7.6.1 Basic
Network Structure
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.
2. Cell structure
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
Of
which:
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.
3. Parameters
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.
5. Coverage
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.
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