Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
  • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
  • H04B7/0682—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission using phase diversity (e.g. phase sweeping)

Definitions

• the present invention relates to a wireless communication system with multiple antennas at both the transmitting and receiving ends of a fading channel, and more specifically to a method and a transmitter for improving the performance of slowly moving mobiles utilizing transmit diversity.
• multiple (or smart) antennas In wireless communication systems, the use of multiple (or smart) antennas is much preferred in order to increase the performance of the system.
• One of the basic utilizations of multiple antennas is transmit diversity where the receiver is provided with copies of the transmitted signal, transmitted from the different antennas. This will improve the robustness of the communication over the fading radio link since one can ideally assume that these copies fade independently.
• This so called transmit diversity is commonly performed by using the so called
• Alamouti code In a document entitled "A simple transmit diversity technique for wireless communication", by S. M. Alamouti, IEEE Journal of selected areas in communication, vol. 16, pp.1451-1458, Oct. 1998, the Alamouti code is described. Every group of two symbols of data to be transmitted are jointly space time encoded and transmitted from two transmitter antennas over two symbol time intervals.
• the Alamouti code is one realization of the so called space time transmit diversity (STTD) encoding and it can be widely applied to both wideband CDMA systems as well as on a narrowband subcarrier in a OFDM system.
• STTD space time transmit diversity
• the number of antennas at the transmitter can be increased to four, thereby enabling the transmission of two parallel Alamouti encoded data streams. This is called double space time transmit diversity (D-STTD).
• D-STTD double space time transmit diversity
• D-STTD Onggosanusi, et.al. Wireless Communication and Networking Conference 2002, pp.194- 199, March 2002, D-STTD is described. While increasing the data throughput, the D-STTD scheme also increases the complexity of the receiver, since the two STTD encoded data streams will interfere with each other. In order to reduce the mutual interference, an interference suppression algorithm is employed which tries to minimize this mutual interference.
• the ability of the receiver to suppress the mutual interference from the two data streams depends on the momentary characteristic of the multiple input, multiple output (MIMO) channel.
• MIMO multiple input, multiple output
• the performance will depend on the separation between the received interfering signal subspace and the received desired signal subspace. This separation changes in a fading channel with a variation dependent on the channel ' s Doppler frequency.
• the subspace separation can thus be in a constant position for a long time, which can lead to an outage for the particular user.
• the drawback of the scheme in the prior art is the necessity of a feedback channel which consumes signaling bandwidth on the reverse channel, i.e. receiver to transmitter channel.
• a method is provided, improving the performance of transmit diversity in a wireless communication system, whereby the method comprises the steps of generating a number of phase shift angle sequence and applying phase shift to one of said a number of transmitter antennas according to said at least one phase shift angle sequence.
• a transmitter comprises the phase sequence generator which is used to creating a sequence of phase shift angles and sends phase shift angles periodically to a multiplier, and a multiplier which is used to multiplying one Alamouti encoded data substream with current phase shift angle received from said phase sequence generator.
• the invention can improve the performance of transmit diversity through applying a sequence of phase shift angles to one of transmitter antennas. Without feedback, the invention needs not to consume signaling bandwidth on the reverse channel.
• Figure 1 shows schematically the D-STTD transmitter scheme with the hopping phase shift ⁇ in CDMA system.
• Figure 2a shows schematically one alternative of the D-STTD transmitter scheme with the hopping phase shift ⁇ in OFDMA system.
• Figure 2b shows schematically another alternative of the D-STTD transmitter scheme with the hopping phase shift ⁇ in OFDMA system.
• Figure 3 shows a diagram comparing the present method to the method without phase hopping.
• Figure 4 shows schematically the transmitter of one embodiment according to the invention.
• Alamouti code is one realization of the so called space time transmit diversity (STTD) encoding, and the present application is concerned with such Alamouti coding.
• STTD space time transmit diversity
• Alamouti coding provides a way to accomplish a two-branch transmit diversity with only one receiver. Two signals, S 1 and s 2; are transmitted simultaneously from the two antennas ai and a 2 , respectively, during a first symbol period. During the following symbol period, signal (- S 1 * ) is sent from antenna a 2 , and signal si is sent from antenna a 1? where * denotes the complex conjugates.
• the use of Alamouti encoding provides enhanced performance in terms of bandwidth and diversity.
• Figure 1 shows schematically the D-STTD transmitter scheme with the hopping phase shift ⁇ in CDMA system.
• a block of information bits are encoded. These coded bits are interleaved using a space time interleaver, and then mapped to a symbol constellation.
• the two streams of data symbols are space time transmit diversity encoded using the Alamouti code, and the output signal from the Alamouti encoder connected to one of the four transmit antennas is multiplied by a phase shift angle ⁇ .
• the phase shift angle ⁇ which is applied to antenna 1 is selected from a predetermined phase hopping sequence.
• the steps of generating phase hopping sequence are as following: (1). Generating K angles, ⁇ i to ⁇ ⁇ , which are in the range [0, 2 ⁇ ]. Preferably, this K angles are uniformly spaced over [0, 2 ⁇ ].
• the value of K is a design parameter, and is a natural number larger than 2. Preferably, K equals to 8.
• phase shift angle ⁇ is held constant during a time which is equal to or longer than the transmission of a group of jointly decoded Alamouti symbols, that is, over at least two consecutive symbol intervals.
• the phase changes are assumed to occur synchronous with the symbol change.
• the same phase shift ⁇ applies to the whole bandwidth in CDMA and WCDMA systems.
• the receiver estimates the channels between the transmitter-receiver antenna pairs and applies some algorithm to separate the two interfering Alamouti encoded substreams using for instance the minimum mean square error (MMSE) algorithm or a Maximum likelihood (ML) decoding algorithm.
• MMSE minimum mean square error
• ML Maximum likelihood
• the receiver should to be able to follow the changes in the concatenated channel from the antenna multiplied with the phase shift.
• One way to do this is to make sure that the receiver also knows the sequence of phase values and also knows which phase value is set at any instant in time, that is, to know the current phase value. Then the receiver can estimate the channel transfer functions from antennas using common pilots, that is, pilots without the phase shift. And then the receiver can apply the instantaneous known phase shift to the channel of antenna multiplied with the phase shift to get the channel it should use for demodulation. Another method is that the receiver estimates the current value of the phase of antenna multiplied with the phase shift and then performs the mentioned creation of the useful channel for the antenna multiplied with the phase shift. The third method is similar to the antenna verification in 3GPP WCDMA closed loop mode 1 TX diversity and is well known.
• FIG. 2a shows schematically the D-STTD transmitter scheme with the hopping phase shift ⁇ in OFDMA system.
• a block of information bits are encoded. These coded bits are interleaved using a space time interleaver, and then mapped to a symbol constellation.
• the two streams of data symbols are space time transmit diversity encoded using the Alamouti code, and are mapped to a subset of the total available subcarriers.
• the output signal connected to one of the four transmit antennas is multiplied by a phase shift angle ⁇ .
• all signals transmitted from antenna 1 is thus by a phase shift angle ⁇ . Hence, this phase shift applies to the whole transmission bandwidth.
• the phase shift angle ⁇ does not apply to the whole bandwidth in OFDMA system, which is also different to that in CDMA system.
• a coded block of data is mapped to a subset of the total available subcarriers, so the phase shift ⁇ can be made a function of both time and subcarrier index (frequency), that is, change phase shift angle every subcarrier or group N adjacent subcarrier together and have the same phase shift angle within each group but change them between the groups.
• the steps of generating phase hopping sequence are as following:
• K angles, O 1 to OK which are in the range of [0, 2 ⁇ ].
• this K angles are uniformly spaced over [0, 2 ⁇ ].
• the number of K is a design parameter.
• K equals to 8.
• sub sequence Si' ⁇ 4, 1, 7, 2, 6, 3, 5, 8, 4, 1, ⁇ for the first subcarrier/group
• sub sequence S 2 ' ⁇ 1, 7, 2, 6, 3, 5, 8, 4, 1, 7, ⁇ for the second subcarrier/group
• sub sequence S 3 ' ⁇ 7, 2, 6, 3, 5, 8, 4, 1, 7, 2, ⁇ for the third subcarrier/group, and so on.
• sub sequence Si ⁇ 4 , G 1 , B 7 , ⁇ 2 , G 6 , O 3 , ⁇ 5 , ⁇ g , ⁇ 4 , ⁇ i... ⁇ for the first subcarrier/group
• phase shift angle ⁇ to the signal of antenna 1 according to the phase hopping sequence for respective subcarrier or group of subcarriers every T symbols, that is, the signal of antenna 1 is multiplied by a new phase shift angle every T symbols and every subcarrier or a group of subcarriers, wherein T is a natural number.
• T is a natural number.
• the rate of change for ⁇ can also be increased to every symbol interval.
• phase shift ⁇ can be limited by the use of some low bandwidth feedback from the receiver. For instance, a system could use sporadic feedback to adjust ⁇ optimally but switch to the random phase hopping scheme when this feedback information is outdated due to time evolution of the channel.
• the invention can be applied with any type of receiver algorithm, such as MMSE 5 ML or iterative turbo.
• the number of receive antennas can be any number larger than or equal to two.
• a generalization of the present invention is to increase the number of parallel substreams to more than two (by increasing the number of transmitter antennas). Assume that M parallel substreams are used, and then M-I pseudo random phase shifts are applied to M-I antennas. The number of receive antennas can in this case be any number larger than M.
• phase shift ⁇ can be applied to all users' signals including a common pilot signal to simplify the receiver operation. If the pilot used for channel estimation is phase modulated by the same angle ⁇ as the data channel, the proposed invention is totally transparent to the receiver operation, under the assumption that the receiver estimates the channel at least once per ⁇ updating-interval.
• Figure 4 shows the transmitter in one embodiment of the invention. As shown in
• the transmitter consists of two
• Two Alamouti encoders perform encoding according to the well known Alamouti algorithm. Each Alamouti encoder is connected to two of the four antennas respectively, as shown in Figure 4, Alamouti encoder 407 is connected to antennas 401 and 402, while Alamouti encoder 408 is connected to antennas 403 and 404.
• the phase sequence generator 405 creates a sequence of phase shift angles and sends phase shift angles in the phase sequence periodically to the multiplier 406.
• the multiplier 406 locates between one of two Alamouti encoders and one of four antennas. In the multiplier 406, the signal of one of the antennas is multiplied W ⁇ he J ⁇ k , where ⁇ k is the current phase angle from the phase generator 405.

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• 2005
  • 2005-11-17 WO PCT/CN2005/001946 patent/WO2007056892A1/en active Application Filing
  • 2005-11-17 CN CN2005800512422A patent/CN101228710B/zh active Active

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