1. Network structure
The basic network structure of WCDMA has been described in the prior chapter. It is divided into core network and access network. This chapter introduces the structure features of UTRAN and some key technologies and network parameters affecting the radio network structure from the perspective of network planning.
Types of areas and the relationships among them
1) Types of areas include:
l Location Areas;
l Routing Areas;
l UTRAN Registration Areas;
l Cell Areas.
2) Relationships among the areas
The classification of location areas and routing areas in the WCDMA system is similar to that in the GSM and GRPS systems.
The Node B of WCDMA system supports omnidirectional, 31, 32, 34, 61, 62 and 64 (combined cabinet) cell configurations. The cell structure of WCDMA is similar to that of GSM with different titles. “Cell” is similar to the “BS” in the GSM, while “SECTOR” is equal to the cell in the GSM system.
For instance: What are 3*1, 3*2, 3*4, 6*1, 6*2 and 6*4?
The first digit is the number of sectors supported by each cell; the second digit is the number of carrier frequencies supported by each sector. 3*1 means the BS supports 3 sectors, each of which has one carrier frequency; 6*4 means the BS supports 6 sectors, each of which has 4 carrier frequencies.
The cell structure planning is to evenly provide high bit rate within the cell area, or the data rate of the cell boundary may be lower than that of the area near the BS, thus the cell area will be larger.
The number of cells is calculated according to the capacity and the link budget. A network may be coverage-limited or capacity-limited. Capacity limited means the maximum cell radius cannot support the total traffic flow. Then, the number of cells may be calculated according to the number of subscribers supported by the cell per sq.km. Coverage limited means there is enough capacity in the cell to support all the traffic flow. Then, the number of required BSs may be calculated according to the maximum cell area.
3. Hierarchical structure of the air interface
According to the protocol, the air interface may functionally form a hierarchical structure. From bottom to top, they are physical layer, link layer and network layer. The physical layer fulfills the coding, modulation and spread spectrum of the physical channel. The link layer may be subdivided into two sub-layers: Medium Access Control (MAC) and Link Access Control (LAC).The former determines the resources provided by the physical layer, while the latter completes the establishment, maintenance and release of the logic link connection. The network layer includes such functions as call control, mobility management and radio resource management.
4. Channel allocation and reconfiguration
The channel allocation includes the following types:
Connection-oriented channel configuration: Fundamental Channel Configuration (FRC) and Dynamic Channel Reconfiguration (DCCC)
Cell-oriented channel configuration: Cell code resource allocation, cell channel resource allocation and uplink scramble allocation
Fundamental channel configuration: Allocate the channel types and bandwidth according to the service request; and configure the parameters of each layer of the channel according to the QoS.
Dynamic channel configuration: During the communication, dynamically change the channel configurations according to current service status, including the channel types and the parameters of each layer of the channel
Cell channel resource allocation: Common channel is the resource in the cell, including RACH, FACH, DSCH and CPCH.
Cell code resource management: Cell downlink code resource allocation policy and code resource maintenance.
Uplink scramble allocation: The uplink scramble includes the scramble reserved for the common channels RACH and CPCH, and the scramble allocated to the UE with dedicated channel.
If RRC is connected or RAB sets up the request, the fundamental channel configuration entity determines the channel type according to the service type and rate requirements, and configures parameters of each layer of the channel according to the QoS. It makes a request to the call admission control entity for admission control according to the channel configuration parameter (QoS).If it is permitted, then go ahead; or the process of the channel setup is failed.
The dedicated channel is used to allocate the uplink scrambles; while the cell code resource maintenance entity is used to allocate the downlink channel code. The common channel is used to allocate the common channel parameters.
If the channel is set up successfully, the dynamic channel configuration entity will monitor the traffic flow for the specific service, and make dynamic adjustment to the channel parameters.
The cell channel resources are allocated according to the current cell service and the load conditions, so as to adjust the resource configuration of the cell common channel and optimize the system performances. The cell code resource management entity is used to maintain the cell code resources.
7.6.2 Hierarchical Network Structure
1. Basic concept of the hierarchical network structure
Similar to the GSM system, the cell may be divided into macro cell (umbellate cell), micro cell and pico cell according to the features and scope of the cell services. The macro cell, micro cell and pico cell form a hierarchical network structure (HCS).
The third generation mobile communication system should be able to support the services with wide coverage in various radio operation environments. The cell type varies with the requirements: Continuous coverage should be ensured for the large cell, while the small cell needs high spectral efficiency and capacity. The cell with small coverage is used to the terminal with low mobility and high capacity, while the cell with large coverage is used to the terminal with high mobility and low capacity. In addition, cells should be able to operate on other cells of different types. The coverage area of micro cell is hundreds meters, while the coverage area of macro is one or just over one kilometers. In the rural area, it can provide the micro cell with continuous coverage and the fast mobile subscribers with services. The pico cell covers an indoor scope with the radius of several meters. The satellite cell provides global continuous coverage. The traffic should be based on the minimum available cell.
There are two methods to design multi-layer cell in the CDMA system: Different hierarchical cells operating on the same band, or different hierarchical cells operating on the different bands. The multi-layer structure also can be applied to the multi-operator environment.
2. Micro cell and macro cell with the same frequency
The frequency reusability factor is 1. The processing gain of the system enables subscribers to bear the interference from the cells of different layers. The intra-layer interference is controlled by the power control, while the inter-layer interference is controlled by the spatial isolation. Generally, the attenuation of the micro cell is larger than that of the macro cell, because its antenna is lower. Soft handover can offset the attenuation valley of the boundary of micro cell.
3. Micro cell and macro cell with different frequencies
It is easy to manage when the cells of different layers adopt different frequencies, because there is no interference among the layers. The disadvantage of this method is that it needs large spectrum. At least 15 MHz bandwidth is required if the WCDMA system is divided into three layers. The impact on the total spectral efficiency from the non-linear power amplifier depends on the neighboring channel interference and the link performance deterioration. The increase of the neighboring channel interference will reduce the spectral efficiency.
Although different carrier frequencies are adopted in the multi-layer cells, interference will occur between neighboring channel carriers if the capacity is high.
4. Antennas selection and parameters setting of the hierarchical network
Similar to the GSM system, the traffic of the hierarchical network should be deployed as much as possible to the cell with minimum coverage area; that is, the macro cell is used to satisfy the requirements of the system for wide coverage, while the micro cell and the pico cell are used to absorb the traffic and the data traffic.
For this purpose, as to the engineering parameters setting, the antenna and the transmit power of the macro cell are high; while the antenna and the transmit power of the micro cell are low; as to the software parameters setting, it is easier for the MS to access the micro cell and the pico cell. Most of the data traffic is convergent in the pico cell, so higher QoS of the pico cell should be ensured for higher service rate.
7.6.3 Mobility Management
1. Cell selection and re-selection
1) MS status
According to the protocol, five statuses are available for the MS (UE): IDLE, CELL_DCH, CELL_FACH, CELL_PCH and URA_PCH.
In the CELL_DCH status, the cell-crossing is judged by the measurement report, and the location is updated by the handover process.
In the CELL_FACH and CELL_PCH statuses, the cell-crossing is judged by the UE cell reselection, and the location is updated by CELL UPDATE.
In the URA_PCH status, the cell-crossing is judged by the UE URA, and the location is updated by URA UPDATE.
2) Mobility management policy in the IDLE status
When the UE starts up, it will carry out PLMN selection, cell selection and location registration.
Upon the completion of the cell selection, the cell re-selection will be carried out. If a new cell is selected to stay, the location registration will be carried out for the new location area entry.
If access and immediate cell evaluation are required, then initiate the access in the optimized cell.
If the MS is in the CELL_DCH status, the UE crosses the cell through the handover flow.
3) Potential subscriber control
The stay cell selection by the MS is determined by adjusting the parameters of the cell reselection, so as to adjust the cell load direction and achieve the load self-adaptive adjustment.
2. Random access procedure
Random access procedure is a process: A MS requests the access system, then the network responses and allocates a service channel to the MS. The random access is carried out when the MS begins to transmit power; or when synchronization loss for some reasons occurs; or when message packets should be transmitted. The random access is fulfilled after following steps are completed: 1) synchronization between the code and frame; 2) search for the cell parameters, such as the random access code; 3) evaluation of the downlink path loss and random access to the initial power level.
The optimal criterion for the random access procedure is the process rate and the low transmission power. The requirement for the random access procedure speed is determined by the requirement for the initial synchronization time. The number of access channels is depending on the involved access load. In addition, it also will be affected by the information transmitted in the random access status. Too high transmission power will reduce the capacity of the CDMA system, and the transmit power in the random access status cannot be controlled by the fast closed loop power control, so it is most important to make the total transmit power in the random access status minimum. If the initial transmission power is lowest, there is a long time for the access attempt. On the other hand, the high transmission power in the initial access will cause interference to other subscribers during the fast synchronization. The least information that needs to be transmitted in the random access attempt is the identity numbers of the MSs of some types. A kind of typical random access information includes the pre-field, the synchronization and the data. The data should include at least the MS identity number, while the pre-field is the unmodulated wideband spread spectrum signal.
3. Call Admission Control
Call Admission Control is a part of the load management.
The call admission control algorithm is used to accept new calls as many as possible on the basis of ensuring the existing QoS. Its principle is: Current Status of the Cell Resources + Service Request Þ YES/NO.
The current status of the cell resources is depending on the uplink interference and the downlink load; while the requested service is depending on the QoS.
4. SNRS migration
The structure shown in the left figure may occur for some reasons, such as the handover, cell update, URA update, RRC reconnection and direct retry. To save Iur interface resource and reduce the time delay, it is required to migrate the Iu interface as shown in Figure 7-8, that is, SNRS migration. The SNRS migration may effectively reduce the traffic of the Iur interface and improve the adaptability of the system.
7.6.4 Factors Affecting the Network Structure
1. Universal antenna and smart antenna (narrow-beam antenna)
Smart antenna is widely used in the WCDMA system. In some relevant articles, it is also called self-adaptive antenna or narrow-beam antenna.
The smart antenna adopts the concept of SDMA, monitoring and extracting the space information of every subscriber via the self-adaptive array antenna. It separates the signals of different directions without any interference according to the differences of the antenna array in the incident signal direction. In fact, it makes the communication resources are no longer restricted by the time domain, the frequency domain and the code domain, and extended to the space domain.
The advantages of the smart antenna are: The result of antenna beam forming is equal to increase the gains of the antenna; the antenna beam forming algorithm may take the multi-path transmission into consideration, avoiding the multi-path communication from affecting the digital wireless communication system with the performance enhanced; the antenna beam forming greatly reduces the multiple access interference. In this way, the communication capacity can be expanded in multiple. It also can improve the channel multiplexing rate of the communication system and the BS coverage area, and solve the increasingly serious interference problems (like common channel and multi-path fading). In addition, it can optimize the network structure.
2. The transmission models of GSM, CDMA and WCDMA and the affects of radio transmission on the system structure
From the perspective of signal transmission, in the same frequency band range, the signals of the GSM, CDMA and WCDMA transmitted in the space have the same features, including the path loss, the slow and fast fading from the transmitter to the receiver.
However, the transmission bandwidth in the WCDMA reaches 5M or more, so the multi-path fading performance is powerful. Its signal frequency band is far larger than the relevant bandwidth of the channel. The multi-path components may be separated, making fully use of the multi-path diversity receiving technology.
The network planning structure is realized by setting proper network engineering parameters and the network functional parameters. Different antenna gains, antenna heights, antenna types, network connection parameters, power control parameters, handover parameters and service rates are set for the cells of different layers of the hierarchical network.
4. Power control
The power control plays an important role in the CDMA network performance and the network capacity.
The maximum cell coverage is determined by the link budget. Besides the data rate and the Eb/N performance, such specific factors as the cable loss, the antenna gain and the receiver noise should be calculated. In addition, the affects of the soft handover gain and asymmetric traffic should be taken into consideration. Different service coverage area has different service rate requirements. The design basis of the hierarchical network is as follows:
Outdoor in the rural: Terminal speed 250 kmph, 144 Kpbs at least, 384 Kpbs optimal
Outdoor in the urban or suburb: Terminal speed 150 Kmph, 384 Kpbs at least, 512 Kpbs optimal
Indoor or outdoor with a small area: Terminal speed 10 Km, 2 Mbps at least.
Real-time fixed time delay: BER--, time delay 20-300 ms.
Non-real time variable time delay: BER--, time delay 150 ms.
Erl/km2 may be adopted for in a geometrical area. The data traffic may be Mbps/km2.
The BS adopts multi-subscriber detection technology to provide a favorable coverage and lower the transmit power of the MS. The increase of the data rate will reduce the coverage area of the uplink. It differs from the narrowband system.
7.6.5 Radio Resource Planning
1. WCDMA frequency resource
The W-CDMA spectral efficiency is related to the link performance. According to the theoretical analysis and the emulation, the uplink capacity is 2 to 2.5 times of the downlink capacity. Besides the antenna diversity in the BS, the uplink adopts the multi-subscriber signal detection technology, which provides almost two times capacity than the common receiver. In the downlink, two BSs transmit to the same MS the signals, which are not orthogonal, but only cause multi-path diversity. For the spectral efficiency, the bandwidth of 15- to 20-MHz is required for each cell to support an effective 2-Mbps subscriber.
2. Relationship between the resource planning and the network structure
The WCDMA carrier interval is 200 KHz, ranging from 4.2 MHz to 5.4 MHz. The carrier intervals are adopted according to the interference to obtain proper protection for the neighboring channels. The bandwidth of 15 MHz may be divided for the use of three cells. The interval between different operators may be longer to avoid interference among them.