HK1089042A - Maximizing allowable flexible slot-to-cell allocation by using adaptive antennas in a tdd system - Google Patents

Description

Maximizing allowable flexible timeslot-cell allocation using adaptive antennas in TDD systems

Background

The present invention relates generally to radio communication systems using Time Division Duplex (TDD) mode. More particularly, the present invention relates to the use of TDD in a radio communication system for timeslot and timeslot-to-cell operation.

A system of cells typically divides the time axis into equal duration intervals called frames. Systems of cells use the TDD scheme to divide frames into a finite number of equal duration intervals, called timeslots, and allow a cell to use some or all of the timeslots for uplink (mobile to base station) or downlink (base to mobile) transmissions. The slot assignment for a cell defines how each slot is used by that cell. There are three possible methods for a cell to use a slot: an uplink transmission; 2) a downlink transmission; or 3) no time slot is used.

The slot assignments for a cell may be varied systematically to accommodate long-term changes in traffic load. For example, the system may modify the operation of a time slot from uplink to downlink if the strength of the downlink traffic increases and the uplink traffic decreases. In addition, different cells of a system typically do not need to have the same timeslot assignment procedure. Accordingly, if the traffic characteristics in one geographic region differ from those in other regions, the cells covering those regions may have different slot assignments to best accommodate local traffic conditions.

In the prior art, a simple method to avoid base station to base station and mobile unit to mobile unit interference is to use the same timeslot for all cells in the same geographic area; only different jobs between explicitly isolated cells from each other are allowed. A significant disadvantage of this approach is that when cells are deployed to provide continuous coverage, as is often the case, it is difficult to detach one cell's subscriber telephone from the other. Unless the use of some slots in some cells is not allowed at all. This ultimately results in a loss of capacity in the system.

Thus, both mobile-to-mobile and base-to-base station interference limit the use of inter-cell independent slot assignments in the same geographic area. What is needed is a system that avoids capacity loss when traffic asymmetry metric changes in a coverage area.

Disclosure of Invention

The present invention establishes a system that preferably uses adaptive antennas at a base station and optionally at Wireless Transmit Receive Units (WTRUs) to mitigate interference due to conflicting slot operations between two neighboring cells. This system will allow a maximum flexibility in time slot-to-cell operation, particularly when two cells or mobile units in the same vicinity are operating in conflicting cells, more particularly, one cell using a time slot operation for uplink and the other cell using the same time slot operation for downlink.

Drawings

A more detailed understanding of the present invention may be derived from the following description of a preferred embodiment, taken in conjunction with the accompanying drawings, in which:

fig. 1 shows an example of a cell partition into zones.

Figure 2 illustrates how a cell can be cut into two simple regions.

FIG. 3 is a look-up table showing conflicting zones.

Figure 4 illustrates the edges of two neighboring cells and a WTRU nearest to the edge.

Detailed Description

The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout.

Although the invention will be described with reference to the example of a hexagonal deployment shown in fig. 1, the invention can readily be extended to other forms of deployment. The system presets administrators and operators to be busy defining areas according to their specific locations. Cell segmentation as shown in fig. 1 is just one type of cell segmentation example, it should be noted that in an actual deployment there are other ways to segment cells into sectors.

As depicted in fig. 1, cell a 100 has been partitioned into six (6) sectors A1-a6 and, as such, neighboring cells B110 and C120 each have been partitioned into six (6) sectors B1-B6 and C1-C6, respectively. A plurality of WTRUs 130, 140 are also randomly configured. In a first embodiment, it is assumed that WTRUs are not equipped with adaptive antennas. A first WTRU 130 is located in sector a6 and a second WTRU 140 is located in sector B3. The method and system of the present invention permits two neighboring cells to use conflicting slot assignments (i.e., a slot is used in one cell on the uplink and in a neighboring cell on the downlink) based on the "zone" concept. A region is a subdivided area of a cell defined by the system operator. A cell is divided into a certain number of (non-overlapping) zones. Two zones (belonging to different cells) are defined to collide with each other if there is a high probability that a first WTRU transmission in one zone causes severe interference to be received by a second WTRU in the second zone. The decision whether two zones are in conflict with each other can be made by an analysis of the cell wiring. Alternatively, a more complex structure is envisioned based on collected measurements made by WTRUs.

Although the method of determining conflicting zones will be described with reference to fig. 3, this embodiment is illustrative and should not be construed as the only method of determining such conflicting zones. The conflicting block lookup table helps determine the conflicting zones in FIG. 3. For example, the first WTRU 130 is located in zone a 6. Zone A6 is the first vertical column set in the conflict lookup table. An X in its corresponding row indicator B3 is a potential conflicting zone and a in the tableAn entry is indicated to correspond to a zone of the same cell (e.g., a1, a2, A3, a4, and a 5). Since a slot cannot be used simultaneously by uplink and downlink in the same cell, blocks belonging to the same cell cannot be allowed to have opposite directions. This indicates that transmissions from the first WTRU 130 in zone a6 have the potential to cause interference to the second WTRU 140 in zone B3. If both WTRUs 130 and 140 use the same time slot for transmission and reception, interference will occur separately.

Having determined the conflicting block, physical resources can now be used more flexibly and efficiently. For example, suppose a given time slot (S) is desired to be used for the uplink of cell a and the downlink of cell B, which has more downlink traffic than cell a. Without the concept of a zone, it is difficult to use time slot S in different directions for cell a and cell B. This is because a WTRU of cell a will create too much interference in time slot S near the edge of cell B that WTRUs cannot receive in time slot S of cell B. However, when using the concept of zones, this problem is overcome in the following way. The time slot S may be used by some of the WTRUs of cells a and B as provided by the inability to be used by two WTRUs simultaneously in a conflicting zone. For example, assume that region a3 of fig. 1 does not collide with any of the regions of cell B. It is also assumed that time slot S is used in the uplink of cell a and the downlink of cell B. A WTRU in region a3 will be allowed to use time slot S (for uplink) even though that time slot may be used for downlink in cell B. This represents an advantage of flexibility and ultimately of capacity, since otherwise the time slots S are completely unavailable for the uplink of cell a.

In the description of fig. 3, it is assumed that the location of the WTRU is known. However, as the WTRU traverses the coverage area, the system must maintain the trajectory of the zone in which it is now located. For example, if a WTRU originates in zone C3 and travels through the coverage area to zone B4, the WTRU travels through zones C3, C4, B3, and B4. From the knowledge of the look-up table showing the collision zones, the system can decide the best usage per time slot (uplink, downlink or no action) per zone, taking into account the collisions between zones and the constraints imposed by the average traffic characteristics. This is known as a Slow Dynamic Channel Allocation (SDCA) method, which occurs over a relatively large time scale (hours, days, weeks or months). The SDCA uses a collision zone lookup table in the decision on the use of this time slot.

Assuming that the configuration of slot usage is determined by the SDCA method, when a WTRU is connected to a system that is assigned at least one downlink channel in one or more slots, the slots may be used for downlink in the block in which they are set. It is also allocated at least one uplink channel in one or more time slots, which can be used for uplink. Whenever a WTRU moves to another zone, the system checks if the channel allocation needs to be changed, and a problem condition may occur if the time slot used by the WTRU is no longer allowed in the particular direction in which it moves into the new zone. This can be achieved by the Fast Dynamic Channel Allocation (FDCA) method. Thus, there is no need to re-consult the collision zone lookup table each time the WTRU moves to a new area. Of course, each zone is associated with a possible use, uplink, downlink or none of the slots, which is determined by the slow DCA based on the look-up table. The system uses the usage data of the time slot when it is necessary to allocate channels to a WTRU. The system then makes appropriate non-interfering uplink and downlink slot assignments.

Another advantage of the present invention is the impact of the use of time slot assignments on load balancing. Referring to fig. 4, if the traffic load in each of two adjacent cells 300, 310 has different asymmetric usage characteristics, (that is, the first cell 300 is primarily downlink traffic and the second cell 310 is primarily uplink traffic), there is no conflicting zone management, and the WTRU 32 transmission causes interference in the first cell 310 if the cells are using the same time slot as the different direction. A fast allocation protocol may attempt to use an escape mechanism, which is a method for a particular WTRU's channel allocation change due to excessive interference as detected by a particular WTRU using the channel. The use of escape mechanisms is unsatisfactory because it causes the use of time slots in a significant part of the cell to be effectively prevented. Conflicting zones allow a mobile unit to traverse through a coverage area, (inclusive zone) with negligible impact on the cell usage characteristics.

In an alternative embodiment according to the present invention, a simple zone segmentation map is used. A cell is split into two regions as shown in fig. 2. The first zone is the outer zone 21, which is defined by the coverage area 24 of the cell, (excluding the inner zone 22); and the second zone is the inner zone 22. The serving base station 20 is located at the center of the inner zone 22.

In one embodiment, the system determines the cell in which the WTRU 28 is located and whether the WTRU is in an inner zone 22 or an outer zone 21. A first measurement of the signal delay and the received signal power is performed. This allows a determination of whether the WTRU 28 is located in an inner or outer zone. In the example of fig. 2, a WTRU 28 is shown located in the inner zone 22 and a WTRU 26 is located in the outer zone 21.

Another method for determining location utilizes additional nearby base stations or other WTRUs. However, because a WTRU requires a fixed trajectory as it passes through the coverage area, there is a need for continuous system sharing and coordination of other base stations, and WTRUs make this a unique resource enhancement technique.

One additional technique to determine location utilizes global positioning satellites of the Global Positioning System (GPS). A GPS receiver is added to the location of each WTRU that has defined the WTRU. The coordinates reported by the WTRU use the coordinates for the base station and system as described above. However, there are some disadvantages with this option. First, the WTRU must be in a favorable position to allow it to correctly receive satellite signals (outer door). Also, because measurements are performed by the WTRU, the WTRU needs to constantly transmit location data to the system, which increases the signal load of the air interface and utilizes precious battery resources.

The consistency and accuracy of zone locations is improved with adaptive antennas. Position measurements including tilt angle and signal level readings are used to determine the position of the WTRU. An advantage of adaptive antennas is that a location is obtained without the need for any other base station or WTRU. Thus, the adaptive antenna provides an efficient and independent method of tracking WTRUs.

The cell system has two types of interfaces to handle, the first being the base station to base station interface and can occur when a first base station downlink is the uplink of another base station and the uplink base station receives the other base station downlink which blocks or reduces the intended uplink signal. The second type of interface, which may occur in a cellular system (mobile-to-mobile interface), occurs when the reception of a first mobile unit is blocked or reduced by the transmission of another mobile unit.

The adaptive antennas may be located at the base stations, at the WTRUs, or at both the base stations and the WTRUs. The completion of the segmentation scheme basically depends on two factors: 1) effective (i.e., confidence and convenience) in that it can determine in which area a user is located and track the ability of a moving user; 2) the size of the neighborhood of a zone. The neighborhood of zone Z is defined as other groups of zones like other cells and conflicts with zone Z. This means that entries that conform to Z and any blocks belonging to the vicinity of Z should be checked in the look-up table of fig. 3.

The use of zones for time slot allocation requires locating the WTRU with reasonable facts. If the accuracy of WTRU positioning increases, a large number of regions per cell may be defined, resulting in higher flexibility and increased system efficiency.

A small neighborhood is a region defined as having a small number of regions. This means that for all zones there is a relatively small number of other zones with which it conflicts. Thus, there is less binding, which allows more flexibility in determining the timeslot usage for each zone. This allows maximum flexibility in assigning different characteristics of uplink to downlink traffic to different cells. Thus, any system that tends to limit the size of the adjacent area results in the attainment of flexibility.

When two regions conflict with each other, this may be due to one or both of the following:

a) the probability of a base station to base station interface is high if the zone uses time slots in the opposite direction

b) The probability of a mobile-to-mobile interface is high if the zone uses time slots in the opposite direction.

When the mobile unit is also equipped with an adaptive antenna, there will be fewer pairs of blocks that collide with each other because the overall probability of mobile unit-to-mobile unit interface is reduced. This is because mobile units equipped with adaptive antennas tend to transmit and receive power from a particular direction, which reduces the likelihood of one mobile interface from another. That is, there will be fewer pairs of conflicting blocks as if the neighborhood size of each block were reduced under the definition of the neighborhood of a block.

The size of the neighborhood depends on several factors, such as the particular geographic deployment or current proliferation. If the mobile unit is equipped with an adaptive antenna, the vicinity of the zone may be limited to fewer zones. In the best example scenario, a neighborhood may even be limited to only one or two zones, with little dispersion around the mobile unit if deployed. This is because the likelihood of mobile unit-to-mobile unit interface is reduced when the mobile unit transmissions use narrow radio waves. Similarly, when adaptive antennas are used at the base station, the size of the neighborhood should be reduced for the same reason.

In another embodiment, the transmission power of the mobile unit is considered. Transmission power is considered an important factor and affects the size of the neighboring area. If a zone Z is geographically defined as being close to the base station, any WTRUs transmitting in that zone will tend to transmit at a lower power level because they are closer to the base station and will require less power than other WTRUs that are further away. They generate fewer interfaces than other WTRUs on their own and are less affected by other mobile units receiving in the same time slot. Therefore, the number of zones colliding with this zone Z tends to be small. In other words, the vicinity of a cell near a serving base station will typically be smaller than the vicinity of a cell near the edge of the cell.

As noted above, mobile units typically transmit at a lower power when near their serving base station. However, because of the advent of new high data rate cell technologies, increases in power are often required to facilitate high data rate switching at the same time a mobile unit is communicating with a base station. In order to accommodate higher data speeds, WTRUs as well as serving base stations will have to increase transmission power even if the WTRU is close to the serving base station. Therefore, it may be necessary to define a neighborhood of service bases.

For example, it is possible that two non-conflicting zones may be present if the WTRUs are using a high speed service, such as a data speed of 384 kbps. In that example, a WTRU may be allowed to use a particular timeslot in a known region if it uses a low speed service.

Although the present invention has been described in detail, it must be understood that the invention is not limited thereto and that various changes may be made without departing from the scope and spirit of the invention, which is defined by the appended claims.

Claims (10)

1. A method of maximizing flexibility of timeslot-to-cell allocation in a TDD (time division duplex) UMTS system, comprising:

defining a plurality of regions contained in a plurality of Node-Bs; and

the use of the decision time slots minimizes interference caused by a conflicting uplink or a conflicting downlink operation on a zone-by-zone basis.

2. The method of claim 1 wherein a plurality of Wireless Transmit Receive Units (WTRUs) are incorporated into the plurality of Node-Bs.

3. The method of claim 1 wherein the location of WTRUs in the area is determined using a signal delay at the Node-Bs.

4. The method of claim 1 wherein the location of WTRUs in the region is determined using a received power at the Node-Bs.

5. A method for maximizing allowable flexible timeslot-to-cell allocation in a TDD (time division duplex) system using adaptive antennas, comprising:

defining a plurality of service dependent neighborhoods for a plurality of localities comprised in a plurality of Node-Bs;

the use of decision time slots minimizes interference caused by colliding uplink or a colliding downlink on a zone-by-zone basis.

6. A system for maximizing flexibility of timeslot-to-cell allocation in a TDD (time division duplex) UMTS system, comprising:

several regions are contained in several Node-Bs; and

the method of slot usage is determined on a zone-by-zone basis to minimize interference caused by a conflicting uplink or a conflicting downlink operation.

7. The system of claim 6 further comprising a plurality of WTRUs.

8. The system of claim 7 wherein the location of WTRUs in the area is determined using a signal delay in the Node-Bs.

9. The system of claim 7 wherein the location of WTRUs in the area is determined using a received power of the Node-Bs.

10. A method of maximizing allowable flexible timeslot-to-cell allocation in a TDD (time division duplex) using adaptive antennas, comprising:

defining a plurality of service dependent neighborhoods for a plurality of localities comprised in a plurality of Bode-Bs; and

the use of the decision time slots minimizes the interference caused by a conflicting uplink or a conflicting downlink operation on a zone-by-zone basis.