KR20050075650A - Method and system for complex transmit diversity - Google Patents

Abstract

The present invention relates to a complex diversity system and method that satisfies both spatio-temporal diversity gain and signal-to-interference ratio (SNR) at the receiving end, and is suitable for high quality and high speed data transmission in downlink. A combined transmit diversity system capable of simultaneously increasing the space-time diversity gain and the SNR at the receiver by combining a STTD-OTD system with a closed-loop MRT system using transmit and receive antenna weights to derive the maximum SNR at the receiver. Provide a method.

Description

METHOD AND SYSTEM FOR COMPLEX TRANSMIT DIVERSITY

The present invention relates to transmission diversity of a mobile communication system, and more particularly, to a hybrid transmission diversity system and method for transmitting high quality and high speed data in downlink.

In general, the major performance deterioration factors that occur in mobile communication systems are fading and multi-user interference. In particular, the fading reduces the amplitude of the received signal from a few dB to several tens of dB. As a result, if the phase of the received signal distorted by fading during data demodulation is not compensated for, the quality of the entire mobile communication service is degraded due to an information error of the received data. Therefore, in order to appropriately cope with signal attenuation due to fading, various methods for realizing various types of diversity in time, frequency, and spatial domains have been studied.

Diversity techniques are generally classified into time diversity and space diversity. Among them, the spatial diversity technique is a method of transmitting and receiving a signal through a plurality of antennas that are not spatially correlated to reduce fading by using independent properties of a plurality of radio channels.

However, in the spatial diversity scheme, since a plurality of antennas should be arranged at wide intervals, a reception diversity scheme is more widely used than a transmit diversity scheme, that is, a reception diversity scheme in which a base station uses multiple reception antennas on a reverse link. It is becoming. This reception diversity technique has been used as an effective performance improvement scheme when the reverse link limits the capacity of the entire system, as in the second generation mobile communication system.

Meanwhile, in the third generation IMT-2000 system, the traffic on the forward link has much more asymmetrical characteristics than the traffic on the reverse link. Due to these characteristics, the forward link is expected to limit the overall capacity of the system. However, considering the requirements of low power, miniaturization and low cost of the terminal, the number of receiving antennas of the terminal should be limited. It's hard to expect

Therefore, in order to overcome this problem, researches on a transmit diversity scheme using a plurality of transmit antennas at the base station are actively conducted. According to this trend, in the 3GPP (Third Generation Partnership Project) IMT-2000 system of the W-CDMA wireless standard, transmission diversity such as Space Time Transmit Diversity (STTD), Time-Switched Transmit Diversity (TSTD), and TxAA (Transmit Adaptive Array) The techniques are already adopted as a standard.

Recently, a maximum ratio transmission (MRT) technique has been proposed as a method similar to the TxAA. Unlike the TxAA, the MRT uses a signal-to-noise ratio at the receiving end by simultaneously using a weight for not only a transmitting antenna but also a receiving antenna; It is a closed loop transmission diversity scheme that maximizes SNR.

STTD is a typical transmission diversity scheme in the form of an open loop. The diversity gain is obtained through space-time coding in which a channel coding technique, which is mainly applied in time, is expanded in space. It is a technique for. The STTD was first proposed by Alamouti. The STTD is a technique using two transmit antennas and a single receive antenna, but may be extended according to an increase in the number of transmit antennas according to the same space-time encoding rules. This extended STTD can improve coding gain and channel capacity without the need for additional bandwidth compared to existing technologies. In addition, since the STTD does not need feedback information from the receiving end to the transmitting end, the STTD has an advantage that there is no change in system performance due to speed and radio channel change.

Space-Time Transmit Diversity-Orthogonal Transmit Diversity (STTD-OTD) is an extension of the STTD scheme using two transmit antennas to four transmit antennas. In the STTD-OTD, the transmitter uses the same space-time coding scheme as that of the STTD, but uses a coding scheme different from that of the Alamouti scheme (using two transmitting antennas) to use four transmitting antennas. In other words, STTD-OTD uses a new space-time coding method that combines space-time coding scheme in the Orthogonal Transmit Diversity (OTD) scheme adopted as the W-CDMA standard of STTD and 3GPP to increase space-time code gain and diversity gain. In particular, when the STTD-OTD is used, the complexity of the receiver is not the same as that of the STTD using two conventional transmission antennas and a single reception antenna.

The MRT system is similar to the TxAA technique, which is one of the closed-loop transmit diversity schemes adopted as the W-CDMA standard of 3GPP.However, unlike the TxAA technique, the MRT system simultaneously uses the weights for the receive antenna as well as the transmit antenna. It is a closed loop transmission diversity scheme that maximizes SNR. To this end, unlike TxAA, which uses the normalized eigenvector corresponding to the maximum eigenvalue of the channel correlation matrix to maximize the received power at the receiver when determining the weight of the transmit antenna as the transmit antenna weight, the MRT system uses a channel to which the transmitted signals are subjected to at the receiver. It shows a simple structure that applies the transmit antenna weight in the form of estimating the information and then feeding it back to the transmitter to compensate for the channel.

In addition, in the case of determining the weight of the receiver, when two or more receive antennas are used, a weight corresponding to each receive antenna cannot be obtained separately. However, in the MRT system, when the single receive antenna is used for the target system, the receiver weight is generally set to a value of 1. Since it is fixed, the receiver structure for the transmission symbol estimation at the receiving end is simplified.

Hereinafter, the operation of the transmission diversity system to which the respective transmission diversity scheme is applied is as follows.

1 is a view for explaining the transmission and reception operation of the STTD-OTD system.

Consecutive transmission symbols ( Assuming that the symbol level scrambler 10 has four input symbols ( Symbol level scrambling is performed on the For this purpose, the symbol level scrambler 10 is a bit. Input symbol according to the combination of Determine the scrambling order. The bits ( ) Is obtained using a long PN code, and symbol level scrambling is performed before the STTD-OTD space-time encoding process. Table 1 shows the bits ( ) Is an example of a symbol sequence generated by a combination of

b ₀ b ₁ Symbol sequence 0 0 s ₁ s ₂ s ₃ s ₄ 0 1 s ₂ s ₁ s ₄ s ₃ 1 0 s ₃ s ₄ s ₁ s ₂ 1 1 s ₄ s ₃ s ₂ s ₁

The STTD-ODT encoder 11 space-time encodes a symbol sequence output from the symbol level scrambler 10, for example, a bit ( , Assuming that the combination of ) Can be represented as shown in [Table 2]. Thus, as shown in Table 2, four space-time coded symbols are transmitted during the four slot period through four transmission antennas Tx1 to Tx4.

Tx 1 Tx 2 Tx 3 Tx 4 Slot 1 s ₁ -s ₂ * s ₃ -s ₄ * Slot 2 s ₁ -s ₂ * -s ₃ s ₄ * Slot 3 s ₂ s ₁ * s ₄ s ₃ * Slot 4 S ₂ s ₁ * -s ₄ -s ₃ *

If the channel component between the transmitter and receiver , The signal received through the single receive antenna Rx1 for 4 slots ( To ) Are expressed as Equation 1 below.

here, Represents Gaussian noise. Therefore, the linear combiner 13 receives the received signal (Equation 1) To ) Is estimated by performing a decoding process as shown in Equation 2 below.

The ML (Maximum Likelihood) estimator 13 calculates the estimated symbol ( To Apply the ML detection method to the final symbol ( To ). In addition, an estimated symbol for deriving the output SNR of the receiver from Equation (2). Is expressed as Equation 3 below.

The symbol estimated by Equation 3 The output SNR of the receiving end obtained from Eq.) Is derived as shown in Equation 4 below.

here, Is the noise component added during one slot period. Noise component per slot Are assumed to have the same variance.

2 is a diagram illustrating a basic algorithm for transmission and reception of an MRT system.

Assuming an MRT system on a MIMO flat fading channel using K transmit antennas and L receive antennas, Is a complex flat fading channel coefficient from the k th transmit antenna to the l th receive antenna, and is assumed to be unchanged for one symbol interval, the channel propagation matrix having a size L × K predicted by a channel predictor (not shown). H can be defined as shown in Equation 5 below.

If the weight vector of the transmission antenna (Tx # 1 ~ Tx # K) The weight vector of the receiving antennas Rx # 1 to Rx # L is Then, the relationship between them can be defined as shown in Equation 6 below.

Where superscript Represents a Hermitian operation, Is a normalization factor and is expressed as shown in [Equation 7].

If the signal (d) transmitted through the K transmit antennas (Tx # 1 to Tx # K) is represented by a vector s of size K × 1, the equation d is obtained from Equation 6 as shown in Equation 8 below. Become.

The signal obtained after the signal transmitted through the channel represented by Equation (5) passes through the correlator banks (L correlators) 20 of the receiver is expressed as a L × 1 size vector as shown in Equation (9). Can be.

here, To Called Gaussian noise (AWGN) input to the first receive antenna, When to be. Therefore, as shown in Equation 10, the equation signal x is multiplied by the weight vector g of the receiving end, and then added by the adder 21, thereby transmitting the transmit symbol ( Estimate of ) Can be obtained.

The symbol thus estimated ( The output SNR of the receiving end obtained from < RTI ID = 0.0 >)< / RTI >

here, In this case, when diversity is not used, that is, the SNR of a single antenna system is Becomes Also here Assume that

Therefore, in order to minimize the BER (Bit Error Ratio), the output SNR of Equation 11 should be maximized. It is like maximizing. The weight vector g of the receiving end that satisfies this condition is expressed by Equation 12 below.

Here, p, q = 1,, L, and using g obtained as described above in [Equation 6] and [Equation 7] Wow You can get it easily. It should be noted that when determining the weight of the receiver from Equation 12, each weight is not calculated separately, but is calculated as a product of weights. Thus, when using two receive antennas (ie L = 2) From Equation 12 Can be easily obtained, but if more than 3 receive antennas are used, it is very difficult to obtain 3 receive weights using Equation (12). .

However, when the Alamouti STTD scheme using two transmit antennas and a single receive antenna, which are typical forms of open-loop transmit diversity, is simply extended to use four transmit antennas, orthogonality between symbols transmitted for each antenna is used. Because of this problem, system performance degradation due to intersymbol interference occurs.

On the other hand, in case of using the STTD-OTD scheme and 4 transmit antennas recently proposed as the W-CDMA standard of 3GPP, a new space-time encoding scheme is combined by combining the existing STTD technique and the OTD technique that guarantees orthogonality between transmission symbols. use. In this case, when transmitting four consecutive transmission symbols using four transmission antennas, the maximum space-time coding gain combining only the advantages of the space-time coding scheme of the STTD technique and the OTD technique can be obtained. However, unlike the closed-loop transmit diversity scheme, the open-loop transmit diversity scheme such as the STTD-OTD technique has a big advantage that there is no change in performance as the terminal speed increases, but in a slow fading channel environment, compared to the closed-loop transmit diversity scheme. It has the disadvantage of poor performance.

The MRT system, which is one of the closed loop transmission diversity schemes, is a system similar to TxAA, which is formally adopted as the closed loop transmission diversity scheme in the 3GPP IMT-2000 system of the W-CDMA radio access standard. Unlike the TxAA system that uses the eigenvector of the channel correlation matrix for the maximum power reception at the receiver as the transmit antenna weight, the MRT system uses the weights for the receive antenna as well as the transmit antenna simultaneously to maximize the signal to noise ratio at the receiver. System.

However, in the case of the MRT system, since the weight is calculated and applied only by the channel information estimated for each transmission antenna, the diversity gain is determined solely by the given channel characteristics, and thus there is a limit to the increase in the diversity gain. In addition, there is a problem in the transmission power reduction according to the weight normalization in the transmission antenna weight calculation. In addition, in case of two receiving antennas in the MRT system, each weight cannot be obtained separately in determining the weight of the receiving antenna and has to be obtained from the product of these weights.

In addition, closed-loop transmit diversity schemes such as MRT systems show better performance on slow fading channels than open-loop transmit diversity schemes such as STTD-OTD systems, but system performance degradation due to feedback of channel information under fast fading channel Has the disadvantage that occurs.

It is an object of the present invention to provide a complex transmit diversity system and method capable of simultaneously satisfying a space-time diversity gain and a signal-to-interference ratio (SNR) at a receiver.

It is another object of the present invention to provide a complex transmission diversity system and method suitable for high quality and high speed data transmission in downlink.

In order to achieve the above object, in a dedicated diversity system using a plurality of transmit antennas, the complex transmit diversity system according to the present invention transmits and receives with an open loop space-time transmit diversity system using four transmit antennas. A closed loop type transmit diversity system using antenna weights has a combined structure.

Preferably, the open-loop space-time transmission diversity system is a space-time transmit-diversity-orthogonal transmit diversity (STTD-OTD) system.

Preferably, the closed loop transmission diversity system is a maximum ratio transmission (MRT) system having a single receive antenna.

In order to achieve the above object, in a dedicated diversity system using a plurality of transmit antennas, the transmit diversity method according to the present invention is an open-loop space-time transmit diversity scheme and antenna weights using four transmit antennas. By combining a closed loop type transmission diversity scheme using a, to increase the output signal-to-noise ratio (SNR) and the space-time diversity gain of the receiver.

Preferably, the open loop transmission diversity scheme is a space-time transmit diversity-orthogonal transmit diversity scheme (STTD-OTD) technique.

Preferably, the closed loop transmission diversity scheme is a maximum ratio transmission (MRT) technique.

Preferably, the closed loop transmit diversity scheme uses a single receive antenna.

Hereinafter, the embodiments of the present invention will be described in detail.

The present invention proposes a complex system for acquiring the number of symbols that can be transmitted simultaneously and increasing the SNR at the receiving end. To this end, the present invention provides a system combining the advantages of the open loop transmit diversity scheme with the advantages of the closed loop transmit diversity scheme. That is, the present invention is a closed-loop MRT system that exhibits excellent performance in a slow channel environment with the advantages of the time-space coding technique for minimizing the interference between transmission symbols of the open-loop STTD-OTD system and the performance change according to the increase of the terminal speed. We propose a system that satisfies the advantages at the same time.

3 is a structure of a composite diversity system according to an embodiment of the present invention.

Referring to FIG. 3, the complex diversity system according to the present invention has an open loop type STTD-OTD system having four transmit antennas and a single receive antenna, and a closed loop MRT system.

In the proposed system, the transmitter has the same structure as the transmitter of the existing STTD-OTD system except that the transmit weight is multiplied by the symbol to be transmitted for each transmit antenna. In addition, in the conventional MRT receiver, a complicated calculation is required for determining the receiver weight value due to the use of a plurality of receive antennas. However, in the proposed system using a single receive antenna, the receiver weight value is set to 1. Therefore, the proposed system has a simple structure in which weights of transmission antennas are applied only at the transmitting end.

Hereinafter, a symbol transmission and reception process in the proposed composite diversity system will be described.

The four consecutive transmission data symbols are output by changing the order of the symbols inputted by the symbol level scrambler 30 of the transmitter as shown in [Table 1], and the changed symbols are displayed in the STTD-OTD encoder 31 [Table 2]. After the space-time encoding as shown in the]] is transmitted through the four transmit antenna (Tx1 ~ Tx4) for each of the four slot interval. In this case, by using Equation 8 which obtains the weight applied to each transmit antenna in the existing MRT system using two transmit antennas, the transmit antenna weight to be applied to the four transmit antennas can be derived as shown in Equation 13. have.

Here, h _j (j = 1, ..., 4) means a channel between each of the transmit antennas Tx1 to Tx4 and the single receive antenna Rx1. Also, a constant In the open loop STTD-OTD system, when the transmit power of each transmit antenna is 1 and the total transmit power is assumed to be 4, the total transmit power of the proposed system is equal to that of the open loop STTD-OTD system. This is the value to do. That is, when it is assumed that the average power of each channel component having a Gaussian distribution is 1, it means a compensation value for maintaining the transmission power for each transmission antenna due to normalization in the transmitter weight. In addition, each transmission antenna weight has a form in which the channel is compensated in advance with the channel information fed back from the terminal to the base station, and the weights of each transmission antenna are normalized by the sum of the magnitudes of four independent complex channel values.

The received signal obtained from the single receive antenna Rx1 of the proposed system can be obtained by the following Equation (14) by applying the same principle of operation of the STTD-OTD system described above.

here Is the signal received during the 4-slot interval, Denotes a Gaussian noise component. And a new variable by multiplying the channel component by the weight of each transmitting antenna Is defined as shown in [Equation 15].

In order to estimate four transmission symbols from the received signal of Equation 14, a decoding process as shown in Equation 16 is performed to estimate the transmitted symbols.

The received signal of [Equation 14] to [Equation 16] Estimation symbol by substituting If we rearrange the equation, it is expressed as [Equation 17].

In order to obtain the output SNR of the receiver with the final estimated symbol at the receiver obtained by [Equation 17], four estimated symbols Estimated symbol We derive the output SNR of the receiver only for. For this purpose, the estimated symbol of [Equation 17] Substituting the variable of [Equation 15] into and arranging it is expressed as the following [Equation 18].

Estimated symbol shown in equation (18) The output SNR of the receiver in the proposed system is derived from Equation 19 below.

here, Is the noise component added during one slot period. Means the variance of, and the noise component per every slot Are assumed to have the same variance.

Therefore, when comparing the output SNR of the receiver of [Equation 4] derived from the existing STTD-OTD system with the output SNR of the receiver of [Equation 19] derived from the proposed system, the system performance in the proposed system is compared. It can be seen that this further improves.

As described above, the present invention combines an open-loop STTD-OTD system having orthogonality between transmission symbols and a closed-loop MRT system that simultaneously uses transmit / receive antenna weights to derive the maximum SNR at the receiving end. Since the SNR can be increased at the same time, the performance of the mobile communication system can be greatly improved.

In other words, the present invention combines the advantages of the open-loop transmit diversity scheme with the advantages of the closed-loop transmit diversity scheme to improve space-time coding and terminal speed increase to minimize interference between transmission symbols in an open-loop STTD-OTD system. The advantages of no change in performance and the superior performance of low-speed channel environment in closed loop MRT system can be obtained simultaneously.

In addition, since the present invention uses a single receive antenna, as in the conventional MRT system, a complicated calculation is not necessary when determining the receive antenna weight.

In addition, the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. . Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a diagram for explaining a transmission / reception operation of a conventional STTD-OTD system having four transmission antennas.

2 is a diagram for explaining a transmission / reception operation of a conventional MRT system.

3 is a block diagram of a composite transmission diversity system according to the present invention having a combined structure of an STTD-OTD system and an MRT system.

** Explanation of symbols on the main parts of the drawing **

10,30: Symbol level scrambler 11,31: STTD-OTD encoder

12,32 Channel predictor 13,33 Linear combiner

20: correlator bank 34: ML detection unit

Claims (7)

In a dedicated diversity system using multiple transmit and receive antennas, A composite transmit diversity system comprising a combined structure of an open loop space-time transmit diversity system using four transmit antennas and a closed loop transmit diversity system using transmit and receive antenna weights. The system of claim 1, wherein the open loop space-time transmission diversity system STTD-OTD (Space-Time Transmit Diversity-Orthogonal Transmit Diversity) system. The method of claim 1, wherein the closed loop transmission diversity system A composite transmit diversity system, characterized in that it is a maximum ratio transmission (MRT) system having a single receive antenna. In a transmission diversity system using a plurality of transmit and receive antennas, By combining open-loop space-time transmit diversity scheme using four transmit antennas and closed-loop transmit diversity scheme using antenna weights, the output signal-to-noise ratio (SNR) and space-time diversity gain of the receiver are increased. Transmission diversity method. 5. The method of claim 4, wherein the open loop transmit diversity scheme A transmission diversity method, which is a Space-Time Transmit Diversity-Orthogonal Transmit Diversity (STTD-OTD) technique. The method of claim 4, wherein the closed loop transmission diversity scheme Transmission diversity method, characterized in that the MRT (Maximum Ratio Transmission) technique. The method of claim 4, wherein the closed loop transmission diversity scheme A transmit diversity method using a single receive antenna.