U.S. patent application number 16/201615 was filed with the patent office on 2019-03-28 for communication support for low capability devices.
The applicant listed for this patent is Samsung Electronics Co., Ltd.. Invention is credited to Hyoung-Ju Ji, Young-Bum Kim, Aris Papasakellariou.
Application Number | 20190098576 16/201615 |
Document ID | / |
Family ID | 48610075 |
Filed Date | 2019-03-28 |
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United States Patent
Application |
20190098576 |
Kind Code |
A1 |
Papasakellariou; Aris ; et
al. |
March 28, 2019 |
COMMUNICATION SUPPORT FOR LOW CAPABILITY DEVICES
Abstract
A method for performing communication by a user equipment (UE)
in a wireless communication system is provided. The method includes
identifying a starting orthogonal frequency division multiplexing
(OFDM) symbol for a downlink (DL) channel in a subframe based on
higher layer signaling; identifying a bandwidth of the DL channel
based on a modulo operation using a cell identity and a
predetermined number; and receiving the DL channel in the subframe
based on the starting OFDM symbol and the bandwidth.
Inventors: |
Papasakellariou; Aris;
(Houston, TX) ; Ji; Hyoung-Ju; (Seoul, KR)
; Kim; Young-Bum; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung Electronics Co., Ltd. |
Gyeonggi-do |
|
KR |
|
|
Family ID: |
48610075 |
Appl. No.: |
16/201615 |
Filed: |
November 27, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15812659 |
Nov 14, 2017 |
10142933 |
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16201615 |
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15397181 |
Jan 3, 2017 |
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15812659 |
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14861596 |
Sep 22, 2015 |
9538521 |
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15397181 |
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13715174 |
Dec 14, 2012 |
9144065 |
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14861596 |
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61576695 |
Dec 16, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 2001/0093 20130101;
H04W 72/06 20130101; H04W 72/0446 20130101; H04L 27/34 20130101;
H04L 1/0025 20130101; H04L 1/0027 20130101; H04L 1/1607 20130101;
H04B 7/0626 20130101; H04L 1/0029 20130101; H04W 24/10 20130101;
H04L 1/0071 20130101; Y02D 70/21 20180101; H04L 1/0072 20130101;
H04L 27/18 20130101; H04W 72/042 20130101; H04L 1/0026 20130101;
H04W 52/0225 20130101; H04W 72/0473 20130101; Y02D 30/70 20200801;
H04L 1/0067 20130101; H04L 1/1812 20130101; H04B 7/0632
20130101 |
International
Class: |
H04W 52/02 20090101
H04W052/02; H04L 1/00 20060101 H04L001/00; H04B 7/06 20060101
H04B007/06; H04L 27/34 20060101 H04L027/34; H04L 27/18 20060101
H04L027/18; H04W 72/04 20090101 H04W072/04; H04W 72/06 20090101
H04W072/06 |
Claims
1. A method for performing communication by a user equipment (UE)
in a wireless communication system, the method comprising:
identifying a starting orthogonal frequency division multiplexing
(OFDM) symbol for a downlink (DL) channel in a subframe based on
higher layer signaling; identifying a bandwidth of the DL channel
based on a modulo operation using a cell identity and a
predetermined number; and receiving the DL channel in the subframe
based on the starting OFDM symbol and the bandwidth.
Description
PRIORITY
[0001] This application is a Continuation Application of U.S.
patent application Ser. No. 15/812,659, which was filed in the U.S.
Patent and Trademark Office on Nov. 14, 2017, now U.S. Pat. No.
10,142,933, issued on Nov. 27, 2018, which is a continuation of
U.S. patent application Ser. No. 15/397,181, which was filed in the
U.S. Patent and Trademark Office on Jan. 3, 2017, which is a
continuation of U.S. patent application Ser. No. 14/861,596, which
was filed in the U.S. Patent and Trademark Office on Sep. 22, 2015,
now U.S. Pat. No. 9,538,521, issued on Jan. 3, 2017, which is a
continuation of U.S. patent application Ser. No. 13/715,174, which
was filed in the U.S. Patent and Trademark Office on Dec. 14, 2012,
now U.S. Pat. No. 9,144,065, issued on Sep. 22, 2015, which claims
priority under 35 U.S.C. .sctn. 119(e) to U.S. Provisional Patent
Application No. 61/576,695, which was filed in the U.S. Patent and
Trademark Office on Dec. 16, 2011, the content of each of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates generally to wireless
communication systems and, more particularly, to the transmission
and reception of control channels and data channels to and from,
respectively, UEs with limited capabilities.
2. Description of the Art
[0003] A communication system includes a DownLink (DL) that conveys
transmission signals from transmission points such as Base Stations
(BSs) (which may also be referred to as NodeBs) to User Equipments
(UEs) and an UpLink (UL) that conveys transmission signals from UEs
to reception points such as NodeBs. A UE, also commonly referred to
as a terminal or a mobile station, may be fixed or mobile, and may
be a device such as a cellular phone, a personal computer device,
etc. A NodeB, which is generally a fixed station, may also be
referred to as an access point or other equivalent terminology.
[0004] DL signals include data signals, which carry information
content, control signals, and Reference Signals (RSs), which are
also known as pilot signals. A NodeB conveys data information to
UEs through respective Physical Downlink Shared CHannels (PDSCHs)
and control information through respective Physical Downlink
Control CHannels (PDCCHs). Multiple RS types may be supported, such
as a Common RS (CRS) transmitted over substantially the entire DL
BandWidth (BW) and the DeModulation RS (DMRS) transmitted in a same
BW as an associated PDSCH.
[0005] UL signals also include data signals, control signals, and
RSs. UEs convey data information to NodeBs through respective
Physical Uplink Shared CHannels (PUSCHs) and control information
through respective Physical Uplink Control CHannels (PUCCHs). A UE
transmitting data information may also convey control information
through a PUSCH. The RS may be a DMRS or a Sounding RS (SRS), which
a UE may transmit independently of a PUSCH.
[0006] Downlink Control Information (DCI) serves several purposes,
and is conveyed through DCI formats in respective PDCCHs. For
example, DCI includes DL Scheduling Assignments (SAs) for PDSCH
reception and UL SAs for PUSCH transmissions. As PDCCHs are a major
part of a total DL overhead, the resources required to transmit
PDCCHs directly reduce DL throughput. One method for reducing PDCCH
overhead is to scale it's the size of the overhead according to the
resources required to transmit the DCI formats during a DL
Transmission Time Interval (TTI). When Orthogonal Frequency
Division Multiple (OFDM) is used as a DL transmission method, a
Control Channel Format Indicator (CCFI) parameter transmitted
through a Physical Control Format Indicator CHannel (PCFICH) can be
used to indicate a number of OFDM symbols occupied by PDCCHs in a
DL TTI.
[0007] FIG. 1 is a diagram illustrating a conventional structure
for PDCCH transmissions in a DL TTI.
[0008] Referring to FIG. 1, a DL TTI includes one subframe having a
number N of OFDM symbols. In the present example, N=14. A DL
control region that includes PDCCH transmissions occupies a first M
OFDM symbols 110. A remaining N-M OFDM symbols are used primarily
for PDSCH transmissions 120. A PCFICH 130 is transmitted in some
sub-carriers, also referred to as Resource Elements (REs), of a
first OFDM symbol and includes 2 bits indicating a DL control
region size of M=1, or M=2, or M=3 OFDM symbols. Moreover, some
OFDM symbols also contain respective RS REs 140 and 150. These RSs
140 and 150 are transmitted substantially over an entire DL
operating BandWidth (BW), and are referred to as Common RSs (CRSS),
as they can be used by each UE to obtain a channel estimate for its
DL channel medium and to perform other measurements. The BW unit
for a PDSCH or a PUSCH over a subframe is referred to as a Physical
Resource Block (PRB). A PRB includes several REs, such as 12 Res,
for example.
[0009] A PDCCH and a PCFICH transmitted with the conventional
structure in FIG. 1 are referred to as C-PDCCH and a C-PCFICH,
respectively. Additional control channels may be transmitted in a
DL control region but are not shown for brevity. For example, when
using a Hybrid Automatic Repeat reQuest (HARQ) process for a
transmission of data Transport Blocks (TBs) in a PUSCH, a NodeB may
transmit HARQ-ACKnowledgement (ACK) information in a Physical
Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether
its previous transmission of each data Transport Block (TB) in a
PUSCH was correctly received (i.e., through an ACK) or incorrectly
received (i.e., through a Negative ACK (NACK)). A PHICH transmitted
with a conventional structure is referred to as C-PHICH. The
aforementioned conventional DL control channels will be jointly
referred to as C-CCHs.
[0010] FIG. 2 is a diagram illustrating a conventional encoding and
transmission process for a DCI format.
[0011] Referring to FIG. 2, a NodeB separately codes and transmits
each DCI format in a respective PDCCH. A Radio Network Temporary
Identifier (RNTI) for a UE, for which a DCI format is intended for,
masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in
order to enable the UE to identify that a particular DCI format is
intended for the UE. The CRC of (non-coded) DCI format bits 210 is
computed using a CRC computation operation 220, and the CRC is then
masked using an exclusive OR (XOR) operation 230 between CRC and
RNTI bits 240. The XOR operation 230 is defined as: XOR(0,0)=0,
XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC bits are
appended to DCI format information bits using a CRC append
operation 250, channel coding is performed using a channel coding
operation 260 (e.g. an operation using a convolutional code),
followed by rate matching operation 270 applied to allocated
resources, and finally, an interleaving and a modulation 280
operation are performed, and the output control signal 290 is
transmitted. In the present example, both a CRC and a RNTI include
16 bits.
[0012] FIG. 3 is a diagram illustrating a conventional reception
and decoding process for a DCI format.
[0013] Referring to FIG. 3, a UE receiver performs a reverse of the
operations performed by the NodeB transmitter in order to determine
whether the UE has a DCI format assignment in a DL subframe. A
received control signal 310 is demodulated and the resulting bits
are de-interleaved at operation 320, a rate matching applied at a
NodeB transmitter is restored through operation 330, and data is
subsequently decoded at operation 440. After decoding the data, DCI
format information bits 360 are obtained after extracting CRC bits
350, which are then de-masked 370 by applying the XOR operation
with a UE RNTI 380. Finally, a UE performs a CRC test 390. If the
CRC test passes, a UE determines that a DCI format corresponding to
the received control signal 310 is valid and determines parameters
for signal reception or signal transmission. If the CRC test does
not pass, a UE disregards the presumed DCI format.
[0014] The DCI format information bits correspond to several
Information Elements (IEs) such as, for example, the Resource
Allocation (RA) IE indicating the part of the DL BW or UL BW
allocated to a UE for PDSCH reception or PUSCH transmission,
respectively, the Modulation and Coding Scheme (MCS) IE indicating
the data MCS, the Transmission Power Control (TPC) IE indicating an
adjustment to the PUSCH transmission power or to the HARQ-ACK
signal transmission power in a PUCCH, the New Data Indicator (NDI)
IE informing a UE whether the scheduled data TB corresponds to a
new transmission or to a retransmission for the same HARQ process,
and so on.
[0015] In order to avoid a C-PDCCH transmission to a UE that blocks
a C-PDCCH transmission to another UE, a location of each C-PDCCH in
the time-frequency domain of a DL control region is not unique.
Therefore, a UE may perform multiple decoding operations per DL
subframe to determine whether there are any C-PDCCHs intended for
the UE in a DL subframe. The REs carrying a PDCCH are grouped into
Control Channel Elements (CCEs) in the logical domain. For a given
number of DCI format bits in FIG. 2, a number of CCEs for a
respective C-PDCCH depends on a channel coding rate (Quadrature
Phase Shift Keying (QPSK) is the modulation scheme in the present
example). A NodeB may use a lower channel coding rate (i.e., more
CCEs) for transmitting PDCCHs to UEs experiencing a low DL
Signal-to-Interference and Noise Ratio (SINR) than to UEs
experiencing a high DL SINR. The CCE aggregation levels may
include, for example, 1, 2, 4, and 8 CCEs.
[0016] For a C-PDCCH decoding process, a UE may determine a search
space for candidate C-PDCCH transmissions after the UE restores the
CCEs in the logical domain according to a common set of CCEs for
all UEs (i.e., a Common Search Space (CSS)) and according to a
UE-dedicated set of CCEs (i.e., a UE-Dedicated Search Space
(UE-DSS)). A CSS may include the first C CCEs in the logical
domain. A UE-DSS may be determined according to a pseudo-random
function having UE-common parameters as inputs, such as the
subframe number or the total number of CCEs in the subframe, and
UE-specific parameters such as the RNTI. For example, for CCE
aggregation levels L.di-elect cons.{1,2,4,8}, the CCEs
corresponding to PDCCH candidate m are given by Equation (1)
CCEs for C-PDCCH candidate m=L{(Y.sub.k+m)mod .left
brkt-bot.N.sub.CCE,k/L.right brkt-bot.}+i. (1)
[0017] In Equation (1), N.sub.CCE,k is the total number of CCEs in
subframe k, i=0, . . . , L-1, m=0, . . . , M.sub.C.sup.(L)-1, and
M.sub.C.sup.(L) is the number of C-PDCCH candidates to monitor in
the search space. Exemplary values of M.sub.C.sup.(L) for
L.di-elect cons.{1,2,4,8} are, respectively, {6, 6, 2, 2}. For the
UE-CSS, Y.sub.k=0. For the UE-DSS, Y.sub.k=(AY.sub.k-1)mod D, where
Y.sub.-1=UE_RNTI.noteq.0, A=39827, and D=65537.
[0018] DCI formats conveying information to multiple UEs are
transmitted in a CSS. Additionally, if enough CCEs remain after the
transmission of DCI formats conveying information to multiple UEs,
a CSS may also convey some DCI formats for PDSCH reception or PUSCH
transmission. A UE-DSS exclusively conveys DCI formats for PDSCH
reception or PUSCH transmission. For example, a CSS may include 16
CCEs and support 2 DCI formats with L=8 CCEs, or 4 DCI formats with
L=4 CCEs, or 1 DCI format with L=8 CCEs and 2 DCI formats with L=4
CCEs. The CCEs for a CSS are placed first in the logical domain
(prior to interleaving).
[0019] FIG. 4 is a diagram illustrating a conventional transmission
process of a DCI format in a respective C-PDCCH.
[0020] Referring to FIG. 4, after channel coding and rate matching
are performed (as described with reference to FIG. 2), encoded DCI
format bits are mapped to C-PDCCH CCEs in the logical domain. The
first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are
used for C-PDCCH transmission to UE1. The next 2 CCEs (L=2), CCE5
411 and CCE6 412, are used for C-PDCCH transmission to UE2. The
next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used for C-PDCCH
transmission to UE3. Finally, the last CCE (L=1), CCE9 431, is used
for C-PDCCH transmission to UE4.
[0021] The DCI format bits are then scrambled, at step 440, by a
binary scrambling code, and the scrambled bits are modulated at
step 450. Each CCE is further divided into Resource Element Groups
(REGs). For example, a CCE including 36 REs can be divided into 9
REGs, such that each REG includes 4 REs. In step 460, interleaving
is applied among REGs in blocks of four QPSK symbols. For example,
a block interleaver may be used where interleaving is performed on
symbol-quadruplets (i.e., four QPSK symbols corresponding to the
four REs of a REG) instead of on individual bits. After
interleaving the REGs, in step 470, a resulting series of QPSK
symbols may be shifted by J symbols, and finally, in step 480, each
QPSK symbol is mapped to an RE in a DL control region. Therefore,
in addition to RSs from NodeB transmitter antennas 491 and 492, and
other control channels, such as a PCFICH 493 and a PHICH (not
shown), REs in a DL control region contain QPSK symbols for PDCCHs
corresponding to DCI formats for UE1 494, UE2 495, UE3 496, and UE4
497.
[0022] The C-PDCCH structure in FIG. 4 uses a maximum of M=3 OFDM
symbols and transmits the signal substantially over a total DL BW.
As a consequence of using such a structure, such a control region
has a limited capacity, and therefore cannot achieve interference
coordination in the frequency domain. There are several cases where
expanded capacity or interference coordination in the frequency
domain is needed for transmission of control signals. One such case
is the extensive use of spatial multiplexing for PDSCH
transmissions, where multiple DL SAs correspond to the same PDSCH
resources. Another case is for heterogeneous networks where DL
transmissions in a first cell experience strong interference from
DL transmissions in a second cell and DL interference coordination
in the frequency domain between the two cells is needed.
[0023] Due to REG-based transmission and interleaving of C-PDCCHs,
the control region cannot be expanded to include more OFDM symbols
while maintaining compatible operation with existing UEs that
cannot be aware of such an expansion. An alternative to the
REG-based transmission and interleaving of C-PDCCHs is to extend
the control region in the PDSCH region and use individual PRBs for
transmitting new PDCCHs, which are referred to as Enhanced PDCCHs
(E-PCCCHs).
[0024] FIG. 5 is a diagram illustrating a conventional E-PDCCH
transmission structure.
[0025] Referring to FIG. 5, although E-PDCCH transmissions start
immediately after C-PDCCH transmissions 510 and are transmitted
over all remaining DL subframe symbols, they may instead always
start at a fixed location, such as the fourth OFDM symbol. E-PDCCH
transmissions occur in four PRBs, 620, 630, 640, and 650, while
remaining PRBs 660, 662, 664, 666, 668 are used for PDSCH
transmissions. As an E-PDCCH transmission over a given number of
subframe symbols may require fewer REs than the number of subframe
symbols available in a PRB, multiple E-PDCCHs may be multiplexed in
a same PRB. The multiplexing can be in any combination of possible
domains (i.e., time domain, frequency domain, or spatial domain)
and, in a manner similar to a PDCCH, an E-PDCCH includes at least
one Enhanced CCE (E-CCE).
[0026] A transmission for an extended control channel (E-PDCCH,
E-PCFICH, E-PHICH) may be in a same PRB, in which case the
transmission is referred to as localized, or over multiple PRBs, in
which case the transmission is referred to as distributed. The
aforementioned Enhanced Control CHannels are jointly referred to as
E-CCHs. The demodulation of information in an E-CCH may be based on
a CRS or on a DMRS.
[0027] FIG. 6 is a diagram illustrating a conventional DMRS
structure.
[0028] Referring to FIG. 6, DMRS REs 610 are placed in some OFDM
symbols of a PRB. When there are two NodeB transmitter antenna
ports, a first DMRS transmission is assumed to apply the Orthogonal
Covering Code (OCC) of {1, 1} over two DMRS REs that are located in
a same frequency position and are successive in the time domain
while a second DMRS transmission is assumed to apply the OCC of {1,
-1}. A UE receiver can estimate the channel experienced by the
signal from each NodeB transmitter antenna port by removing a
respective OCC.
[0029] UL Control Information (UCI) is transmitted from a UE to a
NodeB to facilitate PDSCH transmissions or PUSCH transmissions. UCI
includes HARQ-ACK information associated with a transmission of one
or more Transport Blocks (TBs) in a PDSCH, Channel State
Information (CSI) informing a NodeB about a channel experienced by
DL transmissions to a UE, and Service Request (SR) informing a
NodeB that a UE has data to transmit. CSI may include a Channel
Quality Indicator (CQI), which implicitly or explicitly informs a
NodeB of a wideband or a sub-band SINR experienced by a UE, a
Precoding Matrix Indicator (PMI), which informs of an entry in a
precoding matrix for a NodeB to apply beamforming to a DL signal
transmission, or a Rank Indicator (RI) which informs a NodeB that a
UE can support spatial multiplexing for a DL signal
transmission.
[0030] FIG. 7 is a diagram illustrating a conventional structure
for a HARQ-ACK signal transmission in one of the two subframe slots
of a PUCCH.
[0031] Referring to FIG. 7, HARQ-ACK signals and RS that enable
coherent demodulation of HARQ-ACK signals are transmitted in one
slot 710 of a PUCCH subframe including 2 slots. The transmission in
the other slot can be at a different part of the UL BW. HARQ-ACK
information bits 720 modulate 730 a Zadoff-Chu (ZC) sequence 740,
for example using Binary Phase Shift Keying (BPSK) for 1 HARQ-ACK
bit or QPSK for 2 HARQ-ACK bits, which is then transmitted after
performing a Inverse Fast Fourier Transform (IFFT) operation 750.
Each RS 760 is transmitted using an unmodulated ZC sequence.
[0032] For a UL system BW including N.sub.RB.sup.max,UL RBs, where
each RB includes N.sub.sc.sup.RB=12 REs, a ZC sequence
r.sub.u,v.sup.(.alpha.)(n) can be defined by a Cyclic Shift (CS)
.alpha. of a base ZC sequence r.sub.u,v(n) according to
r.sub.u,v.sup.(.alpha.)(n)=e.sup.j.alpha.nr.sub.u,v(n),
0.ltoreq.n.ltoreq.M.sub.sc.sup.RS, where
M.sub.sc.sup.RS=mN.sub.sc.sup.RB is a length of the ZC sequence,
1.ltoreq.m.ltoreq.N.sub.RB.sup.max,UL, and r.sub.u,v(n)=x.sub.q(n
mod N.sub.ZC.sup.RS) where a q.sup.th root ZC sequence is defined
by
x q ( m ) = exp ( - j .pi. q m ( m + 1 ) N ZC RS ) ,
##EQU00001##
0.ltoreq.m.ltoreq.N.sub.ZC.sup.RS-1 with q given by q=.left
brkt-bot.q+1/2 .right brkt-bot.+v(-1).sup..left brkt-bot.2q.right
brkt-bot. and q given by q=N.sub.ZC.sup.RS(u+1)/31. A length
N.sub.ZC.sup.RS of a ZC sequence is given by a largest prime number
such that N.sub.ZC.sup.RS<M.sub.sc.sup.RS. Multiple RS sequences
can be defined from a single base sequence through different values
of .alpha.. A PUCCH transmission is assumed to be in one RB
(M.sub.sc.sup.RS=N.sub.sc.sup.RB).
[0033] FIG. 8 is a diagram illustrating a conventional structure
for a periodic CSI signal transmission in one of the two subframe
slots of a PUCCH.
[0034] Referring to FIG. 8, CSI signals and RSs that enable
coherent demodulation of CSI signals are transmitted in one slot
810 of a PUCCH subframe including 2 slots. The transmission in the
other slot can be at a different part of the UL BW. After encoding
(using for example a block code) and modulation (using for example
QPSK) which are not shown for brevity, encoded CSI bits 820
modulate 830 a ZC sequence 840 which is then transmitted after
performing an IFFT operation 840. Each RS 850 is transmitted using
an unmodulated ZC sequence.
[0035] FIG. 9 is a diagram illustrating a transmitter for a ZC
sequence for which, without modulation, serves as an RS and with
modulation serves as a HARQ-ACK signal or as a CSI signal.
[0036] Referring to FIG. 9, a mapper 920 maps a ZC sequence 910 to
REs of an assigned transmission BW as they are indicated by RE
selection unit 925. Subsequently, an IFFT is performed by IFFT unit
930, a CS is applied to the output by CS unit 940, followed by
scrambling with a cell-specific sequence using scrambler 950, a
Cyclic Prefix (CP) is inserted by CP insertion unit 960, and the
resulting signal is filtered by filter 970. Finally, a transmission
power P.sub.PUCCH is applied by power amplifier 980 and a ZC
sequence is transmitted 990. The reverse of these operations is
performed at a NodeB receiver.
[0037] Different CSs of a ZC sequence provide orthogonal ZC
sequences. Therefore, different CSs a of a same ZC sequence can be
allocated to different UEs in a same PUCCH RB and achieve
orthogonal multiplexing for HARQ-ACK signals and RS or for CSI
signals and RS. For a RB including N.sub.sc.sup.RB=12 REs, there
are 12 different CSs. A number of usable CSs depends on the channel
dispersion characteristics, and can typically range between 3 and
12 CSs. Orthogonal multiplexing can also be in the time domain
using OCC where PUCCH symbols conveying a same signal type in each
slot are multiplied with elements of an OCC. For example, for the
structure in FIG. 7, a HARQ-ACK signal transmission can be
modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC,
while an RS transmission can be modulated by a length-3 OCC, such
as a DFT OCC. In this manner, the multiplexing capacity is
increased by a factor of 3 (determined by the OCC with the smaller
length N.sub.oc).
[0038] UEs may communicate over a total system BW or over only a
part of the system BW. The former UEs can benefit from most or all
network capabilities for PDSCH receptions or PUSCH transmissions,
are typically used by humans, and are referred to as conventional
UEs herein. The latter UEs have substantially reduced capabilities
compared to the former UEs in order to substantially reduce their
cost, are typically associated with machines, and are referred to
as Machine Type Communication (MTC) UEs herein.
[0039] MTC UEs are low cost devices targeting various low data rate
traffic applications including smart metering, intelligent
transport systems, consumer electronics, and medical devices.
Typical traffic patterns from MTC UEs are characterized by low duty
cycles and small data packets in the order of a few tens or a few
hundred bytes. MTC UEs have typically low mobility, but high
mobility MTCs, such as in motor vehicles, for example, may also
exist. Also, unlike conventional UEs, MTC UEs generate more UL than
DL traffic and a majority of DL traffic is higher layer control
information, such as Radio Resource Control (RRC) information, for
configuration of a communication with a NodeB.
[0040] Unlike conventional UEs, such as for example a smart-phone,
which may have many features, MTC UEs only have a minimum of
necessary features, and the modem is a primary contributor to the
cost of an MTC UE. Therefore, main cost drivers for MTC UEs are
Radio Frequency (RF) components and Digital Base-Band (DBB)
components mainly for the receiver. RF components include the power
amplifier, filters, transceiver radio chains, and possibly a
duplexer (for full duplex FDD operation). DBB components include a
channel estimator, a channel equalizer, a PDCCH decoder, a PDSCH
decoder, and a subframe buffer. For example, a channel estimator
may be based on a Minimum Mean Square Error (MMSE) estimator, a
channel equalizer may be an FFT, a PDCCH decoder may be a decoder
for a Tail Biting Convolutional Code (TBCC), and the data decoder
may be a decoder for a Turbo Code (TC) or a TBCC.
[0041] RF costs are related to implementation and production
methods as well as to design choices. For example, considering
economies of scale, it may be more cost effective to use the same
amplifier for conventional UEs and MTC UEs, which will also ensure
a same UL coverage, while the number of transmitter antennas for
MTC UEs may be limited to one antenna.
[0042] DBB costs are related to the communication capabilities of
MTC UEs and are dominated by the receiver complexity, which is
typically about an order of magnitude larger than the transmitter
complexity. As the channel estimator complexity, the FFT complexity
and the subframe buffering requirements are directly associated to
the reception BW, DL transmissions to MTC UEs may be over a smaller
BW than DL transmissions to conventional UEs. For example, DL
transmissions to MTC UEs may be over a 1.4 MHz BW while DL
transmissions to conventional UEs may be over a 20 MHz BW.
[0043] A complexity of a PDCCH decoder depends on a number of
decoding operations an MTC UE needs to perform per DL subframe. As
MTC UEs do not need to support a same number of transmission modes
(TMs) as conventional UEs, for example MTC UEs may not need to
support spatial multiplexing for PDSCH or for PUSCH, a maximum
number of decoding operations per DL subframe can be significantly
smaller than that for conventional UEs. A complexity of a PDSCH
decoder depends on a maximum supportable data rate. Allowing for a
relatively small maximum data rate for MTC UEs provides a limit to
an associated decoder complexity.
[0044] MTC UEs generally access the communication system in the
same manner as conventional UEs. Synchronization signals are first
acquired to establish synchronization with a NodeB followed by a
detection of a Broadcast CHannel (BCH) that conveys essential
information for subsequent communication between a NodeB and UEs
(conventional or MTC ones). Regardless of a DL BW, synchronization
signals and BCH are transmitted over a minimum DL BW located in the
center of a DL BW, such as in the middle six RBs of a DL BW, and
over a number of OFDM symbols in a subframe, for example. After
establishing communication with a NodeB, a different part of a DL
BW may be allocated to an MTC UE.
[0045] One aspect of supporting communication of MTC UEs is a
design of DL control signaling. As transmissions of C-CCHs are
distributed substantially over a total DL system BW, if MTC UEs
receive DL transmissions only in a BW smaller than the total BW,
the MTC UEs may not be able to decode C-CCHs. The use of E-CCHs can
provide DL control signaling support for MTC UEs, but due to
reduced receiver DBB capabilities of MTC UEs, it may not be
possible to use a same design of E-CCHs for MTC UEs and for
conventional UEs.
[0046] Another aspect of supporting communication of MTC UEs is a
reduction in an overhead associated with DL control signaling and
UL control signaling for MTC UEs. As data TBs associated with MTC
UEs are typically substantially smaller than the ones associated
with conventional UEs, applying same DL or UL control or data
multiplexing mechanisms for MTC UEs as for conventional UEs will
cause resource utilization for MTC UEs to be substantially worse
than the resource utilization for conventional UEs.
[0047] Another aspect of supporting communication of MTC UEs is
applying a set of functionalities associated with signal
transmissions to or from MTC UEs. If signal transmissions for MTC
UEs are over a smaller BW than for conventional UEs, different
functionalities can be often needed for MTC UEs compared to
conventional UEs. Higher power utilization can also be desirable
for MTC UEs.
[0048] Therefore, there is a need to design control signaling and
data or control multiplexing for MTC UEs.
[0049] There is also a need to reduce control overhead for MTC
UEs.
[0050] There is also a need to define, whenever necessary,
different functionalities for signal transmissions to or from MTC
UEs compared to conventional UEs and to increase power savings for
MTC UEs.
SUMMARY OF THE INVENTION
[0051] Accordingly, an aspect of the present invention is to
address at least the aforementioned limitations and problems and
provide at least the advantages described below. An aspect of the
present invention provides methods and apparatus for a UE with
reduced capabilities (MTC UE) to transmit and receive signaling and
to conserve power in a network that also supports conventional
UEs.
[0052] According to an aspect of the present invention, a method
for performing communication by a user equipment (UE) in a wireless
communication system is provided. The method includes identifying a
starting orthogonal frequency division multiplexing (OFDM) symbol
for a downlink (DL) channel in a subframe based on higher layer
signaling; identifying a bandwidth of the DL channel based on a
modulo operation using a cell identity and a predetermined number;
and receiving the DL channel in the subframe based on the starting
OFDM symbol and the bandwidth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] The above and other aspects, features, and advantages of the
present invention will be more apparent from the following detailed
description taken in conjunction with the accompanying drawings, in
which:
[0054] FIG. 1 is a diagram illustrating a conventional structure
for PDCCH transmissions in a DL TTI;
[0055] FIG. 2 is a diagram illustrating a conventional encoding and
transmission process for a DCI format;
[0056] FIG. 3 is a diagram illustrating a conventional reception
and decoding process for a DCI format;
[0057] FIG. 4 is a diagram illustrating a conventional transmission
process of a DCI format in a respective C-PDCCH;
[0058] FIG. 5 is a diagram illustrating a conventional E-PDCCH
transmission structure;
[0059] FIG. 6 is a diagram illustrating a conventional DMRS
structure;
[0060] FIG. 7 is a diagram illustrating a conventional structure
for a HARQ-ACK signal transmission in one of the two subframe slots
of a PUCCH;
[0061] FIG. 8 is a diagram illustrating a conventional structure
for periodic CSI signal transmission in one of the two subframe
slots of a PUCCH;
[0062] FIG. 9 is a diagram illustrating a conventional transmitter
for a ZC sequence, which, without modulation, serves as an RS and
with modulation serves as a HARQ-ACK signal or as a CSI signal;
[0063] FIG. 10 is a diagram illustrating a general principle of DL
signaling to an MTC UE according to an embodiment of the present
invention;
[0064] FIG. 11 is a diagram illustrating a multiplexing of CCHs,
having a same structure as C-CCHs, and PDSCHs for MTC UEs in OFDM
symbols of a DL subframe according to an embodiment of the present
invention;
[0065] FIG. 12 is a diagram illustrating a multiplexing of E-CCHs
and PDSCHs for MTC UEs according to an embodiment of the present
invention;
[0066] FIG. 13 is a diagram illustrating a multiplexing of PDSCHs
and E-CCHs using a granularity of a half PRB according to an
embodiment of the present invention;
[0067] FIG. 14 is a diagram illustrating a CRS transmission only in
a DL BW allocated to an MTC UE according to an embodiment of the
present invention;
[0068] FIG. 15 is a diagram illustrating a first approach to
increase a multiplexing capacity of periodic CQI transmissions from
MTC UEs according to an embodiment of the present invention;
[0069] FIG. 16 is a diagram illustrating a second approach to
increase a multiplexing capacity of periodic CSI transmissions from
MTC UEs according to an embodiment of the present invention;
[0070] FIG. 17 is a diagram illustrating a third approach for
increasing a multiplexing capacity of periodic CSI transmissions
for MTC UEs according to an embodiment of the present
invention;
[0071] FIG. 18 is a diagram illustrating a process for an MTC UE to
perform SRS transmissions according to an embodiment of the present
invention; and
[0072] FIG. 19 is a diagram illustrating a DL BW determination by
an MTC UE after BCH detection according to an embodiment of the
present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0073] Various embodiments of the present invention are described
hereinafter with reference to the accompanying drawings. Throughout
the drawings, the same drawing reference numerals may refer to the
same or similar elements, features and structures. In the following
description, specific details such as detailed configuration and
components are provided to assist the overall understanding of
embodiments of the present invention. Therefore, it should be
apparent to those skilled in the art that various changes and
modifications of the embodiments described herein can be made
without departing from the scope and spirit of the invention. In
addition, descriptions of well-known functions and constructions
may be omitted for clarity and conciseness.
[0074] Additionally, although embodiments of the present invention
are described herein with reference to Orthogonal Frequency
Division Multiplexing (OFDM), embodiments of the present invention
are also are applicable to all Frequency Division Multiplexing
(FDM) transmissions in general and to Discrete Fourier Transform
(DFT)-spread OFDM in particular.
[0075] An embodiment of the invention that considers a DL control
signaling design for MTC UEs is described as follows.
[0076] Higher layer signaling from a NodeB to an MTC UE may
indicate that DL signaling (for control or data) to the MTC UE
begins at any OFDM symbol between a first OFDM symbol of a DL
subframe and an OFDM symbol after a maximum number of OFDM symbols
used for transmissions of C-CCHs. In practice, this is equivalent
to a NodeB informing an MTC UE of a number of OFDM symbols the MTC
UE needs to assume as used for transmissions of C-CCHs (regardless
of whether there are transmissions of C-CCHs) to conventional UEs.
Due to a reduced DL BW capability, an MTC UE cannot generally
correctly decode C-CCHs transmitted over a wider DL BW and intended
to conventional UEs, OFDM symbols conveying C-CCHs should be
dismissed by MTC UEs. Therefore, DL signaling to MTC UEs may be in
a fraction of a DL subframe instead of being distributed over an
entire DL subframe as for conventional UEs.
[0077] A higher layer signaling indication to an MTC UE of a
starting subframe symbol for DL signaling may also be a function of
a DL subframe number as certain DL subframes may convey different
traffic types (for example, unicast or broadcast) and be associated
with a different maximum number of OFDM symbols for transmissions
of C-CCHs. For example, in some DL subframes this maximum number of
OFDM symbols can be three, while in other subframes there is a
maximum of two OFDM symbols. Although a number of OFDM symbols used
for transmitting C-CCHs is never zero, this value may still be
indicated to MTC UEs. It is up to a network to avoid transmitting
any C-CCHs to conventional UEs that do not need to be aware of this
event. However, an MTC UE may need to exclude CRS REs from REs
conveying DL signaling (control or data) as, unlike C-CCH REs, a
NodeB cannot replace signaling of CRS with DL signaling to MTC UEs.
An alternative to higher layer signaling is for MTC UEs to assume a
maximum number of OFDM symbols for transmitting C-CCHs. However, in
many cases, this assumption can be wasteful as transmissions of
C-CCHs may require less than a maximum number of OFDM symbols.
[0078] FIG. 10 is a diagram illustrating a general principle of DL
signaling to an MTC UE according to an embodiment of the present
invention.
[0079] Referring to FIG. 10, an MTC UE is informed, through higher
layer signaling, of OFDM symbols available for DL signaling 1010 at
a given subframe (or the MTC UE may assume a maximum number of OFDM
symbols for transmissions of C-CCHs). OFDM symbols available for DL
signaling to the MTC UE may or may not include all DL subframe
symbols. The MTC UE is also informed, through higher layer
signaling, of a DL BW part allocated for DL signaling 1020.
Different MTC UEs may be informed of different parts of a DL BW,
but all MTC UEs are informed of the same OFDM symbols for DL
signaling. Therefore, higher layer signaling indicating OFDM
symbols for DL signaling may be common to all MTC UEs while higher
layer signaling indicating a DL BW for DL signaling may be specific
to each MTC UE. Conventional UEs have all DL subframe symbols 1030
available and potentially also have all DL BW (if there are no DL
transmissions to MTC UEs) 1040 available, and the conventional UEs
do not need be aware of a presence of MTC UEs.
[0080] After an MTC UE is informed of a number of OFDM symbols and
of a DL BW part for DL signaling, physical structures for
transmissions of control channels and data channels need to be
defined. A PDSCH transmission to an MTC UE can be the same as for a
conventional UE and can occur over a maximum number of PRBs
corresponding to an allocated DL BW part and to a number of
available OFDM symbols.
[0081] In a first approach, DL control signaling support for an MTC
UE is provided through CCHs having the same structure as C-CCHs but
which, unlike C-CCHs, are transmitted only in a DL BW allocated to
the MTC UE instead of being transmitted over substantially the
entire DL BW.
[0082] FIG. 11 is a diagram illustrating a multiplexing of CCHs,
having a same structure as C-CCHs, and PDSCHs for MTC UEs in OFDM
symbols of a DL subframe according to an embodiment of the present
invention.
[0083] Referring to FIG. 11, a first two OFDM symbols available for
DL signaling to an MTC UE, as described in FIG. 10, are used for
transmissions of CCHs 1110. Remaining OFDM symbols of a DL subframe
are used for transmissions of PDSCHs 1120. Transmissions of CCHs to
the MTC UE span an entire DL BW 1130 allocated to the MTC UE.
[0084] A duration for transmitting CCHs to an MTC UE may be
configured by higher layer signaling and, unlike a configuration
for conventional UEs, a respective PCFICH transmission in every
subframe may be avoided. This is because due to a typically small
DL BW, savings from dynamically dimensioning in every subframe CCH
resources for MTC UEs are largely offset by the resource overhead
required for reliable PCFICH detection by all MTC UEs in a same
allocated DL BW.
[0085] Depending on a NodeB scheduler decision, PDSCH transmissions
to MTC UEs may span or may not span all DL BW allocated to MTC UEs.
However, in the latter case, DL BW not used for PDSCH transmissions
to MTC UEs cannot be used in practice for PDSCH transmissions to
conventional UEs as some OFDM symbols always contain CCH
transmissions to MTC UEs. For example, it is likely in practice
that most CCHs for MTC UEs schedule PUSCH transmissions and most of
the DL BW after transmissions of CCHs to MTC UEs remain
unutilized.
[0086] In a second approach according to an embodiment of the
present invention that avoids the above shortcoming associated with
using the structure of C-CCHs for transmitting CCHs to MTC UEs, the
structure of E-CCHs is used for transmitting CCHs to MTC UEs.
Although E-CCHs may also be used for conventional UEs, a different
design is needed for MTC UEs due to aforementioned RF and DBB
limitations.
[0087] FIG. 12 is a diagram illustrating a multiplexing of E-CCHs
and PDSCHs for MTC UEs according to an embodiment of the present
invention.
[0088] Referring to FIG. 12, E-CCHs are transmitted to MTC UEs over
all respective OFDM symbols available for DL transmissions, as
described in FIG. 10, and over two PRBs 1210A and 1210B (of a DL BW
part allocated to MTC UEs) that are provided to respective MTC UEs
through higher layer signaling. Remaining PRBs can be used for
PDSCH transmissions 1220 either to MTC UEs or to conventional
UEs.
[0089] As a PRB granularity may be too large for PDSCH
transmissions to MTC UEs, which typically convey small data packets
or configuration control information by higher layer signaling, a
smaller granularity may be used for transmitting E-CCHs or PDSCHs
to MTC UEs. For example, a minimum resource allocation unit can be
half a PRB or equivalently a second PRB type, which includes half
the REs of a conventional PRB, can be used. The DMRS associated
with each channel (E-CCH or PDSCH) can be transmitted over the
whole PRB, such as illustrated in FIG. 6. Multiplexing PDSCHs to
different MTC UEs in one PRB can also be performed in the same
manner. In this case, a UE may determine which half of a PRB a
respective PDSCH is transmitted, either by configuration from the
network, or by explicit indication by one bit in a DL SA, or by
implicit indication (for example, depending on whether the UE is
configured an odd or even RNTI). The DMRS antenna port associated
with a respective MTC UE may be predetermined as configured to the
MTC UE by higher layer signaling or included in a field of a DCI
format scheduling the PDSCH reception.
[0090] FIG. 13 is a diagram illustrating a multiplexing of PDSCHs
and E-CCHs using a granularity of a half PRB according to an
embodiment of the present invention.
[0091] Referring to FIG. 13, E-CCHs (or PDSCHs) are transmitted in
two half PRBs located at the two edges of a DL BW allocated to MTC
UEs 1310A and 1310B. The other half of each of the 2 PRBs
containing transmissions of E-CCHs is allocated to PDSCH
transmissions to MTC UEs 1320A and 1320B. Remaining PRBs 1330 may
be allocated for PDSCH transmissions to MTC UEs or to conventional
UEs. As typically more UL data traffic than DL data traffic is
associated with MTC UEs, resources allocated to transmissions of
E-CCHs to MTC UEs may be similar to or more than resources
allocated to transmissions of PDSCHs to MTC UEs.
[0092] Multiplexing PUSCHs from different MTC UEs in one PRB can
also be performed in a similar manner as for PDSCHs and the
associated UL DMRS can be transmitted in one PRB. The UL DMRS from
different MTC UEs can be orthogonally multiplexed using different
CS of a respective ZC sequence which may be predetermined as
configured to the MTC UE by higher layer signaling or included in a
field of a DCI format scheduling the PUSCH transmission.
[0093] Due to their reduced DBB capabilities, MTC UEs may not
support both distributed E-CCHs and localized E-CCHs while
conventional UEs may support both E-CCH transmission types. A
transmission type for an E-CCH can be same as for a PDSCH and may
depend on a RF capability of an MTC UE. If a DL BW supported by an
MTC UE is small enough for transmissions of DL signals to not
experience significant frequency selectivity, a detection
performance difference between a localized E-CCH and a distributed
E-CCH will not be significant, as a channel response experienced by
a respective transmission will be similar. Distributed
transmissions for both E-CCHs and PDSCH may offer interference
diversity while leveraging on existing NodeB and UE implementations
using transmitter antenna diversity.
[0094] Conversely, if a DL BW supported by an MTC UE is large
enough for transmissions of DL signals to experience significant
frequency selectivity, a detection performance difference between
localized E-CCHs and distributed E-CCHs can be significant and
depend on an availability of accurate, PRB-based, CSI at a NodeB.
Then, localized PDSCH and E-CCH can be precoded and the NodeB can
select PRBs where an MTC UE experiences large DL SINR. Considering
an UL control overhead required for a CSI feedback to track channel
variations and provide sufficient accuracy for beam-forming or
Frequency Domain Scheduling (FDS) of PDSCH or E-CCH transmissions,
localized PDSCH and E-CCH transmissions may be practically feasible
only for MTC UEs with very limited or no mobility for which
infrequent CSI feedback suffices. Otherwise, if accurate PRB-based
CSI for an MTC UE is not available at a NodeB, distributed
transmissions may substantially outperform localized
transmissions.
[0095] When an E-CCH transmission to an MTC UE is distributed over
multiple PRBs, two alternatives for a respective RS structure for
demodulating a control signal in the E-CCH are described as
follows.
[0096] A first alternative uses a CRS structure as shown in FIG. 1.
However, unlike the CRS structure in FIG. 1, which is wideband and
substantially occupies an entire DL BW, a CRS used by an MTC UE may
be contained only in a DL BW allocated to the MTC UE and may not
extend in a remaining DL BW. Therefore, a CRS may be commonly used
by MTC UEs but may not be used by conventional UEs.
[0097] FIG. 14 is a diagram illustrating a CRS transmission only in
a DL BW allocated to an MTC UE according to an embodiment of the
present invention.
[0098] Referring to FIG. 14, which depicts an example corresponding
to two NodeB transmitter antenna ports, a CRS from a first antenna
port 1410 and a CRS from a second antenna port 1420 are transmitted
in REs of a DL BW allocated to MTC UEs 1430 in same OFDM symbols as
in FIG. 1. CRS transmissions may or may not occur in REs of a DL BW
allocated to conventional UEs 1440A and 1440B.
[0099] If a PRB in a DL BW allocated to MTC UEs is not used for
E-CCH or PDSCH transmissions to MTC UEs, the PRB may be allocated
to a PDSCH transmission to a conventional UE. Then, if CRS is
transmitted in REs of that PRB and if in respective OFDM symbols
there are no CRS transmissions in a DL BW allocated to conventional
UEs, a conventional UE may not be aware of an existence of CRS and
its PDSCH detection will be degraded. This degradation can be
avoided if MTC UEs assume presence of CRS only in the PRB of a
respective E-CCH or PDSCH transmission but this will instead result
to worse channel estimation for MTC UEs and associated E-CCH or
PDSCH performance degradation.
[0100] A second alternative uses a DMRS, which is transmitted only
in PRBs used by a respective E-CCH or PDSCH transmission. This
avoids the above tradeoff from using a CRS and allows flexible use
of PRBs in a DL BW allocated to MTC UEs for transmissions to
conventional UEs. To achieve transmit diversity, different
precoding in different REs or different PRBs may be used for the
DMRS. However, prior to establishing communication with a NodeB,
MTC UEs must detect a BCH, which may be transmitted by a NodeB
using antenna transmitter diversity and received by MTC UEs based
on CRS, MTC UEs should be able to perform channel estimation and
demodulation based on both CRS and DMRS. Therefore, a tradeoff for
the RS structure used between the first alternative (CRS) and the
second alternative (DMRS) is a receiver implementation simplicity
offered by the former versus a more efficient DL resource
utilization offered by the latter.
[0101] Another embodiment of the invention that considers
reductions in control signaling overhead for MTC UEs is described
as follows.
[0102] As sizes of data packets for MTC UEs are typically
substantially smaller than sizes of data packets for conventional
UEs, and as a number of MTC UEs with PDSCH or PUSCH scheduling in a
DL subframe may be greater than a respective number of conventional
UEs, a relative control signaling overhead associated with MTC UEs
may become significant. For example, although a DCI format
scheduling PDSCH or PUSCH to an MTC UE may address a much smaller
BW than a DCI format scheduling PDSCH or PUSCH to a conventional
UE, a reduction in a DCI format size may not be proportional to a
reduction in a BW size due to an existence of fields with fixed
size such as the RNTI/CRC field. Also, it is not possible to reduce
resources for transmitting HARQ-ACK signals conveying single binary
information (ACK or NACK) regarding a reception of a data TB.
[0103] Therefore, if the same design principles as for conventional
UEs are followed for MTC UEs, a control information size relative
to a data information size can become significantly larger for MTC
UEs than for conventional UEs leading to a proportionally much
larger relative control overhead for MTC UEs. A control signaling
overhead for MTC UEs may further increase if respective
transmissions are constrained to be over a smaller BW than the BWs
for conventional UEs. This constraint reduces frequency diversity
or frequency scheduling gains thereby necessitating a use of more
resources (time/frequency/power) in order to maintain same
detection reliability targets.
[0104] A reduction or avoidance of control signaling overhead
associated with HARQ-ACK signal transmissions either from a NodeB
or from an MTC UE in response to, respectively, detections of data
TBs in a PUSCH or a PDSCH is subsequently considered.
[0105] For a dynamically scheduled PUSCH, assuming than an MTC UE
keeps received data for a HARQ process in its buffer until a
respective NDI bit is toggled, an NDI field in a corresponding DCI
format avoids a need for explicit HARQ-ACK signaling. For
semi-persistently scheduled (SPS) PUSCH, associated applications
are typically related to file transfers, which are delay tolerant.
Then, a higher layer ARQ, such as a Radio Link Control (RLC) ARQ,
is sufficient to trigger a retransmission whenever a PUSCH is not
correctly received. A network may also avoid a latency associated
with higher layer ARQ by dynamically scheduling a PUSCH
retransmission, for example, when an overhead for transmitting an
associated DCI format is not a concern, such as during off-peak
hours, for example. Therefore, the NDI field can be preserved in
DCI formats scheduling PUSCH to provide HARQ-ACK information and
explicit HARQ-ACK signaling to MTC UEs through respective PHICHs
may not be supported.
[0106] PDSCH transmissions to MTC UEs are typically used for
configuration of transmission parameters and to provide respective
control information by higher layers. Therefore, PDSCH
transmissions are not as frequent as PUSCH transmissions from MTC
UEs and do not consume as many resources. Such transmissions enable
a network to target a more reliable PDSCH than PUSCH reception. A
network may also derive whether a PDSCH was correctly or
incorrectly received by monitoring a response from an MTC UE to the
configuration information. For example, if a PDSCH configures an
SRS transmission from an MTC UE, a network may detect a presence or
absence of an SRS transmission with configured parameters and
determine that the MTC UE correctly or incorrectly, respectively,
received the PDSCH. Additionally, if a network schedules both a
PDSCH and a PUSCH to an MTC UE in a same subframe, as can be
expected in practice due to UE dominant traffic for MTC UEs, the
MTC UE can include HARQ-ACK information for the PDSCH reception in
the PUSCH transmission. Therefore, in addition to higher layer ARQ,
sufficient means exist for a NodeB to obtain information of whether
an MTC UE correctly or incorrectly received a PDSCH and therefore
separate transmission of a HARQ-ACK signal from an MTC UE is not
needed. This also avoids the need to include TPC commands for
HARQ-ACK signal transmission in a PUCCH in DCI formats scheduling
PDSCH.
[0107] Therefore, embodiments of present the invention consider
that physical layer retransmissions for an HARQ process are
supported for MTC UEs but only through a use of a NDI field when
using a PUSCH transmission or by including HARQ-ACK information in
a PUSCH when using a PDSCH transmission and the respective DCI
formats are accordingly designed. Benefits from avoiding explicit
support of HARQ-ACK signal transmissions in response to a PDSCH or
to a PUSCH detection include an associated control signaling
overhead reduction, avoidance of resource fragmentation, a simpler
system design, and a reduced DBB design complexity for MTC UEs.
[0108] Another embodiment of the invention that considers the
design of different functionalities for signal transmissions to or
from MTC UEs compared to conventional UEs is described as
follows.
[0109] A first functionality that needs to be modified for MTC UEs
compared to conventional UEs is CSI reporting. Conventional UEs
compute a wideband CQI based on an unrestricted observation
interval in time and frequency and derive for each CQI value a CQI
index between 1 and 15 for which a single PDSCH TB with a
combination of modulation scheme and TB size corresponding to the
CQI index, and occupying a group of DL PRBs could be received with
a TB error probability not exceeding 0.1. If this is not possible,
a CQI index of 0 is reported by the conventional UE. An
interpretation of the CQI indices is given in Table 1.
TABLE-US-00001 TABLE 1 4-bit CQI Table CQI CQI Bits index
Modulation code rate .times. 1024 efficiency 0000 0 out of range
0001 1 QPSK 78 0.1523 0010 2 QPSK 120 0.2344 0011 3 QPSK 193 0.3770
0100 4 QPSK 308 0.6016 0101 5 QPSK 449 0.8770 0110 6 QPSK 602
1.1758 0111 7 16QAM 378 1.4766 1000 8 16QAM 490 1.9141 1001 9 16QAM
616 2.4063 1010 10 64QAM 466 2.7305 1011 11 64QAM 567 3.3223 1100
12 64QAM 666 3.9023 1101 13 64QAM 772 4.5234 1110 14 64QAM 873
5.1152 1111 15 64QAM 948 5.5547
[0110] As MTC UEs need to have reduced DBB cost and capabilities,
MTC UEs may support only QPSK modulation for data transmitted in a
PDSCH TB. Therefore, only the first 7 CQI indices in Table 1 are
applicable to wideband CQI reporting from MTC UEs. Alternatively, a
larger granularity for a spectral efficiency may be supported and
only 8 of the 16 values in Table 1 may be indicated. Then, a
respective 3-bit CQI table is given in Table 2 (assuming use of
only QPSK modulation--a similar table can be constructed if only
every other efficiency from Table 1 is reported). Moreover, as MTC
UEs receive PDSCH only in an allocated DL BW, which can be less
than a total DL BW available to conventional UEs, CQI reporting for
MTC UEs should be restricted in frequency only in the allocated DL
BW.
TABLE-US-00002 TABLE 2 3-bit CQI Table CQI CQI Bits index
Modulation code rate .times. 1024 efficiency 000 0 out of range 001
1 QPSK 78 0.1523 010 2 QPSK 120 0.2344 011 3 QPSK 193 0.3770 100 4
QPSK 308 0.6016 101 5 QPSK 449 0.8770 110 6 QPSK 602 1.1758 111 7
QPSK reserved reserved
[0111] Additional differences in CQI reporting functionalities
between conventional UEs and MTC UEs can include reporting support
of sub-band CQI, PMI, and RI. As a conventional UE can receive
PDSCH at any part of a total DL BW, CQI reporting corresponding to
sub-bands of the total DL BW may be configured in order to enable
FDS. Conversely, because MTC UEs can receive PDSCH only in a small
DL BW, support of sub-band CQI is not needed.
[0112] A conventional UE may also support PDSCH spatial
multiplexing and reception of large data TBs and report a support
of a PDSCH transmission rank larger than one if it experiences
favorable DL channel conditions. Conversely, as, according to the
present embodiment of the present invention, MTC UEs do not support
PDSCH spatial multiplexing and received data TBs are typically
small, there is no need for a PDSCH transmission rank reporting
from MTC UEs.
[0113] For CRS-based PDSCH and CCH demodulation at MTC UEs,
respective transmissions are not precoded and, therefore, there is
no need for MTC UEs to report PMI.
[0114] For DMRS-based PDSCH and CCH demodulation at an MTC UE and
an FDD system, respective transmissions may be precoded and the MTC
UE may report a PMI to enable non-random precoding of a signal from
a NodeB. For DMRS-based PDSCH and CCH demodulation at an MTC UE and
a TDD system, respective transmissions may be precoded but, due to
the reciprocity of the DL and UL channels, an MTC UE does not need
to report a PMI to enable precoding of a signal from a NodeB since
this information is obtained by the NodeB through a DMRS or SRS
transmission by the MTC UE.
[0115] When an MTC UE reports only a wideband CQI of 3 bits, a
required SINR to achieve a target detection reliability is
significantly decreased compared to a respective one for a CSI of
4-11 bits that may include wideband CQI, sub-band CQI, and PMI.
This SINR reduction can be exploited to reduce an associated PUCCH
overhead by increasing a multiplexing capacity of MTC UEs per
PRB.
[0116] One approach to increase (double) a multiplexing capacity of
periodic CQI transmissions for MTC UEs is to allow transmission of
periodic CQI over one slot instead of over one subframe. A duration
of a periodic CQI transmission (slot or subframe) can be configured
to an MTC UE by a NodeB through higher layer signaling (in addition
to parameters such as a PUCCH RB, the CS for a ZC sequence, a
transmission period, etc.). MTC UEs that are not
UL-coverage-limited can be configured to transmit in one slot of a
PUCCH subframe. A performance degradation compared to a
conventional periodic CSI structure will include a 3 decibel (dB)
loss, due to reducing a transmission time interval by a factor of
2, and a frequency diversity loss due to limiting a transmission in
same frequency resources. However, a 3 dB loss can be tolerable,
due to a smaller periodic CQI information payload for MTC UEs
compared to conventional UEs (3 bits instead of 4-to-11 bits),
while a frequency diversity loss will typically be small, if a
smaller UL BW is allocated to MTC UEs, and will be further reduced
due to receiver antenna diversity that typically exists at a
NodeB.
[0117] FIG. 15 is a diagram illustrating a first approach to
increase a multiplexing capacity of periodic CQI transmissions from
MTC UEs according to an embodiment of the present invention.
[0118] Referring to FIG. 15 and for PUCCH transmissions over one
RB, a set of K conventional UEs transmit periodic CQI reports by
multiplexing their transmissions in a first slot 1510 and in a
second slot 1520 of a PUCCH subframe using respectively K different
CS of a ZC sequence. A first set of K MTC UEs transmit periodic CQI
reports by multiplexing their transmissions in a first slot 1530 of
a PUCCH subframe using respectively K different CS of a ZC sequence
and a second set of K MTC UEs transmit periodic CQI reports by
multiplexing their transmissions in a second slot 1540 of a PUCCH
subframe using respectively K different CS of a ZC sequence.
Therefore, a multiplexing capacity for MTC UEs is twice the
multiplexing capacity of conventional UEs.
[0119] Another approach to increase (double) a multiplexing
capacity of periodic CQI transmissions for MTC UEs is to restrict
such transmissions to a half RB, instead of one RB for conventional
UEs, and use ZC sequences of a half-length compared to ZC sequences
used by conventional UEs. For example, for a RB including REs, two
ZC sequences of length 6 (which can be the same), can be used for
transmitting periodic CQI by MTC UEs in two half RBs while ZC
sequences used for transmitting periodic CQI by conventional UEs
are of length 12 and periodic CQI transmission is in one RB.
Through this allocation, frequency diversity loss is avoided, and
only an SINR loss of 3 dB exists for periodic CQI transmissions
from MTC UEs compared to SINR losses from conventional UEs.
[0120] FIG. 16 is a diagram illustrating a second approach to
increase a multiplexing capacity of periodic CSI transmissions from
MTC UEs according to an embodiment of the present invention.
[0121] Referring to FIG. 16, a set of K conventional UEs transmit
periodic CQI reports by multiplexing their transmissions over a RB
in a first slot 1610 and in a second slot 1620 of a PUCCH subframe
using respectively K different CS of a ZC sequence. A first set of
K MTC UEs transmit periodic CQI reports by multiplexing their
transmissions over half RB in a first slot 1630A and in a second
slot 1630B of a PUCCH subframe using respectively K different CS of
a ZC sequence and a second set of K MTC UEs transmit periodic CQI
reports by multiplexing their transmissions over half RB in a first
slot 1640A and in a second slot 1640B of a PUCCH subframe using
respectively K different CS of a ZC sequence.
[0122] Another approach to increase a multiplexing capacity of
periodic CQI transmissions for MTC UEs is to further reduce a
number of bits for wideband CQI reports from 3 to 2 and report 4,
instead of 7, CQI indexes. For example, reported indexes can be as
in Table 3 (or by reporting every fourth of the 16 efficiencies in
Table 1). A drawback of this approach is that there will be some
loss in the spectral efficiency of PDSCH transmissions, because a
granularity of wideband CQI reports is increased. However, as for
typical DL SINR distributions, most MTC UEs are able to support a
largest spectral efficiency for QPSK modulation that is still
reported (this largest spectral efficiency captures all other ones
corresponding to the use of QAM16 or QAM64 in Table 1 which are not
applicable for MTC UEs), and therefore, a loss in spectral
efficiency will be small.
[0123] An advantage of this approach is that, in addition to
increased reliability of CQI feedback as a number of bits is
decreased to 2, a PUCCH format used by conventional UEs for
transmitting 2 HARQ-ACK bits can be used by MTC UEs for CQI
reporting according to embodiments of the present invention,
thereby increasing a multiplexing capacity by as much as a factor
of 3, and also reducing UL overhead. Moreover, a single PUCCH
structure can be used by MTC UEs to support HARQ-ACK transmissions,
if necessary, as well as to support SR transmissions. If a UE is to
transmit a CQI report in a same subframe as a HARQ-ACK signal, the
UE can suspend transmission of the CQI report and transmit only the
HARQ-ACK signal. Additionally, CQI feedback may have a nested
structure with a first CQI reporting indicating a first value, as
for example in Table 3, and a second CQI reporting indicating a
second value that includes predetermined values around the first
value. For example, the first CQI reporting (including 2 bits) may
indicate a value of 0.6016 and the second CQI reporting (including
1 bit or 2 bits) may indicate one of the 0.3770 or 0.6016
values.
TABLE-US-00003 TABLE 3 2-bit CQI Table CQI CQI Bits index
Modulation code rate .times. 1024 efficiency 00 0 out of range 01 1
QPSK 120 0.2344 10 2 QPSK 308 0.6016 11 3 QPSK 602 1.1758
[0124] FIG. 17 is a diagram illustrating a third approach for
increasing a multiplexing capacity of periodic CSI transmissions
for MTC UEs according to an embodiment of the present
invention.
[0125] Referring to FIG. 17, a set of K.sub.1 MTC UEs transmit
periodic CQI reports by multiplexing the CQI bits 1710 with a ZC
sequence 1720 and also transmitting an non-modulated ZC sequence
for RS 1730 using the same structure per slot 1740 as conventional
UEs use for HARQ-ACK signal transmissions.
[0126] A second functionality that needs to be modified for MTC UEs
according to embodiments of the present invention compared to
conventional UEs is the one of SRS transmissions that is not
configured with a maximum SRS BW and hop in successive transmission
instances in different BWs within the maximum SRS BW. Conventional
UEs that are not configured, due to low SINR or due to overhead
considerations, to transmit a SRS with a maximum BW can be
configured by a NodeB to transmit SRS with a smaller BW and with
location that is hopping within a maximum SRS BW over successive
SRS transmission instances in order to scan in this way a maximum
SRS BW.
[0127] SRS transmissions according to embodiments of the present
invention may also be supported by an MTC UE in order to assist a
network to determine an appropriate UL BW for PUSCH transmissions,
for example by selecting an UL BW from a predetermined set of UL
BWs where the MTC UE experiences favorable SINR for a signal
transmission. The set of possible BWs, is signaled to the MTC UE by
a NodeB through higher layer signaling. For example, to minimize UL
BW fragmentation experienced for PUSCH transmissions by
conventional UEs due to an allocation of UL BWs to MTC UEs, a
number of possible UL BWs for an MTC UE may be limited towards the
two edges of a total UL BW available for PUSCH transmissions.
Therefore, it is inefficient for an MTC UE to transmit SRS in all
BW parts of a maximum SRS BW configured for conventional UEs as
these BW parts may not be allocated to the MTC UE.
[0128] Consequently, a modification to an SRS hopping pattern used
by a conventional UE can be used for an MTC UE in order to include
only UL BWs that a network may allocate to the MTC UE. A resulting
modified SRS hopping pattern is described in U.S. patent
application Ser. No. 12/986,620 titled "Enhancing Features of
Uplink Reference Signals". When an SRS transmission is activated by
a DCI format, an SRS transmission BW indicated by the DCI format
can only belong to a predetermined set of UL BWs.
[0129] FIG. 18 is a diagram illustrating a process for an MTC UE to
perform SRS transmissions according to an embodiment of the present
invention.
[0130] Referring to FIG. 18, an MTC UE is first informed by a
NodeB, through higher layer signaling, of a set of UL BWs 1810. For
example, a UL BW of 48 RBs can be divided into 8 BW parts of 6 RBs
each, and a respective bit-map of 8 bits can indicate, to an MTC
UE, the BW parts for an SRS transmission (for example, a binary 1
may indicate a BW part for SRS transmission and a binary 0 may
indicate a BW part excluded from an SRS transmission). The MTC UE
is also informed of a set of SRS transmission parameters, such as
the SRS transmission BW, the SRS transmission period, and other
parameters associated with the construction of the SRS, which can
also be based on a ZC sequence 1820. The MTC UE transmits an SRS at
each respective transmission instance only within a BW in the set
of indicated UL BWs 1830 (the SRS transmission BW may not
necessarily be the same as each UL BW in the set of UL BWs).
[0131] Due to RF re-tuning time requirements, which are typically
in the range of a few transmission symbol intervals, it is not
practically possible for an MTC UE to transmit in a same subframe
control or data signals within one BW and SRS within another BW, if
the transmission of control or data signals and the transmission of
an SRS happen to coincide in the same subframe and be in different
UL BWs. Therefore, in such a case, according to an embodiment of
the present invention, an MTC UE suspends an SRS transmission and
performs only transmission of control or data signals.
[0132] Although embodiments of the present invention described
herein above refer to transmissions of control signals and data
signals from an MTC UE within a same allocated BW, embodiments of
the present invention are not limited to these examples, and
transmission of control signals may be within a different allocated
BW than transmission of data signals in accordance with embodiments
of the present invention. A network may then configure an MTC UE to
transmit a PUSCH within a first set of RBs and transmit a PUCCH
within a second set of RBs. An MTC UE can tune its RF at an
appropriate set of RBs and when the MTC UE needs to transmit both
UCI and data information, the MTC UE can multiplex both in a PUSCH.
One reason for configuring an MTC UE different sets of RBs for
transmissions of control signals and data signals is to avoid
congestion when MTC UEs are allocated several, common, RBs for
PUCCH transmissions, since PUCCH multiplexing capacity per RB (up
to 18 or 36 UEs) can be much larger than PUSCH multiplexing
capacity per RB (typically only in the order of one UE). Another
reason is to provide flexibility to a network in reserving a set of
RBs for PUCCH transmissions by MTC UEs and utilize or not utilize
another set of RBs for PUSCH transmissions by MTC UEs while also
considering scheduling of conventional UEs.
[0133] Additionally, although embodiments of the present invention
described herein above refer to MTC UEs completing the initial
communication setup in a subset of the PRBs used for BCH
transmission, such as for example the middle six PRBs in a DL BW,
prior to being informed by higher layer signaling of another
allocated DL BW, embodiments of the present invention are not
necessarily be the case in practice. For example, in order to
support communication in heterogeneous networks, interference
coordination among different cells is needed for PDCCH
transmissions that schedule, to MTC UEs, the reception of PDSCHs
that provide initial configuration information and are less
reliable than BCH transmissions.
[0134] A first alternative is for an MTC UE to implicitly derive a
DL BW for communication after BCH detection and prior to detecting
PDCCHs and respective PDSCHs conveying a higher layer control
signaling that allocates a DL BW and other parameters for
subsequent communication as a function of a NodeB (cell) identity
that is provided by synchronization signals. For example, if a
result of a modulo operation between a cell identity and a
predetermined number (such as a subframe number) is zero, a first
DL BW is used for subsequent DL communication for MTC UEs after BCH
reception; otherwise, a second DL BW is used.
[0135] A second alternative is for an MTC UE to derive PRBs for
communication after BCH detection to be a subset of the BCH PRBs
based again on a cell identity. For example, if a result of a
modulo operation between a cell identity and a predetermined number
is zero, a first half of PRBs of BCH transmission is used for
subsequent PDCCH and PDSCH transmissions; otherwise, a second half
of PRBs of BCH transmission is used.
[0136] FIG. 19 is a diagram illustrating a DL BW determination by
an MTC UE after BCH detection according to an embodiment of the
present invention.
[0137] Referring to FIG. 19, an MTC UE first detects
synchronization signals and the BCH and determines an identity of a
respective cell 1910. Based on an outcome of a modulo operation
between a cell identity and a predetermined number L 1920, the MTC
UE determines a first DL BW for communication after BCH detection
1930 or a second DL BW for communication after BCH detection 1940.
Although the method of FIG. 19 is performed according to the first
of the previous two alternatives described herein above, extension
of the method of FIG. 19 to the second of the previous alternatives
is straightforward. Alternatively, the DL BW for communication
after BCH detection may be indicated by the BCH.
[0138] Finally, as scheduling of PDSCH or PUSCH transmissions to or
from an MTC UE, respectively, may not be needed in every subframe,
as the applications for the MTC UE may not have strict latency
requirements, the subframes possible for scheduling PDSCH or PUSCH
to the MTC UE can be indicated from the NodeB through higher layer
signaling. For example, a network may signal to an MTC UE a bit-map
including X bits indicating respective X subframes where the
network may transmit a DL SA or an UL SA to an MTC UE (for example,
for binary `1` value) and subframes where the MTC UE may not
transmit a DL SA or an UL SA to the MTC UE (for example, for binary
`1` value). The set of X subframes may be determined with respect
to a reference subframe such as for example the first subframe in a
reference radio frame including multiple subframes. This approach
can increase the instances where an MTC UE does not need to
transmit or receive, thereby increasing the associated power
savings.
[0139] While the present invention has been shown and described
with reference to certain embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the present invention as defined by the appended
claims and their equivalents.
* * * * *