KR20010070988A - Cellular communication system with variable channel loading - Google Patents

Abstract

Cellular wireless communication systems operating according to time division multiple access protocols have means for assigning different fractional loading factors to different time slots. In addition to such results, due to the influence of the local delivery environment, different time slots will have different levels of interference. In worst-case delivery environments, such as when near the edge of a cell or when severe shadowing fading occurs, the mobile station may be assigned to a time slot with a low interference level, while in a good delivery environment the mobile station may May be assigned to a time slot with a high interference level. Therefore, the present invention provides improved quality of service in poor delivery environments.

Description

Cellular communication system with variable channel loading {CELLULAR COMMUNICATION SYSTEM WITH VARIABLE CHANNEL LOADING}

In a cellular wireless communication system, the area in which the system operates is subdivided into a plurality of cells, each cell being defined as a communicable area of a base station BS. Each BS may be connected to a system controller that coordinates the operation of the complete system, or system control functions may be distributed among base stations. A mobile station (MS) in a call communicates with the BS of the cell in which it is currently located, the communication being made over a wireless channel in accordance with a defined wireless protocol. One such protocol, based on the Mobile Communications Globalization System (GSM) and the UMTS Time Division Duplex (TDD) standard as an example, is multi-carrier time division multiple access (TDMA). In accordance with this protocol, communication occurs in an assigned time slot defined on a plurality of carrier frequencies.

Many cellular wireless communication systems, such as operating in accordance with the GSM standard, for example, use the concept of frequency re-use, where a particular carrier frequency used in one cell is re-used in other cells according to some repetitive pattern. . The re-use factor for a system is defined as the total number of carrier frequencies used by the system divided by the number of carriers available for use in any one cell. Given a small re-use factor, there are a large number of frequencies available in each cell, but the system is limited by interference due to low re-use factors (because the same frequency may be in use in adjacent cells) ). Given a high re-use factor, the number of users in the system is limited by the smaller number of available channels in each cell.

An important problem with such a system is that it provides a communication link to the MS under worst-case transmission situations where the interference is high and the power received by the MS is low. Such a situation may arise, for example, when the MS is located near a cell boundary or when severe shadowing fading occurs in the MS. Therefore, the choice of re-use factor is generally a compromise between meeting spectral efficiency (and thus system capacity) and worst-case requirements.

Many more sophisticated cell designs and re-use patterns have been proposed to improve system capacity. Examples include the following:

Ring cell structure. The MS in close proximity to the BS uses a carrier frequency other than the carrier frequency used by the MS away from the BS. Such a structure is suitable for both code division multiple access (CDMA) systems and time division multiple access (TDMA) systems. This structure has the disadvantage that additional carrier frequencies need to be assigned to the system.

Multiple re-use patterns. In this structure, different sets of carriers have different re-use patterns, and some carrier frequencies are re-used more frequently than other carrier frequencies. This structure is suitable for TDMA systems using frequency hopping, such as GSM, for example, and improves interference diversity. This means that if the MS has a very high interference level, the MS may hop to a different carrier frequency, and if this carrier has a re-use pattern that is different from the first pattern then the interference level will also be different and likely to be reduced. Because. This structure has the disadvantage that it only reduces the general interference level and the reduction may not be sufficient to handle the worst-case connection.

Fractional loading. This structure is also applicable to TDMA structures using frequency hopping. Only a limited time portion (partial loading factor) and frequency slots are generally assigned on a random basis. Reducing the partial loading factor reduces the average interference level and allows smaller re-use factors. Admission control (the process of determining whether the requested connection is established, which process can be centralized) ensures that the maximum load is not exceeded. This structure also has the disadvantage that it only reduces the overall interference level.

The present invention relates to a cellular wireless communication system having, or operating according to, a time division multiple access protocol or component, and also relates to primary and secondary stations for use in the system, A method of operating the system.

1 illustrates a portion of a cellular wireless communication system.

2 is a schematic block diagram of one cell of a wireless communication system.

3 illustrates subdividing a wireless channel into frames and time slots.

4 is a flow diagram illustrating a method according to the present invention for initiating a call.

It is an object of the present invention to better utilize system capacity while avoiding the above drawbacks.

According to a first aspect of the present invention, there is provided a cellular wireless communication system comprising one first station and a plurality of second stations, wherein the system transmits time slots within a transmission frame in which transmission between the first station and the second station occurs. It operates according to the time division multiple access protocol in which different time slots have different partial loading factors.

Time slots with different partial loading factors will have different interference factors. Thus, a call can be allocated to a time slot based on the carrier-to-interference ratio being sufficient to meet the required quality of service. For example, a time slot with a low interference level can be allocated to the worst-case connection and high interference. Time slots with levels can be assigned to the best-case connection, thereby allowing system capacity to be optimized.

According to a second aspect of the present invention, there is provided a first station for use in a cellular wireless communication system operating according to a time division multiple access protocol wherein the transmission between the first station and the second station is made in time slots within a transmission frame. Means are provided here for applying different partial loading factors to different time slots.

According to a third aspect of the present invention, there is provided a second station for use in a cellular wireless communication system operating according to a time division multiple access protocol wherein the transmission between the first station and the second station occurs in time slots within a transmission frame, Means are provided here to determine the level of interference in different time slots and to request that a connection be assigned to a time slot with the worst carrier to interference ratio so that the quality of service required for a call can be provided. .

According to a fourth aspect of the present invention, a method is provided for operating a cellular wireless communication system, the system comprising one first station and a plurality of second stations, wherein transmission between the first station and the second station is transmitted. It operates according to a time division multiple access protocol made in time slots within a frame, where different partial loading factors apply to different time slots.

The present invention, although not present in the prior art, is based on the recognition that different time slots in a transmission frame may advantageously have different partial loading factors.

Embodiments of the present invention will now be described with reference to the accompanying drawings, by way of example.

A portion of a cellular wireless communication system is shown in FIG. 1, which includes one central controller (CON) 102 and a plurality of cells 104. Each cell includes one primary station (BS) 106 (indicated by A to D), the first station being the center of the cell with the omni-directional transmission pattern or one-way transmission pattern. It may be located at one edge of the cell having. Although cell 104 is shown as having a hexagonal shape, in practice the shape of the cells may not be so regular and will depend on the local wireless environment in each cell. Also, not all cells 104 need to have the same size. In fact, the size of a cell often varies in different places to provide different traffic requirements.

The controller 102 is connected to each BS by connecting means 108, which may comprise, for example, terrestrial communication line connecting means or wireless connecting means. The controller 102 coordinates the operation of the complete system, i.e. performs any necessary coordination and synchronization tasks. In some systems, the functionality of controller 102 may be distributed among a plurality of first stations 106 as an example. In the system according to the present invention, the controller 102 may store information about partial loading and re-use factors for each channel and time slots, which information may be updated. The connection to the PSTN or other suitable network may be made via the controller 102 or each BS 106 may have its own connection.

The arrangement of the devices in a single cell of the above system is shown in FIG. 2, the arrangement comprising one BS 106 and a plurality of secondary stations (MS) 210. . The BS 106 includes a microcontroller (μC) 202, a transceiver means 204 connected to the wireless transmission means 206, and a connection means 108 for connection to the central controller 102. Each MS 210 comprises a transceiver means 214 and a microcontroller (μC) 212 connected to a wireless transmission means 216. Communication from BS 106 to MS 210 is via downlink channel 222, while communication from MS 210 to BS 106 is through uplink channel 224.

The invention also relates to such a cellular wireless communication system operating according to a single or multi-carrier TDMA protocol. An arrangement of radio channels for such a system is shown in FIG. 3. The wireless channel 300 is divided into consecutive frames 302. Each frame 302 is further subdivided into a plurality of time slots 304 through which communication between BS 106 and MS 210 can occur. The wireless channel 300 may be shared between the downlink 222 channel and the uplink 224 channel, or may be dedicated for either downlink or uplink communication.

In the system according to the invention, each time slot 304 may have different partial loading factors and thus different re-use factors. In one example, the first time slot 304 in each frame 302 can have a small partial loading factor (and thus a low re-use factor), while the second time slot 304 has a high partial loading factor (And accordingly high re-use factor). Information about the partial loading and re-use factors is stored in the system controller 102.

To ensure different average interference levels for each time slot, frequency hopping (or equivalent time slot hopping) may be used. There needs to be an explicit coordination between the first stations 106. none. However, if the first station 106 is synchronized via the controller 102, the same time slot for partial loading assignment can be used by each BS 106. Without synchronization between the first stations 106, the loading of each slot should be measured by determining the interference level.

By applying different partial loading factors to different time slots, different re-use factors can also be applied to different time slots. Therefore, each time slot has its own re-use pattern, which is selected by the controller 102 to select a time slot with a sufficiently low interference level for each radio link. Or alternatively, it may be allocated in a coordinated manner between the first stations 106 by using a dynamic channel allocation technique. Frequency hopping is not necessary, because in the system according to the invention, the average of the interference levels caused by frequency hopping is not needed.

Referring to Figure 1, consider the time slot used by BS 106, denoted A. Only those time slots with low partial loading and re-use factor of "1" will also be used by the first station 106 denoted by B. FIG. Time slots with higher partial loading and lower re-use factor will be re-used by the first stations 106 denoted by C and D.

In order to properly use the different partial loading and re-use factor usage in different time slots, the assignment of calls to time slots must be performed, whereby the carrier-to-indirect cost in the selected time slot is It is similar to the carrier-to-interference ratio required to meet the quality of service required for a call. This ensures that time slots with a low interference level are available to the MS 210 with the worst-case propagation path to the desired BS 106. This is true of systems such as Digital Enhanced Cordless Telecommunications (DECT) where new calls are allocated to the available slots with the lowest interference level, although this provides a better carrier-to-interference ratio than is required for successful calls. Remarkably different.

For example, in some applications with requirements for minimal quality of service, it may be necessary to evaluate the appropriate quality of a particular call for time slot allocation. In general, this will require knowledge of the current interference level and available carrier signal level (depending on the delivery situation and transmitter power output) for each time slot. Such information is easy to obtain in a typical mobile wireless system: The transmit power of BS 106 is known (and broadcasted) while the interference level can be measured by MS 210.

A method in accordance with the present invention for causing MS 210 to initiate a call is shown in FIG. The method begins at step 402 when the MS 210 wants to talk. Next, in step 404, the MS 210 captures the broadcast signal from the BS 106 to synchronize with the transmission of the BS 106. The MS 210 may then measure the carrier-to-interference ratio in the available time slots, at step 406. In general, there will be multiple time slots available depending on the range of carrier to interference ratio. By selecting only time slots with sufficient carrier-to-interference ratios for the required quality of service, time slots with better characteristics are left to be used by the MS 210 in a worse delivery environment. Thus, it is desirable for MS 210 to request BS 106 to initiate a connection in a time slot with the worst carrier-to-interference ratio that, in step 408, the quality of service required for the call can be provided. .

An important advantage of the system according to the invention is that it is not necessary to provide additional carrier frequencies in order to provide additional re-use patterns with high re-use factors. This means that the quality of service and / or capacity of an already-deployed TDMA system can be improved without requiring any hardware changes.

If the data rates on the uplink and downlink connections are different, then this will usually result in different loading factors on the uplink and downlink. In addition, for both symmetric and asymmetric traffic in the uplink and downlink, where uplink and downlink slots can be allocated independently, the loading factor between the various time slots is uplink to maximize system capacity. It can be organized differently for and downlink.

The present invention has a TDMA component and is generally applicable to any wireless system including GSM, DECT, UMTS and IMT2000 (although the channel assignment mechanism currently defined for DECT is shown above for some aspects of the present invention). Even if it interferes with use). The present invention is equally applicable to uplink and downlink transmissions.

For applications such as UMTS TDD mode, where CDMA technology is used, partial loading can be implemented simply by assigning more spreading codes to some time slots than other time slots. Also, loading can be adjusted by changing the spreading factor of the assigned code, with higher loadings corresponding to lower spreading factors. In such a CDMA system, more flexibility in resource allocation is possible. As an example, a call can be assigned to a time slot with the best carrier-to-interference ratio, which is possible if the carrier-to-interference ratio does not exceed the loading factor required for the slot. Synchronization between adjacent cells is desirable but will not be necessary for a CDMA-TDD system. Also, although not required, either or both frequency hopping and time slot hopping can be used.

By reading this disclosure, other changes will become apparent to those skilled in the art. Such changes may include other features already known in the cellular wireless communication system and its components, which features may be used in place of or in addition to the features already described herein.

In the present specification and claims, although the expression referred to as a single element is used, it does not exclude the presence of a plurality of such elements. Also, the word “comprising” does not exclude the presence of elements or steps other than the written element or steps.

The present invention can be applied to the field of wireless communication systems such as GSM and UMTS as an example.

Claims (10)

A cellular wireless communication system comprising one primary station and a plurality of secondary stations, the system comprising: The system operates according to a time division multiple access protocol where the transmission between the first and second stations is in time slots within a transmission frame, A cellular wireless communication system in which different time slots have different fractional loading factors. 2. The cellular wireless communication system of claim 1, wherein the call is assigned to the time slot with the worst carrier-to-interference ratio such that the quality of service required for a call can be provided. 3. The cellular radio according to claim 1 or 2, wherein the system operates according to a code division multiple access protocol and partial loading is implemented by assigning different numbers of spreading codes to different time slots. Communication system. A first station for use in a cellular wireless communication system operating according to a time division multiple access protocol wherein transmission between the first and second stations is in time slots within a transmission frame, A first station is provided for applying different partial loading factors to different time slots. 5. The method of claim 4, wherein means for determining the level of interference in different time slots and for allocating a call to the time slot with the worst carrier to interference ratio such that the quality of service required for the call can be provided. A first station, characterized in that the. 6. A system according to claim 4 or 5, characterized in that the system operates according to a code division multiple access protocol and means are provided for assigning different numbers of spreading codes to different time slots. Bureau. A second station for use in a cellular wireless communication system operating in accordance with a time division multiple access protocol, wherein the transmission between the first and second stations is in time slots within a transmission frame, A second station is provided, in which means are provided for determining the level of interference in different time slots and for requesting that a connection be assigned to a time slot with the worst carrier-to-interference ratio such that the quality of service required for the call can be provided. . A method of operating a cellular wireless communication system, The system includes one first station and a plurality of second stations, the system operates according to a time division multiple access protocol wherein the transmission between the first and second stations is in time slots within a transmission frame, 10. A method of operating a cellular wireless communication system, wherein different partial loading factors are applied to different time slots. 10. The method of claim 8, wherein the call is assigned to the time slot with the worst carrier to interference ratio such that the quality of service required for the call can be provided. 10. The system of claim 8 or 9, wherein the system operates according to a code division multiple access protocol and assigns different numbers of spreading codes to different time slots. Way.