System capacity is one of the most important issues in wireless communication systems. In order to increase the capacity of wireless communication systems, antenna arrays, (also known as smart antennas), have been developed. Smart antennas use multiple antennas on a communication unit, such as a wireless transmit/receive unit (WTRU), a base station, or an access point. Many different techniques have also been developed to optimize the performance of the smart antennas. One example of such a technique is beam switching.
A switched beam antenna system is a system where a plurality of fixed beams are defined, and the system selects a particular beam for transmission and reception among a plurality of beams. In selecting a particular beam, the system frequently measures each beam, and selects the beam which provides the best link quality. This measurement could be signal-to-interference ratio (SIR), signal strength indicator (RSSI) or some other measurement that reflects the link quality.
The transmit power control (TPC) is another important concern in wireless communication systems. The objective of power control is to guarantee that a minimum power is received and a maximum power is not exceeded. In a CDMA system, for example, in a reverse link (uplink), a wireless communication system uses a power control to minimize the near-far problem by assuring that all WTRUs achieve the same received power levels at a base station. In the forward link (downlink), power control is used to reduce intra-cell and inter-cell interference. Power control in the uplink also helps the WTRU to optimize battery consumption.
Transmit power is usually initialized to some value while making assumptions regarding the channel conditions. A power control algorithm is then implemented to make any necessary adjustments to reflect the actual channel condition. Several iterations may take place before the transmit power converges to a desired value.
TPC can be either open loop power control or closed loop power control. In the case of open loop power control, a transmitting terminal decides the transmit power based on its own measurements, while in the case of closed loop power control algorithm, the receiving terminal sends feedback to the transmitting terminal in order to adjust the transmit power of the transmitting terminal.
Closed loop power control usually comprises an inner loop power control and an outer loop power control. The inner loop power control makes a decision on whether the receiving terminal should ask a transmitting terminal to increase or decrease the transmit power. For example, in a cdma2000 system, this decision is based on the measured Eb/Nt and the Eb/Nt set-point (target Eb/Nt); in a W-CDMA system, this decision is based on the measured SIR and SIR set-point (SIR target). The outer loop power control is responsible for determining and adjusting the set-point values (i.e., the target Eb/Nt for cdma2000 and the target SIR for W-CDMA).
The WTRU chooses the initial set-point values based on the Quality of Service (QoS) requirements provided by the network (e.g., target frame error rate (FER) for cdma2000 and target block error rate (BLER) for W-CDMA) and other factors. The target set point is then adjusted up or down by the WTRU based on the measured QoS. After several iterations of the inner and outer loop power control, the transmit power converges to a desired value.
As a WTRU moves around and the channel condition changes, the TPC algorithm constantly re-evaluates the received QoS and makes the necessary adjustments. When multiple antenna beams are used by a WTRU, the WTRU measures the link quality using at least one of the plurality of beams. The WTRU then chooses the best beam for reception and transmission. As the WTRU switches from the current beam to a new beam having a better quality, the switching of the beam causes an abrupt change of in the QoS.
When the TPC had converged to an optimum value with the previous beam, the transmit power of the new beam may be too high, therefore cause a near-far problem and decreasing performance of the wireless communication system.