CONCLUSIONES Y RECOMENDACIONES

There are several proposed power control techniques employed in wireless communication, the WCDMA system basically employs two mechanisms to manage power transmission control:

 open-loop power control,

 closed-loop power control, Open- Loop Power Control

The open loop power control function is implemented in both directions (uplink and downlink).

In uplink direction, the open loop power control function sets the initial transmit power levels for the UE and in downlink direction it determines the transmit power levels for the downlink channels. The open loop power control uses the measurement reports of the UE about the received power from the BS, and it then decides how to set the transmit downlink power levels (Sultan, 2010). The open-loop power control is designed to ensure that the received powers from all users are equal in average at the base station. In the open-loop algorithm, the mobile user can compute the required transmit power by using an estimate from the downlink signal (no feedback information is needed). This is because the large-scale propagation loss is reciprocal between uplink and downlink channels Fig. 2.20 illustrates the OPLC applied in the uplink. In this case the UE estimates the transmission signal strength from the received power level of the pilot signal from the base station and make the adjustment of the transmitting power in a certain level that is inversely proportional to the pilot signal strength. Consequently the stronger the received

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pilot signal, the lower the UE transmitted power (Hossain et al, 2007). This is because the large-scale propagation loss is reciprocal between uplink and downlink channels.

Figure 2.20: Open Loop power Control (Hossain et al, 2007)

Fig. 2.21 shows how an open-loop power control algorithm solves the near far problem in the reverse link of a CDMA system (Aditt, 2003).

Figure 2.21 Mechanism of open-loop power control (Adit, 2003)

In Fig. 2.21, user 1 located at distance d1from the base station receives a power level Pr1.This power level is higher than that received by user 2, Pr2, who is located at distance d2 from the base station because d1< d2. Therefore to deliver an equal power received at the base station, user 2 must transmit a higher power level than user 1 or Pt2>Pt1. The procedure to determine the transmit power can be expressed as:

Pt= - Pr+ Poff+ Pp, (2.22)

Where Pt(dBm) is the required transmit power for a mobile user, Pr(dBm) is the received power at the mobile, Poff(dB) is the offset power parameter, and Pp(dB) is the power adjustment parameter. The offset power parameter is used to compensate for different frequency bands, i.e.

P_off= –76 dB for the 1900 MHz and P_off= –73 dB for the 900 MHz frequency band (Adit,

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2003).The power adjustment parameter is used to compensate for differences with regard to different cell sizes and shapes, basestation transmit power, and receiver sensitivities.

However, it is important to obtain a good method of the downlink signal measurement. If the measurement period is too short, rapid fluctuation due to multipath may still exist and may not give an accurate result for the mean power measurement. On the other hand if measurement period is too long it may average out the effect of shadowing and therefore open-loop power control may not compensate for the shadowing effect (Adit, 2003).

Closed-Loop Power Control

Closed-loop power control aims at eliminating the received signal fluctuation due to small scale propagation loss. In contrast to the large-scale propagation loss, the small-scale propagation loss is uncorrelated between uplink and downlink. Therefore, to control the uplink fading, the uplink channel information must be estimated at the base station and then fed back to the mobile station, so that the mobile station can adjust its transmit power according to the fed back information (Adit, 2003).

The CLPC system is further divided into two different processes, both operating in parallel to each other; inner loop and outer loop power control.

The inner loop power control is also known as the fast closed loop power control. The fast closed loop controls the transmission power levels for UE and BS based on the received signal-to-interference ratio (SIR) level at BS and UE, to combat fading characteristics of the radio channel (Sulatan, 2010). The TPC (transmit power control) commands are sent by UE and BS providing the information either to increase or decrease the transmission power levels. In UMTS both in uplink and downlink directions, the UE and BS measures the received SIR to compare it with the target SIR levels, set earlier by outer loop power control. If the measured uplink SIR is greater than the target SIR, the BS requests the UE to lower its transmission power. Similarly, if the measured uplink SIR is lower, then the UE increase its transmission power to attain the target SIR. In downlink the BS changes the transmission power levels in response to the TPC commands from the UE.

The outer loop power control provides the target SIR level for the inner loop power control. The outer loop power control adjusts the target SIR to achieve the desirable BLER (block error rate)

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for the particular service (voice or data) carried out by the UE. The change in the mobile speed or the multipath propagation environment also results in the adjustment of the target SIR (Sulatan, 2010). A closed-loop power control model for the reverse link is shown in Fig. 2.22.

Figure 2.22 Closed-loop power control model (Adit, 2003)

In this model, the signal strength or SIR is first estimated at the base station for every time slot, T_p, which corresponds to one power control interval. In Fig 2.22 this estimated quantity is represented by γest. Then it is compared with the desired or the target level γt.The difference between the estimated SIR or signal strength and the target level is then quantised and sent to the mobile user via the downlink channel as a binary representation of PC command (PCC) bits.

The command bits are multiplexed with the user data. The mobile users then extract the PCC bits from the downlink data stream and use them to adjust their transmit power (Adit, 2003).

Due to the downlink channel impairments, the PCC bits received by the mobile user can be in error. The PCC bit error is modelled as a multiplicative quantity with opposite bit polarity. A delay is also introduced by the control loop. This delay is called the feedback loop delay and is expressed in a multiple, D, of power control interval Tp, where D is an integer. After the PCC bits are recovered by the mobile user, they are used to adjust the transmit power by the required step size, PCC x Δp. Due to feedback delay, however, the mobile transmit power (after adjustment) may not compensate the current channel condition because at the time the mobile

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adjusts the power, channel conditions may have already changed in a fading situation. This problem is what the open-loop PC tends to correct.

To avoid positive feedback, a strength-and-SIR-combined power control scheme is proposed in (Zhang, Zhang, and Ko, 2000). In this scheme, SIR is used to control the desired signal quality, while signal strength is used to control the interference level. For example when a user‟s SIR is below the required threshold but its signal strength is already high (above the threshold), that user cannot increase its transmit power. Alternatively, power control should be operated together with another technique, such as call admission control (CAC) in order to prevent positive feedback by assuming that the maximum system capacity is not exceeded. Another possible technique to reduce the possibility of positive feedback, a soft dropping technique can be used.

With this technique, a user who needs a high transmit power to combat deep fades can decrease its target SIR, which is quite possible for a CDMA system at the expense of a graceful performance degradation (Adit, 2003).