U.S. patent application number 16/654749 was filed with the patent office on 2021-04-22 for efficient detection of primary broadcast channel in 5g-nr.
This patent application is currently assigned to NXP USA, Inc.. The applicant listed for this patent is NXP USA, Inc.. Invention is credited to Jayesh H. Kotecha, Jayakrishnan Cheriyath Mundarath.
Application Number | 20210119751 16/654749 |
Document ID | / |
Family ID | 1000004399592 |
Filed Date | 2021-04-22 |
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United States Patent
Application |
20210119751 |
Kind Code |
A1 |
Mundarath; Jayakrishnan Cheriyath ;
et al. |
April 22, 2021 |
Efficient Detection of Primary Broadcast Channel in 5G-NR
Abstract
A mechanism is provided for detecting and decoding a primary
broadcast channel transmitted by a base station at a user equipment
device. Embodiments improve performance and robustness over prior
methods for detecting and decoding by determining a channel
differential metric for channel estimates derived based on each
candidate demodulation reference symbol (DMRS) sequence and
selecting a set of top DMRS sequence candidates using the
associated channel differential metrics. From the set of top DMRS
candidates, a best DMRS candidate, and consequently, the three
least significant bits of the beam index, can be determined.
Inventors: |
Mundarath; Jayakrishnan
Cheriyath; (Austin, TX) ; Kotecha; Jayesh H.;
(Austin, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NXP USA, Inc. |
Austin |
TX |
US |
|
|
Assignee: |
NXP USA, Inc.
Austin
TX
|
Family ID: |
1000004399592 |
Appl. No.: |
16/654749 |
Filed: |
October 16, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 1/0057 20130101;
H04L 5/0051 20130101; H04L 1/0047 20130101; H04L 25/0204 20130101;
H04L 1/0052 20130101; H04W 72/02 20130101; H04W 72/005
20130101 |
International
Class: |
H04L 5/00 20060101
H04L005/00; H04W 72/00 20060101 H04W072/00; H04W 72/02 20060101
H04W072/02; H04L 25/02 20060101 H04L025/02; H04L 1/00 20060101
H04L001/00 |
Claims
1. A method for detecting and decoding a primary broadcast channel
(PBCH) transmitted by an access node at a user equipment device,
the method comprising: determining channel estimates associated
with multiple candidate demodulation reference symbol (DMRS)
sequences; calculating a channel differential metric for each
candidate DMRS sequence, using the channel estimate associated with
the DMRS sequence; and selecting a set of preferred DMRS sequences
based on the channel differential metric associated with each
candidate DMRS sequence.
2. The method of claim 1 wherein said determining channel estimates
associated with the multiple candidate DMRS sequences comprises:
performing a least squares calculation using a sequence of received
frequency domain subcarriers corresponding to DMRS locations in the
PBCH.
3. The method of claim 1 wherein said calculating the channel
differential metric corresponding to each candidate DMRS sequence
comprises determining a differential power metric.
4. The method of claim 3, wherein a channel estimate based on the
i.sup.th candidate DMRS sequence is h.sub.i(1), h.sub.i(2), . . . ,
h.sub.i(K); and the differential power metric is
r(i)=.SIGMA..sub.n=2.sup.K|h.sub.i(n)-h.sub.i(n-1)|.sup.2.
5. The method of claim 1 wherein said calculating the channel
differential metric for each candidate DMRS sequence comprises
determining a channel phase differential metric.
6. The method of claim 5, wherein a channel estimate based on the
i.sup.th candidate DMRS sequence is given by h.sub.i(1),
h.sub.i(2), . . . , h.sub.i(K); and determining the channel phase
differential metric comprises calculating a phase of the channels
(.theta..sub.i(n)), wherein the phase of the channels is .theta. i
( n ) = h i ( n ) h i ( n ) , ##EQU00005## for n=1,2, . . . , K,
calculating a phase differential metric (d.theta..sub.i(n)),
wherein the phase differential metric is
d.theta..sub.i(n)=.theta..sub.i(n)-.theta..sub.i(n-1), for n=2, 3,
. . . , K, and calculating a variance of the phase differential
metric (r(i)), wherein the variance is
r(i)=var{d.theta..sub.i(2),d.theta..sub.i(3), . . . ,
d.theta..sub.i(K)}.
7. The method of claim 1 wherein said selecting the set of
preferred DMRS sequences comprises selecting a set of DMRS sequence
candidates corresponding to one or more lowest values for the
channel differential metric.
8. The method of claim 7 further comprising determining which
member of the set of DMRS sequence candidates is a valid value.
9. The method of claim 8 wherein said determining which member of
the set of DMRS sequence candidates is a valid value comprises:
interpolating the channel estimate for the DMRS sequence candidate
(i) in the frequency domain to span all DMRS sequences to arrive at
h.sub.f,i; calculating equalization weights (w.sub.i) using
h.sub.f,i; demodulating the PBCH using w.sub.i; polar decoding the
demodulated PBCH for the DMRS subcarrier candidate (i); and
determining whether the decoded PBCH passes an error check.
10. The method of claim 7 further comprising determining three
least significant bits of a downlink beam index associated with the
PBCH based on the selected set of preferred DMRS sequences.
11. A wireless device comprising: a multiple antenna subsystem and
beamformer subsystem; and a digital controller, coupled to the
multiple antenna subsystem and beamformer subsystem, and configured
to detect and decode a primary broadcast channel (PBCH) transmitted
by an access node by being configured to: determine channel
estimates associated with multiple candidate demodulation reference
symbol (DMRS) sequences; calculate a channel differential metric
for each candidate DMRS sequence, using the channel estimate
associated with the DMRS sequence; and select a set of preferred
DMRS sequences using the channel differential metric associated
with each candidate DMRS sequence.
12. The wireless device of claim 11 wherein the digital controller
is configured to determine the channel estimates associated with
the multiple candidate DMRS sequences by being further configured
to perform a least squares calculation using a sequence of received
frequency domain subcarriers corresponding to DMRS locations in the
PBCH.
13. The wireless device of claim 11 wherein the digital controller
is configured to determine the channel differential metric for each
candidate DMRS sequence by being configured to determine a
differential power metric.
14. The wireless device of claim 13, wherein a channel estimate
based on the i.sup.th candidate DMRS sequence is given by
h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K); and the differential
power metric is
r(i)=.SIGMA..sub.n=2.sup.K|h.sub.i(n)-h.sub.i(n-1)|.sup.2.
15. The wireless device of claim 11 wherein the digital controller
is configured to determine the channel differential metric for each
candidate DMRS sequence by being further configured to determine a
channel phase differential metric.
16. The wireless device of claim 15, wherein a channel estimate
based on the i.sup.th candidate DMRS sequence is given by
h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K); and the wireless device
is configured to determine the channel phase differential metric by
being further configured to calculate a phase of the channels
(.theta..sub.i(n)), wherein the phase of the channels is .theta. i
( n ) = h i ( n ) h i ( n ) , ##EQU00006## for n=1,2, . . . , K,
calculate a phase differential metric (d.theta..sub.i(n)), wherein
the phase differential metric is
d.theta..sub.i(n)=.theta..sub.i(n)-.theta..sub.i(n-1), for n=2, 3,
. . . , K, and calculate a variance of the phase differential
metric (r(i)), wherein the variance is r(i)=var{d.theta..sub.i(2),
d.theta..sub.i(3), . . . , d.theta..sub.i(K)}.
17. The wireless device of claim 11 wherein the digital controller
is configured to select the set of preferred DMRS sequences by
being further configured to select a set of DMRS sequence
candidates corresponding to one or more lowest values for the
channel differential metric.
18. The wireless device of claim 17 wherein the digital controller
is further configured to determine which member of the set of DMRS
sequence candidates is a valid value.
19. The wireless device of claim 18 wherein the digital controller
is further configured to determine which member of the set of DMRS
sequence candidates is a valid value by being further configured
to: interpolate the channel estimate for the DMRS sequence
candidate (i) in the frequency domain to span all DMRS sequences to
arrive at h.sub.f,i; calculate equalization weights (w.sub.i) using
h.sub.f,i; demodulate the PBCH using w.sub.i; polar decode the
demodulated PBCH for the DMRS subcarrier candidate (i); and
determine whether the decoded PBCH passes an error check.
20. The wireless device of claim 17 wherein the digital controller
is further configured to determine three least significant bits of
a downlink beam index associated with the PBCH based on the
selected set of preferred DMRS sequences.
Description
BACKGROUND
Field
[0001] This disclosure relates generally to 5G-NR (New Radio)
initial access procedure, and more specifically, to a fast and low
complexity mechanism for primary broadcast channel detection and
decoding.
Related Art
[0002] As mobile computing devices, such as smart phones, provide
greater amounts of computing power, so too does the demand for
greater amounts of data transfer. 5G-NR (New Radio) is the latest
cellular wireless technology following the 3GPP specification
release 15 and above, and has been developed to provide data speeds
as much as ten times that of 4G (LTE) technology. The new
technology also provides updated methods for mobile devices to
connect with a base station (e.g., a cellular tower).
[0003] In some instances of 5G wireless network technologies being
developed and deployed, millimeter wave (mm-wave) data transmission
has emerged as a highly promising approach to achieving gigabit per
second throughput in wireless communication links. This is due to
such transmission providing orders of magnitude more spectrum than
the presently-congested bands in conventional ultra-high frequency
and microwave frequencies below 3 GHz. But, due to very high
carrier frequencies, mm-wave signals can experience poor
propagation and large isotropic path loss. To overcome this large
isotropic path loss, mm-wave signal schemes rely heavily on highly
directional transmissions, provided by, for example, electrically
steerable antenna elements that implement beamforming techniques
for transmitting and receiving communications.
[0004] Use of directional transmissions can significantly
complicate the process of initial access by a user equipment (UE)
client device (i.e., a mobile device). During initial access a UE
discovers a potential cell and establishes a link-layer connection
with the corresponding base station (BS). To enable mutual
detection between the BS and the UE, the initial access procedure
in mm-wave cells provides a mechanism by which both the the UE and
the BS can determine suitable beamforming directions on which
subsequent directional communication can be carried out.
[0005] In one proposal for establishing initial beamforming
directions between a UE and BS, the 5G-NR wireless standard
specifies an initial access scheme in which a BS uses a fixed time
and frequency grid to transmit a plurality of synchronization
signal blocks (SSB) on differentially directed transmit beams to
identify the available transmit beams. SSB blocks carry primary and
secondary synchronization channels (PSS and SSS, respectively) that
enable a UE to acquire timing information for the uplink and
downlink transmissions as well as reference frame and transmission
time interval indices. The SSB also carries a master information
block (MIB) in a physical channel called a primary broadcast
channel (PBCH) that contains information about the network,
including a downlink beam index used to transmit the SSB. The
initial uplink transmission by the UE, which is part of a procedure
to register with the network (known as a RACH procedure), indicates
to the network the downlink beam index on which the UE decoded the
MIB. This provides the base station with information about the
downlink beam to use for that particular UE.
[0006] Downlink beam information is carried in part explicitly and
in part implicitly as information carried on the PBCH. 5G-NR allows
up to 64 downlink beams during SSB transmission, and therefore a
six-bit beam index is needed to identify each beam. Three of the
six bits of the beam index are carried explicitly as part of the
MIB payload. The other three bits of the beam index are signal
implicitly using demodulation reference signals (DMRS). DMRS are
pilot subcarriers embedded in the PBCH transmission to facilitate
estimation of the downlink channel at each UE. One of eight
different DMRS sequences can be used to transmit the pilot
subcarriers based on the three implicit bits corresponding to the
beam index. The UE then reconstructs the six-bit beam index (e.g.,
three bits acquired via demodulation to identify the correct DMRS
sequence and three bits identified by decoding the MIB). Decoding
the MIB and identifying the correct beam index is a critical part
of initial acquisition, and having a robust mechanism for MIB
detection is essential to ensuring connectivity in a 5G network.
Further, there are stringent complexity requirements on the UE so
that the UE uses compute resources efficiently. It is therefore
important to have a robust, low-complexity solution to detect the
beam index as part of the initial acquisition process, and
subsequently during a background cell search process for handovers,
and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Embodiments of the present invention may be better
understood by referencing the accompanying drawings.
[0008] FIG. 1 is a simplified block diagram illustrating a wireless
communication system in which a base station and user equipment are
each equipped, respectively, with transmit/receive multi-antenna
array systems.
[0009] FIG. 2 is a simplified block diagram illustrating a SSB used
in the 5G-NR standard.
[0010] FIG. 3 is a simplified flow diagram illustrating a process
for determining the three least significant bits of the SSB
index.
[0011] FIG. 4 is a simplified flow diagram illustrating a process
for determining the three least significant bits of the SSB index,
in accord with embodiments of the present invention.
[0012] FIG. 5 is a simplified flow diagram illustrating an
alternative process for determining the least significant bits of
the SSB index, in accord with embodiments of the present
invention.
[0013] FIGS. 6A-6D are a set of plot diagrams illustrating
differences in magnitude and phase with correct and incorrect DMRS
selection.
[0014] FIG. 7 is a plot illustrating a probability of error for
index detection over a range of signal-to-noise ratio (SNR).
[0015] FIG. 8 is a plot further illustrating the performance
improvement of the present method of that of the prior method.
[0016] The use of the same reference symbols in different drawings
indicates identical items unless otherwise noted. The figures are
not necessarily drawn to scale.
DETAILED DESCRIPTION
[0017] Embodiments of the present invention provide a mechanism for
detecting and decoding a primary broadcast channel transmitted by a
base station at a user equipment device. Embodiments improve
performance and robustness over prior methods for detecting and
decoding by determining a channel differential metric for channel
estimates derived based on each candidate demodulation reference
symbol (DMRS) sequence and selecting a set of top DMRS sequence
candidates using the associated channel differential metrics. From
the set of top DMRS candidates, a best DMRS candidate, and
consequently, the three least significant bits of the beam index,
is determined.
[0018] Reference is now made to FIG. 1 which is a simplified block
diagram illustrating a wireless communication system 100 in which a
base station (gNB) 101 and user equipment (UE) 110 are each
equipped, respectively, with transmit/receive multi-antenna array
systems 104, 111. In a given geographic area, the wireless
communication system 100 may include one or more base stations 101
to communicate with one or more UEs 110 (e.g., subscriber stations)
by equipping the base stations 101 and UEs 110 with transmitters
and receivers. Base station 101 can use beamforming techniques to
transmit one or more data streams through the transmit antenna
array 104, and UE 110 combines the received signal from the receive
antenna array 111 to reconstruct the transmitted data. This is
accomplished using "beamforming," whereby each data signal s.sub.i
is processed by a digital signal processor 102 and analog/digital
beamformer 103 for transmission by applying a weight vector w.sub.i
to the signal s.sub.i and transmitting the result x.sub.i over the
transmit antenna array 104. The weight vector w.sub.i is used to
directionalize the signal with the objective of enhancing the
signal quality or performance metric, like
signal-to-interference-and-noise ratio (SINR) of the received
signal at the receiver station 110. At UE 110, the received signals
detected at the receive antenna array 111 are processed using a
combining vector v.sub.i that is applied by analog beamformer (AB)
112 and digital signal processor (DP) 113.
[0019] In an example embodiment where the base station 101 has an
array of N transmit antennas 104, the digital signal processor 102
and analog beamformer 103 prepare a transmission signal,
represented by the vector x.sub.i, for each signal s.sub.i. The
transmission signal vector x.sub.i is determined in accordance with
equation x.sub.i=w.sub.is.sub.i, where w.sub.i is the i.sup.th
beamforming, N dimensional transmission weight vector (also
referred to as a "transmit beamformer"), and each coefficient
w.sub.j of weight vector w.sub.i represents a weight and phase
shift on the j.sup.th transmit antenna 104.
[0020] At UE 110, the transmitted signals are received on the
k.sub.i receive antennas in the receive antenna array 111. For
example, the transmission signal vector x.sub.1 is transmitted via
a channel represented by a channel matrix H.sub.1, and is received
at the receiver 110 as a receive signal vector
y.sub.1=H.sub.1.sup.Hx.sub.1+n.sub.1, where n.sub.1 represents
noise and any co-channel interference caused by other subscriber
stations, and "H.sub.1.sup.H" represents the complex conjugate
transpose of the channel matrix correlating the base station 101
and UE 110. The superscript "H" is used herein as a hermitian
operator to represent a complex conjugate transpose operator. The
j.sup.th element of the received signal vector y.sub.1 represents
the signal received on the j.sup.th antenna of UE 110, j.di-elect
cons.{1, 2, . . . , k.sub.i}. To obtain a data signal, z.sub.1,
which is an estimate of the transmitted data s.sub.1, the digital
signal processor (DP) 113 and analog beamformer (AB) 112 at UE 110
combine the signals received on the k; antennas of the array 111
using a combining vector v.sub.1 in accordance with the equation,
z.sub.1=y.sub.1.sup.Hv.sub.1.
[0021] With emerging 5G wireless communications, massive MIMO
techniques are proposed for using antenna arrays 104 and 111 which
include arrays of elements driven by individual signals to create
multiple signal beams over multiple paths. At the location of UE
110, signals from all these paths may be added constructively to
enabling a high data rate by using multichannel phase shifting
techniques to combine the signals from the multiple paths. In
selected analog antenna embodiments, multichannel phase shifting
can be done in the analog domain, by taking the transmit data
stream, dividing it as many ways as there are elements in the
antenna array, and then applying phase shifting to each of them.
But due to the inflexibility of such analog systems that can only
handle one data stream and generate therefrom one signal beam,
another digital antenna approach provides each element of the
antenna array with its own transceiver and set of data converters,
thereby providing the ability to handle multiple data streams and
generate multiple beams from one array.
[0022] With increasing reliance on directional transmit and receive
beamforming techniques for overcoming high isotropic path loss with
next generation mmWave cellular communication systems, such as
5G-NR, there are associated control and setup challenges. For
example, initial access procedures can be significantly delayed due
to the need for the base station and the UE to find the initial
directions of transmission. With typical cell acquisition
techniques where the UE 110 relies on a synchronization signal
block (SSB) to acquire initial access, base station 101 will
transmit, periodically or aperiodically, the SSB on each beamformed
TX beam, thereby forming an SSB burst which has a specified SSB
duration (specifying the total time for transmitting SSBs spanning
the entirety of all supported TX beams/directions) and a specified
SSB period (specifying the periodic repetition rate between SSB
bursts). In the example of FIG. 1, each SSB burst includes SSB
transmissions that are sent on 64 TX beams (e.g., SSB 1-SSB 64).
With each SSB transmission having a fixed length (T.sub.B)
occurring within a time span of fixed duration (T.sub.Ti), the
multiple SSB transmissions (T.sub.T1+T.sub.T2+ . . .
T.sub.T64=T.sub.SSB) in each SSB burst will have a specified SSB
burst duration (e.g., 5 ms) that is less than the SSB burst period,
T.sub.SSBPERIOD, (e.g., 20 ms) for periodically repeating each SSB
burst.
[0023] 5G-NR (New Radio) supports operation in a frequency spectrum
ranging from sub-1 GHz to millimeter wave (mmWave) bands. Two
frequency ranges are defined in the 5G-NR standard: FR1 (450 MHz-6
GHz) and FR2 (24.25 GHz-52.6 GHz). FR2 is commonly referred to as
mmWave. In order to handle such a wide range of frequency spectrum,
5G-NR adopts a flexible subcarrier spacing of 2.sup..mu.2.15 kHz
(p=0,1, . . . , 4), which is scaled from the basic 15 kHz
subcarrier spacing in 4G-LTE. By using such a scalable design, a
wide range of deployment scenarios and carrier frequencies are
allowed. For example, at lower frequencies (FR1), cells can be
larger and subcarrier spacings of 15 kHz and 30 kHz are suitable.
At higher frequencies (FR2/mmWave), phase noise is more
problematic, and 5G-NR supports 60 kHz and 120 kHz for data
channels and 120 kHz and 240 kHz for initial access synchronization
signal block (SSB) transmission.
[0024] A 5G-NR transmission is divided up into frames, which have a
duration of 10 ms and include 10 subframes--the same as for 4G-LTE.
Each subframe includes 2.sup..mu. slots of 14 symbols each. A slot
is a typical unit for transmission upon which scheduling operates,
but 5G-NR can allow transmission to start at any symbol and last
only as many symbols as needed for communication. This facilitates
low latency for critical data transfer. Latency optimization is an
important consideration for 5G-NR, and many features have been
built into the standard to reduce latency. Embodiments of the
present invention provide additional improvements to latency while
at the same time improve robustness for initial beam access.
[0025] As discussed above, a UE is constantly determining whether
the UE is communicating with the base station on the best beam. The
standard facilitates this determination with synchronization signal
block (SSB) transmissions periodically being provided by the base
station. SSB subcarrier spacing can be 15 or 30 kHz in FR1 and 120
or 240 kHz in mmWave. By detecting a SSB, a UE can obtain important
information such as physical cell identity, achieve downlink
synchronization in both time and frequency domain, and acquire
timing for the primary broadcast channel (PBCH). The PBCH carries
basic system information.
[0026] FIG. 2 is a simplified block diagram illustrating a SSB 200
used in the 5G-NR standard. A primary synchronization sequence 210
(PSS) and a secondary synchronization sequence 220 (SSS) are
provided in SSB 200. In addition, four pieces of SSB 200 include
primary broadcast channel (PBCH), 230, 232, 234, and 236. The PBCH
includes a master information block (MIB) that provides some of the
data allowing the UE to determine the carrier on which the UE
should communicate, as will be discussed in greater detail
below.
[0027] An SSB is mapped to 4 symbols in the time domain and 240
contiguous subcarriers in the frequency domain. To support
beamforming for initial access, an SS burst set is provided in
5G-NR to support possible beam sweeping for SSB transmission.
Multiple SSBs are transmitted in a localized burst set in
conjunction with a sparse burst set periodicity (e.g., a default of
20 ms). Within an SS burst set period, up to 64 SSBs can be
transmitted in different beams. Transmission of SSBs within a SS
burst set is limited to a 5 ms window. The set of possible SSB time
locations within an SS burst set depends upon the frequency band,
and the frequency location of a SSB is not necessarily aligned with
a beginning of a resource block in the frequency domain. At mmWave
frequencies, a number of possible candidate SSB locations within a
SS burst set are 64. These 64 candidate locations are the SSB time
index.
[0028] Each SSB in a SS burst set (i.e., all SSBs within the 5 ms
period of SS burst transmission) is assigned a unique number from 0
to 63 in mmWave. This number is reset in the next SS burst set
(i.e., the next 5 ms span after the SS burst set transmission
cycle). This unique number is the SSB time index and the six bits
of this number (at mmWave frequencies) are transmitted to the UE
via two different parts of the SSB. The three least significant
bits of a SSB index are determined from a one-to-one mapping with
an index of demodulation reference symbols (DMRS) transmitted in
the PBCH, while the three most significant bits are determined by
examination of PBCH payload bits.
[0029] A PBCH DMRS is a special type of physical layer signal that
functions as a reference signal for decoding the PBCH. This is not
present in 4G-LTE, and thus creates an issue that needs resolving
in 5G-NR. The equation for generating the PBCH DMRS includes a
pseudo random sequence initialization value that is made up of
components such as physical cell identifier, SSB index, and half
frame number. Thus, by decoding the PBCH DMRS, the UE can determine
the SSB index and identify the half frame that included the SSB. In
addition, this information allows for the decoding of the PBCH
itself and thus a determination of the most significant bits of the
SSB index therein.
[0030] In light of the above, as a part of a process for
demodulating and decoding the SSB, the UE needs to determine which
of the possible 3-bit sequences have been used to transmit the PBCH
DMRS. By doing so, the UE can decode the MIB and thereby set up an
opportunity to connect to the base station.
[0031] Embodiments of the present invention thus relate to
improving demodulation and decoding performance at the UE after the
UE receives the SSB burst set. In a typical UE system, the UE scans
the medium to detect a PSS (e.g., 210) corresponding to a SSB
transmission. This includes continuously correlating the received
waveform across all possible PSS sequences and extracting a
strongest correlation peak. A strong correlation peak indicates the
presence of a SSB on a beam in the beam sweep that was most
effective at directing the signal toward the UE. Upon a PSS
detection, an SSS (e.g., 220) detection is performed. Subcarriers
associated with the SSS are extracted and correlated with each
possible SSS sequence. Indices of the strongest PSS and SSS
sequences are combined to give a physical layer cell identity,
which is used for PBCH DMRS and PBCH processing.
[0032] At this point, in a process similar to SSS search,
subcarriers corresponding to PBCH DMRS are extracted and correlated
with each possible PBCH DMRS sequence. The "correct" DMRS sequence
is first identified--the strongest PBCH DMRS identified determines
the three least significant bits of the SSB index, and is also used
for channel estimation and demodulation of the PBCH as well as for
scrambling initialization.
[0033] FIG. 3 is a simplified flow diagram 300 illustrating the
above process for determining the three least significant bits of
the SSB index as typically performed. An index i is initialized to
zero (305). Then, a least squares channel estimate is calculated
for the DMRS subcarriers within the PBCH (310). The least squares
channel estimation calculates the following:
h.sub.i(j)=r(j)*p.sub.dmrs,i(j), j=1,2, . . . , K
In this equation: [0034] K is the length of the DMRS sequence
(equal to number of DMRS subcarriers within PBCH symbols) [0035]
P.sub.dmrs,i(.) is the sequence of DMRS subcarriers for index, i
[0036] r(.) is the sequence of received frequency domain
subcarriers corresponding to PBCH-DMRS locations [0037] h.sub.i(.)
are the least squares channel estimates for the PBCH subcarrier
locations Thus, the eight DMRS channel estimate candidates are
formed as h.sub.1(.), h.sub.2(.), . . . , h.sub.8(.).
[0038] Once the least squares channel estimate candidates are
determined, the absolute value of the mean of the channel estimate
(315) is used for each index (325), until a mean for each index is
calculated (320), as a metric to choose the best index i (330).
Once the optimal i is chosen, h.sub.i is interpolated in the
frequency domain to span all DMRS subcarriers to determine
h.sub.f,i (335) and equalization weights iv are calculated using
h.sub.f,i (340). Demodulation (345) and decoding (350) of the PBCH
is then performed in light of the equalization weights.
[0039] If the demodulated/decoded PBCH passes a cyclic redundancy
check (355), then i is the proper index (365), and if not, then the
index is incorrect (360) and a new determination needs to be made
to find the appropriate index. Thus, if the selected index is
incorrect (as selected in steps 315-330), then a significant amount
of resources and time are expended before determining that the
index was incorrect, and then the whole process needs to be
repeated. This results in an increase in latency in time to acquire
the proper beam for the UE. The performance of this method degrades
with channel and phase noise and is therefore not a good choice for
additive white gaussian noise channels, which is common in mmWave
applications.
[0040] Embodiments of the present invention improve performance and
are more robust in overcoming influences of channel and phase
noise. Through introduction of a better performing index estimation
algorithm, embodiments improve decoder candidate selection during
initial acquisition, thereby reducing latency during initial
acquisition. Embodiments also provide additional robustness in soft
handoff situations, where the MIB bits for a new base station are
provided to the UE by the current base station, by improving beam
selection performance (e.g., SSB index estimation).
[0041] Embodiments employ a methodology for early detection of the
DMRS, thereby potentially avoiding several of the steps executed in
the method of FIG. 3. This methodology exploits frequency domain
correlation in the channel, allowing for a low complexity algorithm
to detect a correct DMRS sequence.
[0042] FIG. 4 is a simplified flow diagram illustrating a process
for determining the three least significant bits of the SSB index,
in accord with embodiments of the present invention. Embodiments
modify the channel selection portion of FIG. 3 (steps 315-330) and
provide an enhancement (steps 410-440) where a best channel is
selected for demodulation and decoding.
[0043] As discussed above with respect to FIG. 3, a least squares
channel estimation is performed, which provides the eight DMRS
channel estimate candidates are formed as h.sub.1(.), h.sub.2(.), .
. . , h.sub.8(.) (310). The DMRS subcarriers are spread across
three OFDM symbols in four contiguous chunks (e.g., two separate
chunks in symbol 2). For each channel estimate, h.sub.i(.),
K.sub.1, K.sub.2, K.sub.3, and K.sub.4 are the number of DMRS
subcarriers on each chunk, respectively and thus
K=K+K.sub.2+K.sub.3+K.sub.4.
[0044] Embodiments rely upon a difference between how using a
correct and an incorrect DMRS sequence affects the results of the
least squares channel estimation. Incorrect DMRS sequences will
provide channel estimates that are random or uncorrelated in the
frequency domain. The incorrect channel estimates do not transition
smoothly in the frequency domain, but rather appear haphazard. On
the other hand, a correct channel estimate will transition smoothly
across the frequency domain. This is illustrated by way of example
in FIGS. 6A-6D. A comparison can be made between a channel estimate
magnitude when the correct DMRS sequence is applied to for the
least squares channel estimation in chart 610 of FIG. 6A and when
an incorrect DMRS sequence is applied in chart 620 of FIG. 6B. The
curve in chart 610 smoothly follows an arc to a single peak, while
the curve in chart 620 has multiple peaks and valleys. A similar
behavior is presented by the phase charts in 630 of FIG. 6C (with a
correct DMRS sequence) and 640 of FIG. 6D (with an incorrect DMRS
sequence).
[0045] The behavior of the channel estimation results are
characterized mathematically as a differential of the channel
coefficients in the frequency domain. This can be performed using
two different metrics.
[0046] In a first metric, a channel differential power metric is
calculated. h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K) are least
square channel estimates for DMRS subcarriers in a PBCH symbol
based on the i.sup.th candidate DMRS sequence. A differential power
metric for (i) is then calculated using the following formula
(410):
r ( i ) = n = 2 K h i ( n ) - h i ( n - 1 ) 2 ##EQU00001##
This is repeated for each of the (i) indices (420). An optimal (i)
is calculated which has the lowest r(i) (430). A determination is
then made as to whether the selected (i) is good for the signal as
received. This determination is measured by comparing the
differential power metric value against a pre-determined threshold
and checking whether the differential power metric value exceeds
the threshold or not. A "good" result should drive the metric to be
close to zero. Once this determination is made, the selected index
can be used for interpolation and demodulating (steps 335-365), as
discussed above.
[0047] An alternative metric can be used in which a channel phase
differential is calculated and used for determining r(i). As above,
h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K) are the least square
channel estimates for DMRS subcarriers in a PBCH symbol based on
the i.sup.th candidate DMRS sequence. The channel differential
metric for each (i) is then calculated as follows (410). A phase of
the channels can be calculated from the least square channel
estimates:
.theta. i ( n ) = h i ( n ) h i ( n ) , for n = 1 , 2 , , K
##EQU00002##
A phase differential metric can then be generated:
d.theta..sub.i(n)=.theta..sub.i(n)-.theta..sub.i-1(n), for n=2, 3,
. . . , K
Where the variance of the phase differential metric is then:
r(i)=var{d.theta..sub.i(2),d.theta..sub.i(3), . . .
,d.theta..sub.i(K)}
[0048] As above, this is repeated for each index (i) (420) and an
optimal (i) is selected that has the lowest r(i) (430). A
determination is made as to whether the selected (i) is good for
the signal as received, and then the index is passed to the
interpolation and demodulation steps 335-365.
[0049] Embodiments use the above metrics to quickly arrive at a
good index before passing the index to the interpolation and
demodulation steps, thereby reducing the latency introduced by
multiple passes through the interpolation and demodulation process
in traditional methods. Further, by minimizing the differential of
either the signal magnitude or the phase, there is less of a chance
of arriving at a false positive for the selected index than by
taking the mean, as in the traditional methods.
[0050] FIG. 5 is a simplified flow diagram illustrating an
alternative process for determining the least significant bits of
the SSB index, in accord with embodiments of the present invention.
The illustrated embodiment seeks to enhance the probability of
arriving at a correct index (i) more quickly by selecting a set S
that includes the best "m" candidate indices (i) from the above
channel differential metrics (510), rather than just choosing a
"best" one. The set of indices is then passed through the
interpolation, demodulation, and decoding steps (335-350) one index
at a time (520). When an index passes CRC (355), that index is
chosen (365). If an index does not pass CRC, then a determination
is made as to whether that was the last index in set S (530), and,
if not, then a next index in set S is chosen (520) and the process
is repeated until a correct index is found.
[0051] FIG. 7 is a plot illustrating a probability of error for
index detection over a range of signal-to-noise ratio (SNR). As
discussed above, embodiments of the present invention are more
robust to noise in the signal. FIG. 7 illustrates this improvement
over a prior process as illustrated in FIG. 3. As discussed above,
the process from FIG. 3 uses a correlation metric that attempts to
exploit the good auto/cross-correlation properties of the DMRS
sequences themselves. Probability of error plot 710 illustrates
performance of that process of FIG. 3. By comparison, the probility
of error plots 720 and 730 illustrate performance of the two
channel metrics, channel differential power metric and channel
phase differential, respectively. As shown, the channel metrics of
the present disclosure significantly out-perform the prior method,
by as much as -5 dB in most probability regions.
[0052] FIG. 8 is a plot further illustrating the performance
improvement of the present method of that of the prior method. The
curves of FIG. 8 relate to the method illustrated in FIG. 5, and
show decreasing probability of error as the number of indices (M)
in set S are provided to the interpolation, demodulation, and
decoding steps. Again, performance curves 820-880, representing
performance as M increases from 1-8, respectively, show increasing
robustness over the prior method curve 810, starting at a -5 dB
improvement in SNR for a given probability of error.
[0053] By now it should be appreciated that there has been provided
a method for detecting and decoding a primary broadcast channel
(PBCH) transmitted by an access node at a user equipment device.
The method includes determining a channel estimate associated with
multiple candidate demodulation reference symbol (DMRS) sequences,
calculating a channel differential metric for each candidate DMRS
sequence using the channel estimate associated with the DMRS
sequence, and selecting a set of preferred DMRS sequences based on
the channel differential metric associated with each candidate DMRS
sequence.
[0054] In one aspect of the above embodiment, determining a channel
estimate for the multiple candidate DMRS sequences includes
performing a least squares calculation using a sequence of received
frequency domain subcarriers corresponding to DMRS locations in the
PBCH.
[0055] In another aspect of the above embodiment, calculating the
channel differential metric for each candidate DMRS sequence
includes determining a differential power metric. In a further
aspect, the differential power metric is
r(i)=.SIGMA..sub.n=.sup.K|h.sub.i(n)-h.sub.i(n-1)|.sup.2, where the
channel estimate based on the i.sup.th candidate DMRS sequence is
given by h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K). In another
aspect of the above embodiment, calculating the channel
differential metric for each candidate DMRS sequence includes
determining a channel phase differential metric. In a further
aspect, determining the channel phase differential metric includes:
calculating a phase of the channels (.theta..sub.i(n)), where the
phase of the channels
.theta. i ( n ) = h i ( n ) h i ( n ) , ##EQU00003##
for n=1,2, . . . , K and the channel estimate based on the i.sup.th
candidate DMRS sequence is given by h.sub.i(1), h.sub.i(2), . . . ,
h.sub.i(K); calculating a phase differential metric
(d.theta..sub.i(n)), where the phase differential metric is
d.theta..sub.i(n)=.theta..sub.i(n)-.theta..sub.i(n-1), for n=2, 3,
. . . , K; and, calculating a variance of the phase differential
metric (r(n)), where the variance is
r(i)=var{d.theta..sub.i(2),d.theta..sub.i(3), . . . ,
d.theta..sub.i(K)}.
[0056] In another aspect of the above embodiment, selecting the set
of preferred DMRS sequences includes selecting a set of DMRS
sequence candidates corresponding to one or more lowest values for
the channel differential metric. In a further aspect, the method
further includes determining which member of the set of DMRS
sequence candidates is a valid value. In another aspect for
determining validity of a DMRS sequence candidate, the method can
include comparing the corresponding channel differential metric
with a predetermined threshold and rejecting the candidate if it
does not meet the threshold condition. In yet a further aspect,
determining which member of the set of DMRS sequence candidates is
a valid value includes: interpolating the channel estimate for the
DMRS sequence candidate (i) in the frequency domain to span all
DMRS sequences to arrive at h.sub.f,i; calculating equalization
weights (w.sub.i) using h.sub.f,i; demodulating the PBCH using
w.sub.i; polar decoding the demodulated PBCH for the DMRS
subcarrier candidate (i); and determining whether the decoded PBCH
passes an error check. In another further aspect, the method
further includes determining three least significant bits of a
downlink beam index associated with the PBCH based on the selected
set of preferred DMRS sequences.
[0057] Another embodiment of the present invention provides a
wireless device that includes a multiple antenna subsystem and
beamformer subsystem, and a digital controller coupled to the
multiple antenna subsystem and beamformer subsystem. The digital
controller is configured to detect and decode a PBCH transmitted by
an access node by being configured to determine a channel estimate
associated with multiple candidate DMRS sequences, calculate a
channel differential metric for each candidate DMRS sequence using
the channel estimate associated with the DMRS sequence, and select
a set of preferred DMRS sequences using the channel differential
metric associated with each candidate DMRS sequence.
[0058] In one aspect of the above embodiment, the digital
controller is configured to determine the channel estimate for the
multiple candidate DMRS sequences by being further configured to
perform a least squares calculation using a sequence of received
frequency domain subcarriers corresponding to DMRS locations in the
PBCH.
[0059] In another aspect of the above embodiment, the digital
controller is configured to calculate the channel differential
metric for each candidate DMRS sequence by being configured to
determine a differential power metric. In a further aspect, the
differential power metric is
r(i)=.SIGMA..sub.n=2.sup.K|h.sub.i(n)-h.sub.i(n-1)|.sup.2, where
the channel estimate based on the i.sup.th candidate DMRS sequence
is given by h.sub.i(1), h.sub.i(2), . . . , h.sub.i(K). In another
aspect of the above embodiment, the digital controller is
configured to calculate the channel differential metric for each
candidate DMRS sequence by being configured to determine a channel
phase differential metric. In a further aspect, the digital
controller is configure to determine the channel phase differential
metric by being further configured to: calculate a phase of the
channels (.theta..sub.i(n)), where the phase of the channels is
.theta. i ( n ) = h i ( n ) h i ( n ) , ##EQU00004##
for n=1,2, . . . , K and the channel estimate based on the it
candidate DMRS sequence is given by h.sub.i(1), h.sub.i(2), . . . ,
h.sub.i(K); calculate a phase differential metric
(d.theta..sub.i(n)), where the phase differential metric is
d.theta..sub.i(n)=.theta..sub.i(n)-.theta..sub.i(n-1), for n=2, 3,
. . . , K; and, calculate a variance of the phase differential
metric (r(n)), where the variance is
r(i)=var{d.theta..sub.i(2),d.theta..sub.i(3), . . . ,
d.theta..sub.i(K)}.
[0060] In another aspect of the above embodiment, the digital
controller is configured to select the set of preferred DMRS
sequences by being further configured to select a set of DMRS
sequence candidates corresponding to one or more lowest values for
the channel differential metric. In a further aspect, the digital
controller is further configured to determine which member of the
set of DMRS sequence candidates is a valid value. In another aspect
for determining validity of a DMRS sequence candidate, the digital
controller is further configured to compare the corresponding
channel differential metric with a predetermined threshold and to
reject the DMRS sequence candidate if the candidate does not meet
the threshold condition. In still a further aspect, the digital
controller is further configured to determine which member of the
set of DMRS sequence candidates is a valid value by being further
configured to: interpolate the channel estimate for the DMRS
sequence candidate (i) in the frequency domain to span all DMRS
sequences to arrive at h.sub.f,i; calculate equalization weights
(w.sub.i) using h.sub.f,i; demodulate the PBCH using w.sub.i; polar
decode the demodulated PBCH for the DMRS subcarrier candidate (i);
and determine whether the decoded PBCH passes an error check. In
yet another further aspect, the digital controller is further
configured to determine three least significant bits of a downlink
beam index associated with the PBCH based on the selected set of
preferred DMRS sequences.
[0061] In another aspect of the above embodiment, the set of
preferred DMRS sequences may have only one member corresponding to
the "best" candidate as indicated by the channel differential
metric.
[0062] Because the apparatus implementing the present invention is,
for the most part, composed of electronic components and circuits
known to those skilled in the art, circuit details will not be
explained in any greater extent than that considered necessary as
illustrated above, for the understanding and appreciation of the
underlying concepts of the present invention and in order not to
obfuscate or distract from the teachings of the present
invention.
[0063] The term "program," as used herein, is defined as a sequence
of instructions designed for execution on a computer system. A
program, or computer program, may include a subroutine, a function,
a procedure, an object method, an object implementation, an
executable application, an applet, a servlet, a source code, an
object code, a shared library/dynamic load library and/or other
sequence of instructions designed for execution on a computer
system.
[0064] Some of the above embodiments, as applicable, may be
implemented using a variety of different information processing
systems. For example, although FIG. 1 and the discussion thereof
describe an exemplary information processing architecture, this
exemplary architecture is presented merely to provide a useful
reference in discussing various aspects of the invention. Of
course, the description of the architecture has been simplified for
purposes of discussion, and it is just one of many different types
of appropriate architectures that may be used in accordance with
the invention. Those skilled in the art will recognize that the
boundaries between logic blocks are merely illustrative and that
alternative embodiments may merge logic blocks or circuit elements
or impose an alternate decomposition of functionality upon various
logic blocks or circuit elements.
[0065] Thus, it is to be understood that the architectures depicted
herein are merely exemplary, and that in fact many other
architectures can be implemented which achieve the same
functionality. In an abstract, but still definite sense, any
arrangement of components to achieve the same functionality is
effectively "associated" such that the desired functionality is
achieved. Hence, any two components herein combined to achieve a
particular functionality can be seen as "associated with" each
other such that the desired functionality is achieved, irrespective
of architectures or intermedial components. Likewise, any two
components so associated can also be viewed as being "operably
connected," or "operably coupled," to each other to achieve the
desired functionality.
[0066] Furthermore, those skilled in the art will recognize that
boundaries between the functionality of the above described
operations merely illustrative. The functionality of multiple
operations may be combined into a single operation, and/or the
functionality of a single operation may be distributed in
additional operations. Moreover, alternative embodiments may
include multiple instances of a particular operation, and the order
of operations may be altered in various other embodiments.
[0067] All or some of the software described herein may be received
elements of system 10, for example, from computer readable media or
other media on other computer systems. Such computer readable media
may be permanently, removably or remotely coupled to a user
equipment (e.g., UE 110). The computer readable media may include,
for example and without limitation, any number of the following:
magnetic storage media including disk and tape storage media;
optical storage media such as compact disk media (e.g., CD-ROM,
CD-R, etc.) and digital video disk storage media; nonvolatile
memory storage media including semiconductor-based memory units
such as FLASH memory, EEPROM, EPROM, ROM; ferromagnetic digital
memories; MRAM; volatile storage media including registers, buffers
or caches, main memory, RAM, etc.; and data transmission media
including computer networks, point-to-point telecommunication
equipment, and carrier wave transmission media, just to name a
few.
[0068] Although the invention is described herein with reference to
specific embodiments, various modifications and changes can be made
without departing from the scope of the present invention as set
forth in the claims below. Accordingly, the specification and
figures are to be regarded in an illustrative rather than a
restrictive sense, and all such modifications are intended to be
included within the scope of the present invention. Any benefits,
advantages, or solutions to problems that are described herein with
regard to specific embodiments are not intended to be construed as
a critical, required, or essential feature or element of any or all
the claims.
[0069] Furthermore, the terms "a" or "an," as used herein, are
defined as one or more than one. Also, the use of introductory
phrases such as "at least one" and "one or more" in the claims
should not be construed to imply that the introduction of another
claim element by the indefinite articles "a" or "an" limits any
particular claim containing such introduced claim element to
inventions containing only one such element, even when the same
claim includes the introductory phrases "one or more" or "at least
one" and indefinite articles such as "a" or "an." The same holds
true for the use of definite articles.
[0070] Unless stated otherwise, terms such as "first" and "second"
are used to arbitrarily distinguish between the elements such terms
describe. Thus, these terms are not necessarily intended to
indicate temporal or other prioritization of such elements.
* * * * *