Input signal protection device 18. The wireless repeater of claim 17, wherein the repeater output gain on the reverse link is controlled in an increasing direction when the repeater output gain of the forward link increases and in a decreasing direction when the repeater output gain of the forward link decreases. Re-Input Signal Protection Device In the wireless relay method comprising a receiving antenna, an amplifying means, a transmitting antenna, A signal searching step of receiving a signal received by the reception antenna and searching for multiple paths of a pilot channel signal to detect signal magnitude and time difference information between signals; A control step of receiving the output of the signal searching means as a control command to control the transmission output gain of the wireless repeater; Method for preventing re-input signal of wireless repeater comprising repeater output step controlled by the control step 20. The method of claim 19, wherein the signal searching step searches for signals branched on a forward link path in a repeater connected from a base station to a mobile terminal. 20. The method of claim 19, wherein the signal searching step includes a reception signal branching step for branching a signal received by the reception antenna to receive the reception side signal. . 20. The method of claim 19, wherein the signal searching step searches for signal-to-noise ratio strength and time difference information between paths of CDMA pilot signals of various paths. 20. The method of claim 19, wherein the signal searching step separates a signal re-input from the transmitting antenna of the wireless repeater to the receiving antenna and a signal from the base station. 24. The method of claim 23, wherein system delay time measurement information based on a distance between a repeater and an antenna using a measuring device is used to separate the re-input signal from the base station. 24. The method of claim 23, wherein when the re-input signal and the signal from the base station are separated, the input value of the signal searched by the search means changes in proportion to the output value while sequentially changing the output value of the wireless repeater. Method for preventing re-input signal of wireless repeater, characterized in that the signal is defined 20. The method of claim 19, wherein the controlling step of controlling the transmission output gain of the wireless repeater comprises separating a signal re-input from the transmitting antenna of the wireless repeater to the receiving antenna and a signal from the base station. How to avoid input signal 27. The method of claim 26, wherein the system delay time measurement information based on the distance between the repeater and the antenna using a measuring device is used to separate the re-input signal from the base station. 27. The method of claim 26, wherein when the re-input signal and the signal from the base station are separated, the input value of the signal searched by the search means is changed in proportion to the output value while sequentially changing the output value of the wireless repeater. Method for preventing re-input signal of wireless repeater, characterized in that the signal is defined 20. The method of claim 19, wherein the controlling step controls the repeater output step so that the signal-to-noise ratio of the re-input signal measured in the signal searching step is equal to or less than a predetermined value. 20. The wireless repeater of claim 19, wherein the controlling step controls the repeater output step so that the difference value between the signal-to-noise ratio of the re-input signal measured in the signal search step and the signal-to-noise ratio of the largest signal is maintained above a certain magnitude. How to prevent re-input signal 20. The method of claim 19, wherein the controlling step controls the repeater output step so that the signal-to-noise ratio of the largest signal does not fall below a constant level from a value maintained on average. 20. The method of claim 19, wherein the control step controls the repeater output step by using an instantaneous value of the information input in the signal search step. 20. The method of claim 19, wherein the control step controls the repeater output step by using an average value of the information input in the signal search step. 34. The method according to any one of claims 19 to 33, wherein said wireless relay method is a forward link. 35. The method of claim 34, wherein the wireless repeater further includes a reverse link transmitted from a terminal to a base station, and the repeater output gain on the reverse link uses change information of a forward link repeater output gain. How to avoid input signal 36. The wireless repeater of claim 35, wherein the repeater output gain on the reverse link is controlled in an increasing direction when the repeater output gain of the forward link increases and in a decreasing direction when the repeater output gain of the forward link decreases. How to prevent re-input signal