WO2011116833A1

WO2011116833A1 - Heterogeneous cell selection

Info

Publication number: WO2011116833A1
Application number: PCT/EP2010/054008
Authority: WO
Inventors: Juergen Michel; Jacek Gora
Original assignee: Nokia Siemens Networks Oy
Priority date: 2010-03-26
Filing date: 2010-03-26
Publication date: 2011-09-29

Abstract

The present invention provides a method, apparatus, and a computer program product for detecting first cells via phase modulation on a reference symbol (RS), measuring the received power of the detected first cells and a second cell in a communication network, the second cell transmission power being larger than the first cell transmission power, calculating a power difference of measured received power between the first cells and the second cell, setting a parameterized power difference offset value between the first cells and the second cell and selecting if the calculated power difference is positive, from the detected first cells, the cell received with the lowest power difference if the power difference is lower than a power difference offset value.

Description

DESCRIPTION

Heterogeneous Cell Selection Field of the invention

The present invention relates to mobile wireless communica- tions, and more particularly to UE (user equipment) power consumption, network optimization, automated configuration and interference reduction in case of wide area network with pico cell (lower transmission power enhanced eNodeB, LeNB) or femto cell (Home eNB, HeNB) or relay co-channel deployment. The invention is particularly applicable to mo^¬ bile wireless communication networks, such as e.g. 3GPP Long-Term Evolution (LTE and LTE-A) .

The invention considers general low power nodes deployed in a coordinated (planed) or uncoordinated manner. The low power nodes are located under an overlay wide area network (high power nodes, macro network) and are operated on the same frequency layer or at least on an overlapping fre^¬ quency layer as the macro cells.

In this context, femto cells are a base station class with lower maximum transmit power with relation to typical macro eNB and are designed for indoor deployments, like e.g. in private residences or public areas (e.g. office) . As the femto cells are intended to be maintained individually by customers their deployment is uncoordinated. Further, pico cells are a base station class with lower maximum transmit power with relation to typical macro eNB and are typically used to cover indoor and outdoor hotspots. They are de- ployed in a coordinated way by operators similar to relay cells which are planned to be used by operators for cover^¬ age extension and are connected via a wireless link to an eNB.

Background of the invention

In the above described scenarios, where eNBs with different transmission powers are operated on the same carrier or at least on frequency bands which overlap, optimum cell selec^¬ tion for downlink and uplink is different due to the fact that there is a big difference in downlink transmission powers for macro nodes versus pico, femto and relay nodes (typical factor of 40 .. 400) and there is no difference in uplink transmission power in general independent whether a target node is a macro, a pico, a femto or a relay node.

Further, downlink based RSRP (reference signal received power) cell selection is not optimum since user competition is different for macro, femto, pico and relay cells and this is not taken into account.

Therefore, investigations have been carried out on this, with the conclusion that a more optimum cell selection with heterogeneous nodes (eNBs with different transmission power) is that UEs should transmit to the closest node (node with the lowest path loss between UE and node) and not to the strongest node (node with strongest received RSRP at UE) .

Benefits of this strategy are higher macro cell offloading, improved traffic balancing between macro, pico, femto and relay cells, a trade-off between uplink and downlink per- formance and further lower power usage in uplink with less interference for neighbour cells.

From those studies, a promising cell selection strategy for heterogeneous networks is to select a low power eNB (e.g. a hotspot covered by a pico cell, a relay or a femto cell) as soon as the UE can receive control signals of this cell. For LTE control channel design, if the received downlink SINR (signal to interference-plus-noise ratio) is better than about -6 dB, a low power eNB is selected or preferred over a wide area high power eNB.

Equivalent promising performance gains are achieved as well if the UE is served by the cell towards which it has the lowest path-loss. This can be realized with utilizing LTE Release 8 RSRP measurement and applying an offset to RSRP which is in proportion to the power difference of the dif^¬ ferent heterogeneous cells. The performance improvement of this approach is shown in Fig. 1 for a femto scenario with macro overlay network and an offset of 4 dB . The macro cell improvements are promising high and the losses for femto cells are (if any) negligible, especially due to the lower user competition. In LTE, for example, cell selection procedure is done in idle mode after UE cell search and is described in 3GPP specification TS 36.304 "UE procedures in idle mode" (current version 9.1.0) and in principle a UE is allowed to camp on a cell only if specific criteria is fulfilled and normally the maximum RSRP based cell association is used.

In summary, the procedure mainly relies on information about carrier frequencies and optionally cell parameters (SIB (system information block) Type 1 and 2) received and stored from previously detected cells (UE cell search pro^¬ cedure) . If no suitable cell is found using the stored in^¬ formation, the UE starts with the initial cell selection procedure .

In more detail, for evaluation if a cell is still suitable, Srxiev (cell selection receive level) is the criterion de^¬ fined and the criterion is fulfilled when the cell selec^¬ tion receive level is S_rxi_ev > 0. Parameters for the calcu- lation are:

• Qrxlevmeas which is the measured receive level value for this cell, i.e. RSRP as defined in 3GPP specification TS 36.331 "Radio Resource Control (RRC) specifica- tion", current version 9.1.0;

• Qrxievmin which is the minimum required receive level in a cell;

• Qrxlevminoffset, an offset to Qrxievmin that is taken into account for a higher priority PLMN (public land mo- bile network) while camped normally in a visitor PLMN mainly to avoid ping-pong;

• Pcompensation is a parameter dependent on the difference of maximum allowed UE power in a cell to the maximum power a UE is capable from UE power class perspective and the parameter is never lower than zero by definition;

With these parameters, S_rxi_ev is calculated according to the following equation:

^rxlev ^— rxlevraeas - (Qrxlevmin Qrxlevminoffset ) Pcompensation (1)

However, a problem of this maximum RSRP based cell selec^¬ tion in scenarios where there is a wide area overlay net- work with high power macro eNBs and underlying lower power eNBs is that it does not result in very good performance. More optimum performance is achieved if the above described RSPR plus offset method is applied. However, then it needs to be ensured that input to cell selection procedure also considers, in case of low power nodes, cells which are weak and would not be considered if they would be wide area cells . Summary of the Invention

In order to solve these problems, according to the present invention, there are provided a method, apparatus and com^¬ puter program product for heterogeneous cell selection as defined in the claims.

According to an aspect of the invention there is provided a method comprising:

detecting first cells via phase modulation on a refer- ence symbol (RS) ;

measuring a received power of the detected first cells and a second cell in a communication network, the second cell transmission power being larger than the first cell transmission power;

calculating a power difference between the received power of the second cell and first cells;

setting a parameterized power difference offset value between the first cells and the second cell; and

selecting, if the calculated power difference is posi- tive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than the power difference offset value. According to further refinements of the invention as defined under the above aspects,

the detecting of the first cell includes using a de^¬ fined phase relation between a primary synchronization sig- nal (PSS) and/or a secondary synchronization signal (SSS) relative to the RS and/or PSS relative to RS or a defined phase relation between PSS and SSS and RS, and

the method further comprises

reserving at least one primary synchronization signal (PSS) code for the first cells,

reserving a predetermined cell ID range for the first cells, wherein

the first cell is a low power cell like a pico, femto or relay cell having a low transmission power node like a LeNB (local eNodeB) or a HeNB (Home eNodeB) ,

the second cell is a high power cell like a macro cell having a high transmission power node (wide area high power eNB) in a LTE (Long Term Evolution) or a LTE-A (LTE- Advanced) communication network, and

the control channel is a wide area overlay broadcast control channel (BCCH) .

According to another aspect of the invention there is provided an apparatus comprising:

a detecting unit configured to detect first cells via phase modulation on a reference symbol (RS) ;

a measuring unit configured to measure a transmission power of the detected first cells and a second cell in a communication network, the second cell being larger than the first cells;

a calculating unit configured to calculate a power difference between the first cells and the second cell; a setting unit configured to set, based on network signaling, a parameterized power difference offset value between the first cells and the second cell; and

a selecting unit configured to select, if the calcu- lated power difference is positive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than the power difference offset value. According to further refinements of the invention as de^¬ fined under the above aspects,

the detecting unit is further configured to use a de^¬ fined phase relation between a primary synchronization signal (PSS) and/or a secondary synchronization signal (SSS) relative to the RS and/or PSS relative to RS or a defined phase relation between PSS and SSS and RS to indicate the first cells, andthe apparatus further comprises

a reserving unit configured to reserve at least one primary synchronization signal (PSS) code for the first cells,

a reserving unit configured to reserve a predetermined cell ID range for the first cells, wherein

the control channel is a wide area overlay broadcast control channel (BCCH) .

According to a still further aspect of the invention there is provided a computer program product including a program for a processing device, comprising software code portions for performing the steps of the methods as defined above when the program is run on the processing device. According to a still further aspect of the invention there is provided a computer program product as defined above, wherein the computer program product comprises a computer- readable medium on which the software code portions are stored .

According to a still further aspect of the invention there is provided a computer program product as defined above, wherein the program is directly loadable into an internal memory of the processing device.

According to still another aspect of the invention there is provided an apparatus comprising:

detecting means for detecting first cells via phase modulation on a reference symbol (RS) ;

measuring means for measuring a transmission power of the detected first cells and a second cell in a communica^¬ tion network, the second cell being larger than the first cells ;

calculating means for calculating a power difference between the first cells and the second cell;

setting means configured to set, based on network sig^¬ naling, a parameterized power difference offset value be^¬ tween the first cells and the second cell; and

selecting means for selecting, if the calculated power difference is positive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than a power difference offset value . Brief Description of the Drawings

These and other objects, features, details and advantages will become more fully apparent from the following detailed description of embodiments of the present invention which is to be taken in conjunction with the appended drawings, in which:

Fig. 1 is a table illustrating uplink and downlink perform- ance with improved cell selection method;

Fig. 2 is a block diagram showing an apparatus which may be a user equipment of a communication network according to an example of the present invention.

Fig. 3 is a flow chart illustrating a method of cell selec^¬ tion according to an example of the present invention.

Detailed Description

In the following, embodiments of the present invention are described by referring to general and specific examples of the embodiments. It is to be understood, however, that the description is given by way of example only, and that the described embodiments are by no means to be understood as limiting the present invention thereto.

According to an embodiment of the present invention, in order to ensure proper input for cell selection procedure (adapt output from cell search procedure according to eNBs power class) and provide a battery consumption efficient and early method for distinguishing between lower power (femto, pico or relay) cells and high power (macro) cells, the network configures UEs with one of the following meth- ods where the configuration signalling and information can be conveyed over the wide area cells via the BCCH (broad^¬ cast control channel) and can also include the details of the configured distinction method.

A) According to a first method, one (or two) PSS (primary synchronization signal) code(s) for low power cells are reserved and a parameterized power difference (difference be^¬ tween high power and low power cells) is signalled. With this, the low power cells can be identified already in the first stage of the UE cell search. UEs can then avoid going into above described detailed evaluation steps of cell search only if the received RSRP power plus a parameterized power difference is lower than the used threshold for wide area cell selection.

This method is particularly beneficial in cases where there is a need for a large number of low power cells and the number of utilized low power cell IDs is, e.g. close to 168 (1 reserved PSS for low power cells) or greater than 168 and less than 337 (ideally close to 337, i.e. 2 reserved PSS codes for low power cells) .

B) According to a second method, a certain (e.g. consecu- tive) range of cell IDs for low power cells is reserved and the reserved range and the power difference is signalled via wide area overlay BCCH either globally for all low power IDs or for each low power ID specifically. In this method, a cell type (= eNB power class) is identified via a PSS and SSS (secondary synchronization signal) pair, thus a full cell search is required.

This method would be beneficial in cases where the number of low power cells and the required number of lower power cell IDs is relatively small i.e. much smaller than 168. Another scenario is the case that the number of low power cell IDs needs to be larger than 168 but less than 337, be^¬ cause the remaining IDs are required for other purposes, i.e. it is neither possible to reserve only one PSS code

(too few IDs) nor to fully reserve two PSS codes (too many IDs, or more specifically too few IDs remaining for other issues) . In this case it is already possible to determine the cell type for two of the three PSS codes. Only for the third PSS, also the SSS needs to be investigated, so the extra complexity of detecting the SSS is only required in a subset of the cases. This can be used during deployment, i.e. the "unique" PSS should be prioritized (used more of^¬ ten) when assigning PSS and SSS to cells during cell con- figuration.

C) According to a third method, a parameterized power dif^¬ ference (difference between high power and low power cells) is signalled on a wide area overlay BCCH and a low power cell is detected via a phase modulation on the RS (refer^¬ ence symbol) or via a different scrambling RS sequence.

In this method, a low power cell can be detected after cell search via hypotheses testing on the RS . The advantage of this method is that all 504 cell IDs (as much many as the current number of all cell IDs) can be provided for low power cells without additional or reserved PSS/SSS codes. It can be beneficial when the required number of cell IDs for low power cells is greater than 336. Alternatively, only a certain range of PSS/SSSs can be used for low power cells in which case, the blind detection on the RS would be executed only for these specific PSS/SSS pairs to check if this cell is a low power cell or a conventional macro (high power) cell, while all other PSS/SSS pairs would indicate a macro cell.

D) According to a fourth method, a parameterized power dif- ference (difference between high power and low power cells) is signalled on a wide area overlay BCCH and a defined phase relation between PSS and/or SSS relative to RS and/or PSS relative to RS or a defined phase relation between PSS and SSS and RS is used to indicate a low power cell.

Using a defined phase relation between the PSS and SSS is similarly as transmit diversity is indicated in UMTS where a defined phase relation between PSC and SSC relative to PICH is utilized (these correspond to PSS, SSS and RS for LTE) .

It is noted that any combination of the above first to fourth methods A to D may be employed. For example, a combination of method C and/or D with method A and/or B, i.e. a combination of phase modulation of the RS which gives a totally new PCI (physical cell identifica^¬ tion) range for low power nodes with "PCI pooling" (matching different ranges of new PCI with different transmission power offsets related to the macro transmission power) may be employed. This would in general give the possibility for the UE to know the path-loss to cells that it may camp on, even if nodes with a different transmission power co-exist. It is an advantage of the above described methods that is ensured that detected low power cells from cell search are included in the set of detected cells even if they are weak and would not be considered if they would be high power cells which results in a more optimum cell selection. In more detail, low power cells that would currently not be considered for evaluation of above described detailed cell search equation are equipped with an offset broadcast on wide area BCCH. With above described methods A to D or a suitable combination thereof, low power cells are identi^¬ fied as soon as UE cell search procedure is done and the offset can be applied to low power cells to ensure that these cells are evaluated in cell selection procedure even if their power would be already to low if they would be high power (wide area) cells.

In the following, a specific example of the invention will be described with reference to Figs 2 and 3. Fig. 2 is a block diagram showing an apparatus which may be a user equipment of a communication network according to an example of the present invention.

Fig. 3 is a flow chart illustrating a method of cell selec- tion according to an example of the present invention.

As shown in Fig. 2, the apparatus like, e.g. a user equip^¬ ment, comprises a detecting unit 11, a measuring unit 12, a calculating unit 13, a setting unit 14 and a selecting unit 15.

The detecting unit 11 detects low power cells (first cells, e.g. pico, femto or relay cells) via a phase modulation on the reference symbol (RS) (step Sll) . For example, the phase modulation on the RS may be relative to the PSS (pri^¬ mary synchronization signal) , the SSS (secondary synchronization signal) or the phase of data symbols on the broad^¬ cast control channel. In such a case, the power difference can be detected after cell search via hypothesis testing on the reference symbols e.g. decoding the broadcast control channel using RS under the assumption that the reference symbol was modulated with symbol 1 or symbol -1 before transmission. Further the RS sequence can be modulated with different pseudo random sequences resulting in different RS sequences for indicating low power cells or power levels of low power cells. However, the operation of the detecting unit is not limited to this but the detecting unit 11 may also detect the low power cells via a different scrambling RS sequence.

The measuring unit 12 measures the received power of the low power cells which have been detected by the detecting unit 11 and also measures the received power of a high power cell (second cell, macro cell) (step S12) . The low power base stations are located under an overlay wide area network with high power base stations. The pico, femto or relay cells are operated on the same frequency layer or at least on an overlapping frequency layer as the macro cells.

The calculating unit 13 calculates a power difference be^¬ tween the measured received power of the high power cells and the measured received power of the low power cells which have been measured by the measuring unit 12 (step S13) . The setting unit 14, which is configured by network signaling, sets a parameterized power difference offset value between the high power cells and the low power cells (step S14) . Then, the selecting unit 15 selects, from the detected cells, the low power cell with the highest re- ceived measured power, if the calculated power difference to the high power cell with the highest received power is positive and lower than the power difference off^¬ set/threshold value (e.g. 4 dB) (step S15) . Otherwise, the strongest received high power cell the UE receives is se^¬ lected .

According to a further example of the present invention, the detecting unit 12 may use a defined phase relation be^¬ tween a primary synchronization signal (PSS) and/or a secondary synchronization signal (SSS) relative to the RS and/or PSS relative to RS or a defined phase relation be^¬ tween PSS and SSS and RS to identify the low power cells.

The apparatus may further comprise a reserving unit which is configured by a network entity (e.g. a LTE eNB) that re^¬ serves at least one primary synchronization signal (PSS) code for identifying the low power cells or that reserves a predetermined cell ID range for identifying low power cells .

The present invention is particularly applicable to LTE environments and self organizing networks (SON) . In such a case, the first cell is a low power cell like a pico, femto or relay cell having a low transmission power node like a LeNB (local eNodeB) or a HeNB (Home eNodeB) , the second cell is a high power cell like a macro cell having a high transmission power node (wide area high power eNB) in a LTE (Long Term Evolution) or a LTE-A (LTE-Advanced) communica^¬ tion network, and the control channel is a wide area over^¬ lay broadcast control channel (BCCH) .

In the foregoing exemplary description of the user equip- ment, only the units that are relevant for understanding the principles of the invention have been described using functional blocks. The user equipment may comprise further units that are necessary for their operation as user equipment, respectively. However, a description of these units is omitted in this specification. The arrangement of the functional blocks of the devices is not construed to limit the invention, and the functions may be performed by one block or further split into sub-blocks. For example, the measuring unit and the calculating unit can be combined into a single block performing the operations of both blocks, respectively.

For the purpose of the present invention as described herein above, it should be noted that

- method steps likely to be implemented as software code portions and being run using a processor at a network control element or terminal (as examples of devices, appara^¬ tuses and/or modules thereof, or as examples of entities including apparatuses and/or modules therefore), are soft^¬ ware code independent and can be specified using any known or future developed programming language as long as the functionality defined by the method steps is preserved;

- generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the embodiments and its modification in terms of the function^¬ ality implemented;

- method steps and/or devices, units or means likely to be implemented as hardware components at the above-defined ap- paratuses, or any module (s) thereof, (e.g., devices carry^¬ ing out the functions of the apparatuses according to the embodiments as described above, UE, eNode-B etc. as de^¬ scribed above) are hardware independent and can be imple^¬ mented using any known or future developed hardware tech- nology or any hybrids of these, such as MOS (Metal Oxide Semiconductor), CMOS (Complementary MOS), BiMOS (Bipolar MOS), BiCMOS (Bipolar CMOS), ECL (Emitter Coupled Logic), TTL (Transistor-Transistor Logic), etc., using for example ASIC (Application Specific IC (Integrated Circuit)) compo- nents, FPGA (Field-programmable Gate Arrays) components, CPLD (Complex Programmable Logic Device) components or DSP (Digital Signal Processor) components;

- devices, units or means (e.g. the above-defined appara- tuses, or any one of their respective units/means) can be implemented as individual devices, units or means, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device, unit or means is preserved;

- an apparatus may be represented by a semiconductor chip, a chipset, or a (hardware) module comprising such chip or chipset; this, however, does not exclude the possibility that a functionality of an apparatus or module, instead of being hardware implemented, be implemented as software in a (software) module such as a computer program or a computer program product comprising executable software code por^¬ tions for execution/being run on a processor;

- a device may be regarded as an apparatus or as an assem^¬ bly of more than one apparatus, whether functionally in co- operation with each other or functionally independently of each other but in a same device housing, for example.

It is noted that the embodiments and general and specific examples described above are provided for illustrative pur- poses only and are in no way intended that the present in^¬ vention is restricted thereto. Rather, it is the intention that all variations and modifications be included which fall within the scope of the appended claims.

Claims

1. A method comprising:

detecting first cells via phase modulation on a reference symbol (RS) ;

selecting, if the calculated power difference is posi^¬ tive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than the power difference offset value.

2. The method according to claim 1, wherein

the detecting of the first cell includes using a de^¬ fined phase relation between a primary synchronization signal (PSS) and/or a secondary synchronization signal (SSS) relative to the RS and/or PSS relative to RS or a defined phase relation between PSS and SSS and RS .

3. The method according to claim 1 or 2, further comprising reserving at least one primary synchronization signal (PSS) code for the first cells.

4. The method according to claim 1 or 2, further comprising reserving a predetermined cell ID range for the first cells .

5. The method according to any one of claims 1 to 4, wherein

the control channel is a wide area overlay broadcast control channel (BCCH) .

6. An apparatus comprising:

a selecting unit configured to select, if the calcu^¬ lated power difference is positive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than the power difference offset value.

7. The apparatus according to claim 6, wherein

the detecting unit is further configured to use a de^¬ fined phase relation between a primary synchronization signal (PSS) and/or a secondary synchronization signal (SSS) relative to the RS and/or PSS relative to RS or a defined phase relation between PSS and SSS and RS to indicate the first cells.

8. The apparatus according to claim 6 or 7, further comprising

a reserving unit configured to reserve at least one primary synchronization signal (PSS) code for the first cells .

9. The apparatus according to claim 6 or 7, further comprising

a reserving unit configured to reserve a predetermined cell ID range for the first cells.

10. The apparatus according to any one of claims 6 to 9, wherein

the control channel is a wide area overlay broadcast control channel (BCCH) .

11. A computer program product including a program for a processing device, comprising software code portions for performing the steps of any one of claims 1 to 5 when the program is run on the processing device.

12. The computer program product according to claim 11, wherein the computer program product comprises a computer- readable medium on which the software code portions are stored .

13. The computer program product according to claim 11, wherein the program is directly loadable into an internal memory of the processing device.

14. An apparatus comprising:

selecting means for selecting, if the calculated power difference is positive, from the detected first cells, the cell received with the lowest power difference, if the power difference is lower than a power difference offset value .