1. Field of the Invention
The present invention relates to a mobile station apparatus and a control method for the mobile station apparatus and particularly to a mobile station apparatus, which performs wireless communication in which transmission diversity is applied, and a control method for the same.
2. Description of the Related Art
During wireless communication, a phenomenon called “fading” generally occurs and transmission performances, that is, bit error rate characteristics are greatly deteriorated by fading.
A generally-known method for compensating such deterioration of transmission performances caused by fading is “transmission diversity.” Following is a description of a type of transmission diversity called “closed-loop transmission diversity mode 1.” (For example, see 3GPP “TS25.214 V5.8.0 Physical layer procedures (FDD).”)
FIG. 1 shows a construction of a wireless base station apparatus (a transmission unit) and FIG. 2 shows a construction of a mobile station apparatus (a receiving unit), both in closed-loop transmission diversity.
As shown in FIG. 1, in the wireless base station apparatus 200 (the transmission unit), an antenna weight generator 183 generates complex weights:W1=A1eiφ1 W2=A2eiφ2 based upon feedback information (FBI) bits from the mobile station apparatus 100. Thereafter, a weighting unit 181 multiplies two transmission data sequences coded by a channel coder 180 with the complex weights:W1=A1eiφ1 W2=A2eiφ2 A spreading unit 182 then carries out a spreading process using a spreading code generated by a spreading code generator 184.
Thereafter, two antennas transmit CPICHs (Common Pilot Channels) with the same carrier phase. The CPICHs transmitted from the two antennas are spread with the same spreading code and orthogonalization is realized by changing pilot symbols.
Meanwhile, as shown in FIG. 2, in the mobile station apparatus 100 (the receiving unit), a CPICH despreading unit 110 despreads the CPICHs transmitted by the two antennas, and a phase comparing unit 120 compares a received carrier phase difference between the separated signals. Thereafter, a FBI bit generating unit 160 generates a FBI bit which controls the receipt carrier phase difference and transmits the FBI bit through a DPCCH (Dedicated Physical Control Channel), one of dedicated uplink physical channels DPCHs (Dedicated Physical Channels).
The wireless base station apparatus 200 (the transmission unit) multiplies transmission data sequences of the two antennas with transmission antenna weights generated based upon the FBI bit from the mobile station apparatus 100, and transmits the multiplied transmission data sequences. As stated above, transmission carrier phases of two antennas are controlled by using a FBI bit from the mobile station apparatus, thus reducing bit errors attributed to a decline in received signals power due to fading.
The closed-loop transmission diversity mode 1 specified by 3GPP is a method for controlling a transmission carrier phase of a dedicated physical channel DPCH for the second antenna with a carrier phase resolution of π/4 so that signals from the two antennas and received at the mobile station have almost the same phase. Further descriptions are given below regarding operations when the closed-loop transmission diversity mode 1 is applied to the dedicated physical channel DPCH.
The transmission amplitude of the two antennas in a slot n is represented by the following equation.
      A          1      ,      n        =            A              2.        ⁢        n              =          1              2            The transmission carrier phases of the two antennas are represented by the following equations.φ1,n=0φ2,n={±π/4,±3π/4}In the mobile station apparatus 100, the phase comparing unit 120 estimates received carrier phases of CPICHs transmitted from the two antennas and the FBI bit generating unit 160 generates a FBI bit in the slot n. The estimated values of received carrier phases represented by the followings:θ1.nCP θ2.nCP are expressed by the following equation for an even slot n:if −π/2≦(θ1.nCP−θ2.nCP)≦π/2 then bn=0, otherwise bn=1and by the following equation for an odd slot n:if 0≦(θ1.nCP−2.nCP)≦π then bn=0, otherwise bn=1A result of decoding using the FBI bit, represented by:{circumflex over (b)}n (where no FBI bit error is found, the decoding result is expressed by the following equation:{circumflex over (b)}n=bn )is used by the wireless base station apparatus 200 to determine an interim transmission carrier phase in a DPCH slot (n+1) of the second antenna:φ2,(n+1) The interim transmission carrier phase is determined as follows: where n is an even number, the interim transmission carrier phase is determined by the following equation:if {circumflex over (b)}n=0 then φ2.(n+1)=0, otherwise φ2.(n+1)=πand, where n is an odd number, the interim transmission carrier phase is determined by the following equation:if {circumflex over (b)}n=0 then φ2.(n+1)=π/2, otherwise φ2.(n+1)=−π/2Thereafter, from the interim transmission carrier phases in the slots n and (n+1), a transmission carrier phase in the slot (n+1) of the second antenna represented byφ2,(n+1) is ultimately obtained by the following equation:φ2,(n+1)=(φ2,n+φ2,(n+1))/2
A FBI bit may be erroneous in an uplink. In such a case, the wireless base station apparatus 200 transmits signals with carrier phases that are different from a control command from the mobile station apparatus 100, and appropriate phase control cannot be carried out, resulting in an increase in an error rate. In order to solve this problem, antenna verification is conducted in the mobile station apparatus 100 to estimate a transmission weight (a transmission carrier phase) in each slot of DPCH. An example of antenna verification is described in, for example, TS25.214 Annex A.1 Antenna verification.
In an uplink, it is general that transmission power is controlled to ensure a consistent performance. As a result, the probability of occurrence of FBI bit errors is also consistent. Therefore, with abovementioned antenna verification process, downlink characteristics are improved.
Antenna verification is a function of correcting a phase control error due to an erroneous FBI bit in an uplink. However, antenna verification may mistakenly lead to a determination that there is a phase control error even though a FBI bit is correct in an uplink, and downlink transmission is carried out with an appropriate phase. In such a case, the mobile station apparatus 100 receives signals based on wrong determination information about a phase even though transmission is carried out with an appropriate phase, resulting in an increment in error rate. In other words, where there is an antenna verification error as above, a downlink wireless quality is deteriorated.
For a control method for the mobile station apparatus, the foregoing antenna verification may or may not be performed. When antenna verification is not performed, the mobile station apparatus assumes that there is no error in a FBI bit transmitted by itself, and receives signals in a downlink.
The descriptions above are about operations when transmission diversity is applied to a downlink dedicated physical channel DPCH. Now, descriptions below are about operations when closed-loop transmission diversity mode 1 is applied to a downlink shared channel, HS-PDSCH (High Speed Physical Downlink Shared Channel).
A HS-PDSCH is a shared physical channel for conveying data by a transmission scheme HSDPA (for example, see 3GPP “TS25.848 V4.0.0 Physical Layer Aspects of UTRA High Speed Downlink Packet Access.”) for high-speed downlink data transmission. Other physical channels include a shared control channel HS-SCCH (High Speed Shared Control Channel), A-DPCH (Associated Dedicated Physical Channel), an associated dedicated channel set for each mobile station, and the like.
Similarly to a dedicated channel, transmission diversity in a HSDPA generates a FBI bit from a CPICH phase difference, transmits the FBI bit via an uplink DPCH, and controls a phase of HS-PDSCH from the second antenna. However, since HS-PDSCH does not have dedicated pilot symbols, antenna verification in the mobile station apparatus is carried out using dedicated pilot symbols of A-DPCH.
As described above, antenna verification of transmission diversity in HSDPA is carried out using a dedicated pilot of A-DPCH, an associated dedicated channel.
However, HS-PDSCH and A-DPCH have different TTI lengths, modulation schemes and the like. In addition, a coding method, a coding rate and the like of mapped HS-DSCH are largely different from those of DCH mapped to A-DPCH. Therefore, when antenna verification is carried out using a dedicated pilot of A-DPCH, deterioration may occur. To be more specific, there has been a problem in that a quality of a dedicated pilot of A-DPCH is not sufficiently high, and antenna verification errors thus increase, resulting in deterioration of qualify characteristics.
The present invention has accomplished in the light of the above problem, and an objective thereof is to provide a mobile station apparatus and a control method for the same, which prevent deterioration of characteristics due to an antenna verification error.