Concentric Cell1 OverviewWith the development of network, the number of subscribers is on the increase, and the contradiction between insufficient frequency resources and great demand becomes more and more prominent.To improve the system capacity, the dense frequency reuse technology should be used so as to improve the spectrum efficiency. However, the dense frequency reuse will increase radio interference considerably and will possibly exert great influence upon the communication quality. Therefore, in the dense frequency reuse pattern, the key problem to solve is how to avoid or reduce the radio interference so as to guarantee the voice quality. The Concentric technology divides ordinary cells into two service layer: big subcell and small subcell. For mobile phones within the big subcell, the frequencies of sparse frequency reuse will be preferably allocated (such as BCCH frequency), while for mobiles within the small subcell, the frequencies with dense frequency resue will be allocated as much as possible (such as non-BCCH frequencies). The system capacity can be efficiently improved by the dense reuse pattern of frequencies within the small subcell. As shown in the following figure, the mobile phone within the small subcell is quite far from the interference source, so although the dense reuse frequency is used, the voice quality can still be guaranteed. For the mobile phone within the big subcell, since the sparse reuse frequency is used, the voice quality can also be guaranteed.

Cell A Signal

Cell B Interference

Overlaid subcell

Underlaid subcell

Figure 1 Schematic diagram of dense frequency resue for Concentric Cell If the overlaid subcell is overlaid with the underlaid subcell or overlaid subcell of another cell, then the MS in the overlaid subcell can be directly handed over to the other cell. Thus, the congestion of the underlaid subcell can be efficiently mitigated. The Concentric technology has divided the cell coverage area into the overlaid subcell and underlaid subcell so that different frequency reuse pattern can be used for TRXs of the overlaid and underlaid subcells respectively. For an overlaid subcell, due to its small coverage, the dense frequency reuse pattern can be used, such as the 1*3 frequency reused Pattern. The sparse frequency reuse pattern

can be adopted for the underlaid subcell, such as the reuse pattern of 4*3. Due to the factors listed above, the Concentric technology, with tightly frequency reused pattern adopted for the overlaid subcell, can provide more network capacity than the MRP do. while guaranteeing the network quality. For example, suppose the underlaid subcell is only configured with one TRX (BCCH), and the 4*3 frequency reuse pattern is adopted. If all the other TCH TRXs are configured within the overlaid subcell with the 1*3 frequency reuse pattern, the Concentric Cell would be completely the same as the 1*3 cell, and the average frequency reuse degree would also be the same as that of the 1*3 reuse pattern. In this condition, the Concentric can efficiently reduce the interference upon the whole netwowrk and obtain better network quality than 1*3 without reducing the network capacity. By employing BCCH with high power amplifier,, the coverage of cell can be widened. However, the actual power provided by the BCCH TRX is greater than that provided by other TRXs, therefore, different TRXs will have different coverage distances. When configuring large base station, such as station type S6/6/6, since the Huawei antenna &feeder system uses the combining mode of CDU and SCU (or the mode of EDU+CDU as will be used in the future), combiners of different TRXs may have different losses. Thus, different TRX antenna feeder interfaces in the same cell will provide different powers, which leads to different coverage distances physically. However, the cell coverage is determined by the TRX with small coverage, so the cell coverage is greatly limited. With the application of the Concentric technology, the TRX with wide coverage can serve as the big subcell of the Concentric and the TRX with small coverage can serve as the small subcell so as to ensure the capacity at the near end of the site. Thuse, the cell radius is increased.

Overlaid subcell Underlaid subcell

Figure 2 Schematic diagram of wide Concentric Cell coverage The following table indicates the increased cell coverage area after the application of Concentric Cell in typical station type, The data is a theoretic one which is based on the analysis of the loss for different combining mode:Number of TRXs in the cell Combining mode Low-loss TRX loss High-loss TRX loss Coverage area increased after the applicatio n of Concentri c Cell 3 4, 5 4, 5 5, 6 CDU + CDU CDU + CDU + SCU CDU + CDU + CDU CDU + CDU + SCU 1.0dB 1.0dB 1.0dB 4.5dB 4.5dB 8.0dB 4.5dB 8.0dB 27% 60% 27% 27%

Table 1 Change of coverage after the application of Concentric Cell in typical station type

2 System ArchitectureThe Concentric Cell function mainly involves the radio resource management module and the handover decision, which is integrated with the existing radio resource management and handover decision functions of the modules. The background data controls whether the Concentric Cell function will be used and at the same time controls relevant operational parameters. Duirng the call setup, the Concentric Cell channel allocation controls the allocation of radio channels of appropriate Concentric layer (overlaid subcell or underlaid subcell) to different calls and the handover decision controls the handover of calls between the overlaid and underlaid subcells. Thus, it is guaranteed that the appropriate Concentric layer will provide appropriate services all along during the call process.OMC MSC

DB

OS

AIR

Channel Allocation of UO cell RR RRM

BTSM

ABIS BSSOMAP BSC LAPD UnderlayOverlay HO

BTS

Figure 3 The position of Concentric Cell in the BSC software system

3 System Function3.1 Interface Function of Concentric CellFrom the background, the user can control whether the Concentric Cell function will be used and can configure relevant parameters. Also the traffic index after the

Concentric is in service can be obtained to guide the adjustment of network optimization parameters.

3.2 Technical Characteristics of Concentric CellThe division of Huawei Concentric Cell into overlaid and underlaid subcells is based upon the downlink receiving level and timing advance of mobile phones (as shown in the following figure). Both the receiving level threshold and the timing advance threshold can be configured in the background. Therefore, the borders of the overlaid and underlaid subcells can be adjusted flexibly so that the overlaid and underlaid subcells can share the traffic reasonably under the precondition of network performance.

Figure 4 Division of Concentric Cell into overlaid and underlaid subcells

3.2.1 Channel Allocation Technology for Concentric CellThe channel allocation technology for Concentric Cell means that different allocation policies are used in different channel allocation conditions with full consideration of the characteristics of the Concentric Cell, which mainly includes the following conditions: 1. Instant assignment Without receiving level and TA for reference when the instant assignment is carried out, . the SDCCH channel on the underlaid subcell will be allocated preferably to guarantee the QoS,. Only when there is no available signaling channel on the underlaid subcell will the signaling channels on the overlaid subcell be allocated. 2. Assignment The Concentric channel allocation policy is used. When in the overlaid subcell, a subscriber should be allocated with the overlaid subcell channel, or with underlaid subcell channel only when overlaid subcell channels is not available.. Similarly, when in the underlaid subcell, a subscriber should be allocated with the underlaid subcell channel, or with the overlaid subcell channel only when there is no available underlaid subcell channel. Thus the appropriate service layer will provide the subscriber with quality service. 3. Intra-BSC Handover

It is applicable to non-Concentric handover and direct handover of overlaid subcell to the neighboring cell. The intra-BSC handover adopts the Concentric channel allocation policy so that the appropriate service layer will provide service for mobile phones handed over. 4. Inter-BSC Handover Since the receiving level and TA of the neighboring cell cannot be obtained, the inter-BSC handover selects the following modes through switch: preferably selecting the underlaid subcell, preferably selecting the overlaid subcell, or without policy.

3.2.2 Concentric Cell Handover TechnologyThe Concentric handover decision technology expands Huawei handover algorithm so that the BSC can guide the traffic intelligently for the reasonable utilization of the frequency resources. Based upon the existing handover algorithm, the Concentric Cell handover technology has been added with the Concentric handover decision function to implement the ordinary Concentric technology. When a mobile phone is crossing the border of the underlaid and ovrlaid subcells, the Concentric handover can be initiated so that the mobile phone can set up call connection in an appropriate service layer. Yet if the handover target layer is congested, the handover will not be initiated. For example: a handover from the Overlaid subcell to the Underlaid subcell needs to be initiated according to the handover decision. If upon checking it is found that there is no idle channel for assignment in the Underlaid subcell, then the handover will not be initiated.

4 Technical ParametersPlease refer to the following table for the data configurations and descriptions related to Concentric Cell:Data table Data item Meaning Value range and unit Suggestions on value selection Can dynamic setting be made, command word

Handover/IC HO data table

U/O Signal intensity difference

The difference of powers between the overlaid and underlaid subcells and the difference of path losses between them cause different signal strength mobile phones receive. The

0 to 63(dB)

Power difference between overlaid and underlaid amplifiers + Difference between combiner insersion losses + Difference

It can be dynamicall y set. The command word is Configure handover data and the parameter is the cell ID.

parameter indicates the value for compensating the powers of the overlaid and underlaid subcells.

between path losses caused by different antennas + Difference between path losses caused by different frequency selectivities: Please read the value onsite and select multiple measurement points if the overlaid and underlaid subcells use different antennas.

Rx level threshold

The receiving level threshold, the receiving level hysteresis, TA threshold and TA hysteresis work together to determine the area of the overlaid and underlaid subcells.

0 to 63 (dBm)

Edge handover threshold + Signal strength difference between overlaid and underlaid subcells

It can be dynamicall y set. The command word isConfigur e handover data and the parameter is the cell ID

Rx level hysteresis

The receivinglevel hysteresis, the receiving level threshold, TA threshold and TA hysteresiswork together to

0 to 63(dBm)