Apparatus and method for transmission diversity using more...

Pulse or digital communications – Spread spectrum – Direct sequence

Reexamination Certificate

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C375S147000

Reexamination Certificate

active

06690712

ABSTRACT:

PRIORITY
This application claims priority to an application entitled “Apparatus and Method for Transmission Diversity Using More Than Two Antennas” filed in the Korean Industrial Property Office on May 25, 2000 and assigned Ser. No. 2000-29136, to an application entitled “Apparatus and Method for Transmission Diversity Using More Than Two Antennas” filed in the Korean Industrial Property Office on Aug. 24, 2000 and assigned Serial No. 2000-49259, and to an application entitled “Apparatus and Method for Transmission Diversity Using More Than Two Antennas” filed in the Korean Industrial Property Office on Aug. 28, 2000 and assigned Ser. No. 2000-47913, the contents of each of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a transmission diversity system, and in particular, to a system where a UTRAN (UMTS Terrestrial Radio Access Network) operates compatibly with a mobile station (MS) supporting a different transmission diversity technique.
2. Description of the Related Art
A third generation mobile communication system has been developed for high-speed data transmission along with the rapid advance of mobile communication technology and the increase of the amount of data being transmitted. W-CDMA (Wideband Code Division Multiple Access), an asynchronous scheme between UTRANs, is standardized as the third generation mobile communication system for Europe, and CDMA-2000, a synchronous scheme between base stations, is standardized as the third generation mobile communication system for North America. In the mobile communication systems, a plurality of MSs communicate through one base station. When data is transmitted at a high rate, the phase of a received signal is distorted due to fading on a radio channel. The fading reduces the amplitude of the received signal by several decibels to tens of decibels. If the distortion is not compensated for in data demodulation, mobile communication quality is deteriorated. Thus, many diversity techniques have been used to overcome fading.
CDMA usually employs a rake receiver for receiving a signal with diversity utilizing the delay spread of a channel. While the reception diversity relying on the delay spread is applied to the rake receiver, the rake receiver does not operate if the delay spread is less than a threshold. Time diversity relying on interleaving and coding is used for a Doppler spread channel. The time diversity, however, is difficult to apply to a slow Doppler spread channel.
Therefore, space diversity applies to a channel with a small spread delay and a slow Doppler spread channel to overcome fading. For the space diversity, at least two transmission/reception antennas are used. Although the strength of a signal transmitted through one antenna is reduced due to fading, a signal transmitted through the other antenna is received. The space diversity is divided into reception diversity using reception antennas and transmission diversity using transmission antennas. Because it is difficult to install a plurality of antennas in an MS for the reception diversity in terms of cost and terminal size, it is recommended to implement the transmission diversity technique in a UTRAN with a plurality of antennas.
The transmission diversity technique is implemented in an algorithm for receiving a downlink signal and obtaining a diversity gain. The algorithm is generally divided into an open loop mode and a closed loop mode. In the open loop mode, if a UTRAN encodes a data signal and transmits the coded signal through diversity antennas, an MS receives the signal from the UTRAN and obtains a diversity gain by decoding it. In the closed loop mode, if the MS estimates channel environments that signals transmitted through transmission antennas of the base station will experience, calculates weights that maximize the power of reception signals for the transmission antennas based on the estimated values, and transmits the weights as signals to the UTRAN on an uplink channel, the UTRAN adjusts the weights of the antennas based on the weight signals received from the MS. To help the MS estimate the channels, the UTRAN transmits pilot signals through the respective transmission antennas to the MS. Then, the MS estimates the channels according to the pilot signals and acquires optimum weights based on the channel information.
Transmission diversity is applied in a feed-back mode in U.S. Pat. No. 5,634,199 entitled “Method of Subspace Beamforming Using Adaptive Transmitting Antennas with Feed-Back” and U.S. Pat. No. 5,471,647 entitled “Method for Minimizing Cross-talk in Adaptive Transmission Antennas”. While the former proposes channel estimation and feed-back in a perturbation algorithm and a gain matrix, this is a blind scheme that is not suitable for a system with pilots due to a slow convergence speed for channel estimation and difficulty in obtaining accurate weights.
The 3GPP (3
rd
Generation Partnership Project) specification (Release 99) for UMTS (Universal Mobile Telecommunications System) has suggested quantization and feedback of weights for two antennas. It describes only the case in which an MS supports 2-antenna transmission diversity. The specification made no comment on signal transmission from a UTRAN with transmission antennas and signal transmission and reception in the case where a 2-antenna transmission diversity MS coexists with a 4-antenna transmission diversity MS. Expansion to four antennas by adaptively using a conventional method of expanding signal transmission through one antenna to signal transmission through two antennas is not valid for the 2-antenna transmission diversity MS. Simultaneous use of a signal transmission method using two antennas and a signal transmission method using four antennas also has the problem of power imbalance between the antennas.
Different pilot signals can be transmitted through a plurality of antennas by time division multiplexing, frequency division multiplexing, and code division multiplexing. In W-CDMA, code division multiplexing can be performed with the use of multiple scrambling codes, channelization codes, or multiple orthogonal pilot symbol patterns in order to transmit different pilot signals through the antennas.
In general, a high diversity gain and an SNR (Signal to Noise Ratio) gain of up to 3 dB are acquired by using two transmission antennas, as compared to a conventional system using a single transmission antenna. If transmission diversity is implemented with more than two antennas, an additional diversity gain is obtained besides the diversity gain in a two antenna-transmitter and an SNR gain increases in proportion to the number of antennas. The additional diversity gain is less than that obtained from the 2-antenna transmission diversity but since the diversity order increases, the diversity gain is very high if the SNR (Eb/No) increases.
The 3GPP specification (Release 99) describes a UMTS system operated with 2-antenna transmission diversity but considers the need of transmission diversity using more than two antennas. Consideration should also be given to a transmission/reception framework for a mobile telecommunication system where an existing MS receiving signals from two transmission antennas coexists with an MS receiving signals from more than two antennas. That is, even if an MS designed to communicate with a UTRAN with 2-antenna transmission diversity is located within the coverage area of a UTRAN supporting more than 2-antenna transmission diversity, the MS should operate normally, and vice versa for an MS designed to communicate the UTRAN with more than 2-antenna transmission diversity. It is also necessary to ensure compatible operation of the more than 2-antenna transmission diversity UTRAN with the 2-antenna transmission diversity MS.
The need for compatibility is more pressing for a common pilot channel (CPICH) and a common data channel (CDCH). While a dedicated channel transmits a signal adaptively to a given number of antennas according to the characteristics and

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