WO2010039076A1

WO2010039076A1 - Joint detection for a code division multiple access communication network

Info

Publication number: WO2010039076A1
Application number: PCT/SE2008/051151
Authority: WO
Inventors: Jie Mao; Lei Xiao; Huaisong Zhu
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-10-03
Filing date: 2008-10-08
Publication date: 2010-04-08

Abstract

The invention relates to joint detection for a Code Division Multiple Access communications network. A method, a joint detector receiver, a node, a computer program product and a communications network are disclosed. One or more active user devices are determined from a plurality of cells associated with a node. The one or more user devices being operable via a first frequency and at a first time slot. A joint detection operation is then performed for the one or more user devices to distinguish user devices operating at the first frequency.

Description

Joint Detection for a Code Division Multiple Access Communications Network

Technical Field The invention relates to joint detection for a Code Division Multiple Access communications network, and in particular to a method, a joint detector receiver, a node, a computer program product and a communications network to provide joint detection for a Code Division Multiple Access communications network.

Background

A Code Division Multiple Access (CDMA) communications network permits different user devices to operate at the same frequency in a given cell. However, CDMA communications networks suffer from Multiple Access Interference (MAI) whereby user devices from the same cell, or from neighbouring or nearby cells may cause interference problems. With third generation CDMA communications networks, such as a Time Division Synchronous Code Division Multiple Access Network (TD-SCDMA), interference between user devices from a neighbouring or nearby cell, known as intra- frequency interference, is an obstacle for increasing the network capability. For example, TD-SCDMA may utilise several carriers in one cell, such that there are several time slots in one carrier and a maximum of sixteen spreading codes in one time slot. Since the maximum number of spreading codes and scramble code length is sixteen chips, the spreading gain, up to a maximum of 2⁴, may not be sufficient to suppress the intra-frequency interference. Such interference may reduce the network capability which is undesirable. It is known to provide a Single Cell Joint Detection (SCJD) operation in an attempt to overcome the problem of MAI. A problem with the SCJD operation is that it can only process signals from active user devices operating at one frequency at one time slot and in one cell only. All signals from user devices of neighbouring cells are treated as noise. Accordingly, intra-frequency interference may be large when using the SCJD operation.

It is further known to provide a Multi-Cell Joint Detection (MCJD) operation which uses a blind detection technique in an attempt to distinguish user devices from neighbouring cells. With the MCJD operation each mobile cell is provided with a joint detection device for each mobile frequency used in each cell. Details of active user devices in a particular cell are input to the joint detection device associated with that cell, and active user devices from other cells are blindly detected. A joint detection operation is then performed on the active user devices to separate them.

There are several problems associated with the MCJD operation. The need for blind detection means that each joint detection device cannot know with a reasonable certainty which user devices are active. Each joint detection device effectively listens to any mobile radio frequencies that may be in use and guesses which user devices are active. Such guessing introduces an inaccuracy to the joint detection operation which is undesirable.

Implementing the MCJD operation has a further problem because blind detection requires that some criterion is used to judge whether user devices are active or not. If the criteria is chosen to be a mobile signal power level it is necessary to set a threshold power level below which user devices can be defined as inactive, and above which they can be defined as active. Choosing a suitable threshold level may be difficult because if it is too high then few user devices will be detected from neighbouring cells, whereas setting it too low will means that too many user devices will be selected. Overall the use of blind detection introduces complexity and uncertainty into the process of joint detection which has an undesirable impact on performance and processing complexity.

Summary An object of the invention is to provide an improved way of providing joint detection for a CDMA communications network.

According to a first aspect of the invention there is provided a method of performing joint detection for a Code Division Multiple Access communications network. The method comprising determining one or more active user devices from a plurality of cells associated with a node. The one or more user devices operating via a first frequency and at a first time slot. The method further including performing a joint detection operation for the one or more user devices to distinguish user devices operating at the first frequency.

Such a method eliminates the need to perform blind detection that is required with the prior art Multi Cell Joint Detection (MCJD) operation. This is because all active users are known and identified from the plurality of cells according to the invention. This leads to a reduced uncertainty of performance for a network implementing the method such that processing complexity due to wrongly detected users is substantially eliminated. A further advantage of the invention is that the user devices from a plurality of mobile cells are known and use the same frequency source. Hence the user devices from the plurality of mobile cells may be substantially frequency-synchronised.

The method may further include determining the one or more active user devices, and/or performing the joint detection operation at the node.

Preferably the method further includes determining active user devices operating via a plurality of different frequencies from the plurality of cells. Preferably the method further includes performing the joint detection operation for the active user devices operating via the plurality of different frequencies.

Preferably the method further includes determining the one or more user devices operating at a plurality of different time slots. Preferably the method further includes performing the joint detection operation for the active user devices operating at the plurality of different time slots.

According to a second aspect of the invention there is provided a joint detection receiver for a Code Division Multiple Access communications network. The joint detection receiver comprising at least one input operable to receive data from one or more user devices from a plurality of cells associated with a node. The one or more user devices operable via a first frequency and at a first time slot. The joint detection receiver operable to perform a joint detection operation on the received data to distinguish user devices operating at the first frequency.

Such a joint detection receiver eliminates the need to perform blind detection that is required with the prior art Multi Cell Joint Detection (MCJD) operation due to the fact that all active users are known and identified from the plurality of cells according to the invention. This leads to a reduced uncertainty of performance for a network using the joint detection receiver such that processing complexity due to wrongly detected users is substantially eliminated. A further advantage of the invention is that the user devices from a plurality of mobile cells are known by the joint detection receiver and use the same frequency source. Hence the user devices from the plurality of mobile cells may be substantially frequency-synchronised.

Preferably the joint detection receiver is operable to receive data from the one or more user devices operating via a plurality of different frequencies from the plurality of cells. Preferably the joint detection receiver is operable to perform the joint detection operation for the one or more user devices operating via the plurality of frequencies.

Preferably the joint detection receiver is operable to receive data from one or more user devices operating at a plurality of different time slots. Preferably the joint detection receiver is operable to perform the joint detection operation for the one or more user devices operating at the plurality of different time slots. According to a third aspect of the invention there is provided a node including a joint detection receiver according to the second aspect of the invention.

According to a fourth aspect of the invention there is provided a computer program product operable to perform a method according to the first aspect of the invention, or operable to control a joint detection receiver according to the second aspect of the invention, or operable to control a node according to the third aspect of the invention.

According to a fifth aspect of the invention there is provided a communications network configured to use a method according to the first aspect of the invention, or including a joint detection receiver of the second aspect of the invention, or including a node according to the third aspect of the invention, or arranged to implement a computer program product according to the fourth aspect of the invention.

It will be appreciated that any preferred or optional features of one aspect of the invention may also be preferred or optional feature of other aspects of the invention.

Brief Description of the Drawings

Other features of the invention will be apparent from the following description of preferred embodiments shown by way of example only with reference to the accompanying drawings, in which;

Figure 1 shows a graph to illustrate a Time Division Synchronous Code Division Multiple Access frame structure; Figure 2 shows a network performing a joint detection operation according to an embodiment of the invention;

Figure 3 shows a node shown in Figure 2 according to an embodiment of the present invention; and Figure 4 shows a flow diagram to describing a method according to an embodiment of the present invention.

Detailed Description

Figure I shows a graph to illustrate a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) frame structure, generally designated 10, useful in understanding embodiments of the invention. The graph has an x-axis 12 to illustrate time, which shows that there are seven time slots TSl, TS2, TS3, TS4, TS5, TS5, TS6 and TSO that are available for user data in the frame structure 10. Time slots TS 1 and TS2 are nominally reserved for uplink data, whereas time slots TS3, TS4, TS5, TS5, TS6, and TSO are nominally reserved for downlink data. A further time slot 14 is also provided for overhead data. The graph also has a y-axis 16 to illustrate that there are sixteen available spaces in each time slot for sixteen possible spreading codes as shown at 18. The sixteen spreading codes 18 permit sixteen user devices to operate in each time slot. In the frame structure 10 shading illustrates that a user device is occupying an available space at a particular time slot, such that for example, at TS5 there are six user devices, and ten available spaces. The graph also has a z-axis 20 which illustrates frequency and shows that there may be three of more carrier frequencies 22, 24, 26 to accommodate additional users. Figure 2 shows a network performing a joint detection operation according to an embodiment of the invention, generally designated 30. The network 30 is used as a reference network for describing embodiments of the present invention, and it will be appreciated that the network 30 is a simplified version of a real life network. The network 30 comprises a wider communications network 32 in communication with a NodeB 34, which is an access node. The NodeB 34 typically serves three sectors having a respective cell 36, 38, 40 with several carriers. In this example each cell has three frequencies fl, f2 and f3 such that a different frequency may be assigned to be a primary carrier for a particular cell 36, 38, 40. Each cell 36, 38, 40 has a respective antenna 42, 44, 46. In Figure 2 solid lines represent physical links along which traffic can flow, whereas dashed lines are used to show the partition lines between cells 36, 38, 40. It will be understood that there may be many additional nodes or network devices between the NodeB 34 and the wider communications network 32, which have been omitted for the purposes of clarity.

Figure 2 also shows mobile devices 48, 50, 52, 54, 56, 58 in cell 36, mobile devices 60, 62, 64 in cell 38, and mobile devices 66, 68, 70, 72, 74 in cell 40. The mobile devices 48, 50, 52 are operating at frequency fl and time slot TSl, the mobile devices 54, 56 are operating at frequency f2 and time slot TSl, and the mobile device 58 is operating at frequency β and time slot TS 1. The mobile devices 60, 62 are operating at frequency fl and time slot TS 1 , and the mobile device 64 is operating at frequency fl and time slot TSl. The mobile device 66 is operating at frequency fl and time slot TSl , and the mobile devices 68, 70, 72, 74 are operating at frequency f2 and time slot TSl . It will be appreciated that there may be many more mobile devices operating in the cells 36, 38, 40 at other time slots but these have been omitted for the purposes of clarity. Furthermore the mobile devices are one example of user devices that may be operable in the cells 36, 38, 40.

In the uplink direction the NodeB 34 performs a joint detection operation for all devices operating at frequency fl and time slot TSl, such that the mobile devices 48, 50, 52, 60, 62, 66 are detected together, and six signals are sent in the uplink direction corresponding to the mobile devices 48, 50, 52, 60, 62, 66. The NodeB 34 also performs a joint detection operation for all devices operating at frequency f2 and time slot TS 1 , such that the mobile devices 54, 56, 64, 68, 70, 72, 74 are detected together, and seven signals are sent in the uplink direction corresponding to the mobile devices 54, 56, 64, 68, 70, 72, 74. The NodeB 34 also performs a joint detection operation for all devices operating at frequency O and time slot TSl, such that only the mobile device 58 is detected, and one signal is sent in the uplink direction corresponding to the mobile device 58. In the context of the present invention the skilled person will understand that a joint detection operation such as Zero-Forcing (ZF) joint detection, or Minimum Mean Square Error (MMSE) joint detection may be used as required. Other joint detection algorithms may be used as appropriate, and the invention is not limited to any particular kind of joint detection.

With the arrangement of Figure 2 the number of active user devices are known such that there can be a maximum of 16 X 3 = 48 spreading codes from the configuration of the three cells 36, 38, 40. Accordingly there is a reduced uncertainty of performance with the network 30 when compared to the prior art Multi-Cell Joint Detection (MCJD) operation, and processing complexity due to wrongly detected users using blind detection is substantially eliminated.

Figure 3 shows a node 34 shown in Figure 2 according to an embodiment of the present invention. Like features to the embodiment of Figure 2 are shown with like reference numerals. In Figure 3 the uplink signals are shown running from left to right. Each respective antennae 42, 44, 46 from the cells 36, 38, 40 are shown in communication with a respective joint detection device 82, 84, 86. Each joint detection device 82, 84, 86 has three inputs which are able to receive data from one or more of the respective antennae 42, 44, 46. The joint detection devices 82, 84, 86 may be traffic cards of the NodeB 34 having one or more inputs operable to accept uplink data from each antenna 42, 44, 46. The inputs may be ports of the traffic cards or any other arrangement capable of receiving the uplink signals. Since each joint detection device 82, 84, 86 has a direct connection to the antennas 42, 44, 46 the active users in the cells 36, 38, 40 operating on frequencies fl, f2, f3 are known to the joint detection devices 82, 84, 86 as shown at 88 and illustrated by respective dotted arrows to the joint detection device 82, 84, 86. The joint detection devices 82, 84, 86 perform a joint detection operation to separate users operating at the same frequency and at the same time slot from the three cells 36, 38, 40.

Referring to Figures 2 and 3 together the joint detection device 82 will detect mobile devices 48, 50, 52, 60, 62, 66 together because they are operating at fl and at time slot TSl. The joint detection device 82 will then output six data streams to a decoder 90 where the six data steams are decoded and output at 96 to the wider communications network 32. The joint detection device 84 will detect mobile devices 54, 56, 64, 68, 70, 72, 74 together because they are operating at β and at time slot TSl. The joint detection device 84 will then output seven data streams to a decoder 92 where the seven data steams are decoded and output at 97 to the wider communications network 32. The joint detection device 86 will only detect mobile device 58 because it is the only mobile device operating at f3 and at time slot TSl from all three cells 36, 38, 40. The joint detection device 86 will then output one data stream to a decoder 94 where the data steam is decoded and output at 98 to the wider communications network 32.

It will be appreciated that the NodeB 34 shown in Figure 2 represents a new NodeB structure such that the NodeB 34 is operational for level baseband signal processing and for receiving signals from active user devices. Such a new NodeB structure means that the baseband signal processing combines all received signals and processes them jointly.

Figure 4 shows a diagram illustrating a method according to an embodiment of the present invention, generally designated 100, for performing joint detection for a CDMA communications network. The method 100 starts by determining one or more active user devices from a plurality of cells as shown at step 102. The plurality of cells are associated with one NodeB 34, and the user devices are operating via a first frequency and at a first time slot. The method then performs a joint detection operation for the one or more user devices as shown at step 104 to distinguish user devices operating at the first frequency. It will be appreciated that the joint detection operation may be performed at the NodeB 34, or once the various data has been gathered it may be sent to a remote device to perform the joint detection. An advantage of the embodiment is that joint detection is performed for active user devices from the plurality of cells 36, 38, 40. This is due to the signals from the user devices 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74 being sent to all antennae 42, 44, 46 of the NodeB 34 which are combined together and detected together using the joint detection operation.

The method 100 may also include determining active user devices operating via a plurality of different frequencies from the plurality of cells as shown at step 106. The method may then perform the joint detection operation 104 if the method has determined active user devices operating via a plurality of difference frequencies. The method 100 may also include determining the one or more user devices operating at a plurality of different time slots as shown at step 108. The method may then perform the joint detection operation 104 if the method has determined active user devices operating at the plurality of different time slots.

Whereas the prior art MCJD operation may provide some improvement over the prior art Single Cell Joint Detection (SCJD) operation it introduces problems due to the need to use blind detection. The blind detection technique means that joint detection devices have no knowledge of the received spreading code sequences or training data. Blind detection relies on detecting the midamble code, but the mapping between the midamble code and the spreading code is not unique so it is possible that a NodeB may use the wrong spreading code when implementing the MCJD operation. This limits the usage of MCJD or requires very complex algorithms to determine the mapping between the midamble code and the spreading code. These problems are substantially avoided according to the above described embodiments of the invention because the need for blind detection is eliminated which means that the mapping between the midamble code and the spreading code is known by the NodeB 34 since all users belong to it. Overall the embodiments of the invention described above lead to a reduced uncertainty of performance for a network. Processing complexity due to wrongly detected users is substantially eliminated when compared to the prior art MCJD operation. Performance is also improved with the embodiments of the invention when compared to the Single Cell Joint Detection (SCJD) operation because intra-frequency interference from user devices is considered.

With the prior art MCJD operation problems may also be encounter due to frequency synchronisation between the user devices in different mobile cells undergoing joint detection. This is because user devices in a particular cell are substantially frequency- synchronised with that cell. A frequency deviation may therefore occur using MCJD such that user devices may suffer signal degradation. These problems are substantially avoided according to the above described embodiments of the invention because the user devices from a plurality of mobile cells are known by the joint detection device and use the same frequency source. Hence the user devices from the plurality of mobile cells are substantially frequency-synchronised using the embodiments of the invention.

Furthermore, with the prior art MCJD operation the detected signals from neighbouring cells are discarded after joint detection because they do not belong to the particular cell undergoing the MCJD operation. Such detection and discarding of signals wastes time, information processing, and computing costs. These problems are minimised according to the above described embodiments of the invention because the signals from active user devices are from a plurality of mobile cells that are known to the joint detection devices 82, 84, 86. Accordingly the decoders 92, 94, 96 output user device signals in one time slot from a plurality of cells together. Hence there are no discarded signals which avoids wasting time, information processing, and computing costs.

It will also be appreciated by those skilled in the art that the embodiments of the invention described provides a reduced complexity of the NodeB 34 when compared to the prior art MCJD and SCJD operations. Embodiments of the invention require fewer joint detection devices when compared to the MCJD and the SCJD operations. Furthermore any decoders used with the prior art MCJD and SCJD operation may only output a user device signal in one time slot from one cell only. In contrast the decoders 92, 94, 96 of embodiments of the invention can output user device signals in one time slot from a plurality of cells together.

A further advantage of the embodiments of the invention is provided due to a gain in diversity of the network 30. The NodeB 34 uses all antennae 42, 44, 46 to receive signals from active user devices, and combines the signals with a technique such as Maximum Ratio Combination during the joint detection operation. Such combining may reduce fading and provide a gain in diversity. In contrast, the known MCJD and SCJD operations do not use such a combining technique, and can only transmit signals from one cell in the upstream direction. Accordingly, the embodiments of the present invention provide an overall diversity gain. In the prior art the intra-frequency interference between cells associated with one NodeB is normally more significant than intra-frequency interference from two cells associated with two respective NodeBs. Typically the interference from user devices from two respective NodeBs only exists at a boundary between sectors of the two NodeBs. The embodiments of the invention may provide a further advantage because such interference associated between cells associated with one NodeB is determined to be a useful signal which may provide a further performance improvement over the known SCJD and MCJD operations.

According to other embodiments of the invention a computer program product may be operable to perform the method described above, or operable to control the joint detection receiver described above, or operable to control the NodeB described above. Furthermore, a communications network may be configured to use the method, or use the joint detection receiver, or use the NodeB, or arranged to implement the computer program product.

The skilled person will appreciate that the uplink performance in a TD-SCDMA system is usually a limiting factor of the network, and hence any improvements made to the NodeB according to the embodiments of the invention may have an advantageous effect on the uplink performance which can directly improve the capacity for the whole network. The skilled person would also appreciate that because the invention detects active user devices in a plurality of cells associated with one NodeB, any handovers between adjacent NodeBs may also be improved.

Claims

1. A method (100) of performing joint detection for a Code Division Multiple Access communications network (30), comprising: determining one or more active user devices from a plurality of cells (102) associated with a node (34), the one or more user devices operating via a first frequency (22) and at a first time slot; and performing a joint detection operation (104) for the one or more user devices to distinguish user devices operating at the first frequency.

2. A method according to claim 1, and further including determining the one or more active user devices (102), and/or performing the joint detection operation (104) at the node (34).

3. A method according to claim 1 or claim 2, and further including determining active user devices operating via a plurality of different frequencies (106) from the plurality of cells.

4. A method according to claim 3, and further including performing the joint detection operation (104) for the active user devices operating via the plurality of different frequencies.

5. A method according to any preceding claim, and further including determining the one or more user devices operating at a plurality of different time slots (108).

6. A method according to claim 5, and further including performing the joint detection operation (104) for the active user devices operating at the plurality of different time slots.

7. A joint detection receiver (82, 84, 86) for a Code Division Multiple Access communications network (30), the joint detection receiver comprising: at least one input operable to receive data from one or more user devices (48, 50, 52, 60, 62, 66) from a plurality of cells (36, 38, 40) associated with a node (34), the one or more user devices operable via a first frequency (22) and at a first time slot; the joint detection receiver (82, 84, 86) operable to perform a joint detection operation (104) on the received data to distinguish user devices operating at the first frequency.

8. A joint detection receiver according to claim 7, operable to receive data from the one or more user devices (48, 50, 52, 60, 62, 66; 54, 56, 64, 68, 70, 72, 74; 58) operating via a plurality of different frequencies (22, 24, 26) from the plurality of cells (36, 38, 40).

9. A joint detection receiver according to claim 8, operable to perform the joint detection operation (104) for the one or more user devices operating via the plurality of frequencies.

10. A joint detection receiver according to any of claims 7 - 9, operable to receive data from one or more user devices operating at a plurality of different time slots.

11. A joint detection receiver according to claim 10, operable to perform the joint detection operation ( 104) for the one or more user devices operating at the plurality of different time slots.

12. A node (34) including a joint detection receiver (82, 84, 86) according to any of claims 7 - 1 1.

13. A computer program product operable to perform a method (100) according to any of claims 1 - 6, or operable to control a joint detection receiver (82, 84, 86) of any of claims 7 - 11, or operable to control a node (34) according to claim 12.

14. A communications network configured to use a method (100) according to any of claims 1 - 6, or including a joint detection receiver (82, 84, 86) of any of claims 7 - 11, or including a node (34) according to claim 12, or arranged to implement a computer program product according to claim 13.