ZTE´s Strategy of WCDMA Radio Resources Management

Release Date:2005-09-21  Author:Zhang Baosong, Wang Yong  Click:

Embedded with various advanced algorithms and concepts, WCDMA Radio Resources Management developed by ZTE presents excellent performance in efficient coverage, capacity, QoS and the band utilization ratio.
ZTE´s RRM solution includes power control, handover control, load control, access control and dynamic channel allocation.

WCDMA Radio Resources Management (RRM) is in charge of the allocation and utilization of the whole resources of air interfaces. Its goal is to obtain the maximal coverage and largest wireless capacity on the condition that Quality of Service (QoS) is guaranteed. RRM has a great influence on the performance of a WCDMA system, and is the soul of system control. RRM includes power control, handover control, load control, access control and dynamic channel allocation.

    ZTE Corporation has made a comprehensive and deep research on WCDMA RRM and got fruitful results. These achievements help implement the stable performance of the WCDMA system. ZTE´s RRM solution will be discussed in this article.

1 Power Control
The WCDMA network is an interference-limited system. Due to the Far-Near Effect, its system capacity is limited to the interference of each mobile phone and Node B.  Therefore, if the signal of each mobile maintains the minimum Signal-to-Interference Ratio (SIR) needed for quality communication when it comes to the base station, the capacity of a WCDMA system will be up to the maximum. Power control is the technology to overcome the Far-Near Effect. Based on the appraisal of energy and SIR of received signal, this technology equalises the fading occurring in wireless channels and thus not only maintains the high quality of communication but also won´t produce extra interference on the system. In a WCDMA system, three types of power control exist: open-loop power control, inner-loop power control and outer-loop power control.

1.1 Open-loop Power Control
The basic principle of open-loop power control is to suppose that the fading of up/down link is close. A mobile station obtains the transmitting power of a system from the broadcast message of the system, measures its received power, calculates the path loss in the transmitting process, and thus gets the transmitting power of uplink. Such a method would be much too inaccurate. The prime reason for this is that the fast fading is essentially not correlated between the up- and downlink, due to the large frequency separation of the uplink/downlink bands of the WCDMA Frequency Division Duplex (FDD) mode.

    Figure 1 shows the simulation testing graphs of open-loop power control in ZTE´s WCDMA system. (the graph on the left is without open-loop power control). It is seen that the open-loop power control is able to provide a relatively accurate power setting of the mobile station at the beginning of a connection, improve the ratio of convergence, and effectively reduce the impact on system load.

1.2 Inner-loop Power Control
Inner-loop power control is a closed-loop power control. In the open-loop power control, the adjustment of transmitting power of the mobile station is based on the strength of the forward channel. Since the up- and downlink are not correlated, open-loop power control is only a preliminary adjustment on transmitting power. To equalize the power for the fast fading channel, inner-loop power control is needed. It is also called fast
power control.

    Inner-loop power control is divided into inner-loop power control of the uplink and that of the downlink. In the former, the mobile station adjusts its transmitting power according to the power control command sent by the Node B. The base station frequently measures the received SIR and compares it to a target SIR. If the measured SIR is higher than the target SIR, the base station will command the mobile station to lower the power. In reverse, it will command the mobile station to increase its powers when the measured SIR is too low.

    The instruction of power control of Node B is transmitted through the power control bit. When it is 1, the mobile station is asked to increase the transmitting power; and when it is 0, to lower transmitting power. The power adjustment can be 1 dB or 2 dB according to different algorithms.

    Inner-loop power control is a method of fast power control, at the rate of 1 500 times per second (1.5 kHz) for each mobile station. Therefore, it has a more obvious effect than that of the CDMA and GSM systems.
With many simulation tests, the Radio Network Control (RNC) of ZTE Corporation is proven to have the excellent performance of power control. The fluctuation of power is within the scope of 0.25 dB and thus increases
10-15% of the system capacity.

1.3 Outer-loop Power Control
Outer-loop power control is a slow, closed-loop power control. Its goal is to select a suitable target SIR for
closed-loop power control and guarantee the quality of communication required. The SIR should be neither too high (which wastes system capacity) nor too low (which lowers the quality of communication). Both the uplink and the downlink utilize the inner-loop power control, as well as the outer-loop power control.

    Outer-loop power control is the power control on physical layer, which receives the Block Error Rate (BLER) through Cyclic Redundancy Code (CRC) verification and statistics, compares it with the target value and modifies the SIR of inner-loop power control.

    The basic flow of outer-loop power control is illustrated in Figure 2.

    Outer-loop power control of ZTE Corporation can make the WCDMA system come to the theory maximal value, on the premise of guaranteeing the quality of communication.

2 Handover Control
Handover control is a basic function of the WCDMA system and plays an important part in the QoS guarantee. When the quality of calling drops due to the mobility of the mobile station or the changes of wireless environments, the original wireless channels must be transferred into new free ones to protect the process and quality of communication.
There are two types of handover: soft handover and hard handover. Soft handover is carried out in the same frequency, keeps the links simultaneously connected with multiple Node B´s, and implements the seamless handover. Hard handover has to disconnect the original base station before establishing a new connection. As it is difficult to immediately get a new connection, hard handover will impair the quality of calling to some extent.

    With regard to handover control, ZTE´s WCDMA system has the following strategies:

  • Trying to avoid handover, especially the inter-system handover, since soft handover will consume large system resources.
  • Preferentially using soft handover when the handover is necessary, since softer handover can provide better quality of communication.
  • Using the one-directional handover when 3G handovers to 2G, to avoid the ping-pong effect.
  • Coming back to the WCDMA network through cell reselection when 2G handovers to 3G.

3 Load Control
In the process of system measurement, it is found that the cell load may occur over the threshold in one period and comes to the unstable status. At this moment, the load control is needed. The basic principle of load control is to allow the maximal service access on the premise that the system is running stable, in order to get a high-efficient operation.

    When system load is near or over the system threshold, the load control module of the system will be launched.

  • Load control of the downlink will refuse accepting the mobile´s instruction of increasing downlink power;
  • Load control of the uplink will lower the target value of Eb/No used in the uplink fast power control;
  • Lowering data throughput of packet service is mainly implemented through lowering service rate;
  • Handover to carrier frequency of other WCDMA;
  • Handover to other Radio Access Technology (RAT) system;
  • Decreasing the rate of the real-time service, such as Adaptive Multi-Rate (AMR) voice rate;
  • Carrying out dropping call operation.

    Actually, the load control of both the uplink and downlink are implemented in Node B. The transmitting power of each User Equipment (UE) may be decreased by decreasing the target value of receiving Eb/No in each channel. This decreases the total receiving power of the base station and alleviates the pressure of the uplink load. When the actual transmitting power is close to the allowed maximum value, the transmitting power of each downlink channel will be decreased to screen the requirement of power increasing. For services in Circuit Switching (CS) domain, service data decreasing will impair the quality of communications, while for the services in Packet Switching (PS) domain, users will be aware of the changing of data communication.

    Figure 3 shows the performance simulation of dynamic adjustment of AMR voice rate in load control of ZTE´s WCDMA system. It is shown that through load control the system coverage gain is increased by 2.2 dB and coverage area is by 30%.

4 Access Control
If there were no mechanism to limit the access of subscribers, the air interfaces would be overloaded. Furthermore, the coverage of cell would be decreased greatly comparing with the planning area, and the quality of services accessed would be difficult to provide.

    Therefore, before a subscriber accesses the WCDMA system, a measure is necessary to be taken to verify whether such an access will impair the performance of the network and then decide to permit or refuse the request of this access. This function is implemented in the access control module.

    Access control in the WCDMA system is defined in two algorithms: access control based on power and access control based on throughput.

     (1) Access Control Based on Power
    Access control based on the power of the uplink is:
I Total_old +ΔI >IThreshold 
    where I is the receiving power of Node B.
Then, the uplink is:
PTotal_old +ΔP >PThreshold
where P is the transmitting power of Node B.

    (2) Access Control Based on Throughput
Access control based on the throughput of the uplink is:
ηUL+ΔL >ηUL_Threshold 
Then, the uplink is:
ηDL+ΔL >ηDL_Threshold

    Where ηUL, ηDL are the load factors respectively in the uplink and the downlink before access, ΔL is the load factor of the new subscriber.

    In ZTE´s WCDMA system, access control is divided into new subscriber access and handover access. The latter access is prior to the former one. In actual test, the number of accessed 12.2 kb/s voice subscribers is up to 123, close to the theory limitation value.

5 Dynamic Channel Allocation
Dynamic channel allocation exists in two phases. One is channel selection while calling; the other is channel reselection after the call is established to guarantee the quality of service. Dynamic channel allocation includes two aspects:

    (1) When there is a new call, the system will evaluate the call according to the access algorithm. If the call access is permitted, the system will allocate a suitable channel for the user according to the situation of channel resources of
the time.

    (2) In the process of communication, due to the deterioration of the wireless environment or the changes of channel resources needed by users, the system will adjust the wireless channel to guarantee the quality of service.

    Two main aspects are taken into consideration of the dynamic channel allocation of ZTE´s WCDMA system: utilization ratio and complexity. Utilization ratio refers to reduce the number of code resources blocked due to allocation; complexity is mainly used to decrease the complexity of code resources allocation and the allocation cost.
Fox example, for the load capacity of a single-code C4, 1 is equal to the bearing capacity of dual-code(C8,1, C8,2). The complexity of the system will go up with the increase of multi-code transmission. Therefore, the less the multi-code transmission is allocated, the better the system works.

    In the actual testing of ZTE´s WCDMA system, it is found that utilizing dynamic channel allocation algorithm may implement the bandwidth allocation according to the changes of the actual service flow. While service flow is small, the bandwidth is decreased to avoid the resource waste and improve the utilization ratio of wireless resources. Moreover, the bandwidth is increased, while the service flow is large, to quickly send the temporarily stored data and improve the quality of service.

    RRM plays a critical role in the WCDMA system. Its operation directly influences the performance of the system. Embedded with various advanced algorithms and concepts, RRM developed by ZTE presents excellent performance in efficient coverage, capacity, QoS and the band utilization ratio.

Manuscript received: 2005-06-20


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