The application of smart antennas in TD-LTE

[Abstract] article introduced from the technical aspects of a smart antenna technology, beamforming and adaptive algorithm for single-stream beamforming smart antenna in TD-LTE, the Shuangliu beamforming technology and related algorithms, analysis The application of smart antennas in TD-LTE, the last brief development trend of smart antenna technology.

[Keywords:] an overview of the TD-LTE smart antenna beamforming smart antenna (Smart Antenna technology in microwave technology, automatic control theory, adaptive antenna technology, digital signal processing DSP (Digital Signal Processing technology and software radio technology, multi-disciplinary made on the basis of comprehensive development of a new technology. intelligent adaptive antenna array antenna is a certain degree of intelligence. early smart antenna used in the military field, since the 3G era towards civil communication in today's TD-LTE trial network and commercial networks, smart antenna technology has made rapid development.

Smart antenna technology using the signal transmission space coherence, the weight value of the transmission signal by adjusting the array element of the antenna array, a predetermined beam generating space, and the specific direction of the radio signal guide, so that the main lobe beam adaptively tracking the direction of the user the main signal arrives, sidelobe or zero parallel notch the alignment interference signal reaches a direction to achieve the full and efficient use of the mobile user signal, delete, or the dual purpose of suppressing interference signals. smart antenna can achieve the spatial filter and the positioning of the signal, in a plurality of points to the different users. under the control of the antenna beams, can significantly reduce the mutual interference between the user signal.

Smart antenna is usually used in the base station side, in the downlink transmit signal pre-weighted to achieve selective sending, can also be in the uplink received signal aliasing different weighted combination of the corresponding waveform. Smart antenna because of its increase system capacity, improve communication quality and expand cell coverage, have been widely used in TD-SCDMA and TD-LTE network. certainly is the scenario, miniaturization, power transfer technology, combining broadband and integrated smart antennas, will play an irreplaceable role in the TD-LTE and the latter part of the evolution of the system.

2 smart antenna Introduction [1]

Negative factors affect the complexity and uncertainty due to the wireless mobile communication channel transmission environment, mainly by multipath fading, delay spread, exist crosstalk between symbols, environment deterioration communication between co-channel interference and multiple access interference, direct reduces the link performance and system capacity, and smart antenna is one of the important means to solve these problems.

2.1 intelligent antenna signal model of Figure 1 is a smart antenna receiving portion thumbs, by the element, is composed of three parts of the weighted and merge attenuation and delay of user transmission signal through the multipath channel, and reaches the antenna array element is all the transmission signal and each delay a copy of the superposition.

The assumption that the system has K users, the array has M array element, for simplicity, the uniform linear array model, then the received signal vector of a certain time k-th user signals reaching the array may be expressed as:
(1
Wherein, βk, l l-path fading amplitude, τk, l l-path delay time, sk (t for the k-th transmission signal of the user. @ (Θk, l is the array response vector, and the corresponding to the k-th user channel l diameter reaches angle θk, l, can be expressed as:
(2
Where, f is the signal frequency, and satisfy fc-B / 2 ≤ f ≦ fc + B / 2, fc is the carrier frequency, B is the signal bandwidth, τ is caused due to the finite propagation speed of the signal in the adjacent antenna elements on the delay, it with the angle of arrival of the signal related to the array element intervals and the signal propagation speed can be expressed as τ = (dsinθk L / c, d for the array element spacing, usually take λc of / 2, λc of carrier wavelength, c is the signal propagation speed.

Since the receiving antenna is the superposition of all the signals of the users, so (a formula can be expressed as:

(3
Wherein η (t for the receiving end of the added white noise vector.
Array having a directivity, it is shown in Figure 1, each array element Weighted wk, according to certain criteria and signal detection requirements, and calculated by the array signal processing module, the available array beamforming weighted merged vector output as follows:
(4
(4 is the basic model of the smart antenna beamforming signal, but also smart antenna technology foundation.
2.2 beam shaping technology-(4 is the the array beamforming mathematical expression, array signal pre-processing technology, of which the weights wk merely need to match the channel slow change, DOA DOA (Direction Of Arrival and average road loss. during beamforming can also not necessary to use terminal feedback information needed to wave direction and path loss information, but rather in the base station side through the uplink reception signal, which can reduce air interface transmission burden, and the weights are calculated parameters can be easily obtained. Further, in order to obtain the beamforming gain, it is necessary to use more of the antenna unit, the LTE only up can use the four common pilot frequency, and can not support more than 4 sub antenna unit antenna array beamforming, so the beamforming also need to use a dedicated pilot.

Figure 2 is the basic principle of the beamforming process: DOA estimates obtained from the uplink signal of the antenna array, the generated weight value to the antenna weight controller, then the weight value fed back to the antenna array, the shaped beam is formed by the antenna array. Apparently , beam-forming process, the key problem can be simply stated as: (1 based on system performance (such as bit error rate, wrong frame rate requirements to determine the optimization criterion (cost function, that is, the weight vector functions and related parameters, (2 a certain method to obtain the required parameters, and (3 selects a certain algorithm for solving the optimization criterion under optimal weight vector value.

2.3 The adaptive algorithm is well known that the smart antenna is actually one including a variety of advanced technology, systems engineering, but its core technology is adaptive algorithm the typical blind adaptive algorithm and non-blind adaptive two categories. Latter method is based on the training sequence, such as the minimum mean square (LMS method, the recursive least squares (RLS method and the sample matrix inversion (SMI method, etc., the former is no training sequence, such as basic DOA estimation method, characterized in value recovery and despread re-expansion method, the DOA estimation method is a direct (4 in delayed addition method. following brief several algorithms.

(1 minimum mean square LMS algorithm: follow the minimum mean square error (MMSE criterion, according to (4-type iterative update method of the weight vector can be expressed as the estimated quadratic surface (ie, the squared error gradient on the right values, the right values ​​along the delivery degree negative direction to move a step length constant, thereby iteratively, i.e.:

Estimated output:

Errors result in:

Coefficient update:

Wherein, y (n is known the desired response sample, x (n is the received signal vector sampled sample, μ is the step length of the root distribution of the LMS algorithm convergence speed and stability with the input signal x (n covariance matrix characterized closely related to the general characteristics of the root spread is not large when LMS algorithm converges faster.

(2 recursive least squares RLS algorithm: the algorithm always make runs from the filter to the moment of total square error to a minimum, with the LMS algorithm, RLS follow guidelines is the minimum variance (LSE. set,,,, are:

Least squares error update:
The RLS algorithm convergence and correlation matrix eigenvalue expansion has nothing to do with the values ​​of λ (less than or equal to 1.

3 smart antennas in TD-LTE TD-LTE for smart antenna applications specialized standardized design, dedicated transmission mode is defined. The 3GPP R8 supports transfer mode frequency single-port 5 guide TM7 3GPP R9 support the transmission mode based on the port 7 and the port 8 dedicated pilot frequency MT8, respectively, support a single stream beamforming technology and dual beamforming technology. eNode B side, in the LTE system according to the 3GPP protocol, although the FDD and TDD are used dedicated pilot to realize beamforming, but in terms of the terminal, only the TD-LTE terminal mandatory requirements must have the ability of the demodulated beamformed data.

Practice has proved that the TD-LTE system using smart antenna can improve the system's peak rate, to enhance edge user throughput, improve cell coverage. Especially the combination of smart antennas and MIMO multi-antenna the Shuangliu beamforming technology, single-user single-user beamforming allows spatial multiplexing gain, will enable the system to multi-user diversity gain in multi-user beamforming so predictable, smart antenna technology TD-LTE system The wide range of applications, can significantly improve system performance.

3.1 TD-LTE beamforming technology [2]

(A single streaming beam shaping technology: LTE R8 defined transmission mode TM7 support during transmission, the UE through the dedicated pilot measurements based on smart antenna beamforming dedicated pilot, i.e. single current beamforming technology. to estimate beamforming after the equivalent letter Road, and coherent detection in order to able to estimate after beamforming transmission by the experience of the channel, the base station must send a and data simultaneously transmitted beam Fu-shaped reference signal, the reference signal is the UE dedicated, also called UE Proprietary pilot take the antenna port 5, traffic demodulation for the transmission mode 7 in the single stream beamformed process shown in Figure 3, layer mapping and precoding is simply a behind generated beamforming of course, is also relatively simple to-one mapping.

(2 Shuangliu beamforming technologies: LTE R9 specification defines the port 7 and port 8 two dedicated pilot transmission mode for the traffic channel demodulation TM8. Also introduced a new control signaling and antenna configuration (8 × 2, beamforming extended to Shuangliu transmission Beamforming MIMO spatial multiplexing combination Shuangliu beamforming technology. Shuangliu beamforming applications can be divided into single-user beamforming -shaped and multi-user beamforming, as shown in Figure 4 is a single stream, the Shuangliu single user and Shuangliu three multi-user, the situation of the beamforming.

ENode Bs after a single user Shuangliu beam imprinting technology: eNode Bs B of measuring uplink letter Road, to obtain an uplink channel status information based on the uplink channel information to calculate the two shaped vector, using the excipients vector to be transmitted two data stream downlink forming a single-user Shuangliu beamforming technology allows a single user at a time can be two data streaming, while shaping gain and spatial multiplexing gain access to more than the single-stream beamforming technology transmission rate, thereby increasing the system capacity.

The the over 2 users Shuangliu beam forming technology: of eNode B uplink channel information or UE feedback of the results of a multi-user matching multi-user match completed, in accordance with certain criteria to generate the beam-forming vector, the use of the resulting beam-forming vector for every A UE, each stream is shaped multi-user the Shuangliu beamforming technique utilizes a smart antenna beam orientation principle, to achieve a multi-user SDMA.

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based on TD-LTE beamforming technology, an important application is the use of space selectively to support Space Division Multiple Access (SDMA Spatial Division Multiple Access. limited application scenarios and terminal size and the number of antennas, single user, is often difficult to support the high Rank data transmission. Rank is channel matrix EBB exploded characteristic value is not 0 in the number of feature vectors, the UE will be measured Rank value RI (Rank Indicator reported to of eNode B, eNode B according to the RI can be distinguished in the space independent and unrelated to each other the number of channels when more number of system users, eNode B total can be found in the independence of the channel space two strong UE eNode B is equipped with a multi-antenna beamforming signal spatial isolation of the parallel transmission of multiple UE, which is a multi-user MIMO technology, or beamforming technology TD-LTE and MIMO technology The organic combination Therefore, the only multi-UE, Shuangliu beamforming before filling SDMA function.

3.2 TD-LTE in the smart antenna algorithm [3]

Single-stream beamforming is actually ordinary smart antenna beam forming in LTE applications, Shuangliu beamforming simply said is the typical application of multi-antenna channel singular value decomposition algorithm and its implementation mechanism have been basically mature, but the algorithm optimization there is a lot of work we need to continue efforts to this end, we have to understand some of the more typically used in LTE beamforming algorithm.

(A single-stream beamforming shaped algorithm: single-stream beamforming can be divided into long-term beamforming and short-term beamforming Shaped vector obtained, which is the most common short-term beamforming based on singular value decomposition SVD ( Singular is Value Decomposition beam Fu-shaped, long-term beam Fu shaped usually referred to wave direction DOA's beam Fu shaped in the SVD method, the transmission side from the uplink sounding guide frequency (Sounding estimate out the channel information, then the user channel carried SVD decomposition calculate a corresponding pre-coding unitary matrix. wherein SVD decomposition operation is: assuming antenna to send the number to M, the number of receiving antennas to N, then the space of the channel matrix H of dimension N × M, space channel matrix H SVD decomposition as:

H = UAVH (5

Wherein U and V are unitary matrix of dimension N x N and M × M, A is a dimension of the N × M matrix, whose diagonal elements are non-negative real number, the non-diagonal wireless is 0, and A diagonal elements λ1> = λ2> = ...> = λn, that is, in accordance with the matrix H after the size sorting singular values, wherein n is the minimum value in the M and N. obtained after Singular Value Decomposition, the unitary matrix V is the linear pre-coding.

DOA beamforming weighted vector is based on the measurement of the period of time is much larger than the channel coherence time of the channel, without the training sequence i.e. the traditional blind adaptive method, commonly used is delayed addition method.

(2 double single-user beamforming algorithm: When the transmission of the single-user, with a time-frequency resource block is assigned only to a user, the base station end of the useful signal only to perform beamforming, and enhance the power of the useful signal, a typical algorithm has a characteristic value beamforming the EBB (Eigenvalue Based Beamformin, its beamforming matrix is ​​calculated as follows:

The installed base station the number of transmission antennas for nr, the mobile station receives the number of antennas is Mr. base station to the i-th user's channel matrix Hi. The i-th user support of independent data streams for RI (RI ≤ mR.

Hi conducted SVD decomposition, we get:

(6
Wherein the descending order of nonzero singular values ​​corresponding to the feature vectors are represented by vi 1 (vi, 1, Vi, 2 (vi 2, ..., and Vi, mR, (Vi first mR column taken before ri right singular vectors expressed as a single-user multi-stream beamforming matrix:
(7
(3 Shuangliu beamforming multi-user algorithm: multi user Shuangliu beamforming algorithm such as of zero forcing ZF (Zero Forcing block angle BD (Block Diagonalization need to meet the constraints: the total number of matching user receiving antenna ≤ sent the total number of antennas. conditions limit matching the number of users, especially when the number of receiving antennas> 1, the number of paired user will be limited to the paired user of the total number of receiving antennas, so that will affect the performance of the joint scheduling gain. Currently, a better multi-user beamforming algorithm, multi-user characteristic pattern of transmission MET (the Multiuser Eigenmode Transmission algorithm DB relax the restrictions of the algorithm: the total number of matching the user's total number of data streams ≤ sending antenna, namely:
Wherein, M represents the number of paired user.
When the user data stream <number of receiving antennas, this algorithm can provide more orthogonal user pairing higher performance increase compared to BD algorithm the main steps of the algorithm:
1 compression user channel matrix: SVD decomposition on the i th user's channel matrix Hi, such as formula (6 taken ui first ri column vectors conjugated transpose, then:
A channel matrix of the user when the number of the user's data stream Ri <receiving antenna number NR, is compressed by nR rows RI line.

Suppress interference between users (to build "one for all, all for one" harmony signal transmission

Definition:

SVD decomposition

0 singular value corresponding eigenvectors which multi-beam forming matrix has been able to ensure that interfere with the user in the user signal zero limit.

3 at the same time assure paired-user interference, maximize the useful signal strength.
Beamforming algorithm will be looking for a more optimized, while suppressing the interference between users, maximizing the useful signal strength, and then once beamforming of the useful signal, the SVD decomposition, to obtain:
Which, taken before ri right singular vectors. Thought the beamforming matrix interference cancellation algorithm can guarantee the complete elimination of interference, but also the useful signal power boost and optimize system performance.

So, multi-user beamforming matrix is ​​expressed as:

(8
In short, the smart antenna is the beginning of the application of TD-LTE, has been combined with MIMO technology. Supports only single-stream beamforming in LTE R8 TM7, surface, but the eNode B can be "transparent" way or a plurality of UE scheduling in the same time-frequency resources, so as to constitute a multi-user MIMO transmission, because only the definition of a specific pilot port, so to support only a single stream beamforming eNode B is defined in the LTE R9 TM8 two UE dedicated pilot port, eNode B through the downlink control signaling, the indication of transmission of two Rank1 occupied, respectively, a pair of mutually orthogonal dedicated pilot port, to avoid the UE interference between the impact of the specific pilot channel estimation, but also ensure Multi-user MIMO has a better transmission quality.

The direction of development of the smart antenna [4]

With the evolution of TD-LTE systems, smart antennas will be made to the scenario, miniaturization, power transfer, broadband, integrated, and a fast, efficient, simple, adaptive algorithms DBF firmware direction.

Scenarios: either to adapt to the outdoor environment features beautiful antenna surface, but also to adapt to the specific environment beamforming

Miniaturization: media type smart antenna dielectric resonator instead of the conventional antenna array is ideal for small pathway,

ESC: to achieve virtual array of vertical and horizontal adjustment beam corresponding to the lower inclination and azimuth purpose by adjusting the weight of the shaped beam,
Broadband: the band can cover a variety of heterogeneous network band GSM, TD-SCDMA, TD-LTE, WLAN, WiMAX etc.
Integration: not only beautify the environment but also save resources to adapt to a variety of heterogeneous network standard, operators can simultaneously access a number of common-tower type smart antennas.

In addition, the evolution of TD-LTE system, smart antenna may also support the use of multiple cell joint debugging method, it can enable the same allocation of resources in different directions beam, use different resources to avoid interference in order to achieve the purpose of the same direction of the beam, and consider in the single cell shaped outer cell interference to the user by a nulling method to avoid the interference of the external cell-user interference suppression, also supported with a plurality of cells at the same time in some special cases for a user to perform beamforming, to achieve the purpose to enhance the signal strength short TD-LTE system smart antenna smart features will be increasingly high.

5 summarizes the smart antenna technology TD-LTE system, although only in the test network and minimal foreign commercial network, but has been widely used in TD-SCDMA, 3G systems mature technology smart antenna use strong directionality of strong spatial channel correlation and wave interference principle to form a very clear radiation pattern, its main lobe adaptive to point users to wave direction, not only greatly improve the signal-to-noise ratio of the user to obtain obvious array gain, and also enables the network to expand the coverage and improve the edge throughput and interference suppression performance.

TD-LTE network MIMO multi-antenna technology is eNode B and UE both parties are using a plurality of antennas transmitting and receiving, through appropriate emission signal form and receiver design, is not significant with the increase in system cost the same time, improve the system capacity, the array gain power gain and interference suppression gain, spatial diversity gain, spatial multiplexing gain other advantages, to bring a higher rate for the network, the effect of better coverage. combined smart antenna technology MIMO technology, TD-LTE network experience can provide users with high-speed, high-bandwidth, high-performance and short delay perception. Shuangliu beamforming is the combination of smart antennas and MIMO technology, with the advent of good algorithm, multi-stream beamforming technology will TD -LTE and the Evolution system provides better wireless communication performance.

References:

[1] to Ping smart antenna technology in IEEE 802.16e OFDMA system (AAS research [D] Hangzhou: Zhejiang University, 2006.

[2] peak Zehua, and so on. TD-LTE technical standards and practice [M] Beijing: People's Posts and Telecommunications Press, 2011.

[3] Jin Jing, WANG Qi-Star, TD-LTE multi-user Shuangliu beamforming analysis and evaluation [J]., Mobile Communications, 2012 (Z2.

[4] Yao Meiling Li. Direction of development of the smart antenna Analysis [J] Mobile Communications, 2012 (1.

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