WO2016039786A1

WO2016039786A1 - Multi-user mimo with degenerate mimo channel and pilot design

Info

Publication number: WO2016039786A1
Application number: PCT/US2014/064198
Authority: WO
Inventors: Yi Zheng
Original assignee: Commscope Technologies Llc
Priority date: 2014-09-08
Filing date: 2014-11-06
Publication date: 2016-03-17

Abstract

In certain embodiments, a communications system includes (i) a base station having multiple base station antennas and (ii) one or more users, each user having one or more user antennas, wherein at least a first user has multiple user antennas. In the downlink direction, the base station transmits one or more data streams to each user with at least two data streams transmitted to the first user. The base station applies a different pilot signal to each different data stream, such that at least two different pilot signals get applied to the at least two different data streams transmitted to the first user. Each user uses its one or more corresponding pilot signals to perform channel estimation for its one or more data streams, such that the first user uses its two or more corresponding pilot signals to perform channel estimation for its two or more data streams.

Description

MULTI-USER MIMO WITH DEGENERATE MIMO CHANNEL AND PILOT DESIGN Cross-Reference to Related Applications [0001] This application claims the benefit of the filing date of U.S. provisional application no. 62/047,196, filed on 09/08/14, the teachings of which are incorporated herein by reference in their entirety. BACKGROUND

Field of the Invention [0002] The present invention relates to communications systems and, more specifically but not exclusively, to wireless multiple in, multiple out (MIMO) communications system. Description of the Related Art [0003] This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art. [0004] The physical limitation of wireless channels presents a fundamental challenge for reliable communications. Channels are susceptible to time-varying noise, interference, and multipaths. Diversity is perhaps one of the most-important techniques to provide reliable communications. Diversity includes time, frequency, and space diversity. MIMO

communications systems in which both the base stations and the wireless units have multiple transmit and receive antennas, employ at least space diversity to improve communications. [0005] Massive MIMO is one of the major technologies for the next-generation wireless network (5G). In massive MIMO, the number of antennas at each base station can be tens to hundreds. For time-division duplexing (TDD) systems, by exploiting channel reciprocity, the channel state information at the transmitters (CSIT) can be obtained from uplink training. In this case, the reference signal overhead scales linearly with the number of active users per cell, but is independent of the number of cooperating antennas at the base stations (BS). For massive MIMO, because of the large number of antennas at the base station, the complexity for signal processing and system overhead becomes high. The existing literature supports a single data stream per user, which limits the throughput per user even though the user equipment may have more than one antenna. BRIEF DESCRIPTION OF THE DRAWINGS [0006] Other embodiments of the invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. [0007] FIG.1 is a diagram that represents an exemplary communications system of the present disclosure; [0008] FIG.2 shows a block diagram of the signal processing associated with downlink transmissions from the base station to the K users in the exemplary communications system of FIG.1; [0009] FIG.3 is a simplified block diagram of each channel estimation block of FIG.2; [0010] FIG.4 is a block diagram of each soft MIMO detector of FIG.2; and [0011] FIG.5 represents a constellation for the communications system of FIG.1. DETAILED DESCRIPTION [0012] In certain embodiments of this disclosure, in order to support multiple data streams per user for users equipped with multiple antennas, for each data stream, there is a different pilot pattern. For a specific user, the pilot patterns for different data streams are orthogonal to each other. The pilot patterns are independent among users. The precoding matrix for one user is in the null space of the channel matrices of the other users. For a specific user, the user estimates the effective channels from the data streams to the receive antennas through the pilot patterns associated with the data streams. With the channel estimation, demodulation and decoding is performed. With decoded information, the user can re-estimate the channel and perform another round of decoding, such that the channel estimation and decoding can be iterative. Turbo codes, LDPC, and other error-correcting codes can be used for channel coding. The base station can feed back the covariance matrix of the interference matrix to the specific user to further enhance the decoding performance. [0013] FIG.1 is a diagram that represents an exemplary communications system 100 of the present disclosure. Communications system 100 includes MIMO base station 110, having M antennas 112, and a number (K>=1) of wireless units (aka users) 120(1)-120(K), where the jth user 120(j) has Nj antennas 122, where M > 1 and Nj >= 1. In a massive MIMO implementation, M >>1. For the jth user 120(j) equipped with antennas 122, data streams are

supported, where

[0014] In the downlink direction, each base station antenna 112 transmits downlink signals that are received at each user antenna 122. Similarly, in the uplink direction, each user antenna 122 transmits uplink signals that are received at each base station antenna 112.

Although FIG.1 shows these downlink/uplink paths only for user 120(1), similar paths exist for each other user in communications system 100. [0015] FIG.2 shows a block diagram of the signal processing associated with downlink transmissions from base station 110 to the K users 120(1)-120(K) in exemplary communications system 100 of FIG.1. The top half of FIG.2 represents the processing within the base station, while the bottom half of FIG.2 represents the processing within the K users. The front-end of the base station processing has K similar branches 210(1)-210(K), one for each different user 120. [0016] For the jth branch 210(j), the source data 212 to be transmitted to the jth user 120(j) is applied to channel encoder 214, which, depending on the particular implementation, could generate channel codes such as bit-interleaved coded modulation (BICM) codes, trellis-coded modulation (TCM) codes, turbo codes, convolutional codes, low-density parity-check (LDPC) codes, etc. The resulting binary, coded symbols are then mapped to an M-ary constellation by block 216. The resulting M-ary constellation symbols are then interleaved by symbol interleaver 218. [0017] If the jth user 120(j) has multiple (Nj>1) antennas 122 and if the base station is to transmit more than one

data streams to the jth user, then some appropriate coding scheme is applied to convert the serial stream of interleaved symbols into ^_^ parallel coded data streams. For example, space time codes or space frequency codes may be added to the interleaved symbols by space time/frequency codes encoder 220, where the results are separated into ^_^ data streams, where is the number of different data streams to be

transmitted simultaneously to the jth user from the base station. [0018] Into each of the data streams, a different pilot pattern (stream) is inserted in block

222, where the pilot patterns are orthogonal to each other. Note that the pilot patterns

across the users 120 may be independent and may have different frequencies. For example, for fast-moving users, the pilot frequency may be relatively high, while, for static users, the pilot frequency may be relatively low. [0019] The resulting data streams with inserted pilot patterns are precoded by

precoder in block 224 to generate M precoded streams. The size of each precoder

depends on the number of data streams for the jth user and the number M of base station

antennas 112. [0020] The resulting M precoded streams for the K users are respectively combined at multiplexing block 226 to generate M combined streams b. That is, the first precoded streams from the K different branches 210 are combined to form a first combined stream, the second precoded streams from the K different branches 210 are combined to form a second combined stream, and so on for all M sets of streams. The antenna mapper 228 respectively maps the M combined streams b for transmission by the M base station antennas 112. That is, the first combined stream x₁ in b is forwarded to the first antenna 112(1), the second combined stream x₂ in b is forwarded to the second antenna 112(2), and so on for all M combined streams and antennas. Each antenna stream x_i is modulated at a corresponding modulator 230 (e.g., using orthogonal frequency-division multiplexing (OFDM) or some other suitable modulation technique) before being transmitted by the corresponding base station antenna 112. [0021] The M transmitted signals are received at the Nj antennas 122 of the jth user 120(j) after transmission over the air, where each received antenna signal is a superposition of the M transmitted signals after traversing the over-the-air channel. The Nj received signals at the jth user are (e.g., OFDM) de-modulated at demodulator 240 in accordance with the modulation applied at the base station modulator 230. The resulting Nj demodulated signals ^_^ are applied to channel estimation block 242, which uses those demodulated signals, possibly along with feedback signals 253 (e.g., log-likelihood ratio (LLR) values) from hard decision block 252, to generate estimated channel characteristics 243 of the Nj channels corresponding to the Nj demodulated signals. Those channel estimates 243 along with the Nj demodulated signals

are applied to soft MIMO detector 244, which generates L_j streams of soft outputs, one for each transmitted signal stream to the jth user 120(j). If Lj>1, then the Li streams of soft outputs are applied to an appropriate decoder, such as space time/frequency codes decoder 246, which generates a single decoded stream, which is applied to symbol de-interleaver 248, channel decoder 250, and hard decision block 252 to generate the recovered data 254 for the jth user. [0022] The data transmitted from the MIMO base station 110 is where s the jth

^{precoding matrix, and are the}

^{pilot-inserted data streams for the jth user. The}

received signal for the jth user 120(j) is:

where: is the "over-the-air" channel matrix for the wireless

transmission path from the base station to the jth user,

is the channel coefficient from the mth base station antenna 112 to the nth antenna 122 of the jth user 120(j),

& is the pre-coding coefficient for the lth pilot-inserted data stream of the jth user

for the mth base station antenna 112,

is channel noise.

Equation (1) for the received signal can be rewritten as follows:

where:

[_0023] The effective channel for the jth user is an ^_^ by ^_^ MIMO channel with ^_^ receive antennas, ^_^ transmit antennas, and colored noise with covariance matrix 2_^34^ given by:

where H represents the matrix Hermitian operator, 9^: is the receiver noise variance, and

is the identity matrix. [0024] The covariance matrix

can be fed from the MIMO base station to the jth user either through a signaling channel or a data channel to improve the decoding performance at the user. [0025] The following sections provide further description of some of the processing blocks of FIG.2. Precoder [0026] The jth user 120(j) sends one or more pilot signals in uplink data streams, and the MIMO base station 110 estimates the channel ^^_^ using conventional uplink channel estimation techniques. With channel estimate

using eigenvalue decomposition, the eigenvalues of can be obtained. The number of data streams ^for the jth user can be

d^{ecided by the magnitudes of the eigenvalues with preset threshold with empirical values. The} _{^^ precoding vectors could be determined as follows:} Step 1. Construct an orthonormal subspace

spanned by the channel estimates for the K different users such that:

Step 2. Denote

as the ith row of which is the effective channel from the

MIMO base station to the receive antenna of the user. Project channel

row vectors of

onto ^

and obtain Compare the magnitudes of

and choose the largest one. Then, ^^is the beamforming vector of the first data stream

for the jth user. ^{Step 3. Denote}

^{to be the mth iteration constructing subspace with}

a_{nd the beamforming vectors from previous steps such that:}

Project the remaining channel row vectors

on

and obtain Compare the

magnitudes of and choose the largest one.

[0027] Step 3 is repeated until

beamforming vectors are determined. For precoder

other methods like matched filtering, zero-forcing, or MMSE can be employed alternatively. Initial channel estimation [0028] FIG.3 is a simplified block diagram of each channel estimation block 242 of FIG.2. As shown in FIG.3, channel estimation block 242 has a switch 302 and two processing blocks: an initial channel estimation block 304 and a channel re-estimation block 306, one of which is selected depending on the state of switch 302. For the initial processing, switch 302 is configured to select initial channel estimation block 304, which performs channel estimation using the pilot pattern in the data streams, e.g., using pilot-pattern matched filtering. Denote the pilot pattern for the mth data stream of the jth user as

where:

where N is the pilot sequence length. Denote as the received pilot sequence and

[0029] Right-multiplying both sides by and dividing by N yields:

[0030] After the initial pilot-based channel estimation, switch 302 is re-configured to select channel re-estimation block 306, which is described further below. Soft MIMO detector [0031] FIG.4 is a block diagram of each soft MIMO detector 244 of FIG.2. As shown in FIG.4, soft MIMO detector comprises MMSE (minimum mean square error) receiver block 402, symbol log likelihood computation log f(Y|X) block 404, and bit log likelihood computation block 406. [0032] Based on the initial estimated channel from initial channel estimation block 304

of FIG.3 and the covariance matrix eceived from the base station, the MMSE receiver

block 402 generates the initial processing output from the received data for the jth

user as follows:

where:

[0033] After the channel re-estimation, is updated, and the MMSE receiver 402 uses the

updated to evaluate Equation (12) and then applies the result from Equation (12) to evaluate

Equation (11). [0034] For symbol log likelihood computation block 404,

where X is the modulation constellation symbols (QPSK, 16QAM, 64QAM, etc.) and Y is the output from MMSE receiver 402. can be obtained from the covariance matrix .

[0035] For bit log likelihood computation block 406, the symbol likelihoods output from block 404 are transformed into bit log-likelihood ratios (LLRs) as follows:

( ) where represents the set of symbols whose nth bit is 1, and represents the set of

symbols whose nth bit is 1. An illustration of the constellation is shown in FIG.5. Symbol deinterleaver, channel decoder, and hard decision block [0036] The output from MIMO soft detector 244 of FIG.2 or optionally from space time/frequency codes decoder 246 is de-interleaved by symbol deinterleaver 248 and fed to the channel decoder 250. The output of the channel decoder 250 is fed to the hard decision block 252, which maps the decoded signals from channel decoder 250 into decoded bits. After hard decision block 252, the decoded bits are fed back to the channel estimation block 242, where channel re-estimation block 306 of FIG.3 is used to re-estimate the channel as

From the decoded output, the transmitted encoded symbols ^_^ can be reconstructed, denoted as

Assuming that the channel from the MIMO base station to the jth user is constant for n consecutive time slots, then:

where is noise in the ith time slot for the jth user. [0037] There are different ways to perform the channel re-estimation. Matched filtering and signal model reorganization techniques are described below. Another technique is an Expectation Maximization (EM) algorithm. Channel re-estimation using matched filtering [0038] To perform channel re-estimation using matched filtering, channel re-estimation block 306 of FIG.3 uses the pilots along with the decoded bits as known information and reconstructs the transmitted symbols denoted as Let N denote the decoded sequence

length including the pilots, and denote as the corresponding received signal with pilot

s_{equence and}

where is the estimated user data embedded with pilot stream for the lth data stream of the jth user. _[0039] Right-multiplying both sides of Equation (16) by and dividing by N yields:

where denotes the channel re-estimation using matched filtering.

Channel re-estimation using signal model reorganization

[0040] The second method reorganizes the signal model into an estimation problem as follows:

[0041] Reformulating Equation (17) to whiten the noise gives:

[0042] The channel re-estimation and decoding process can have several iterations to obtain good performance. Extension of space time/frequency coding to MIMO base station [0043] Space time codes or space frequency codes can be included as shown in the optional functional blocks 220 and 246 of FIG.2. Consider encoder block 220 for two data streams using Alamouti codes, for example. In that case, the input data stream is denoted in sequence as Applying the Alamouti codes, the output becomes

where * denotes complex conjugate. [0044] For four data streams

the output from the space time/frequency codes encoder 220 for rate ½ code is:

[0045] For the details of the corresponding decoder 246, refer to Vahid Tarokh, Nambi Seshadri, and A. R. Calderbank, "Space–time codes for high data rate wireless communication: Performance analysis and code construction," IEEE Transactions on Information Theory 44 (2): 744–765 (March 1998). [0046] Embodiments of the invention may be implemented as (analog, digital, or a hybrid of both analog and digital) circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing blocks in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, general-purpose computer, or other processor. [0047] As used herein in reference to an element and a standard, the term "compatible" means that the element communicates with other elements in a manner wholly or partially specified by the standard, and would be recognized by other elements as sufficiently capable of communicating with the other elements in the manner specified by the standard. The compatible element does not need to operate internally in a manner specified by the standard. [0048] The functions of the various elements shown in the figures, including any functional blocks labeled as "processors," may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term "processor" or "controller" should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. [0049] It should be appreciated by those of ordinary skill in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. [0050] Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word "about" or "approximately" preceded the value or range. [0051] It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain embodiments of this invention may be made by those skilled in the art without departing from embodiments of the invention encompassed by the following claims. [0052] In this specification including any claims, the term "each" may be used to refer to one or more specified characteristics of a plurality of previously recited elements or steps. When used with the open-ended term "comprising," the recitation of the term "each" does not exclude additional, unrecited elements or steps. Thus, it will be understood that an apparatus may have additional, unrecited elements and a method may have additional, unrecited steps, where the additional, unrecited elements or steps do not have the one or more specified characteristics. [0053] The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures. [0054] It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the invention. [0055] Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. [0056] Reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term "implementation." [0057] The embodiments covered by the claims in this application are limited to

embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non-enabled embodiments and embodiments that correspond to non-statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims.

Claims

CLAIMS What is claimed is:

1. A communications system comprising:

a base station having multiple base station antennas; and

5 one or more users, each user having one or more user antennas, wherein at least a first user has multiple user antennas, wherein, in a downlink direction from the base station to the one or more users:

the base station transmits one or more data streams to each user with at least two data streams transmitted to the first user;

10 the base station applies a different pilot signal to each different data stream, such that at least two different pilot signals get applied to the at least two different data streams transmitted to the first user; and

each user uses its one or more corresponding pilot signals to perform channel estimation for its one or more data streams, such that the first user uses its two or more corresponding pilot 15 signals to perform channel estimation for its two or more data streams.

2. The invention of claim 1, wherein the base station assigns at least two orthogonal pilot signals for the at least two data streams of the first user. 20

3. The invention of claim 1, wherein at least first and second users are assigned pilot signals having different pilot frequencies.

4. The invention of claim 3, wherein the different pilot frequencies are assigned based on different characteristics of the different users.

25

5. The invention of claim 4, wherein the system determines the different pilot frequencies based on determined motion of the users.

6. The invention of claim 3, wherein the base station assigns at least two orthogonal pilot 30 signals having the same frequency for the at least two data streams of the first user.

7. The invention of claim 1, wherein:

the base station applies an encoder to convert a serial data stream into the at least two data streams for the first user; and

35 the first user recovers at least two received data streams and applies a corresponding

decoder to convert the at least two received data streams into a serial received data stream.

8. The invention of claim 7, wherein:

the encoder is a space time/frequency codes encoder; and

the decoder is a space time/frequency codes decoder.

5

9. The invention of claim 7, wherein:

the base station inserts a different pilot signal into each of the at least two data streams for the first user; and

the base station applies pre-coding to the at least two pilot-inserted data streams for the first 10 user to generate M pre-coded data streams for the first user, where M is the number of base station antennas.

10. The invention of claim 9, wherein:

the MIMO system comprises multiple users;

15 the base station generates M pre-coded data streams for each user;

the base station combines corresponding pre-coded data streams for the multiple users to generate M combined transmit signals, one combined transmit signal for each base station antenna;

the base station applies a modulation scheme to each different combined transmit signal prior 20 to transmission from the corresponding base station antenna;

each user applies a corresponding demodulation scheme to recover a received data stream for each user antenna, wherein the first user recovers at least two received data streams;

each user performs channel estimation and soft detection to convert the one or more received data streams into a soft data stream for each data stream transmitted by the base station for the 25 user, wherein the first user generates at least two soft data streams; and

the first user applies the corresponding decoder to convert the at least two soft data streams into a serial soft data stream for further processing used to generate a single hard data stream for the first user. 30

11. The base station for the communications system of claim 1.

12. The base station for the communications system of claim 10.

13. The first user for the communications system of claim 1.

35

14. The first user for the communications system of claim 10.