U.S. patent application number 14/925170 was filed with the patent office on 2017-05-04 for beam-scan time indicator.
The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Johan Axnas, Kumar Balachandran, Icaro L. J. da Silva, Dennis Hui, Andres Reial, Johan Rune, Henrik Sahlin.
Application Number | 20170127367 14/925170 |
Document ID | / |
Family ID | 54838404 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170127367 |
Kind Code |
A1 |
Axnas; Johan ; et
al. |
May 4, 2017 |
Beam-Scan Time Indicator
Abstract
The present disclosure relates to transmitting synchronization
signals and in particular to so called beam sweep. In particular
the disclosure relates to methods for providing synchronization
using synchronization sequences that are transmitted at different
points in time. The disclosure also relates to corresponding
devices and computer programs. A method in a network node, for
transmitting synchronization sequences of a synchronization signal
to one or more receiving wireless devices, comprises determining
multiple synchronization sequences, such that each synchronization
sequence comprises a respective timing indication, whereby each
synchronization sequence enables determination of a time of an
event in a receiving wireless device and transmitting the
synchronization sequences to the one or more wireless devices, at
different points in time.
Inventors: |
Axnas; Johan; (Solna,
SE) ; Balachandran; Kumar; (Pleasanton, CA) ;
da Silva; Icaro L. J.; (Bromma, SE) ; Hui;
Dennis; (Sunnyvale, CA) ; Reial; Andres;
(Malmo, SE) ; Rune; Johan; (Lidingo, SE) ;
Sahlin; Henrik; (Molnlycke, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stockholm |
|
SE |
|
|
Family ID: |
54838404 |
Appl. No.: |
14/925170 |
Filed: |
October 28, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 72/0446 20130101;
H04W 56/0015 20130101 |
International
Class: |
H04W 56/00 20060101
H04W056/00; H04W 72/04 20060101 H04W072/04 |
Claims
1. A method implemented by a network node, for transmitting
synchronization sequences of a synchronization signal to one or
more receiving wireless devices, the method comprising: determining
multiple synchronization sequences, such that each synchronization
sequence comprises a respective timing indication, whereby each
synchronization sequence enables determination of a time of an
event in a receiving wireless device; and transmitting the
synchronization sequences to the one or more wireless devices, at
different points in time.
2. The method of claim 1, wherein the multiple synchronization
sequences are time dependent versions of a synchronization signal
referring to one particular event.
3. The method of claim 1, further comprising determining a time of
the event.
4. The method of claim 1, wherein the synchronization sequences are
transmitted in different directions.
5. The method of claim 4, wherein the transmission of the
synchronization sequences constitutes a beam sweep.
6. The method of claim 1, wherein the timing indications are
relative to a time of transmission of the respective
synchronization sequence.
7. The method of claim 1, wherein the timing indications are
relative to a reference clock.
8. The method of claim 1, wherein the event is a time of a reserved
time slot where the wireless device is allowed to transmit.
9. The method of claim 8, wherein the reserved time slot is a
random access window.
10. The method of claim 1, wherein the event is a time of a further
transmission from the network node.
11. The method of claim 1, wherein the timing indications are
encoded into the synchronization sequence.
12. The method of claim 1, wherein the synchronization sequences
are determined to comprise a synchronization sequence from a set of
dissimilar synchronization sequences, and wherein each of the
dissimilar synchronization sequences in the set is mapped to a
respective point in time or to a timing.
13. The method of claim 12, wherein each synchronization sequence
is mapped to a time or timing by an index.
14. A non-transitory computer-readable medium storing a computer
program comprising program instructions that, when executed by
processing circuitry in a network node, configures the network node
for transmitting synchronization sequences of a synchronization
signal to one or more receiving wireless devices, the computer
program comprising program instructions causing the network node
to: determine multiple synchronization sequences, such that each
synchronization sequence comprises a respective timing indication,
whereby each synchronization sequence enables determination of a
time of an event in a receiving wireless device; and transmit the
synchronization sequences to the one or more wireless devices, at
different points in time.
15. A method implemented by a wireless device, for receiving one or
more synchronization sequences of a synchronization signal, the
method comprising: monitoring a spectrum for synchronisation
sequences; and when a first synchronisation sequence is detected
then: obtaining, by analysing the content of the detected first
synchronisation sequence, a timing indication defining a time of an
event.
16. The method of claim 15, wherein the method comprises receiving
a second synchronization sequence, wherein the first and the second
synchronization sequences define the same time.
17. The method of claim 15, comprising performing a transceiver
operation at the time defined by the timing indication.
18. The method of claim 15, wherein the timing indications are
relative times to a time of a transmission of the respective
synchronization sequence.
19. The method of claim 15, wherein the timing indications are
relative to a reference clock.
20. The method of claim 15, wherein the event is a time of a
reserved time slot where the wireless device is allowed to
transmit.
21. The method of claim 20, wherein the reserved time slot is a
random access window.
22. The method of claim 15, wherein the event is a time when the
wireless device is requested to listen for further transmission
from the network node.
23. The method of claim 15, wherein the timing indications are
encoded into the synchronization sequences and wherein the
analyzing comprises decoding the synchronization sequence.
24. The method of claim 15, wherein the wireless device monitors
the spectrum for several dissimilar synchronization sequences, and
wherein each of the dissimilar synchronization sequences is mapped
to a respective point in time or to a timing.
25. The method of claim 24, wherein each synchronization sequence
is mapped to a time or timing by an index and wherein the obtaining
comprises retrieving a time of timing using the index.
26. A non-transitory computer-readable medium storing a computer
program comprising program instructions that, when executed by
processing circuitry in a wireless device, configures the wireless
device for receiving one or more synchronization sequences of a
synchronization signal, the computer program comprising program
instructions causing the wireless device to: monitor a spectrum for
synchronisation sequences; and when a first synchronisation
sequence is detected then: obtain, by analysing the content of the
detected first synchronisation sequence, a timing indication
defining a time of an event.
27. A network node in a cellular communication network configured
for transmitting synchronization sequences of a synchronization
signal to one or more receiving wireless devices, the network node,
comprising: a communication interface; and processing circuitry
configured to cause the network node to: determine multiple
synchronization sequences, such that each synchronization sequence
comprises a respective timing indication, whereby each
synchronization sequence enables determination of a time of an
event in a receiving wireless device; and transmit the
synchronization sequences to the one or more wireless devices, at
different points in time.
28. A wireless device being configured for receiving one or more
synchronization sequences of a synchronization signal, the wireless
device comprising: circuitry communication interface; and
processing circuitry configured to cause the wireless device to:
monitor a spectrum for synchronisation sequences; and when a first
synchronisation sequence is detected then: obtain, by analysing the
content of the detected first synchronisation sequence, a timing
indication defining a time of an event.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to transmitting
synchronization signals and in particular to so called beam sweep.
In particular the disclosure relates to methods for providing
synchronization using synchronization sequences that are
transmitted at different points in time. The disclosure also
relates to corresponding devices and computer programs.
BACKGROUND
[0002] The 3rd Generation Partnership Project, 3GPP, is responsible
for the standardization of the Universal Mobile Telecommunication
System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is
also referred to as Evolved Universal Terrestrial Access Network,
E-UTRAN. LTE is a technology for realizing high-speed packet-based
communication that can reach high data rates both in the downlink
and in the uplink, and is thought of as a next generation mobile
communication system relative to UMTS. In order to support high
data rates, LTE allows for a system bandwidth of 20 MHz, or up to
100 MHz when carrier aggregation is employed. LTE is also able to
operate in different frequency bands and can operate in at least
Frequency Division Duplex (FDD) and Time Division Duplex (TDD)
modes.
[0003] When a User Equipment, UE, wishes to connect to a wireless
communication system, for example after power-on of the UE or when
waking up after an extended sleep period, it goes through an
initial-access procedure. The first step of this procedure is
typically that the UE searches for and detects a synchronization
signal, comprising synchronization sequences that are regularly
broadcasted by the network access nodes, ANs, also referred to as
base stations or network nodes. Note that synchronization signal is
not unambiguously used in prior art. In this disclosure the term
synchronization signal is used to refer to all the synchronization
sequences that are regularly broadcasted by the access nodes. In
other words, the synchronization signal is the sum of the
periodically repeated sequences. The synchronization signal allows
the UE to align with the network in time and frequency, i.e. learn
where the boundaries in time between the symbols (e.g. OFDM
symbols) are, and ensure that it uses, within a small tolerance,
the same carrier frequency as the network. Such alignment in time
and frequency is essential for subsequent communication. In LTE, on
synchronization sequence may be sufficient for the UE to align but
in some cases the UE will need to use several synchronization
sequences. To use several synchronizations sequences is not a
problem since the synchronization sequences are retransmitted
regularly. After successful alignment, the UE may, depending on the
type of system, be supposed to listen for additional information
from the network, e.g. so-called system information, and/or respond
with a request to join the network, often referred to as physical
random access channel message, or Physical Random Access Channel,
PRACH message. The UE is typically not allowed to send the request
to join at an arbitrary time, since that could conflict with other
transmissions in the system, but should rather send it at a
predefined time interval after the synchronization signal was
received. The UE typically also knows at what time interval after
the synchronization signal it could expect to find the additional
information (if any), thereby reducing the complexity of the search
and detection of the additional information.
[0004] In some systems, the UE might not respond directly with a
request to join the network, but might instead only request from
the network to send some additional system information, or only
send an Uplink, UL, synchronization signal to achieve
synchronization on the UL (in a system with a significant
propagation delay, Downlink, DL, synchronization does not
automatically guarantee UL synchronization). For generality, we
will henceforth refer to any UL signaling in response to the DL
synchronization signal (including, but not limited to a request to
join the network, request for additional system information, or an
UL synchronization signal) as the UL signal.
[0005] A procedure similar to the initial-access procedure may also
be performed when a UE wants to make a handover, i.e. it is already
connected to the system, but wishes to connect to another access
node, AN.
[0006] Future systems are expected to make heavy use of high-gain
narrow beamforming, which will enable high-data-rate transmission
coverage also to very distant users and/or in higher frequency
bands which would not realistically be covered with normal
sector-wide beams, which have lower antenna gain.
[0007] In order for the initial-access procedure not to be the
coverage-limiting factor in such systems, the synchronization
signal will typically also have to use high-gain narrow beams. This
means that the AN will typically have to transmit the
synchronization signal multiple times, in different directions, to
cover the geographical area to be served by an access node, AN.
With typical antenna configurations envisioned for the next
generation communication systems, sometimes referred to as 5G
systems, a narrow beam may cover only a small fraction of the
entire geographical area (e.g. 1%) at a time, and consequently it
may take substantial time to transmit the beam in all directions
needed, one or a few directions at a time.
[0008] The AN could in principle, depending on hardware
configuration, transmit the synchronization signal in many
directions at the same time, but given a maximum total output power
of the AN, such simultaneous transmission would be at the expense
of proportionally reduced power per beam, i.e. effectively reduce
the coverage. This could be compensated for by over-dimensioning
the hardware such that excessive total output power is available,
but this would undesirably increase the cost of the equipment. The
procedure of sequentially transmitting the beam in all necessary
directions is referred to as a beam sweep or beam scan. "Necessary
directions" here means all directions where coverage is
desired.
[0009] The UE may hear any of the many transmissions of the
synchronization signal during the beam sweep, and the network will
not know which one the UE heard. This means that if the UE is
supposed to send a system access request, e.g. using PRACH, a
certain time after hearing a synchronization beam transmission,
which is a typical random access request procedure, then the
network has to listen for a UL signal at multiple time instances in
a given direction, and/or the UE has to transmit its UL signal at
multiple time instances. Also, it means that the UE has to listen
for any additional information necessary to access the system, e.g.
system information, at multiple time instances and/or the network
has to transmit additional information at multiple time instances.
All the mentioned cases lead to inefficient use of radio resources.
In particular, this is the case since a node may at any one time
typically only listen to a limited number of signals, and in
half-duplex TDD systems (a typical choice for future wireless
communication systems) the node cannot transmit any signals at all
while listening.
[0010] The article "Directional Cell Search for Millimeter Wave
Cellular Systems" by C. Nicolas Barati et. al. has addressed the
problem of the detection by the mobile terminal of a beam-based
synchronization signal. The authors propose the base station to
periodically transmit synchronization signals in random directions
to scan the angular space and a detection algorithm based on
maximum likelihood is proposed where the mobile can detect the
strongest direction.
[0011] However, known references do not address the joint problem
of the AN transmitting synchronization signals using beamforming,
the mobile terminal detecting these signals and transmitting system
access requests, e.g. RACH request, and the AN detecting these
attempts from the terminal.
SUMMARY
[0012] An object of the present disclosure is to provide methods
and devices which seek to mitigate, alleviate, or eliminate one or
more of the above-identified deficiencies in the art and
disadvantages singly or in any combination.
[0013] This is obtained by a method for use in a network node, for
transmitting synchronization sequences of a synchronization signal
to one or more receiving wireless devices. The method comprises
determining multiple synchronization sequences, such that each
synchronization sequence comprises a respective timing indication.
Thereby, each synchronization sequence enables determination of a
time of an event in a receiving wireless device and transmitting
the synchronization sequences to the one or more wireless devices,
at different points in time. By providing a timing indication in
each synchronization sequence, the wireless device can derive a
more precise time of an event, so that it can react accordingly. In
a system using beam sweeps, the timing indication provides a way
for the wireless device to synchronize to the network node. The
event is for example a network node listening for a system access
request from the wireless device. In that particular case, the
wireless device uses the timing indication to determine when to
send the system access request. The wireless device does then not
have to transmit its uplink, UL, signal at multiple time instances.
It also does not need to listen for any additional information
necessary to access the system, e.g. system information, at
multiple time instances and/or the network does not have to
transmit additional information at multiple time instances.
[0014] According to some aspects, the multiple synchronization
sequences are time dependent versions of a synchronization signal
referring to one particular event. By providing time dependent
versions of a synchronization signal, the fact that the beam sweep
transmits signals at different times can be compensated for.
[0015] According to some aspects, the method further comprises
determining a time of the event. The time may be used when
providing the timing indication in the multiple synchronization
sequences.
[0016] According to some aspects, the synchronization sequences are
transmitted in different directions. Thus, wireless devices located
at different directions from the network node receives the
synchronization signal in the form of synchronization sequences,
which have been transmitted in different directions. The wireless
devices can then use the timing indication in the synchronization
sequence to synchronize the event with the network node.
[0017] According to some aspects, the transmission of the
synchronization sequences constitutes a beam sweep, i.e. the
synchronization sequences are transmitted in several directions
from the network node. The network thus uses high-gain narrow
beamforming, which will enable high-data-rate transmission coverage
also to very distant users which would not realistically be covered
with normal sector-wide beams, which have lower antenna gain.
[0018] According to some aspects, the timing indications are
relative to a time of transmission of the respective
synchronization sequence. When the timing indication is relative to
the time of transmission it is easy for the wireless device to
determine when the time of the event is.
[0019] According to some aspects, the timing indications are
relative to a reference clock. When using a reference clock, it is
possible to give a precise timing indication by referring to the
reference clock.
[0020] According to some aspects, the event is a time of a reserved
time slot where the wireless device is allowed to transmit. I.e.
the event is a time when the network node listens for a UL signal
from the wireless device. Thus, the wireless device is informed of
when it is possible to transmit to the network node.
[0021] According to some aspects, the reserved time slot is a
random access window. Hence, the wireless device is informed of
when to send random access packets.
[0022] According to some aspects, the event is a time of a further
transmission from the network node.
[0023] According to some aspects, the timing indications are
encoded into the synchronization sequence. According to some
aspects, the synchronization sequences are determined to comprise a
synchronization sequence from a set of dissimilar synchronization
sequences, and wherein each of the dissimilar synchronization
sequences in the set is mapped to a respective point in time or to
a timing. When different synchronization sequences are transmitted
in different directions and at different times and when they are
dissimilar, the receiving wireless device can infer a value for the
timing indication. In other words, the timing indication is
implicit in the distinct synchronization sequences.
[0024] According to some aspects, each synchronization sequence is
mapped to a time or timing by an index. It is then possible to
determine the time or timing by looking up the index in for example
a table.
[0025] According to some aspects, the disclosure also relates to a
network node in a cellular communication network configured for
transmitting synchronization sequences of a synchronization signal
to one or more receiving wireless devices. The network node
comprises a communication interface and processing circuitry. The
processing circuitry is configured to cause the network node to
determine multiple synchronization sequences, such that each
synchronization sequence comprises a respective timing indication,
whereby each synchronization sequence enables determination of a
time of an event in a receiving wireless device and to transmit the
synchronization sequences to the one or more wireless devices, at
different points in time. The network node is further configured to
perform all the aspects of the method in a network node described
above and below.
[0026] According to some aspects, the disclosure also relates to a
computer program comprising computer program code which, when
executed in a programmable controller of a network node, causes the
network node to execute the methods described above and below.
[0027] The object of the disclosure is further obtained by a method
for use in a wireless device, for receiving one or more
synchronization sequences of a synchronization signal. The method
comprises monitoring a spectrum for synchronisation sequences and
when a first synchronisation sequence is detected then obtaining,
by analysing the content of the detected first synchronisation
sequence, a timing indication defining a time of an event. Then the
wireless device is informed of the time of an event so that it can
react accordingly. As previously discussed, this enables that the
wireless device does not then have to transmit its uplink, UL,
signal at multiple time instances. It also does not need to listen
for any additional information necessary to access the system, e.g.
system information, at multiple time instances and/or the network
does not have to transmit additional information at multiple time
instances.
[0028] According to some aspects, the method comprises receiving a
second synchronization sequence, wherein the first and the second
synchronization sequences define the same time. The synchronization
sequences may comprise different timing indications but define the
same time. This is useful for example if the event is an event
which occurs at the same time for all wireless devices in all
directions from the network node.
[0029] According to some aspects, the method comprises performing a
transceiver operation at the time defined by the timing
indication.
[0030] According to some aspects, the timing indications are
relative times to a time of a transmission of the respective
synchronization sequence. When the timing indication is relative to
the time of transmission it is easy for the wireless device to
determine when the time of the event is.
[0031] According to some aspects, the timing indications are
relative to a reference clock. When using a reference clock, it is
possible to give a precise timing indication by referring to the
reference clock.
[0032] According to some aspects, the event is a time of a reserved
time slot where the wireless device is allowed to transmit. I.e.
the event is a time when the network node listens for a UL signal
from the wireless device. Thus, the wireless device is informed of
when it is possible to transmit to the network node.
[0033] According to some aspects, the reserved time slot is a
random access window. Hence, the wireless device is informed of
when to send random access packets.
[0034] According to some aspects, the event is a time when the
wireless device is requested to listen for further transmission
from the network node. Thus, the wireless device does not need to
listen for any additional information at multiple time
instances.
[0035] According to some aspects, the timing indications are
encoded into the synchronization sequences and wherein the
analyzing comprises decoding the synchronization sequence.
[0036] According to some aspects, the wireless device monitors the
spectrum for several dissimilar synchronization sequences, and
wherein each of the dissimilar synchronization sequences is mapped
to a respective point in time or to a timing. Since the wireless
device does not initially know in which direction from the network
node it is located in, and thus which synchronization sequence
transmission it may receive, it monitors a spectrum of several
possible transmissions.
[0037] According to some aspects, each synchronization sequence is
mapped to a time or timing by an index and wherein the obtaining
comprises retrieving a time of timing using the index. It is then
possible to determine the time or timing by looking up the index in
for example a table. By knowing the time or timing, the time of the
event is given.
[0038] According to some aspects, the disclosure also relates to a
wireless device being configured for receiving one or more
synchronization sequences of a synchronization signal. The wireless
device comprises a communication interface and processing
circuitry. The processing circuitry is configured to cause the
wireless device to monitor a spectrum for synchronisation
sequences; and when a first synchronisation sequence is detected
then to obtain, by analysing the content of the detected first
synchronisation sequence, a timing indication defining a time of an
event. The wireless device is further configured to perform all the
aspects of the method in a wireless device described above and
below.
[0039] According to some aspects, the disclosure also relates to a
computer program comprising computer program code which, when
executed in a programmable controller of a wireless device, causes
the wireless device to execute the methods described above and
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] The foregoing will be apparent from the following more
particular description of the example embodiments, as illustrated
in the accompanying drawings in which like reference characters
refer to the same parts throughout the different views. The
drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the example embodiments.
[0041] FIG. 1 is illustrating a network node transmitting in
different directions and two wireless devices receiving different
beams;
[0042] FIG. 2 is an example configuration of a wireless device,
according to some of the example embodiments;
[0043] FIG. 3 is an example node configuration of a network node,
according to some of the example embodiments;
[0044] FIG. 4 is a flowchart illustrating embodiments of method
steps in a network node;
[0045] FIG. 5 is a flowchart illustrating embodiments of method
steps in a wireless device;
[0046] FIG. 6 shows PSS and SSS frame and slot structure in time
domain in the FDD case;
[0047] FIG. 7 shows an example of PRACH configurations informed via
SIB2;
[0048] FIG. 8 shows the PBCH structure in LTE. The drawing at the
bottom is an enlargement of one subframe, marked with a thick
square in the figure.
[0049] FIG. 9 shows an example of a downlink synchronization
signal, in the form of a set of synchronization sequences, with
countdown indicator (countdown field).
[0050] FIG. 10 is an illustration of a synchronization signal.
DETAILED DESCRIPTION
[0051] Aspects of the present disclosure will be described more
fully hereinafter with reference to the accompanying drawings. The
apparatuses and methods disclosed herein can, however, be realized
in many different forms and should not be construed as being
limited to the aspects set forth herein. Like numbers in the
drawings refer to like elements throughout.
[0052] The terminology used herein is for the purpose of describing
particular aspects of the disclosure only, and is not intended to
limit the disclosure. As used herein, the singular forms "a", "an"
and "the" are intended to include the plural forms as well, unless
the context clearly indicates otherwise.
[0053] Access node, AN, radio network node and network node are
used interchangeably throughout the disclosure.
[0054] A synchronization signal is a predefined signal that allows
a receiving device to align (its master clock) with the
transmitting device in time and/or frequency, i.e. learn where the
boundaries in time between the symbols (e.g. OFDM symbols) are, and
ensure that the receiver uses, within a small tolerance, the same
carrier frequency as the transmitter. Such alignment in time and
frequency is essential for digital radio communication.
[0055] As discussed in the background, future systems are expected
to make heavy use of high-gain narrow beamforming. When using beam
forming, a wireless device communicating with the network node
using beam forming will not know when to listen or transmit to the
network node; i.e. when the beam is directed towards the wireless
device. An example of a network node 20 transmitting several
directed beams 1, 2, 3, 4 in a beam sweap and two receiving
wireless devices 10a, 10b are illustrated in FIG. 1.
[0056] The beam sweep may serve other purposes than just time and
frequency synchronization; in particular, the sweep may also serve
the purpose of determining the best beam direction for data
transmission to the new UE. In such cases, the beam may contain
some information that uniquely identifies the synchronization beam,
so that the UE can report to the AN which beam that was best
received. Here, the best beam can be characterized by several
alternative measures, for example the one received with highest
power, largest signal to noise ratio, smallest time of arrival
(indicating closest AN) or first received power over a threshold.
This can be seen as a sort of spatial synchronization. For
simplicity, we will henceforth collectively refer to signals for
time and frequency synchronization as well as beam identification
as just synchronization signals, which comprises synchronization
sequences.
[0057] In LTE the synchronization signals comprise synchronization
sequences i.e. pre-defined sequences of complex symbols that are
repeated in predefined patterns. Each synchronization sequence
informs the receiving device about an event, such as a random
access window.
[0058] If a synchronization signal comprising synchronization
sequences is transmitted in a beam sweap, the synchronization
sequences will be repeated in each of the beams 1, 2, 3, 4.
[0059] Typically the beams and thereby the synchronization
sequences will be transmitted at different points in time, i.e. the
synchronization sequences are time dependent. This type of time
shifted synchronization sequences are in this disclosure referred
to as "time dependent" and/or "time shifted".
[0060] FIG. 1 illustrates that a wireless device 10a will receive
the second 2 transmission and the wireless device 10b will receive
the fourth 4 transmission of a time dependent synchronization
sequence at different points in time. In LTE the time of the random
access window is dependent on the time of the synchronization
sequence, which implies that the network node needs to listen for
random access requests in multiple time slots corresponding to the
different beams.
[0061] The disclosure proposes to include, in each of the
synchronization sequences transmitted from a network node, a timing
indication that indicates to the wireless device, or user
equipment, UE, 10 when to, for example, listen for additional
information and/or send the uplink signal. The timing indication
would, according to some aspects, be an integer indicating the
number of OFDM symbols until the time when additional downlink
transmission and/or uplink signal should occur. For example, if one
beam direction is transmitted in each OFDM symbol, the
time-indication number would for example in each successive beam be
one smaller than in the previous beam, and could therefore be
referred to as a countdown indicator or countdown field.
[0062] The invention comprises several example embodiments
describing details of how the timing indication can be efficiently
encoded by signals transmitted in a synchronization sequences.
According to some aspects, a method is proposed in which there is a
predefined mapping from synchronization sequence index to time
indicator, i.e. by detecting which synchronization sequence out of
a set of predefined possible synchronization sequences that was
transmitted, the receiver can infer a value for the timing
indication.
[0063] FIGS. 2 and 3 illustrates examples of a wireless device 10
and a network node 20 which may incorporate some of the example
node operation embodiments discussed below. The network node is
e.g. an eNodeB. As shown the figures, the wireless device 10 and
the network node 20 may comprise a radio communication interface
11, 21 respectively, configured to receive and transmit any form of
communications or control signals within a network. It should be
appreciated that the radio communication interface 11, 21 may be
comprised as any number of transceiving, receiving, and/or
transmitting units or circuitry. It should further be appreciated
that the radio communication interface 11, 21 may be in the form of
any input/output communications port known in the art. The radio
communication interface 11, 21 may comprise RF circuitry and
baseband processing circuitry (not shown). Furthermore, the network
node 20 may comprise a network communication interface 23
configured to exchange any form of communications or control
signals with a core network and/or with other network nodes. The
network communication is typically referred to as a backhaul.
[0064] The wireless device 10 and the network node 20 may further
comprise at least one memory unit or circuitry 14, 24 respectively
that may be in communication with the radio communication interface
11, 21. The memory 14, 24 may be configured to store received or
transmitted data and/or executable program instructions. The memory
14, 24 may also be configured to store any form of beam-forming
information, reference signals, and/or feedback data or
information. The memory 14, 24 may be any suitable type of computer
readable memory and may be of volatile and/or non-volatile type.
According to some aspects, the disclosure relates to a computer
program comprising computer program code or instruction sets which,
when executed in a wireless device, causes the first wireless
device to execute any aspect of the example node operations
described below. According to some aspects, the disclosure relates
to a computer program comprising computer program code or
instruction sets which, when executed in a network node, causes the
network node to execute any aspect of the example node operations
described below.
[0065] The wireless device 10 and the network node 20 may further
respectively comprise a controller or processing circuitry 12, 22.
The processing circuitry 12, 22 may be any suitable type of
computation unit, e.g. a microprocessor, digital signal processor,
DSP, field programmable gate array, FPGA, or application specific
integrated circuit, ASIC, or any other form of circuitry. It should
be appreciated that the processing circuitry need not be provided
as a single unit but may be provided as any number of units or
circuitry. The processing circuitry is further adapted to perform
all the aspects of the method in a network node described above and
below.
[0066] FIGS. 4 and 5 illustrates a concept of the proposed
technique implemented in a wireless device 10 in FIG. 2 and in a
network node 20 in FIG. 3.
[0067] It should be appreciated that FIGS. 4 and 5 comprises some
operations which are illustrated with a solid border and some
operations which are illustrated with a dashed border. The
operations which are comprised in a solid border are operations
which are comprised in the broader example embodiment. The
operations which are comprised in a dashed border are example
embodiments which may be comprised in, or a part of, or are further
operations which may be taken in addition to the operations of the
solid border example embodiments. It should be appreciated that the
operations need not be performed in order. Furthermore, it should
be appreciated that not all of the operations need to be performed.
The example operations may be performed in any suitable order and
in any combination.
[0068] The disclosure provides for a method for use in a network
node, for transmitting synchronization sequences of a
synchronization signal to one or more receiving wireless devices,
see FIG. 4. The method comprises determining S2 multiple
synchronization sequences, such that each synchronization sequence
comprises a respective timing indication.
[0069] In other words, a synchronization sequences are not only
repeated at the different points in time, instead the time
dependent versions of the synchronization signal are selected or
determined such that they are different. Thus, additional
information is included in the time dependent versions, which
defines the actual time of the event. Hence, all of the time
dependent versions of the synchronization signal now refer to the
same point in time, i.e. the time of an event such as a random
access window. This implies that the synchronization sequences are
in some sense similar, or the same, as they refer to the same
event. However, the actual sequence of complex numbers that is
transmitted may be different, which will be further described
below. Hence, when using the proposed technique, each
synchronization sequence enables determination of a time of an
event in a receiving wireless device. According to some aspects the
synchronization sequences is a code word, e.g. using a Reed-Muller
code.
[0070] The method further comprises transmitting S3 the
synchronization sequences to the one or more wireless devices, at
different points in time. The synchronization sequences, which are
parts of the synchronization signal, are transmitted at different
points in time to one or several wireless devices. In other words,
the synchronization sequences that are referring to the same event
are retransmitted at different points in time. The processing
circuitry 22 of the network node is configured to determine S2 the
multiple synchronization sequences and to transmit S3, via the
communication interface 21, the synchronization sequence to one or
more wireless device. According to some aspects, the processing
circuitry comprises a determiner 222 for determining the
synchronization sequence and a transmitter 223 for transmitting the
synchronization sequence.
[0071] According to some aspects, the multiple synchronization
sequences are time dependent versions of a synchronization signal
referring to one particular event. By providing time dependent
synchronization sequences, the fact that the beam sweep transmits
signals at different times can be compensated for. By providing a
timing indication in each synchronization sequence, the wireless
device is informed of the time of an event so that it can react
accordingly. In a system using beam sweeps, the timing indication
provides a way for the wireless device to synchronize to the
network node. The event is for example when the network node
listens for a system access request from the wireless device. In
that particular case, the wireless device uses the timing
indication to determine when to send the system access request. The
wireless device does not then have to transmit its UL signal at
multiple time instances. It also does not need to listen for any
additional information necessary to access the system, e.g. system
information, at multiple time instances and/or the network does not
have to transmit additional information at multiple time instances.
The timing indication is described below with examples.
[0072] The time of the event may not always be predetermined. So,
according to some aspects, the method further comprises determining
S1 a time of the event. The processing circuitry 22 of the network
node is configured to determine S1 the time. According to some
aspects, the processing circuitry comprises a determiner 221 for
determining the time. The time is used when providing the timing
indication in the multiple synchronization sequences. To determine
the time of the event comprises for example to determine a duration
of time between the transmission of a synchronization sequence and
the event. Another example is to determine an absolute time of an
event as given by a reference clock. Examples will be further
discussed below.
[0073] As can be seen in FIG. 1, the synchronization sequences are,
according to some aspects, transmitted in different directions.
Thus, wireless devices located at different directions from the
network node receives synchronization sequences which have been
transmitted in different directions, for example 2 and 4 of FIG. 1.
The wireless devices can then use the timing indication to
synchronize the event with the network node. Different directions
here means in different spatial directions from the network node as
can be seen in the FIG. 1. Transmitting in different directions is
for example accomplished by so called beam forming using co located
antennas, also called antenna arrays. According to some aspects,
the transmission of the synchronization sequences constitutes a
beam sweep. I.e. the synchronization sequences are transmitted in
several directions, at consecutive points in time, from the network
node. The network thus uses high-gain narrow beamforming, which
will enable high-data-rate transmission coverage also to very
distant users which would not realistically be covered with normal
sector-wide beams, which have lower antenna gain.
[0074] When transmitting in different directions or at different
times, the timing indications need to be well defined. According to
some aspects, the timing indications are relative to a time of
transmission of the respective synchronization sequence. In other
words, the timing indication depends on the time of transmission.
According to some aspects, the timing indication comprises a time
length from the time of transmission. When the timing indication is
relative to the time of transmission it is easy for the wireless
device to determine when the time of the event is. According to
some aspects, the timing indications are relative to a reference
clock; for example a reference clock in the network node. When
using a reference clock, it is possible to give a precise timing
indication by referring to the reference clock. The reference clock
is a clock that both the wireless device and the network node has
access to. According to some aspects, the timing indication is
indication of the current time with respect to the frame structure,
e.g. the number of OFDM symbols since the start of the current
super-frame, from which the wireless device can derive the time
until the time intended for additional information and/or uplink
signal.
[0075] According to some aspects, the timing indication is a
pseudo-random number which is used once and discarded until all the
sequences in the current signal are handled. The wireless device
would beforehand have been provided with a translation method for
converting the pseudo-random numbers into time indications as
needed. One possible implementation of the embodiment is to use a
sequence of predetermined states of a linear feedback shift
register with a known starting state.
[0076] There are several examples of events when the present
disclosure is useful. According to some aspects, the event is a
time of a reserved time slot where the wireless device is allowed
to transmit. I.e. the event is a time when the network node listens
for an uplink, UL, signal from the wireless device. Thus, the
wireless device is informed of when it is possible to transmit to
the network node. According to some aspects, the reserved time slot
is a random access, RA, window. Hence, the wireless device is
informed of when to send random access message. The event for
example defines the beginning of the RA window or the beginning of
another action. According to some aspects the time is a start time
of a reserved time slot which may be useful in, for example, the
case that the event is a rather long time window, whose length is
variable but has been communicated by other means.
[0077] According to some aspects, different timing indications are
transmitted in different synchronization sequences in order to
reduce congestion on the uplink. These different timing indictors,
and their corresponding synchronizations, are typically transmitted
in different directions from the network node. Hence, the timing
indications may refer to different points in time, which enables
use of several RA windows. In this way, wireless devices which are
located in different directions from the network nodes, will use
different RA windows. This approach is useful if many wireless
devices are included in the covereage of a network node, i.e. many
devices can receive the synchronization signals. With many devices
detecting synchronization signals follows that also many devices
will transmit a random access signal. The risk of too many
simultaneous RA transmissions, such that the network node cannot
receive all of them correctly, is reduced with a directional
separation of devices into at least two groups transmitting in
different RA windows. This is an adaptation possibility to control
the Random Access, RA, resource reservation vs. congestion
trade-off.
[0078] According to some aspects, the time slots for additional
information and/or uplink signal are not periodically occurring,
but rather dynamically decided by the network based on traffic
demands and/or behavior of other wireless devices.
[0079] In LTE the network sends Primary and Secondary
Synchronisation Sequences, PSS/SSS, in an a priori known frequency
allocation (6 central resource blocks of the downlink frequency
band) in potentially known time domain slots both from an OFDM
symbol and subframes point of views. In other words, the UE knows
that PSS/SSS come respectively in OFDM symbols #6 and #5 (for
normal CP) being repeated in subframe#0 and subframe#5. After
detecting both the PSS and the SSS the UE is DL synchronized both
from an OFDM symbol and subframe perspectives. This is shown in
FIG. 6.
[0080] In addition to the synchronization the UE detects the
physical cell identity, PCI, that is encoded in the PSS/SS. Based
on that the UE is capable of using the cell-specific reference
signals, CRS, in order to estimate the channel and decode the
system information which contains the most basic information the UE
should be aware of before it attempts to access the system. This
information is organized in what we call master information blocks,
MIB, and system information blocks, SIBs.
[0081] The physical channel which this information is transmitted
on differs from block to block. For example, the master information
blocks, MIB, are transmitted over the Physical Broadcast Channel,
PBCH, while the other SIBs are transmitted over the Physical
Downlink Shared Channel, PDSCH, so they can be flexibly scheduled
in other portions of the frequency band. The PBCH structure in LTE
is illustrated in FIG. 8.
[0082] In order to access the system the UE needs to start a random
access procedure. This is triggered by sending a random access
preamble over the physical random access channel, PRACH. The PRACH
can be multiplexed over the uplink band with the physical uplink
control channel, PUCCH, also used for channel status reports,
acknowledgements and/or scheduling requests.
[0083] Before the UE can transmit the random access preamble it has
to acquire information about the how PRACH is multiplexed in the UL
band. This is informed in SIB2 in the IE prach-Configuration Index
which goes from 0 to 63 and contains among other things the
following parameters (details can be found in TS 36.211 version
11.2.0): Preamble format, Subframe Sequence number and Subframe
number.
[0084] The configuration index basically indicates in which
time-domain resource(s) in PUCCH the UE should send a random access
preamble (i.e. on the PRACH). FIG. 7 shows some examples of
configurations for a given preamble format. Details can be found in
TS 36.211 (chapter 5.7.1), version 11.2.0.
[0085] In summary, in LTE the UE needs to decode the whole payload
from SIB2 (after getting DL synchronized and being capable of
performing channel estimation) in order to get the necessary
information to send a system access request. This relies on the
fact that information is sent in all directions in well-defined
time-domain structures (OFDM symbols, subframes, radio frame,
etc.)
[0086] Another example is when the event is a time of a further
transmission from the network node. In other words, the event is a
time that the wireless device is to listen for another transmission
from the network node. This enables the wireless device to be more
efficient since it does not have to listen for transmission on
other times than the time defined in the synchronization
sequence.
[0087] According to some aspects, the mapping from synchronization
signal index to time is informed via the system information which
could be obtained via another method such as the one in LTE or via
the system control plane, SCP, access concept. In the case of LTE,
the SIB2 message could contain a mapping between synchronization
sequence to be beamformed and the time-frequency allocation where
the UL request signal should be transmitted and/or where additional
information would be expected. In the case of the SCP access
concept, this mapping could be informed via the access information
table, AIT, and later pointed to by a system signature sequence,
SSI. An UL system request signal in this context could be e.g. a
PRACH preamble, a request for further system information to be
transmitted or an UL synchronization message without major higher
layer meaning.
[0088] In one embodiment, the "system information" can contain
information about the system bandwidth and/or other information
similar to the information contained in the system information in
LTE, in particular the contents of the MIB, SIB1 and SIB2, such as
system frame number and barring information.
[0089] There are several possible ways to provide the timing
indication. According to some aspects, the timing indications are
encoded into the synchronization sequence. In other words, the
timing indication is implicitly encoded. The system would then, for
example, have multiple different synchronization sequences
predefined, and each synchronization sequence would have an index
associated with it. The synchronization sequences themselves may
have arbitrary construction; normally, the sequences may be
optimized to have good auto- and cross-correlation properties
and/or good Euclidean or Hamming distance properties. Each such
index would then correspond to a timing indication according to a
predefined mapping, which will be referred to as a countdown index
mapping. In general, multiple sequence indices/synchronization
sequences may correspond to the same timing indication.
[0090] This is particularly useful in a variant of the proposed
technique where multiple beams are transmitted at the same time,
from a single network node or from multiple network nodes, since
one may then want the wireless device to be able to distinguish
between the different beams (for example for later reporting to the
network), while still receiving the same (or different) timing
indication. It will be readily apparent, that the synchronization
sequence thus defined would be equivalent to an encoding of the
index. One example of this variant is an access node that transmits
two beams (called a and b) simultaneous in different directions.
However, for some reason, it only wants to listen in one direction
at the time. Hence, the access node wants to make sure that UEs in
different directions respond at different points in time.
[0091] Hence according to a variant, of this disclosure proposes a
method for use in a network node, for transmitting synchronization
sequences of a synchronization signal to one or more receiving
wireless devices. The method comprises determining multiple
synchronization sequences, such that each synchronization sequence
comprises a respective timing indication, whereby each
synchronization sequence enables determination of a respective (or
the same) time of an event in a receiving wireless device; and
transmitting the synchronization sequences to the one or more
wireless devices, at least partly at the same point in time but in
different directions.
[0092] Another example of this variant is an access node may
transmit one beam in direction a and another in direction b.
However, a UE can for some reason only transmit in one direction at
the time, which is known by the access node. Then the access node
wants to make sure that the UE can respond in one direction at the
time.
[0093] Now turning back to FIG. 4. According to some aspects of the
method, each one of the synchronization sequences are determined to
comprise a synchronization sequence from a set of dissimilar
synchronization sequences, and wherein each of the dissimilar
synchronization sequences in the set is mapped to a respective
point in time or to a timing. E.g. wherein the timing indication
corresponds to a synchronization signal index from which the
receiving wireless device can infer a value for the timing
indication. When different synchronization sequences are
transmitted in different directions and when they are dissimilar,
i.e. distinct, the receiving wireless device can infer a value for
the timing indication. In other words, the timing indication is
implicit in the distinct synchronization sequences. Another example
is that each synchronization sequence is mapped to a time or timing
by an index. Please note that multiple different sequence indices
can be mapped to the same timing indication. It is then possible to
determine the time or timing by looking up the index in for example
a table.
[0094] According to some aspects, in the case of implicit encoding,
the multiple different synchronization sequences may correspond to
different frequency shifted version of the same basic
synchronization sequence, e.g. by transmitting the same
synchronization sequence over different subcarriers in different
time slots. The frequency at which the sequence is transmitted can
be mapped into a timing indicator.
[0095] According to some aspects, the uplink signal can be sent on
a physical random-access channel, PRACH, wherein the uplink signal
could e.g. be a random access preamble, or an uplink
synchronization channel, USS, wherein the uplink signal could e.g.
be an uplink synchronization sequence/signal.
Example Embodiments
[0096] In one example embodiment of the disclosure, the downlink
synchronization signals (sequences) are indexed, such that the
indices form a consecutive number series. This could be described
in the system information or possibly even standardized. When
putting together a set of downlink synchronization signals for the
above non-pre-configured scenarios, the access node would choose a
set of downlink synchronization signals whose indices would form a
consecutive number series. The downlink synchronization signals
would be transmitted in an order such that the corresponding
indices would form a consecutive decreasing number series. At the
point in time where the uplink signal, e.g. a system access request
such as a random access request, is to be transmitted, the number
series of the indices, as indicated by the potential transmission
of a downlink synchronization signal, should have reached a number
where a certain number of the least significant bits are zero. We
may refer to this downlink synchronization signal as the end-of-set
downlink synchronization signal and its index the end-of-set index.
The term "potential transmission" is here used for the end-of-set
downlink synchronization signal, simply because it is not certain
that it is transmitted. If the access node wants the set of
synchronization beam transmissions to end a longer time before the
uplink signal, then the decreasing consecutive series of indices
will be interrupted before it reaches the end-of-set index.
[0097] With this principle, each downlink synchronization sequence
would, via its index, indicate to the wireless device exactly the
time distance from the downlink synchronization sequence
transmission until the uplink signal transmission, i.e. it would
serve the purpose of a countdown indicator. For instance, if the
format of an end-of-set index is defined as an index ending with
0000, a set of downlink synchronization sequences could consist of
8 downlink synchronization sequences with the binary index series
(assuming 8 bit indices) 01010111, 01010110, 01010101, 01010100,
01010011, 01010010, 01010001, 01010000 (i.e. decimal 87, 86, 85,
84, 83, 82, 81, 80), see last 8 indices of FIG. 9. In another
example where the end-of-set downlink synchronization sequences is
not transmitted, because a longer time to the uplink signal is
desired, there could be downlink synchronization sequences with
indices 01011001, 01011000, 01010111, 01010110, 01010101, 01010100,
01010011, 01010010 (i.e. decimal 89, 88, 87, 86, 85, 84, 83, 82).
The latter example is illustrated by FIG. 9, showing the point in
time for the uplink signal defined to occur, denoted TUSS (where
USS denotes the uplink signal), after the (hypothetical/potential)
transmission of the end of set downlink synchronization sequences.
An example of an UL signal could be an uplink synchronization
signal (USS), a system access request, or something else. In FIG. 9
each box pictures a Mobility Reference Signal, MRS, with a specific
index, where the MRS here denotes the downlink synchronization
sequence, and the Uplink Synchronization Channel, USS, the uplink
signal. The left vertical arrow indicates the hypothetical
transmission of end-of-set MRS, whereas the right vertical arrow
indicates the point in time for the uplink signal defined to occur
TUSS.
[0098] In another embodiment, according to some aspects, each
transmitted beam consists of at least two parts, one part that
serves as training (pilot/reference/synchronization) sequence, e.g.
a predefined sequence e.g. of QPSK-modulated symbols or a
Zadoff-Chu sequence, allowing the wireless device to synchronize,
detect the beam, and perform channel estimation, and one part that
contains the timing indication encoded using some channel code,
e.g. a Reed-Muller code. The two parts can be separated for example
by using different, but typically adjacent, time and/or frequency
resources. FIG. 10 is an example illustration of a synchronization
signal where each OFDM symbol consists of two parts separated in
time, nA and nB, where n is the OFDM symbol number. The horizontal
axis represents the OFDM symbol number and the vertical axis
represents the frequency.
[0099] As already mentioned, the timing indication can be
implicitly encoded in the synchronization sequence. A variant of
this approach could be to let each transmitted beam consist of two
or more parts, separated in time and/or frequency, where both parts
consist of some kind of synchronization sequence, but where the two
synchronization sequences are not the same.
[0100] For example, the synchronization sequence in one part may be
the same for many beam directions and/or time instances, whereas
the sequence in the other part may be different for different beam
directions and/or time instances. Such an arrangement may be useful
to reduce the computational complexity in the wireless device: For
the first part, only a few possible synchronization sequences may
be needed in the system, reducing the search space and hence
computational complexity in the wireless device, whereas for the
second part, a larger number of synchronization sequences may be
used in the system while still keeping the complexity of the
wireless device moderate, since the wireless device already has a
rather good channel estimate from the first part.
[0101] According to some aspects, certain beam directions are
repeated two or more times, e.g. to allow the receiver to perform
receive beam scanning, but with different countdown indicator
values.
[0102] According to some aspects, the disclosure also relates to a
computer program comprising computer program code which, when
executed in a programmable controller of a network node, causes the
network node to execute the methods described above and below. In
other words, the disclosure also relates to a computer readable
storage medium, having stored there on a computer program which,
when executed in a programmable controller of a network node,
causes the network node to execute the methods described above and
below.
[0103] The disclosure provides a corresponding method in the
wireless device, which will now be described referring to FIG. 5.
The disclosure provides a method for use in a wireless device 10,
for receiving one or more synchronization sequences of a
synchronization signal. The method comprises monitoring S11 a
spectrum for synchronisation sequences. Monitoring a spectrum here
means receiving a radio signal and based on the received signal,
and possibly additional assumptions about e.g. noise and
interference levels, estimating for one or more predefined
synchronization sequences a quality measure, e.g. the likelihood
that it has been transmitted. Such a quality measure can typically
be based on a matched-filter approach where the received signal is
correlated with each one of the one or more predefined
synchronization sequences, e.g. random access preambles. Typically
the monitoring goes on until a match is found, i.e. until the
correlation is above a certain threshold.
[0104] When a first synchronisation sequence is detected then the
method comprises obtaining S12, by analysing the content of the
detected first synchronisation sequence, a timing indication
defining a time of an event. In the description of the network node
many examples are given regarding how the timing indication can be
included.
[0105] The processing circuitry 12 of the wireless device is
configured to monitor S11, via the communication circuitry 11, the
spectrum and to obtain S12 the timing indication. According to some
aspects, the processing circuitry comprises a monitoring unit 121
for monitoring and an obtainer 122 for obtaining the timing
indication. The timing indication has been included in the
synchronization sequence by a transmitting network node.
[0106] Accordingly, the wireless device is informed of the time of
an event so that it can react accordingly. As previously discussed,
this for example enables that the wireless device does not then
have to transmit its uplink, UL, signal at multiple time instances.
It also, for example, does not need to listen for any additional
information necessary to access the system, e.g. system
information, at multiple time instances and/or the network does not
have to transmit additional information at multiple time
instances.
[0107] According to some aspects, in system implementations where
the network node provides the wireless device with a list of beam
identifiers or sequence indices to monitor, the provided index for
a given beam may contain (1) the initial part of the full index
sequence, and (2) the number of countdown hypotheses, i.e. timer
values. The wireless device may then add the timer bits to the
initial part of the sequence to form a full sequence and fetch the
corresponding reference sequence, which will then be searched for
in the received signal.
[0108] As previously discussed, the multiple synchronization
sequences are, according to some aspects, time dependent versions
of a synchronization signal referring to one particular event.
Analyzing the content of the detected first synchronization
sequence comprises for example to decode the first synchronization
sequence and searching for a timing indication.
[0109] Depending on the location of the wireless device 10, the
wireless device may receive more than one synchronization sequence.
According to some aspects, the method comprises receiving S11b a
second synchronization sequence, wherein the first and the second
synchronization sequences define the same time. In the example when
the timing indication is a relative time from the time of
transmission, the relative time is different in the synchronization
sequences but the time the indications define is the same time.
According to some aspects, it is enough that the wireless device
receives one synchronization signal, but there might be the case
that it needs several for sufficient reliability. According to some
aspects the network node has communicated beforehand to the
wireless device how the device should handle such a situation. An
example is that the network node has communicated to the wireless
device that it will repeat the synchronization sequence N number of
times, in each beam direction, with the same sequence every time
except for a countdown field that is reduced with 1 for each
repetition to indicate which of the repetitions it is. According to
some aspects, the timing indications are relative times to a time
of a transmission of the respective synchronization sequence. When
the timing indication is relative to the time of transmission it is
easy for the wireless device to determine when the time of the
event is. The synchronization sequences may thus comprise different
timing indications but define the same time. This is useful for
example if the event is an event which occurs at the same time for
all wireless devices in all directions from the network node. As
also discussed when discussing the method of the network node, the
timing indications are, according to some aspects, relative to a
reference clock or the event is a time of a reserved time slot
where the wireless device is allowed to transmit. According to some
aspects, the reserved time slot is a random access window. The
specifics of these examples have been previously discussed.
[0110] According to some aspects, the method comprises performing
S13 a transceiver operation at the time defined by the timing
indication. In other words, the wireless device receives and/or
transmits at a time defined in the synchronization sequence.
[0111] The processing circuitry 12 is configured to perform S13 the
multiple synchronization sequences. According to some aspects, the
processing circuitry comprises a determiner 123 for determining.
The transceiver operation is for example to transmit a random
access preamble in an indicated RACH slot. According to some
aspects, the event is a time when the wireless device is requested
to listen for further transmission from the network node. Thus, the
wireless device does not need to listen for any additional
information at multiple time instances.
[0112] As also previously discussed when discussing the method of
the network node, the timing indications are, according to some
aspects, encoded into the synchronization sequences and wherein the
analyzing comprises decoding the synchronization sequence. If the
timing indication is encoded in the synchronization sequence, then
the wireless device may need to decode the synchronization sequence
in order to detect the time of the event.
[0113] According to some aspects, the wireless device monitors the
spectrum for several dissimilar synchronization sequences, and
wherein each of the dissimilar synchronization sequences is mapped
to a respective point in time or to a timing. Since the wireless
device does not initially know in which direction from the network
node it is located, and thus which synchronization sequence
transmission it may receive, it monitors a spectrum of several
possible transmissions. The distinct synchronization sequences each
provide a point in time or a timing of the event. If there are
different synchronization sequences being mapped to different
events, then the mere detection of the particular sequence (or
preamble) tells the wireless device where to e.g. send its random
access request.
[0114] As previously discussed, this time or timing is either
relative to something else or absolute. According to some aspects,
each synchronization sequence is mapped to a time or timing by an
index and wherein the obtaining 12 comprises retrieving a time of
timing using the index. Note that multiple different sequence
indices may be mapped to the same timing indication. It is then
possible to determine the time or timing by looking up the index in
for example a table. Thus, the time or timing defines a time of the
event.
[0115] According to some aspects, the disclosure also relates to a
computer program comprising computer program code which, when
executed in a programmable controller of a wireless device, causes
the wireless device to execute the methods described above and
below. In other words, the disclosure also relates to a computer
readable storage medium, having stored there on a computer program
which, when executed in a programmable controller of a wireless
device, causes the wireless device to execute the methods described
above and below.
[0116] The description above consistently considers a wireless
device that should synchronize to a network of network nodes, but
the same techniques can be used in other synchronization and/or
beam finding situations, for example a newly deployed network node
that needs to synchronize to another pre-existing network nodes (or
even wireless device) in the network, especially in networks using
self-backhauling. The description also applies to device-2-device,
D2D, initial synchronization. Hence, the techniques described above
may in many cases apply equally well when "uplink" is replaced by
"downlink" and vice versa. Furthermore the description above
focuses on initial access, but similar techniques can be used to
synchronize and/or find beams in, e.g., handover situations.
[0117] The disclosure above primarily considers joint
synchronization and beam finding, but the techniques are applicable
also in situations where only beam finding is needed, for example
because synchronization has already been achieved before by other
means.
[0118] The disclosure above often mentions OFDM as an example.
However, the techniques described here are applicable also to
several variants of OFDM as well as to many other multiplexing
schemes, including e.g. DFT-spread OFDM, filtered OFDM, filter-bank
multicarrier (FBMC), single-carrier frequency-division multiple
access (SC-FDMA), etc. The techniques are also applicable to other
types of RATs, TDMA, CDMA etc.
[0119] The value of the detected countdown indicator may also
somehow be embedded in an uplink response (e.g. system access
request) from the UE. This may be useful to help the network
determine which beam the UE heard.
[0120] The disclosure results in a significant reduction in
resource allocation overhead. There is a reduction in needed uplink
resources compared to having a fixed time between downlink
synchronization signal and uplink signal. A reduction in downlink
signaling is achieved as compared to transmitting a detailed system
configuration (system information) in all downlink beams. A timing
indication included in the synchronization sequences is a highly
flexible way of indicating the proper timing for the uplink signal.
The system can with this approach configure how often the uplink
signals, e.g. in the form of a packet random access using e.g. a
PRACH, are allowed. This method is in contrast to having a fixed
(or semi-static) configuration of time intervals for uplink access
(PRACH). The network node can select if several beams and wireless
devices, or UEs, are given the same timing indication, in order to
save uplink resources, or if the different beams and UEs should be
configured to separate uplink time intervals, in order to reduce
congestion.
[0121] The advantages may be particularly pronounced in systems
where analog beamforming is employed in the Network node and/or in
the wireless device. There are several reasons for this, and here
is provided just one example: An analog beam former would typically
use digitally controlled phase shifters to define the beam
configuration, and the shape of the resultant pattern would depend
on the particular phasing chosen for the antenna elements or
antenna ports. Thus a beam pattern with multiple main lobes can be
labelled with the same timing indication for several directions,
making the uplink signals from various directions coincide with the
same value of the timing indication. This uplink signal may occur
from random access signals from one or more wireless device in any
advantaged position.
[0122] Within the context of this disclosure, the terms "wireless
terminal" or "wireless device" encompass any terminal which is able
to communicate wirelessly with another device, as well as,
optionally, with an access node of a wireless network) by
transmitting and/or receiving wireless signals. Thus, the term
"wireless terminal" encompasses, but is not limited to: a user
equipment, e.g. an LTE UE, a mobile terminal, a stationary or
mobile wireless device for machine-to-machine communication, an
integrated or embedded wireless card, an externally plugged in
wireless card, a dongle etc. Throughout this disclosure, the term
"user equipment" is sometimes used to exemplify various
embodiments. However, this should not be construed as limiting, as
the concepts illustrated herein are equally applicable to other
wireless nodes. Hence, whenever a "user equipment" or "UE" is
referred to in this disclosure, this should be understood as
encompassing any wireless terminal as defined above.
[0123] Aspects of the disclosure are described with reference to
the drawings, e.g., block diagrams and/or flowcharts. It is
understood that several entities in the drawings, e.g., blocks of
the block diagrams, and also combinations of entities in the
drawings, can be implemented by computer program instructions,
which instructions can be stored in a computer-readable memory, and
also loaded onto a computer or other programmable data processing
apparatus. Such computer program instructions can be provided to a
processor of a general purpose computer, a special purpose computer
and/or other programmable data processing apparatus to produce a
machine, such that the instructions, which execute via the
processor of the computer and/or other programmable data processing
apparatus, create means for implementing the functions/acts
specified in the block diagrams and/or flowchart block or
blocks.
[0124] In some implementations and according to some aspects of the
disclosure, the functions or steps noted in the blocks can occur
out of the order noted in the operational illustrations. For
example, two blocks shown in succession can in fact be executed
substantially concurrently or the blocks can sometimes be executed
in the reverse order, depending upon the functionality/acts
involved. Also, the functions or steps noted in the blocks can
according to some aspects of the disclosure be executed
continuously in a loop.
[0125] In the drawings and specification, there have been disclosed
exemplary aspects of the disclosure. However, many variations and
modifications can be made to these aspects without substantially
departing from the principles of the present disclosure. Thus, the
disclosure should be regarded as illustrative rather than
restrictive, and not as being limited to the particular aspects
discussed above. Accordingly, although specific terms are employed,
they are used in a generic and descriptive sense only and not for
purposes of limitation.
[0126] The description of the example embodiments provided herein
have been presented for purposes of illustration. The description
is not intended to be exhaustive or to limit example embodiments to
the precise form disclosed, and modifications and variations are
possible in light of the above teachings or may be acquired from
practice of various alternatives to the provided embodiments. The
examples discussed herein were chosen and described in order to
explain the principles and the nature of various example
embodiments and its practical application to enable one skilled in
the art to utilize the example embodiments in various manners and
with various modifications as are suited to the particular use
contemplated. The features of the embodiments described herein may
be combined in all possible combinations of methods, apparatus,
modules, systems, and computer program products. It should be
appreciated that the example embodiments presented herein may be
practiced in any combination with each other.
[0127] It should be noted that the word "comprising" does not
necessarily exclude the presence of other elements or steps than
those listed and the words "a" or "an" preceding an element do not
exclude the presence of a plurality of such elements. It should
further be noted that any reference signs do not limit the scope of
the claims, that the example embodiments may be implemented at
least in part by means of both hardware and software, and that
several "means", "units" or "devices" may be represented by the
same item of hardware.
[0128] The various example embodiments described herein are
described in the general context of method steps or processes,
which may be implemented in one aspect by a computer program
product, embodied in a computer-readable medium, including
computer-executable instructions, such as program code, executed by
computers in networked environments. A computer-readable medium may
include removable and non-removable storage devices including, but
not limited to, Read Only Memory (ROM), Random Access Memory (RAM),
compact discs (CDs), digital versatile discs (DVD), etc. Generally,
program modules may include routines, programs, objects,
components, data structures, etc. that performs particular tasks or
implement particular abstract data types. Computer-executable
instructions, associated data structures, and program modules
represent examples of program code for executing steps of the
methods disclosed herein. The particular sequence of such
executable instructions or associated data structures represents
examples of corresponding acts for implementing the functions
described in such steps or processes.
[0129] In the drawings and specification, there have been disclosed
exemplary embodiments. However, many variations and modifications
can be made to these embodiments. Accordingly, although specific
terms are employed, they are used in a generic and descriptive
sense only and not for purposes of limitation, the scope of the
embodiments being defined by the following claims.
* * * * *