WO2017208282A1

WO2017208282A1 - Base station, directional reception method and medium

Info

Publication number: WO2017208282A1
Application number: PCT/JP2016/002652
Authority: WO
Inventors: Kumar DILEEP; Naoto Ishii
Original assignee: Nec Corporation
Priority date: 2016-06-01
Filing date: 2016-06-01
Publication date: 2017-12-07
Also published as: JP2019522411A; JP6658920B2

Abstract

A base station comprises: a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals; a probability function generator configured to generate a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region among the plurality of regions, in which one of the plurality of user terminals has been found in a previous scan; a beam sweep controller configured to scan the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and a power comparator configured to compare a received power level from a scanned region with a predetermined threshold value during the scanning by the beam sweep controller, and, if the received power level from the scanned region is greater than the predetermined threshold value, update the candidate region with the scanned region and stop scanning by the beam sweep controller over a remaining part of the plurality of regions.

Description

BASE STATION, DIRECTIONAL RECEPTION METHOD AND MEDIUM

The present invention is related to a base station, a directional reception method and a medium. In particular, the present invention is related to a base station, a directional reception method and a medium for discovery of low power user terminals in a mobile communication system using directional reception.

Background

In a mobile communication system, a user terminal performs initial access procedure to establish a connection with a network before transmission of actual data. The initial access procedure comprises: finding and achieving synchronization to a cell within the network (cell search); receiving and decoding information (system information extraction); and requesting connection (random access). In general, this access procedure is performed using omnidirectional transmission/reception at a base station.

As a related art, PTL 1 describes a communication system, in which a base station determines a plurality of candidate orientations with satisfactory receiving quality from a mobile terminal, and the mobile terminal selects an optimum orientation from the plurality of candidate orientations.

Japanese Patent Kokai Publication No. H11-252614A WO2013/122440A1

Non-Patent Literature

V. Desai, L. Krzymien, P. Sartori, W. Xiao, A. Soong and A. Alkhateeb, "Initial beamforming for mmWave communications," Signals, Systems and Computers, 2014 48th Asilomar Conference on, Pacific Grove, CA, 2014, pp. 1926-1930.

Summary

Since the mobile networks are evolving very rapidly, it is foreseen that future mobile communication systems will become more heterogeneous and comprise a variety of user terminals including low power IoT (Internet of Things) apparatuses and sensors with diverse applications and network requirements. These user terminals cannot be discovered in the network, if the transmitted signals from these user terminals are not reachable to the base station as illustrated in Fig. 1.

One possible solution is to increase transmission powers of the user terminals because they are directly related to an achievable transmission range. However, it is unrealistic not only for low power terminals but also for high speed mobile apparatuses because more battery drainage is required. On the other hand, transmit antenna gain for these terminals cannot be increased significantly, because most of the IoT apparatuses and sensors have one antenna and transmits in omnidirectional. In this context, massive antenna array beamforming at the base station is considered as a possible option to enhance the link budget margin. Directional reception will provide necessary gain required to receive initial access request from the low power user terminals. However, the reception area will be confined to a very narrow solid angle. In case of Non-Line of Sight (NLOS), larger electronic steering of the beam will be required to align the main beam toward the strongest direction of reception.

Therefore, in order to provide a high gain path to the low power terminals, future base stations are required to use a directional reception sweeping through all possible directions for scanning the service area and discover active terminals. One such technique is based on an exhaustive search algorithm, in which a base station transmits/receives by a narrow beam in a time division multiplexing fashion in all possible direction as shown in Fig. 2. This method may achieve maximum coverage and may be simply implemented. However, a major drawback of this method is that it requires a long time to scan the region, which may results in significant degradation of QoS (Quality of Service) in terms of latency.

PTL 2 proposes a feedback mechanism to avoid iterative scanning. After finding the best transmit/receive beam for each user terminal using a conventional exhaustive search method, the base station updates the best transmit/receive beam in downlink/uplink for each user terminal in the system. This update mechanism is based on a beam tracking that is performed periodically or at fixed intervals or based on the channel characteristics and user movements. This feedback mechanism is not applicable to IoT apparatus or sensors because of their low power constraint. This feedback mechanism may also result in significant battery drainage for high speed mobile terminals because of continuous transmission/reception of feedback messages to keep beam align. Further in case of densely deployed user terminals, tracking the best beam for each individual terminal may introduce unnecessary computations and result in an unfeasible practical system for the base station. Therefore, there is a need in the art for a mechanism that does not require a feedback and that are applicable in a case where no prior knowledge on terminal location or channel characteristics is available at the base station.

A mechanism based on a hierarchical beam search procedure is presented in NPL 1. Initially, a base station receives signals using a wider beam width, this search procedure is refined iteratively by using the best direction found in a previous step, and narrower and narrower beams are used for reception until maximum directional pattern is achieved at the base station. Although this hierarchal based initial search procedure provides a fast terminal discovery, its performance is worse as compared to an exhaustive cell search because of much less gain in initial stages. Further, if a base station chooses an inaccurate beam after first step, the base station cannot provide a beam with acceptable SINR (Signal-to-Interference-plus-Noise Ratio) in the following stages.

Based on the above discussion, previously proposed methods reduce the initial search time at the cost of either increase in computational complexity or in miss-detection probability. This problem has not been carefully considered yet in the literature. However, this will surely become an important research topic for telecom society in the near future.

In the conventional approach such as an exhaustive search or beam switching, a base station transmits/receives signals with a narrow beam in a time division multiplexed fashion over all possible predefined directions to discover active users, and provides high gain path required to receive an initial access request from end terminals. In reality, however, scanning all the coverage area is not equally important. An example for such case is a sensor network, where very few sensors are active for longer duration. Another possible example is slowly moving user terminals or IoT apparatuses, because their mobility region is constrained to some small area. Although the conventional method provides maximum coverage and highest detection probability, the major drawback of the method is the time required to scan the complete coverage area, most of which is not important.

In addition to that, the base station receives signals from all possible directions and compares the received power levels to decide a best beam. Hence, selecting an optimal beam pattern involves a large number of calculations, which increases exponentially with an antenna array size. This may be computationally infeasible for a practical system.

Further, according to PTL 1, in which an optimum orientation from a plurality of candidate orientations is selected not by a base station but by a mobile terminal, a load on the mobile terminal increases and extra communication between the base station and the mobile terminal is required.

The present invention aims at solving the above mentioned problem. Therefore, an object of the present invention is to reduce the search time by finding and avoiding unimportant scan region. Furthermore, it is also desirable to create a set of candidate regions where active users can be discovered with comparatively higher probabilities and avoid excess computations.

According to a first aspect of the present invention, there is provided a base station, comprising: a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals; a probability function generator configured to generate a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region among the plurality of regions, in which one of the plurality of user terminals has been found in a previous scan; a beam sweep controller configured to scan the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and a power comparator configured to compare a received power level from a scanned region with a predetermined threshold value during the scanning by the beam sweep controller, and, if the received power level from the scanned region is greater than the predetermined threshold value, update the candidate region with the scanned region and stop scanning by the beam sweep controller over a remaining part of the plurality of regions.

According to a second aspect of the present invention, there is provided a directional reception method for a base station including a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals, the method comprising: generating a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region among the plurality of regions, in which one of the plurality of user terminals has been found in a previous scan; scanning the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and comparing a received power level from a scanned region with a predetermined threshold value during the scanning, and, if the received power level from the scanned region is greater than the predetermined threshold value, updating the candidate region with the scanned region and stopping scanning over a remaining part of the plurality of regions.

According to a third aspect of the present invention, there is provided a non-transitory computer-readable recording medium storing a program that causes a computer, provided on a base station including a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals, to execute: generating a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region, in which one of the plurality user terminals has been found in a previous scan; scanning the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and comparing a received power level from a scanned region with a predetermined threshold value during the scanning, and, if the received power level from the scanned region is greater than the predetermined threshold value, updating the candidate region with the scanned region and stopping scanning over a remaining part of the plurality of regions. The non-transitory computer-readable storage medium may be provided as a program product.

Advantageous Effect of Invention

According to the base station, the directional reception method, and the non-transitory computer-readable recording medium of the present invention, the search time can be reduced by finding and avoiding unimportant scan region.

A mobile communication system according to a related art comprising a base station and multiple user terminals. A mobile communication system according to a related art with directional transmission and reception at a base station. A beamforming operation at a base station according to a related art. An exemplary flow chart showing an operation of a base station with directional transmission and reception functionality according to a related art. An example of a block diagram for a base station according to a related art with directional transmission and reception functionality. An example of a block diagram for a base station according to a first exemplary embodiment of the present invention. An example of a flow chart showing an operation of the base station according to the first exemplary embodiment of the present invention. An example of an operation of a probabilistic function generator in the base station according to the first exemplary embodiment of the present invention. An example of an operation of the probabilistic function generator in the base station according to the first exemplary embodiment of the present invention. An example of an operation of the probabilistic function generator in the base station according to the first exemplary embodiment of the present invention. An example of an operation of the probabilistic function generator in the base station according to the first exemplary embodiment of the present invention.

A communication system according to an exemplary embodiment of the present invention comprises a base station and a plurality of user terminals that perform communication with each other. More particularly, the communication system aims at improving an initial search procedure by creating a set of candidate scanning regions from a radio coverage area in order to reduce the search time and also reduce the computational complexity while maintaining the same detection probability and improving the system performance.

The present invention provides a method for a base station in a mobile communication system that comprises at least one base station and a plurality of user terminals. The base station includes an antenna array system with digital beamforming functionality, which enables more than one user to be serviced at a time. Assuming no prior knowledge on user density and location, our proposed method starts with considering a uniform probability function i.e., probability of finding an active user in a coverage area is equally probable. The base station chooses a random/fixed starting point and sequentially scans complete coverage area because all regions are initially assumed to have an active user with equal probability, which is equivalent to a conventional exhaustive search with sequential scanning method. After one complete scan, the base station defines candidate sectors by maximizing the probability function over the regions where the received power levels were comparatively higher in previous scanning. The probability function is assumed to be symmetrically decreasing with a peak at a candidate sector (or region) and decreasing with the increase in the distance from the candidate sector as shown in Fig. 8B. In the next iteration, instead of random/fixed starting point, the base station starts scanning from a candidate scan region in a zig-zag fashion in order of decreasing probability values. The base station stops further scanning if any active terminal is discovered in order to avoid redundant scanning and reduce computations complexity. Iteratively, a new probability function is defined by shifting the peaks to a new location where active users have been discovered in order to reduce the search time for the next iteration.

According to the exemplary embodiment, the initial search time required to discover active user terminals can be reduced. This meritorious effect can be provided by considering unequal probability distribution function with a peak at a location of a previously detected active user, the function symmetrically decreasing with distance to avoid scanning unimportant regions with smaller probabilities. On the other hand, instead of random/fixed starting point and sequential scanning, search time in the sensor network with low mobility terminals can be further reduced by selecting a start point at a region, in which a user terminal has been previously discovered, and scanning with respect to decreasing order of probability. The reason is that most of these terminals are moving with low speeds and exist within a limited region(s). Moreover, our proposed method will incurs less computational complexity in comparison to the conventional method because scanning is performed in a relatively smaller region and the proposed method stops further scanning when any active user terminal is discovered. In contrast, in the conventional method, power levels received from all predefined direction are compared, which requires unimportant scanning and also more computations. Finally, consider an extreme case, in which no active user has been discovered in the coverage area after initial scanning. In this case, performance of our proposed method is equivalent to that of the conventional exhaustive search method because the coverage area maintains the same uniform probability distribution function and scan is performed sequentially with some random/fixed starting point, which is equivalent to the conventional exhaustive search method. However, for all other cases our proposed method results in an improved search time and reduced complexity.

The present invention and its advantages can further be understood with the help of following description of embodiments accompanying　illustrative drawings. In the following description, embodiments are constructed by considering its application to a mobile communication system. Note that the reason for assuming a mobile system in the example is to simplify the illustration. However, our present invention can be applied to any wireless communication system that uses direction transmission/reception for the initial access procedure. The exact structure of initial access message is not the scope of the present invention. Rather our focus here is on the access procedure.

In mobile communication systems, directional transmission/reception is generally used to increase a link budget margin by concentrating the reach areas in a specific direction and hence providing a high gain path for communication. However, the major drawback of this method is that the transmission/reception area is confined to a very small solid angle, which results in a significant delay for sequentially scanning the coverage area and results in excess calculations for aligning the best transmit/receive beam.

First, a mobile communication system and a base station that are used in common throughout our discussion in explanation of the present invention are described. Fig. 1 shows an example of a mobile communication system according to a related art that comprises a base station 1 and a plurality of user terminals 2 located in a radio coverage area 11 of the base station. In the mobile communication system, the base station 1 considers an isotropic or omnidirectional transmission/reception 12 for communication with user terminals 2. More specifically, all the initial access procedure and the reference signals are transmitted/received without applying beamforming at the base station 1. However, with evolution of smart apparatuses and diverse applications, wide variety of end terminals, such as low power IoT (Internet of Things) apparatuses and sensors, also participate in the mobile network. The transmitted uplink signals 21 from these terminals may not reach to the base station 1 because of their limited transmission power as show in Fig. 1.

A directional transmission/reception 13 using an antenna array system 15 at the base station 1A is one of possible options to provide gain required to receive the low power uplink signals 21. Fig. 2 illustrates a mobile communication system with the directional transmission/reception 13 applied at the base station 1A. It is assumed that the base station 1A can generate one or more active beams for each sector in order to support one or more user terminals 2. More specifically, the base station 1A can communicate with multiple user terminals 2 at the same frequency and time domain radio resource.

Fig. 3 illustrates an example of a case where the base station 1A forms one or more active transmit/receive beams 14A simultaneously in different directions at the same time or with a time difference while sweeping them 14B. For example, the base station 1A generates N transmit/receive beams in N different directions for N time units 14A (N: a positive integer). In another case, the base station 1A sequentially generates N transmit/receive beams in N different directions using N time units 14B with only one active beam at a time. Alternatively, in a hybrid case, more than one active beams are generated and N directions are scanned in less than N time units.

Fig. 4 shows an example of a block diagram of the base station 1A that comprises an antenna array system 15, an antenna array processing circuit 17, a digital signal processor 18 and a signal detector 19, a storage/memory 110, a power comparator 111, a candidate beam selector 112, a beamforming controller 113 and a beam sweep controller 114. The antenna array system 15 includes N antenna elements used for receiving uplink signals from the user terminals 2 and also for transmitting precoded user data in downlink. Each antenna element is connected to its respective delay line and that is digitally controllable by the beamforming controller 113 and the beam sweep controller 114 by introducing appropriate phase shifts between antenna elements. The beamforming controller 113 implements all the techniques discussed with reference to Fig. 3. The beamforming controller 113 and the beam sweep controller 114, respectively, control the beam refinement and sequential sector sweeping. The N antennas share the antenna array processing circuit 17, which includes all components such as an encoder, a modulator, an analog to digital convertor block etc. The digital signal processor 18 is used to reconstruct the base band signals followed by a signal detector 19 to estimate the received power levels. The power comparator 111 compares the received power levels from all predefined directions. The candidate beam selector 112 finds the best beam for data transmission based on the comparison result of the power comparator 111. The candidate beam selector 112 defines a beam with highest received power level as a candidate beam for all uplink/downlink data transmission.

Fig. 5 shows an exemplary operation of the base station 1A. The base station 1A performs an exhaustive search procedure. At first, the base station 1A scans the coverage area and receives uplink signals (step S1101). Then, the base station 1A compares power levels from all received signals (step S1102) to find a best beam for data transmission and reception (step S1103). The base station 1A chooses a beam with highest received power level and starts actual communication with the end terminals (user terminals).

Based on the above explanation for the mobile communication system according to a related art, we will now explain details specific to embodiment of the present invention in the following.

<First Exemplary Embodiment>
Next, a mobile communication system and a method in the mobile communication system according to a first exemplary embodiment of the present invention will be explained with reference to the drawings.

The mobile communication system comprises: a base station 1B using an antenna array system 15 for directional transmission/reception; and a plurality of low power IoT (Internet of Things) apparatuses or sensors as user terminals 2. The user terminals 2 are designed with one or few antenna element(s) and hence support only omnidirectional transmission/reception of signals to avoid cost and complexity. Generally, sensors and IoT apparatuses are very densely deployed, but only few of these terminals are active at any time for relatively long duration. However, the present invention is not restricted to such cases with few active terminals. The present invention can also be applied to any number active terminals in the mobile communication system. Usually, for most of the applications, IoT apparatuses or sensor are stationary or move within a relatively limited small area. However, the present invention is not restricted to these types of terminals. IoT apparatuses and sensor are generally very small in size and designed to span very long duration. Therefore, the transmitted power is restricted to few milli-watts to enhance overall life span. To discover these low power terminals and provide them with services, a highly directional path is required for the base station. To meet the needs, an antenna array system at the base station is employed. In the following, details of the first exemplary embodiment are described making reference to Figs. 6, 7 and 8A to 8D.

Fig. 6 depicts a block diagram of the base station 1B in the present exemplary embodiment with additional features and functionality. With reference to Fig. 6, the base station 1B in the present exemplary embodiment further comprises a probability function generator 115. The details for operations of

blocks

15, 17, 18, 19, and 112 have been explained above. Therefore, explanations for these units are omitted here for conciseness.

In the related art, the base station 1A receives signals from all predefined direction and the power comparator 111 compares the received power levels to decide a best beam and establish a connection for data transmission. This process not only results in redundant scanning but also results in unnecessary computation and storage/memory 110 for comparing the power levels and deciding best direction for data. To reduce the computational complexity and improve system performance, we propose a first beam search algorithm where the received power levels from each receive direction is compared with a predetermined threshold value in power comparator 111’. More specifically, the base station 1B compares the received power level from each receive direction with a predetermined threshold value. If the received power level is below the predetermined threshold value, the base station 1B switches to a next beam and scans an adjacent region. Otherwise, the base station 1B stops further scanning and starts data transmission using the first beam satisfying the threshold requirement. The memory/storage unit 110’ is also equipped with forgetting functionality to avoid loop scanning and enable renewal of candidate regions after scanning. The probability function generator 115 sets different priorities for scanning a radio coverage area of the base station 1B. More specifically, the probability function generator 115 defines an exponentially or linearly decreasing probability function with a peak at a location where an active terminal was discovered in a previous scanning. The beam sweep controller 114’ computes how beamforming set should be segmented and determines the starting location for scanning. Instead of sequential beam sweeping, the beam sweep controller 114' performs scanning in a zig-zag fashion in decreasing order of the probabilities.

Fig. 7 shows an exemplary operation of the base station 1B. In the beginning, the base station 1B performs a similar procedure as the conventional exhaustive search described above with reference to Fig 4. The base station 1B performs a sequential scanning to receive uplink signals from all direction (step S1101), and compares the received power levels (step S1102) to discover a best beam with a highest received signal power level (step S1103). The base station 1B can generate one or more active beams in each sector and performs scanning by sequential beam switching using multiple time slots as explained previously with reference to Fig. 3. More specifically, the probability of finding an active terminal is assumed initially to be uniform in the radio coverage area 11 of the base station 1B. The base station 1B starts scanning from some fixed/random initial point and completes scanning the coverage area by sequentially moving to adjacent region in each time slot in a same way as the conventional exhaustive search method.

After one complete scan, the base station 1B updates its probability function (step S1104) for each active beam based on received power levels. The base station 1B defines a candidate region for each active beam by maximizing the probability on a region where an active terminal has been discovered previously.

From the second iteration and thereafter, instead of choosing a predefined fixed or random starting point and performing an exhaustive search operation, the base station 1B scans a region based on the values of probability obtained in the previous scanning. Namely, the base station 1B starts scanning from a region with a highest probability and scans in decreasing order of probability values (step S1202).

In order to avoid redundant calculations and reduce complexity, the base station 1B stops further scanning (step S1204) when a power level of an uplink received signal is greater than a predetermined threshold value ("Yes" in step S1203), or, in other words, when an active terminal is discovered. The base station 1B updates the probability function (step S1104) for each active beam by shifting the peak to the new location and this follows iteratively for the subsequent scanning.

Specifically, the complete process and operation of probability function generator 115 can be understood with a help of the following example and with reference to Figs. 8A to 8D.

Assume as an example the following system configurations:
for simplicity, a base station has only one active beam and seven scan regions within a radio coverage area of the base station;
there are two active user terminals 2-1 and 2-2 in the radio coverage area of the base station;
the user terminals 2-1 and 2-2 support only omnidirectional transmission or reception of signals;
no prior knowledge on location and density of the user terminals 2-1 and 2-2 is available at the base station;
at most one user terminal is serviced by each beam at a time; and
uplink received signals from the user terminals 2-1 and 2-2 always satisfy the predetermined threshold requirement at the base station (i.e., power levels of the received signals are greater than the predetermined threshold value).

Since no prior knowledge on user location and user density is available at the base station 1B, our proposed method starts scanning by assuming that a probability function for finding an active terminal is uniformly distributed in the radio coverage area of the base station 1B. In this case, each scan region has an equal probability i.e., 1/7 as shown in Fig. 8A. Further, since we assume that the base station 1B has one active beam at a time, the base station 1B scans seven regions by sequential beam switching using seven time units. The sector (region) with a highest received power level is defined as a candidate sector (region) (step S1103 of Fig. 7) after comparing the received power levels from all seven directions (step S1102 of Fig. 7). This operation is equivalent to a conventional exhaustive search method.

Case 1: Moving user terminals
As case 1, we consider a case in which both user terminal 2-1 and 2-2 are not stationary and transmit uplink signals, and power level of signals received from the user terminal 2-1 is relatively higher than that received from the user terminal 2-2.

The probability function generator 115 updates the probability distributions based on the results of previous scanning. The probability function generator 115 defines a non-uniform probability function. More specifically, the probability function generator 115 generates a symmetric linearly or exponentially decreasing function (such as a Gaussian function) with a peak value at the candidate region and decreases as the distance from the candidate region increases as shown in Fig. 8B. For the next iteration ("No" in step S1201 of Fig. 7), the base station 1B performs scanning based on the probability values. Namely, the base station 1B starts scanning from the candidate region and scans in decreasing order of the probability values in zig-zag fashion until the received power is greater than a threshold value ("Yes" in step S1203 of Fig. 7) as shown in Fig. 8C. Generally, in case of moving user terminals, it is likely that an active user terminal can be found in a vicinity of the candidate region where an active terminal has been discovered previously. Iteratively, after each scanning the probability function generator 115 updates probabilities values (step S1104 of Fig. 7) in the same manner to avoid redundant scanning and computations.

Case 2: Stationary user terminals transmitting alternatively
As case 2, we consider a case in which both user terminals 2-1 and 2-2 are stationary and transmit uplink signal alternatively in their respective turns. Namely, the first user terminal 2-1 transmits while the second user terminal 2-2 waits for its turn, and vice versa.

After the initial exhaustive scanning, the base station 1B only receives signal power from the user terminal 2-1 while signal power received from the user terminal 2-2 remains zero. The probability function generator 115 updates the probability distribution as explained above with reference to Fig. 8B. Since the user terminals 2-1 and 2-2 transmit alternatively in the present case, in the second scanning iteration, only the user terminal 2-2 transmits uplink signals. However, since exact location information of the user terminal 2-2 is not known or available at the base station 1B, the base station 1B cannot directly steer receive beam. Therefore, the base station 1B performs scanning based on the probability values. More specifically, the base station 1B starts from a region with a highest probability value and scans in decreasing order of probability in a zig-zag fashion (step S1202 of Fig. 7) as shown in Fig. 8C and stops further scanning (step S1204) when the received uplink power satisfies the threshold requirement ("Yes" in step S1203 of Fig. 7). Since the user terminals 2-1 and 2-2 are stationary, and they transmit alternatively, the base stations 2-1 and 2-2 define a new probability function with two equal peaks each for one user terminal, which symmetrically decrease as the distance from the candidate regions increases as shown in Fig. 8D to avoid all intermediate redundant scanning. Forgetting feature is employed in the storage/memory 110'. Namely, the storage/memory 110' stores time information for aligning receive beam toward each user terminal in its respective turn to avoid a loop scanning, which may occur if any user terminal does not transmit for some predefined duration.

In order to simplify the explanation, one active beam with several scan regions and two user terminals are assumed in the above examples. However, the present invention can be applied to a system with N active beams (N: a positive integer) and a plurality of user terminals that transmit uplink signals simultaneously for the initial access procedure.

Based on the above explanation of the first exemplary embodiment, it can be concluded that initial search time required for discovering low power active terminals in the radio coverage area of the base station can be reduced by sorting the scanning regions. More specifically, the base station defines an unequal probability distribution function with a peak values at a candidate region where an active terminal has been discovered in a previous scanning and the function linearly/exponentially decreasing as the distance increases from the candidate region. The next scanning is performed in decreasing order of probability values to avoid redundant scanning. Further, first best beam search concept can avoid extra complexity and also reduce scan time and computations by choosing the first best beam satisfying a threshold requirement. The present invention results in substantial reduction of computational complexity and the initial search time that has been conventionally required in comparing the power levels obtained by sequential scanning and then deciding best beam for data transmission.

Note that the above exemplary embodiment may be realized by a program and hardware resources such as CPU (Central Processing Unit) and memory of a computer provided on a base station. Namely, the program may cause the computer to execute the above processes performed by at least part of units (or modules) as described in Fig. 6.

The disclosure of the aforementioned Patent Literatures and Non-Patent Literature is incorporated herein by reference thereto. Modifications and adjustments of the exemplary embodiment are possible within the scope of the overall disclosure (including the claims) of the present invention and based on the basic technical concept of the present invention. Various combinations and selections of various disclosed elements (including each element of each claim, each element of each exemplary embodiment, each element of each drawing, etc.) are possible within the scope of the claims of the present invention. That is, the present invention of course includes various variations and modifications that could be made by those skilled in the art according to the overall disclosure including the claims and the technical concept. Particularly, any numerical range disclosed herein should be interpreted that any intermediate values or subranges falling within the disclosed range are also concretely disclosed even without specific recital thereof.

1, 1A base station
2, 2-1, 2-2 user terminal
11 radio coverage area
12 omnidirectional transmission/reception
13 directional transmission/reception
14A simultaneously generated transmit/receive beams
14B sweeping over transmit/receive beams
15 antenna array system
17 antenna array processing circuit
18 digital signal processor
19 signal detector
21 uplink signals
110, 110' storage/memory
111, 111' power comparator
112 candidate beam selector
113, 113' beam forming controller
114, 114' beam sweep controller
115 probability function generator

Claims

A base station, comprising:
a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals;
a probability function generator configured to generate a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region among the plurality of regions, in which one of the plurality of user terminals has been found in a previous scan;
a beam sweep controller configured to scan the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and
a power comparator configured to compare a received power level from a scanned region with a predetermined threshold value during the scanning by the beam sweep controller, and, if the received power level from the scanned region is greater than the predetermined threshold value, update the candidate region with the scanned region and stop scanning by the beam sweep controller over a remaining part of the plurality of regions.
The base station according to Claim 1, wherein
the probability function generator generates the probability function, which is symmetric and decreases linearly or exponentially as a distance from the candidate region increases.
The base station according to Claim 1 or 2, wherein
the probability function generator generates the probability function, which has multiple peaks corresponding, respectively, to multiple candidate regions, in which multiple of the plurality of user terminals have been found in a previous scan(s).
The base station according to any one of Claims 1 to 3, further comprising:
a storage/memory configured to store the candidate region(s) and, after generating the probability function using the candidate region(s), delete the candidate region(s).
A directional reception method for a base station including a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals, the method comprising:
generating a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region among the plurality of regions, in which one of the plurality of user terminals has been found in a previous scan;
scanning the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and
comparing a received power level from a scanned region with a predetermined threshold value during the scanning, and, if the received power level from the scanned region is greater than the predetermined threshold value, updating the candidate region with the scanned region and stopping scanning over a remaining part of the plurality of regions.
The directional reception method according to Claim 5, further comprising:
generating the probability function, which is symmetric and decreases linearly or exponentially as a distance from the candidate region increases.
The directional reception method according to Claim 5 or 6, further comprising:
generating the probability function, which has multiple peaks corresponding, respectively, to multiple candidate regions, in which multiple of the plurality of user terminals have been found in a previous scan(s).
The directional reception method according to any one of Claims 5 to 7, further comprising:
storing the candidate region(s) in a storage/memory and, after generating the probability function using the candidate region(s), deleting the candidate region(s) from the storage/memory.
A non-transitory computer-readable recording medium storing a program that causes a computer, provided on a base station including a plurality of antennas configured to perform directional reception of initial access requests from a plurality of user terminals, to execute:
generating a probability function for a plurality of regions within a radio coverage area of the base station, the probability function having a peak at a candidate region, in which one of the plurality user terminals has been found in a previous scan;
scanning the plurality of regions in descending order of probabilities given by the probability function for the plurality of regions; and
comparing a received power level from a scanned region with a predetermined threshold value during the scanning, and, if the received power level from the scanned region is greater than the predetermined threshold value, updating the candidate region with the scanned region and stopping scanning over a remaining part of the plurality of regions.