This disclosure relates to wireless communication systems.
Terrestrial wireless communication systems based on Code Division Multiple Access (CDMA) employ mobile transmit power control to mitigate the effects of the so-called “near-far” problem. The near-far problem arises when a base station is in communication with multiple mobile stations located at different distances from the base station. The difference in power level received at the base station from one mobile unit located near the base station and another mobile unit located farther away can be huge because of the high path loss associated with terrestrial radio propagation. Ideally, the use of orthogonal spreading codes in CDMA should allow multiple transmitters to co-exist within the same frequency band without introducing mutual interference, irrespective of the received power level. However, due to a lack of synchronization among the mobile transmitters, as well as other factors such as time dispersion, such perfect isolation of the signals received from different mobile stations cannot be achieved in practice.
The presence of multiple signals arriving at the base station antenna simultaneously also causes an effect called multipath. Signals that are in phase will add while signals out of phase will subtract. Shadowing effects, as well as rapid fading caused by multipath propagation, further increases the variation in received power. The multipath fading is caused by a variation of the amplitude or relative phase of one or more of the frequency components in the received signal. In particular, multipath fading may result in the received power falling 20-30 dB below the average level. Successive minima occur roughly every half of the carrier frequency wavelength. This can be approximately 8 cm in a 1900 MHz Personal Communications Systems (PCS) band.
A well-known remedy to the near-far problem is to control the transmit power of each mobile in such a way that all the signals arrive at the base station with approximately the same Signal-to-Interference Ratio (SIR), independent of where the mobile stations are located. Furthermore, since the total interference level generated by all the mobile transmitters determines the system capacity (i.e. maximum number of simultaneous calls), it is desirable to set the target SIR value to no higher than required to ensure the desired Quality of Service (QoS). The QoS is commonly measured in terms of Frame Error Rate (FER). In practice, the requested QoS and thus the target SIR may vary from one mobile unit to another.
The North American CDMA system, as specified by Telecommunications Industry Association (TIA) standard IS-95, and its future evolution IS-2000, uses two fundamentally different mechanisms for power control. The first is “open-loop” power control, intended to compensate for large-scale signal strength variations caused by propagation path loss and shadowing effects. Such variations can be considered as being frequency-independent. As a result, the large-scale variations in the forward link (i.e. base-to-mobile) and the reverse link (i.e. mobile-to-base) can be considered identical, even when the two links operate in different frequency bands. In open-loop power control, the mobile takes advantage of this particular fact by adjusting its transmit power level autonomously in inverse proportion to the power it receives from the base station. To ensure that only large-scale variations are accounted for, open-loop power control is based on a long-term average of the measured received power.
The second power control mechanism is “closed-loop” power control. The closed-loop power control aims to compensate for the rapid signal strength variations caused by multipath propagation and sudden shadowing effects that cannot be compensated for by the slower open loop power control. The closed-loop power control also compensates for changes in interference level.
The closed-loop power control 100 involves both the base station 102 and the mobile station 104 in a feedback loop arrangement, as illustrated in FIG. 1. The system time is divided into basic power control (PC) periods with duration TPC. In each such PC period, the base station 102 computes a short-term average of the power received from the mobile 104, as well as the power from interfering transmitters. The ratio of these two measurements constitutes the measured SIR value 106 for that period. The measured SIR value 106 is then compared to the target SIR value 108 for that period. Based on this comparison, the base station 102 computes a suitable power correction command 110, which is then transmitted back to the mobile 104 over the forward link. The mobile 104 will thus adjust its transmit power once every PC period.
In order to maintain the lowest possible delay in the PC loop, power correction commands are not protected by error correction coding. Moreover, in order to minimize the forward link capacity loss due to correction commands, it is desirable to encode each command as a single bit. Depending on the value of the received PC bit, the mobile station 104 will then either increase or decrease its transmit power level by a predetermined amount, referred to as the PC step size.
The ability of the closed power control loop 100 to respond to rapid changes in received power level is limited by the PC bit rate 1/TPC, the step size, and the loop delay. The loop delay is the time elapsed between the generation of a PC bit in the base station 102 and the observation of the corresponding change of received power on the base station side.
However, substantial compensation of the multipath fading is only possible at comparatively low fading rates. FIG. 2 illustrates this point for the case of IS-95/IS-2000 with a two-path Rayleigh fading channel and maximal-ratio combining in the base station receiver. The two independently fading paths are simulated using Jake's multipath model, with the mean power of the second path being set to 3 dB below that of the first path and a maximum Doppler frequency (fD) of 20 Hz. Jake's model configures fading amplitude as a Rayleigh random variable. In Jake's model, the distribution of received power along the Doppler frequency axis takes on a U-shape from −fD to +fD. This gives a fading rate of roughly two times fD (40 Hz).
FIG. 2 shows the transmitted and received power and the inverted fading amplitude. The target received power level is 0 dB. In IS-95/IS-2000, the base station transmits a power control bit every TPC=1.25 ms, at a rate of 800 bits/s. The step size is 0.25, 0.5 or 1.0 dB, controlled by the base station via messaging. The loop delay is determined by implementation-dependent factors in the mobile station as well as the round-trip propagation delay. The delay may even be time varying, due to the fact that the power control bit transmission times are pseudo-randomized. Typically, the loop delay is between one and two PC periods.
For the example of FIG. 2, the step size is set to 1 dB and the loop delay to one PC period (1.25 ms). Even with closed-loop power control, deep fades 200 occur frequently. Further, it can be observed that after each deep fade, there is a considerable overshoot 202 in the received power, due to the delayed response of the PC loop. However, the FER is primarily determined by the frequency and duration of the fades, which is equivalent to the time spent below the target power level. Hence, these overshoots 202 constitute a significant waste of transmit power for the mobile station, while contributing to the total interference level on the base station side.