Patent Application
20050152279
The disclosure relates generally to wireless communications, and more particularly to downlink power control in the presence of time varying interference in wireless communications networks, for example, in 3GPP W-CDMA communications networks, including methods in wireless communications devices connected to the network.
In W-CDMA communications systems, for example, 3rd Generation Partnership Project (3GPP) Universal Mobile Telephone System (UMTS) wireless communications systems, mobile user equipment (UE) transmits a power control command to the network for use in controlling transmission power. The power control command is based on an inner-loop power control algorithm that estimates a Signal-to-Interference Ratio (SIR) at the output of the rake receiver.
It is known to estimate the signal-to-interference ratio (SIR) based on the downlink Dedicated Physical Control Channel (DPCCH), which is part of the Dedicated Physical Channel (DPCH). The DPCH is, however, subject to interference from the Primary and Secondary Synchronization Channels, since the codes used on the Synchronization Channels are not orthogonal to those of the DPCH and the Synchronization Channels are transmitted at power levels typically much higher than that of the power-controlled DPCH. How Primary and Secondary Synchronization Channel interference affects the SIR measurement depends the frame offset between the Dedicated Physical Channel (DPCH) and the Common Pilot Channel (CPICH). Interference on the DPCH results in inaccuracies in the estimation of the SIR.
In 3GPP, the interleaving of transport channels onto the DPCH is not highly randomized. Thus the grouping of transport channel data tends to be bunched together instead of randomly and widely dispersed. If the corresponding portion of each slot is corrupted by interference, for example, by cross-correlation of the Primary or Secondary Synchronization Channels and the DPCH, the corrupted transport channel data is less likely to be decoded successfully. This also affects the accuracy of the SIR estimation.
In 3GPP Downlink Power Control (DLPC), the UE requests the base station (BS) to lower the transmit level of the downlink DPCH as low as possible while maintaining the base station prescribed Block Error Rate (BLER). This algorithm is based on the estimated SIR and the measured BLER of specific transport channels. Inaccuracies in the SIR estimate may thus cause the UE to request a lower power level than required to meet the requested BLER.
In applications where Blind Rate Detection (BRD) is employed, for example, in some 3GPP UMTS networks, BRD may compound problems discussed above. BRD is a method where the UE determines the combination of formats sent on all transport channels without guidance from the BS. The UE, however, cannot be sure that erroneous decoding on the transport channels is due to an exceedingly low SIR or to the absence of data, for example, lack of data resulting from a discontinuous data transmission (DTX).
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.
In
The power control command is based on a power control algorithm, residing on the wireless device, that estimates a Signal-to-Interference Ratio (SIR) based on rake receiver outputs as is known generally in the art.
The received signal generally includes multiple channels. These channels may be separate physical channel and/or separate logical channels, all of which may have different symbol rates and/or spread factors depending upon the particular communications protocol to which the signal conforms.
SIR estimation is based generally on signals at the rake finger output. Instantaneous power measurements are obtained on a slot-by-slot basis, e.g., every slot, wherein the signal and noise components of the SIR are obtained from filtered estimates of signal and noise power. In one embodiment, the signal amplitude is estimated from data at the output of a data rate processor (DRP) embodying the functionality of the processing block 211-216 in
In one embodiment the estimated SIR is a composite SIR based on a first SIR computed on a first channel and a second SIR computed on a second channel. In the process diagram 400 of
In one embodiment, the first SIR is estimated on a dedicated data channel, for example, on a Dedicate Physical Date Channel (DPDCH) of the type discussed above in connection with the exemplary signal structure of
DPCH—SIR=p*DPCCH—SIR+(1−p)*DPDCH—SIR Eq. (1)
Estimation of the SIR on a data channel requires distinguishing signal, e.g., symbols having data, from noise, since the signal is required to estimate the signal power component of the SIR. In some applications, the estimation of SIR on the data channel may be further complicated by interruptions in signal transmissions, for example, discontinued transmissions (DTX) during signal fading.
In one embodiment, SIR is estimated on a data channel only if the signal on the channel satisfies a condition, i.e., if the received symbols contain data. In one embodiment, the determination is made whether a data channel symbol contains data by estimating the amplitude of the symbol, for example, based on bit amplitude. The estimated signal amplitude is compared to a reference.
In one embodiment, the reference is obtained by averaging symbol amplitudes known to contain data, for example, upon successfully decoding received symbols. In the exemplary UMTS downlink signal structure 300 in
Since the DPCH_SIR is a ratio of signal to interference energy:
DPCH—SIR(t)=DPCH_signal_energy(t)/DPCH_interference_energy(t) Eq. (2)
According to the relation in Eq. (2), if interference is increased, the signal energy must also be increased by a multiplicative factor to maintain a constant DPCH_SIR. The network can command the UE to maintain an exceedingly low or high block error rate (BLER) target. In 3GPP, for example, these values can range from 100% to 0.00005%. At these exemplary extremes, the DPCH_SIR required to obtain a high BLER target can be quite small, for example 0 dB or less. Conversely, the DPCH_SIR required to obtain a low BLER target can be comparatively large, for example, 10-15 dB.
The transport channel is characterized by several parameters, including block length. Since downlink power control is based on a block error rate, the block error probability must be calculated. The block error probability computation required knowledge of the block length. If a bit error has a probability of “p” and a block is N bits long, then the probability of a block error (assuming the occurrence of a bit error is an independent random variable) may be expressed as:
(1−(1−p){circumflex over ( )}N) Eq. (3)
Eq. (3) is indicative of the probability of 1 or more bits being in error out of N, where: “p” is the probability of a bit error, 0<=p<=1, “1−p” is the probability of a bit not being in error, and “(1−p){circumflex over ( )}N” is the probability of N consecutive bits not being in error. Since the probability of a bit error is widely described as a function of the DPCH_SIR, the probability of a block error may be expressed as follows:
(1−(1−f(DPCH_SIR)){circumflex over ( )}N) Eq. (4)
So for a fixed DPCH_SIR, changing the value of the N will change the probability of a block error.
Another transport channel parameter is the coding type. There are three types of coding allowed: turbo coding, convolutional coding, and no coding. Each of coding types has a different error rate performance as they implement different decoding techniques, for example, MAP, MLSE, HARD DECISION, etc., with differing amounts of error correction.
Depending on the block length, coding type, and rate matching parameters, the UE may be required to puncture or repeat the data of each decoded stream in differing amounts. If a particular transport channel was punctured, this means that a certain percentage of the DL data was not transmitted. This will degrade decoding performance as the UE then insert zeros to compensate for this missing data and rely more on the error-correcting capabilities of the channel decoder to account for the lack of data. If a particular transport channel was repeated, certain symbols in the downlink were transmitted multiple times. This improve decoding performance of the corresponding transport channel as there are some symbols which have multiple (at least twice) redundancy relative to the reliability of the original data set.
In another embodiment, the reference is obtained from a using a DPCCH_PWR_OFFSET. In one embodiment, for example, a candidate level for the DPDCH amplitude is generated using measured amplitude of the DPCCH, e.g., using TPC and PILOT information, and applying the DPCCH_POWER_OFFSET. This candidate amplitude can then be used to generate a threshold by which each DPDCH symbol can be compared to determine existence or absence of signal energy.
In
In implementations where the DRP includes digital gain stages, for example, after each of the despreaders 212, 213 in
While the present disclosure and what is considered presently to be the best mode thereof have been described in a manner that establishes possession by the inventors and that enables those of ordinary skill in the art to make and use the inventions, it will be understood and appreciated that there are many equivalents to the exemplary embodiments disclosed herein and that myriad modifications and variations may be made thereto without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.
