ZTE´s WCDMA Radio Resource Management Strategy

1 Power Control
The WCDMA system is limited by interference. Because of the "Near-Far Effect", the system capacity is limited by the interference between the mobile stations and the Node Bs. As a result, if each mobile station’s signal stays at the minimal S/N ratio required to maintain the QoS when arriving at the Node B, the WCDMA system capacity would attain its maximum. Power control is designed to overcome the "Near-Far Effect". Power Control evaluates the energy or the S/N ratio of the received signal, and compensates the radio channel fading in times of need. It not only maintains the communication quality, but also averts generation of extra interference for the system. There are three types of power control in the WCDMA system: 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: assuming the uplink loss and downlink loss are similar, the mobile terminal acquires the system transmission power from the system broadcast message whereby it measures the actual received power, works out the link loss, and then gets the uplink transmission power. The asymmetry of the uplink & downlink are not considered in the open loop power control, so the power control accuracy cannot be ensured.

  Figure 1 shows the results of open loop power control simulation test of ZTE’s WCDMA system. We can see the open loop power control significantly converges to the initial transmission power of the system, and effectively reduces the impact on the system loads.

1.2 Inner Loop Power Control
Inner loop power control is a kind of closed loop power control. In open loop power control, the adjustment of mobile terminal transmission power is based on the signal intensity of the forward channels. However, since the uplink and downlink are unrelated to each other, the open loop power control is just a rough adjustment. If we want to have power compensation of the fast-fading channels, we need use inner loop power control, also called fast power control sometimes.

  Inner loop power control falls into two categories: inner loop uplink power control, and inner loop downlink power control. In uplink inner loop power control, the mobile terminal adjusts its own transmission power according to the power control instructions sent by the Node B. The Node B then measures and compares the S/N ratio of its received signal with the correspondent S/N ratio threshold. If the measured S/N ratio is too low, it will request the mobile terminal to increase the transmission power; otherwise, it will request the mobile terminal to decrease the transmission power.

  Node B’s power control instruction is sent in the power control byte. If the power control byte is 0, it implies that the mobile terminal is requested to increase the transmission power; if the power control byte is 1, the mobile terminal is requested to decrease the transmission power. The power adjustment step length can be set as 1 dB or 2 dB according to different algorithms.

  The frequency of inner loop power control is as high as 1.5 kHz. Compared with CDMA or GSM, the WCDMA power control effect is more considerable.

  A large number of simulation tests and on-site operations have verified the excellent power control performance of ZTE’s Radio Network Controller (RNC): It can control the power fluctuations within 0.25 dB, and thus increase the system performance by 10-15%.

1.3 Outer Loop Power Control
Outer loop power control is a kind of slow closed loop power control. It selects an appropriate target control value for inner loop power control, so as to ensure the desired communication quality. The purpose of outer loop power control is to have an appropriate S/N ratio: neither too high, nor too low. If it is too high, the system capacity will be wasted; if it is too low, the communication quality will deteriorate. Since inner loop power control is used in the uplink & downlink of the WCDMA system, corresponding outer loop power control needs to be applied in both uplink and downlink. Outer loop power control is the power control in the physical layer. It adjusts the target S/N ratio of inner loop power control by checking Block Error Rate (BLER) of the received data through Cyclic Redundancy Code (CRC) check and comparing it with the target value.

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

  Outer loop power control can make the system capacity reach its theoretical maximum while ensuring the communication quality.

2 Handoff Control
Handoff control is the basic function of the WCDMA system, and is an important step to guarantee QoS. When the call quality decreases because the mobile terminal moves or the radio environment changes, the call should be transferred from the original radio channel to a new idle channel, so as to maintain the communication process and quality.

  There is soft handoff and hard handoff in the WCDMA system. Soft handoff is the real seamless handoff performed in the same frequency, and in the handoff process, the mobile terminal is connected with several Node Bs. In hard handoff, the connection between the mobile terminal and the original Node B is disconnected before the mobile terminal is connected to the new Node B. Usually, the new connection cannot be established immediately after the original connection is disconnected, and as a result, hard handoff impairs the call quality of the system.

  ZTE’s WCDMA system uses the following strategic algorithm for handoff control:

• Try to avoid the occurrence of handoff, especially handoff between systems. Since soft handoff consumes a lot of system resource, the handoff rate should be controlled appropriately.
• Use soft handoff and softer handoff in priority. Soft handoff should be used in priority for better communication quality it guarantees.
• One-way handoff is used for handoff from 3G to 2G, which effectively prevents the Ping-Pong Effect.
• For handoff from 2G to 3G, cell reselection is used to return to the WCDMA network.
  

3 Load Control
If the system becomes unstable when the cell load exceeds the load threshold, we need load control for the system. The basic idea of load control is to connect as many services as possible while ensuring the stable running of the system, so as to increase the system effectiveness.

  When the system load approximates or exceeds the system threshold, the load control module will start the load control:

• Downlink load control: refuse to accept the mobile terminal’s instruction to increase the downlink power;
• Uplink load control: reduce the Eb/No target used in uplink fast power control;
• Reduce the data throughput of the packet service, mainly by reducing the service rate;
• Handoff to other WCDMA carrier frequencies;
• Handoff to other Radio Access Technology (RAT) systems;
• Reduce real-time service rate, like the Adaptive Multi-Rate (AMR) speech codec;
• Drop calls.
  

  Actually, the first two measures are implemented on the Node B: reduce the received Eb/No target in the channels to reduce the transmission power of the User Equipment (UE), and thus reduce the total received power of the Node B to release the uplink load pressure; If the actual transmission power approximates the allowed maximal transmission power, reduce the transmission power of the downlink channels to screen the requirement to increase power. For Circuit Switching (CS) domain services, service rate decrease will impair the communication quality. For Packet Switching (PS) domain services, the subscriber will sense the change of the data transmission rate. The QoS is sacrificed for the stability of the system.

  Figure 3 shows the performance simulation of the AMR speech codec dynamic adjustment under load control. We can see that through load control, the system coverage gain can be increased by around 2.2 dB, and the coverage area can be increased by around 30%.

4 Admission Control
If the connections of new subscribers are not limited, the air interfaces will be overloaded, the actual coverage of a cell will diminish significantly, and the quality of connected services cannot be guaranteed. As a result, we need to check whether the addition of a new subscriber will impair the network performance before admitting the connection request. The module that performs that function is the admission control module.

  In WCDMA admission control, there are two algorithms: admission control based on power, and admission control based on throughput:

• Admission control based on power
  

  Uplink admission control based on power can be denoted in the following formula:
 I Total_old +ΔI >I Threshold   
(I : Node B receiving power)
Uplink:
 PTotal_old  +ΔP >P Threshold   

(P : Node B transmission power)

• Admission control based on throughput
  

  Uplink admission control based on throughput can be denoted in the following formula:
η UL+ΔL >η UL_Threshold
Uplink:
η DL+ΔL >η DL_Threshold
η DL andη DL are respectively the load factors of uplink and downlink before the admission. ΔL is the load factor of new subscribers.

  In ZTE’s WCDMA system, the admission control falls into two categories: new subscriber admission and handoff admission. Handoff admission is prior to new subscriber admission. During testing, the admitted subscribers in 12.2 kb/s could reach as high as 123, approximately the theoretical limit.

5 Dynamic Channel Allocation
Dynamic Channel Allocation (DCA) has two phases: channel selection in calling, and channel reselection for QoS after the call is established. As a result, dynamic channel allocation has two aspects in implementation:

• When a new call is started, the system will judge according to the admission algorithm whether to admit the call. If it is allowed to be admitted, the system will allocate an appropriate channel to the subscriber according to the condition of the available channel resources at that moment;
• In the communication process, when the radio environment deteriorates or the subscriber’s channel resource needs changes, the system will adjust the radio channels to ensure reliable service quality.
  

  For the dynamic channel allocation, we mainly consider two aspects: usage and complexity. By considering usage, we try to reduce the amount of code resources blocked in allocation. By considering complexity, we try to reduce the complexity in code resource allocation, and reduce the system’s allocation consumption. For example, if the load of a single code C4,1 is equivalent to that of a double code (C8,1, C8,2), but multi-code transmission will increase the system complexity, we should try to avoid multi-code transmission.

  While testing ZTE’s WCDMA system, we found that using the dynamic channel allocation algorithm can effectively and dynamically allocate the bandwidth according to the actual traffic change. When the traffic is low, we can reduce the bandwidth to increase the usage of the radio resources; when the traffic is high, we can increase the bandwidth to transmit the data in the buffer as soon as possible to increase the service quality.

  RRM plays a very important role in the WCDMA system, and is directly related to the system performance. ZTE integrates many advanced algorithms and ideas in the RRM for the WCDMA system, which results in the excellent performance in effective coverage, capacity, QoS, and bandwidth usage.

Manuscript received: 2004-12-20