Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-07-20
2003-11-11
Ton, Dang (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S480000, C370S491000
Reexamination Certificate
active
06647003
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for making channel estimation (propagation path estimation) of data symbols from a pilot symbol sequence parallel to a data symbol sequence, and a CDMA (Code Division Multiple Access) receiver and CDMA transmitter with the device.
BACKGROUND ART
In a mobile communications environment, amplitude and phase fluctuations in a traffic channel can occur because of Rayleigh fading due to changes in the relative location between a mobile station and a base station. Thus, in a conventional phase modulation scheme that transmits data (information) by the phase of a carrier, it is common for a transmitting side to carry out differential encoding of transmitted data for impressing the data on relative phases of neighboring symbols, and for a receiving side to discriminate and decide the data by differential detection.
However, since the transmitted data is subjected to the differential encoding as mentioned above, a one-bit error in a radio section appears as a two-bit error in the differential detection, thereby increasing the receiving error rate by 3 dB in terms of the SNIR (Signal-to-Noise Interference power Ratio) as compared with coherent detection like binary phase-shift keyed modulation (BPSK modulation).
On the other hand, although absolute coherent detection, which discriminates and decides the phase of a received signal using the absolute phase of each data symbol, has a highly efficient receiving characteristic, it is difficult under the Rayleigh fading environment to decide the absolute phase of the reception.
In regard to this matter, Sadayuki Abeta, et al., “DS/CDMA Coherent Detection System with a Suppressed Pilot Channel”, IEEE GLOBECOM'94, pp. 1622-1626, 1994, proposes a method of estimating fading distortion by inserting, in parallel with a data channel for transmitting data, a pilot channel which is orthogonal to the data channel and has known phases, thereby compensating for the fading distortion.
FIG. 13
illustrates a channel estimation method disclosed in this paper. In
FIG. 13
, the channel estimation is carried out using a pilot symbol sequence parallel with a data symbol sequence. To reduce a power loss, the power of the pilot symbol sequence is set less than that of the data symbol sequence.
In addition, to follow instantaneous Rayleigh fluctuations, the transmission power control is carried out on a slot by slot basis. Accordingly, as shown in
FIG. 13
, the amplitudes (powers) of the data symbol sequence and pilot symbol sequence vary slot by slot, and their phases also vary slightly due to the operation of amplifiers during transmission. Such transmission power control enables a reverse channel of the DS-CDMA (Direct Sequence CDMA) to maintain the SNIR against interference signals due to cross-correlation from other users.
The channel estimation of data symbols obtains its channel estimates by averaging (coherently adding) pilot symbols (estimated complex fading envelope) in a section (slot, in this case) to which the data symbols belong. The channel estimation with high SNIR is carried out in this manner. The estimates are employed to detect with the pilot symbols in the data symbol sections the received signal of a path of each user, to measure the amplitude and phase of the signal of each path, and to estimate and compensate for the channel fluctuations in the data symbol sections.
However, it is difficult for the foregoing method disclosed in the paper to achieve highly accurate channel estimation. This is because the method obtains the channel estimates by only averaging the pilot symbols in the slot including the data symbols to be subjected to the channel estimation.
Furthermore, in an actual mobile transmission environment, thermal noise (reducing the transmission power as low as possible creates a noise-limited environment, particularly at cell borders) and interference signals from other users due to cross-correlation are added to a desired signal of the channel to be received, and the phase and amplitude of the received signal vary every moment because of fading, which degrades the channel estimation accuracy. In summary, it is difficult for the method disclosed in the foregoing paper, which carries out the channel estimation of the data symbols using only the pilot symbols in the slot containing the data symbols, to achieve highly accurate channel estimation.
DISCLOSURE OF THE INVENTION
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to achieve highly accurate channel estimation by obtaining highly accurate channel estimates by calculating a sum of appropriately weighted pilot symbols when carrying out the channel estimation of the data symbols.
Furthermore, the present invention can achieve higher accuracy channel estimation by carrying out the channel estimation of the data symbols using pilot symbols not only in the slot containing the data symbols, but also in other slots.
The highly accurate channel estimation and compensation for channel fluctuations in the data symbols based on the channel estimation make it possible for the absolute coherent detection to decide the absolute phase of each data symbol even in the Rayleigh fading environment, which can reduce the SNIR for achieving desired receiving quality (receiving error rate). This can reduce the transmission power, and increase the capacity of a system in terms of the number of simultaneous
In order to accomplish the object aforementioned in the first aspect of the present invention, there is provided a channel estimation unit for obtaining channel estimates of data symbols from a pilot symbol sequence which is parallel to a data symbol sequence that comprises:
means for generating a plurality of pilot blocks from the pilot symbol sequence; and
means for obtaining the channel estimates of the data symbols by calculating a weighted sum of averages of the pilot symbols in the individual pilot blocks.
In the second aspect of the present invention, there is provided a CDMA receiver which receives a data symbol sequence that is spread, and a pilot symbol sequence that is spread and parallel to the data symbol sequence, and which generates a data sequence by demodulating the spread data symbol sequence by using the spread pilot symbol sequence, and which comprises:
means for receiving the spread data symbol sequence and the spread pilot symbol sequence;
means for generating a data symbol sequence by despreading the spread data symbol sequence;
means for generating a pilot symbol sequence by despreading the spread pilot symbol sequence;
means for generating from the pilot symbol sequence a plurality of pilot blocks;
means for obtaining channel estimates of the data symbols by calculating a weighted sum of averages of the pilot symbols in the individual pilot blocks;
means for compensating for channel fluctuations in the data symbol sequence by using the channel estimates of the data symbols; and
means for generating the data sequence by demodulating the data symbol sequence compensated for.
Here, the spread data symbol sequence may have been spread using a first spreading code, the spread pilot symbol sequence may have been spread using a second spreading code, the means for generating the data symbol sequence may despread the spread data symbol sequence which has been spread using the first spreading code, and the means for generating the pilot symbol sequence may despread the spread pilot sequence which has been spread using the second spreading code, and the first spreading code and the second spreading code may be orthogonal to each other.
In the third aspect of the present invention, there is provided a CDMA transceiver have a transmitting processor and a receiving processor, the transmitting processor generate a spread data symbol sequence by modulating a data sequence, and transmits the spread data symbol sequence with a spread pilot symbol sequence which is spread in parallel with the data symbol sequence, and the receiving processor receives the spr
Abeta Sadayuki
Adachi Fumiyuki
Sawahashi Mamoru
Brown & Raysman Millstein Felder & Steiner LLP
NTT Mobile Commmunications Network, Inc.
Tran Phuc
LandOfFree
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