JP3306843B2 - Mobile communication method using microcell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a communication area in which a plurality of microcells are integrated by repeatedly using a plurality of frequencies or spreading codes, and a base station is provided in each of the microcells. The present invention relates to a mobile communication method for performing communication between a mobile station and a mobile station.

[0002]

2. Description of the Related Art Hitherto, various methods of configuring a microcell have been proposed in order to improve the frequency use efficiency. Regarding these, as described in, for example, “Digital Mobile Communication” (Moriji Kuwahara, published by Kagaku Shimbun), pp. 123 to 136, a configuration method based on a regular hexagonal cell model has been proposed, and necessary repetition methods have been proposed. The number of frequencies was determined so as to satisfy the required CIR (carrier-to-interference-wave power ratio) required for system design, as described on pages 136 to 143 of the same book.

In digital mobile radio communication using the TDMA (time division multiple access) system, as seen in a PHS (Personal Handy Horn System),
A method has been proposed for reducing the interference of time-division multiplex signals between microcells by autonomously distributing synchronization between base stations in a slotted TDMA frame. PH
In S, for example, RCR STD-28 “Second Generation Cordless Telephone System Standard”, Chapter 4, FIG. 4.2.4
As described in the above section, eight slots are prepared in a TDMA frame, and one slot is allocated to each physical slot for uplink control and one physical slot for downlink control, and three slots are allocated to each physical slot for communication. PHS
Is a circuit-switched system, and except for the control physical slot, the communication physical slot is occupied by the communication between the base station and the mobile station from the start to the end of the call. For this reason, the once allocated physical slot for communication becomes a channel with little interference from others, but is not released until the completion of the call, during which the set line (frequency channel) is used. Therefore, in the transmission of a burst signal generated discretely such as a wireless packet, the frequency channel is constrained, but many empty slots that do not transmit information are generated, and the transmission efficiency, that is, the frequency utilization efficiency is reduced. .

As for the random access system using a slotted TDMA frame, for example, the ALOHA system with a slot is adopted for the physical slot for uplink control of the PHS. However, in an area composed of a plurality of microcells using the same frequency, communication efficiency of the random access control is reduced because slot allocation control is not performed cooperatively among the microcells.

[0005] This will be briefly described with reference to the drawings. That is, FIG. 4A shows an example of a zone configuration using seven frequencies repeatedly using a regular hexagonal cell model. The microcells 10, 11, 12, 13, 14, 15, 16 respectively have a first frequency a, a second frequency b, a third frequency c, a fourth frequency d, a fifth frequency e, and a sixth frequency e. The frequency f and the seventh frequency g are assigned, and the microcells 17 to 22 are the microcells to which the first frequency a arranged closest to the microcell 10 is assigned. The microcell 10 receives major interference from a plurality of microcells 17 to 22 which are present around the same and assigned the same frequency a. FIG. 4A shows a configuration in which the channel is divided on the frequency axis. However, when the channel is divided by a spreading code having orthogonality, it is read as a microcell using a spreading code consisting of a and g. The same description will be applied below. Here, as the spreading code having orthogonality, an M sequence is typically used. Although perfect orthogonality is not guaranteed, a Gold code can be applied as a code having low correlation.

The distance between the microcells 10 is adjusted so that the total amount of interference received by the microcells 10 from the surrounding microcells 17 to 22 satisfies the communication quality requirements of the microcells 10. Interference amount or C
If the IR has to be kept low, the distance between microcells (hereinafter referred to as the repetition distance) using the same frequency or the same spreading code must be increased, so that more frequency channels or spreading codes are required. Therefore, this mobile communication service occupies a wide frequency band or a large number of spreading codes.

FIG. 4B shows an example of use of radio channels in the conventional microcell 10 and microcell 17 on a time axis. This figure is an example in which the time axis is continuously used, and the microcells 10 and 17 perform burst transmission at an arbitrary timing. The shaded time zone 2 of the time zones 21 to 28 in which the burst occupies the wireless medium.
Since 1 and 22 overlap with 25 and 24 with 27 and 28 on the time axis, the CIR is degraded and the quality is an unstable burst. On the other hand, the time zones 23 and 26 are bursts of stable quality because there is no overlap on the time axis.

Conventionally, as shown in FIG. 4C, the time axis is divided into slots, and for example, one frame is divided into A, B, C
And the microcells 10 and 1
In 7, the frame and the slot are synchronized with each other, and burst transmission is performed at the start timing of the slot. There is a possibility that a delay of up to one slot occurs between the burst generation timing and the transmission start timing, and the generated burst is temporarily stored in the buffer. In the time zones 31 to 38 in which the burst occupies the wireless medium, the hatched areas 32 and 35 and the areas 34 and 37 overlap on the time axis, so that the CIR is degraded and the burst is of unstable quality. . On the other hand, the time zones 31, 33, 36, and 38 are bursts of stable quality because there is no overlap on the time axis. In FIG. 4B, the burst occurs at an arbitrary time, whereas in FIG. 4C, the burst is restricted to the slot timing.
Burst collision probability is reduced, and as a result, the time rate at which CIR deteriorates can be suppressed. In other words, in packet transmission, if even one bit of a packet overlaps, both packets cause interference, and therefore the overlap of the packets in FIG. 4C is smaller than in FIG. 4B and the interference is smaller. However, as shown in FIG. 4C, the slot assignment between the cells 10 and 17 has not been considered so as not to interfere with each other, so that the interference could not be sufficiently reduced yet.

[0009] Further, the synchronization between the base stations of the microcells can be determined by, for example, as described in RCR STD-28 "Second generation cordless telephone system standard", Chapter 3, 3.2.15 slot transmission conditions. Is to detect the timing and vacancy of the slot to be used by carrier sensing the preceding and succeeding bursts of the slot to be used as a physical slot for downlink control, and to establish autonomously and decentralized frame synchronization with the adjacent base station. Was. For this reason, interference from a base station or a mobile station (a so-called hidden terminal) for which a communication path with the base station has not been established is not detected, resulting in deterioration of communication quality.

[0010]

When a communication area is formed by using microcells, slot synchronization is achieved between microcells using the same frequency, but cooperative random access control is performed between microcells. Therefore, in digital wireless packet communication in which data occurs in bursts, the frequency utilization efficiency has been reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a digital radio communication method which solves the problem of low use efficiency of frequency or spread signal in radio packet communication for transmitting bursty traffic.

[0012]

According to the present invention, a beacon is used for synchronization between base stations in a microcell in order to efficiently synchronize a slotted TDMA frame between microcells using the same frequency or spreading code. Send out,
After the slot synchronization is established, the allocation priority of the used slots is made different for each base station.

That is, according to the present invention, the same frequency or
Is the TDM between microcells using a spreading code channel
Since the A time slot is synchronized and different allocation orders are set for the slots in the TDMA frame for each microcell, the probability of burst signal collision in the same slot can be reduced. The occurrence of traffic between microcells is random, has little statistical overlap, and even if they occur at the same time, if no slot is used in any microcell, the slots used are different from each other. The temporal overlap of traffic between cells is reduced, and the requirements for the CIR required for the system are also reduced, so that repeated use with a smaller number of frequencies or spreading codes is possible, and high frequency use efficiency is realized.

[0014]

FIG. 1A shows an embodiment of the present invention. This embodiment is based on the cell arrangement shown in FIG. 4A,
Each frame is a case where a slotted channel is divided into three groups A, B, and C. The division into three groups is for the purpose of explaining the concept of the present invention, and it is easily understood that grouping into any number is possible. In each microcell, allocation of 0 to 3 slots is controlled for each frame in accordance with traffic. In other words, the traffic is zero and the slot allocation is 0. If one traffic occurs, one slot is allocated, and if two traffics occur at the same time, two slots are allocated.
One slot, and so on. Further, the microcell 10 performs burst transmission by using the slot B as the first priority slot and sequentially using the slots C and A in addition to the slot B when traffic is temporally concentrated. on the other hand,
The microcell 17 is assigned C as the first priority slot, and performs burst transmission using slots A and B sequentially in addition to slot C when traffic is concentrated in time. As described above, the use priority of the slot is made different depending on the microcell. The time periods 41 to 48 in which the burst occupies the wireless medium are as shown in FIG.
When the traffic is relatively small (the burst is intermittent), the temporal overlap of the bursts 41 to 48 can be suppressed.

As a method of determining the order in which each micro cell uses slots from a plurality of slot groups, a method of fixedly allocating in advance is applied as a simple means, but is determined in an independent and distributed manner. Applying such an algorithm is also effective in reducing the control load of the system and ensuring expandability. A representative example of such an autonomous distributed algorithm is a random selection method. More specifically, at the start of operation of the system, the time stamp or the identification code of the base station is applied to an appropriate random number generation function, and the slot allocation order is used in the generated random number sequence, that is, all available slots are used in order. This can be realized by a method of specifying whether

According to the first aspect of the present invention, synchronization between micro cells is established. FIG. 1B shows a configuration example of a TDMA frame used in each microcell for establishing this synchronization.
Guard time period for carrying out the adjustment of the TDMA slot timing, 51 and 54, T in each of the following 57
Carrier sense periods 52, 55, 58 for transmitting and receiving a group of beacons (synchronization reference burst signals) serving as information for adjusting the DMA slot timing,
Further, burst data transmission / reception periods 53, 56, and 59 follow. The carrier sense period and the burst data transmission / reception period have a fixed time length, but a guard time period is used to absorb a timing difference from other microcells and a beacon jitter. In this example, 3
One beacon signal is transmitted at a fixed period.

FIG. 2 shows how slots are synchronized between base stations (micro cells) in carrier sense period 5 in FIG. 1B.
2 is shown enlarged. The beacon signals 61 to 63 are transmitted from the microcell 10 base station, the beacon signals 65 to 67 are transmitted from the microcell 17 base station, and the beacon signals 69 to 71 are transmitted from the microcell 18 base station. The burst data transmission / reception period of each base station at 10, 17, and 18 is 64, 6
8,72. Although the number of beacons in one TDMA frame is three for the purpose of explaining this concept, any number of beacons of two or more can be applied, and the selection of the number is a design matter of each system.

For example, in FIG. 2A, in the microcell 10, after the base station transmits the beacon 61, the beacon 65 is received from the microcell 17 and the beacon 70 is received from the microcell 18. Since the beacon 69 transmitted from the microcell 18 arrives at the base station of the microcell 10 before the beacon 61 is transmitted, it is not received. That is, each base station starts transmitting a beacon signal from its own station and then starts receiving a beacon signal from another base station. Similarly, after the base station of the microcell 10 transmits the beacon 62, the base station receives the beacon signals 66 and 71 from the microcells 17 and 18, respectively. After the base station of the microcell 10 transmits the beacon signal 63, only the beacon signal 67 from the microcell 17 is received.

In FIG. 2B, the microcell 17
In this state, the slots of the microcell 18 and the microcell 18 are synchronized. In the microcell 10, after the base station transmits the beacon 61, the beacons 66 and 70 from the microcells 17 and 18 are superimposed and received. Similarly, beacon 6
After transmitting 2, the beacons 67 and 71 from the microcells 17 and 18 are superimposed and received. Micro cell 10
After the base station transmits the beacon 63, no beacons from the microcells 17 and 18 are received.
In FIG. 2C, the microcell 10, the microcell 1
7 and the slots of the microcells 18 are synchronized, and the timings of the beacons transmitted by the microcells 10, 17, and 18 match.

According to the first aspect of the present invention, a beacon transmitted from each microcell for establishing synchronization is transmitted from any mobile station synchronized with each base station in synchronization with the base station. The mobile station needs to establish synchronization with the base station in order to communicate with the base station, receives a synchronization beacon sent from the base station, and after synchronization is established, the mobile station also establishes synchronization with the base station. The transmitter of the mobile station is set to transmit a synchronous beacon at the beacon timing. In this case, the synchronization beacon transmitted from each microcell is transmitted not only from the base station but also from a mobile station in the microcell. The beacon from the mobile station is superimposed and received, and the CIR is increased accordingly, so that the synchronous beacon of the microcell can be more clearly captured.
In this case, beacons are transmitted from a plurality of transmitters (base station and mobile station) having different propagation distances.
At the receiver, individual beacons may be received out of sync. Therefore, in the receiver, synchronization adjustment may be performed with the timing at which the peak power of the envelope of the beacon group is given as the timing of the beacon. It is possible to converge the synchronization by setting the guard time between slots to an appropriate value as the jitter of the beacon described above as the synchronization jitter. For example, in an environment where the propagation path difference is 30 m, jitter of about 100 nanoseconds may occur, and it is possible to absorb the influence of jitter by inserting a guard time of 100 nanoseconds or more between slots. .

FIG. 3 shows an algorithm for adjusting the TDMA slot timing. This will be described with reference to the example shown in FIG. In the microcell 10, after the base station transmits the beacon 61, the other station beacon detected first is 65 transmitted from the microcell 17, and each station transmits the first beacon from its own station. In order to start the synchronization operation, the base station of the microcell 10 ignores the beacons 70 and 71 from the base station of the microcell 18 in which the phase is ahead of its own and the beacon 61 is transmitted and only two beacons are received. Between beacon 61 and 65,
The time difference between the beacons 62 and 66 and the time difference between the beacons 63 and 67 are measured, and if necessary, the time difference is averaged or the like, and then the time slot difference between the beacon of the microcell 17 and the base station is calculated. to (9 2). Since the base station of the microcell 10 detects the same number of beacons as the number of transmitted beacons, the base station of the microcell 10 determines that the beacon of the own station is advanced in time (93), and The beacon transmission timing is not corrected.

On the other hand, in the microcell 17, after the base station transmits the beacon 65, the other station beacon first detected is the beacon 70 transmitted from the microcell 18.
Therefore, the base station of the microcell 18 ignores the beacons 62 and 63 transmitted from the base station of the microcell 10 and measures the time difference between the beacons 65 and 70 and between the beacons 66 and 71, and determines these time differences. Then, after performing processing such as averaging, the time slot difference between the microcell 18 and the beacon of the base station is calculated (92). Since the base station of the microcell 17 did not detect the beacon from the microcell 18 after the last beacon 67 of the own station, it judges that the beacon of the own station is later in time (93). The beacon transmission timing of the own station is corrected (94). As a result, as shown in FIG. 2B, the slot timing of the base station of the microcell 10 remains the same, but the base station of the microcell 17 is synchronized with the slot timing of the base station of the microcell 18. . Further, the adjustment is performed in the same manner again in the next frame, and the base station of the microcell 10 can synchronize with the base station of the microcell 18 as shown in FIG. 2C.

[0023]

According to the present invention, slots are synchronized between base stations and the priorities of used slots in one frame are made different from each other (or are given randomly). Signals such as packets generated at random with each other are less likely to be transmitted simultaneously by base stations using the same frequency or spreading code, and thus are less likely to cause interference. Therefore, the repetition distance of the frequency or spread code can be shortened, the required number of channels can be reduced, and the use efficiency of the frequency or spread code can be increased. In this case, synchronizing the transmission of the packet with the slot further eliminates the possibility of interference.

By transmitting a synchronization beacon from the base station, slot synchronization can be relatively easily achieved between the base stations. When the mobile station also transmits a beacon in synchronization with the base station of the serving cell, the CIR of beacon reception at another base station can be increased.

[Brief description of the drawings]

FIG. 1A shows cells 10, 17 in an embodiment of the present invention.
FIG. 3B is a diagram showing an example of a slot occupancy state, and FIG. 3B is a diagram showing an example of a TDMA frame configuration used in the present invention.

FIG. 2 is a diagram showing transmission and reception states of synchronization beacons in cells 10, 17, and 18, and transitions to be synchronized with each other.

FIG. 3 is a flowchart illustrating an example of a slot synchronization processing procedure.

FIG. 4A is a diagram illustrating an example of a seven-frequency repetitive configuration using regular hexagonal cells in mobile communication, and FIGS. 4B and 4C are diagrams illustrating an example of a wireless medium occupation state of cells 10 and 17 in conventional packet transmission.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-59151 (JP, A) JP-A-7-236173 (JP, A) JP-A-5-235833 (JP, A) JP-A-5-235833 110498 (JP, A) JP-A-58-36034 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04Q ^7/ 00-7/38 H04B ⁷ /24-7/26

Claims (2)

(57) [Claims] 1. A communication area is configured by integrating a plurality of microcell areas by repeatedly using a plurality of frequency channels or spreading codes, and fixed to each of the microcells .
A base station to which a frequency channel or a spreading code is assigned is installed, and at least one data is transmitted between the base station and the mobile station.
In a mobile communication method using a micro cell for performing bursty wireless communication using a transfer slot, a nearby base station using the same frequency channel or spreading code and a mobile station in the base station are provided with a slotted TD.
In synchronization with the MA frame, each of the base stations is fixed within the TDMA frame.
A fixed short beacon is sent to multiple base stations in a cycle.
The beacon and the beacon,
By sensing the beacon sent by the base station,
Measures the time difference from the beacon and converges to the minimum time difference
A plurality of data transfer slots are prepared in the TDMA frame, and the allocation order of the data transfer slots for each microcell is determined by the same frequency channel or spreading code.
A mobile communication method using a micro cell, wherein the control is performed so as to be different from the allocation order in other micro cells using the micro cell. 2. A mobile station residing in a microcell synchronizes to a beacon radiated by a base station of the microcell, and
The mobile communication method using the micro-cell according to claim 1, characterized in that sending multiple short beacon at a constant cycle in a DMA frame.