U.S. patent application number 13/049172 was filed with the patent office on 2011-09-15 for smooth hard handover method and mobile station adapted for the method.
Invention is credited to Gyorgy Miklos, Andras Racz.
Application Number | 20110223965 13/049172 |
Document ID | / |
Family ID | 35510151 |
Filed Date | 2011-09-15 |
United States Patent
Application |
20110223965 |
Kind Code |
A1 |
Miklos; Gyorgy ; et
al. |
September 15, 2011 |
Smooth Hard Handover Method and Mobile Station Adapted For The
Method
Abstract
A cellular radio communications system, method, mobile station,
and base station for a smooth hard handover. Selected base stations
prepare radio links for potential communication with a mobile
station although only one radio link is actively used for
transmission. A handover is then performed from the active radio
link to one of the prepared radio links. The smooth hard handover
is faster and more secure than a conventional hard handover, while
requiring a less complex system architecture than a conventional
soft handover.
Inventors: |
Miklos; Gyorgy; (Budapest,
HU) ; Racz; Andras; (Budapest, HU) |
Family ID: |
35510151 |
Appl. No.: |
13/049172 |
Filed: |
March 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11570613 |
Dec 14, 2006 |
7933600 |
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PCT/SE2004/000981 |
Jun 15, 2004 |
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13049172 |
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Current U.S.
Class: |
455/525 |
Current CPC
Class: |
H04W 36/18 20130101 |
Class at
Publication: |
455/525 |
International
Class: |
H04B 1/40 20060101
H04B001/40; H04B 7/26 20060101 H04B007/26 |
Claims
1-18. (canceled)
19. A mobile station comprising: a receiver chain; a transmitter
chain; wherein the receiver and transmitter chains are respectively
adapted to receive and transmit radio signals over an exchangeable
first radio link served by a first base station, characterized by
further comprising: a receiver chain controller that controls the
receiver chain to prepare a second radio link for transceiving with
a second base station, while communication continues on the first
radio link, wherein the receiver chain is adapted to synchronize
reception to the second base station by receiving the control
information from the second radio base station over a control
channel.
20. The mobile station according to claim 19, wherein the receiver
chain controller controls the receiver chain to prepare the second
radio channel in response to a command received over the first
radio link.
21. The mobile station according to claim 19, wherein the receiver
chain controller is adapted to select the second base station for
the preparation of the second radio link based on measurements made
by the mobile station on the signal strengths received from base
stations neighboring the first base station.
22. The mobile station according to claim 19, wherein the receiver
chain is adapted to receive user data over the first and second
radio links in parallel when the user data on the two radio links
are different.
23. The mobile station according to claim 19, wherein the receiver
chain includes: a receiver unit that receives radio signals; a
descrambling and demultiplexing unit having an input from the
receiver unit and adapted to descramble and demultiplex the first
and the second radio links in parallel and to output the
descrambled and demultiplexed first and second radio links
separately; a demodulation, de-spreading, and channel decoding unit
having at least one input from the descrambling and demultiplexing
unit that separately receives the first and second radio links and
demodulates, de-spreads, and channel decodes dedicated data
channels received on the first and second radio links,
respectively, and provides on at least one output, base band
signals of the dedicated data channels received on the first and
second radio links; and a radio link control processor unit with at
least one input from the demodulation, de-spreading, and channel
decoding unit that separately receives the base band signals of the
data channels received on the first and second radio links,
respectively, and receives control commands and forwards a command
regarding the preparation of the second radio link to the receiver
chain controller.
24-26. (canceled)
27. A handover method performed by a mobile station having a
receiver chain and a transmitter chain, each adapted to receive and
transmit radio signals over an exchangeable first radio link served
by a first base station, the method comprising the steps of:
preparing by the receiver chain, a second radio link for
transceiving with a second base station while communication
continues on the first radio link; and synchronizing by the
receiver chain, reception to the second base station by receiving
the control information from the second radio base station over a
control channel.
28. The handover method according to claim 27, wherein a receiver
chain controller controls the receiver chain to prepare the second
radio link in response to a command received over the first radio
link.
29. The handover method according to claim 27, wherein the step of
preparing the second radio link includes selecting the second base
station by the receiver chain controller based on measurements made
by the mobile station on the signal strengths received from base
stations neighboring the first base station.
30. The handover method according to claim 27, wherein the step of
preparing the second radio link for transceiving with a second base
station while communication continues on the first radio link
includes receiving user data over the first and second radio links
in parallel when the user data on the two radio links are
different.
31. The handover method according to claim 30, wherein the step of
receiving user data over the first and second radio links in
parallel includes: receiving the radio signals over the first and
second radio links by a receiver unit; forwarding the received
radio signals to a descrambling and demultiplexing unit;
descrambling and demultiplexing the first and the second radio
links in parallel; forwarding the descrambled and demultiplexed
first and second radio links separately to a demodulation,
de-spreading, and channel decoding unit; demodulating,
de-spreading, and channel decoding dedicated data channels received
on the first and second radio links, respectively; separately
forwarding to a radio link control processor unit, base band
signals of the dedicated data channels received on the respectively
first and second radio links; and forwarding to the receiver chain
controller, a command from the radio link control processor unit
regarding the preparation of the second radio link to the receiver
chain controller.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 11/570,613 filed Dec. 14, 2006, which is a 371
of International Application No. PCT/SE2004/000981, filed Jun. 15,
2004.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a cellular radio
communications system and especially a method, a mobile station and
a base station for smooth hard handover.
DESCRIPTION OF RELATED ART
[0003] WCDMA (Wideband Code Division Multiple Access) is a cellular
radio communications system standardised by 3GPP. A network based
in CDMA standard normally comprises some RNCs (Radio Network
Controllers), and a greater number of base stations. The base
stations provide radio service for mobile stations in respective
geographical areas called cells. The cells may partly overlap. The
RNCs control a respective group of base stations and have
connections to a core network for setting up communication from a
mobile station via a base station, and the RNC itself to the core
network for further connection.
[0004] When the user of the mobile terminal is moving while having
a connection to another terminal, the RNC controls that the right
base station provides the radio link to the mobile station. This
may include change of the base station, during the connection and
that is referred to as handover.
[0005] WCDMA supports soft handover, which means that more than one
base station provides the mobile station with a radio link. When a
radio link is released there is always at least one other radio
link established over a corresponding base station that support the
communication. The group of radio base stations providing links in
parallel to the same mobile station are named active set in the
WCDMA standard. An advantage of the soft handover is that the
connection to the mobile station is safe, and provides good
quality. A disadvantage is though it is very complicated to
implement. The parallel links are not only necessary over the radio
link but are also needed within the fixed network to a central
connecting and splitting point. The parallel links also occupies
double or triple transmission resources, depending on the number of
parallel links.
[0006] Hard handover is an alternative to the soft handover and by
which is meant that communication over the old link is stopped
before the new radio link is established. Hard handover is easier
to implement than the soft handover and is used in first and second
generation cellular networks while also being supported by the
third generations standards as WCDMA. A disadvantage is though that
the end users are disturbed by the interruption caused by the
handover. This is especially the case when the delay is long before
the new link is established. An additional disadvantage is that the
new link occasionally may not be established after the release of
the old link. Because of these disadvantages, it should be avoided
to perform hard handovers frequently. Accordingly, the hard
handover is not performed until the target base station performs a
certain level better quality than the old base station. Until the
level is hit, the mobile station may cause interference on the
radio spectrum because it is not under control of the optimum base
station.
SUMMARY OF THE INVENTION
[0007] The present invention addresses the hard handover problem of
delay until the new radio link can be established and the resulting
interruption of the communication.
[0008] The present invention solves the problem by setting up a two
or more radio links over respective base stations with a mobile
station similar to the case of soft handover. However, active
transceiving is made over one of the radio links, while the other
radio links setup between the mobile station and the other one or
more radio base stations are prepared for transmission. The radio
link used for transceiving can then be rapidly changed to any of
the prepared radio links when the mobile station is handed over to
a second base station from a first base station.
[0009] The process of keeping a number of prepared radio links over
a set of base station is made similar to the set up of an active
set of base stations. However, it is a process separated from the
handover itself. Candidates for the handover are though only the
set of base stations providing prepared radio links.
[0010] An advantage of the present invention is that the handover
from a first to a second base station is fast and that the user
experiences high quality of the reception. Moreover, the handover
is safe because the chance of succeeding with establishing active
communication is high when the radio link has been prepared. For
this reasons the smooth hard handover can advantageously replace
the soft handover process.
[0011] When compared to the soft handover it is an advantage of the
present invention that no splitter combiner node is needed in the
network. The network architecture can thus be simplified and also
less transmission capacity in the network is needed. The splitting
and combiner node needed in the soft handover case also results in
long delays in case data need to be retransmitted. Consequently,
also retransmission delays will decrease with the smooth hard
handover.
DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 block diagram illustrating cells and base
stations.
[0013] FIG. 2 flowchart of an inventive method.
[0014] FIG. 3 a view over WCDMA RAN architecture.
[0015] FIG. 4 is a chart of the signals transmitted during the
radio link addition according to a prior art soft handover.
[0016] FIGS. 5a and 5b are one chart, split over two pages, of
signals sent during the prior art hard handover.
[0017] FIG. 6 is a chart of signals sent during the prior art soft
handover link addition.
[0018] FIG. 7 is a chart of signals sent during the prior art soft
handover radio link removal.
[0019] FIGS. 8a and 8b is one chart, split over two pages, of
signals sent during the prior art soft handover combined radio link
addition and removal.
[0020] FIGS. 9a and 9b is one chart, split over two pages, of
signals sent during the inventive smooth hard handover.
[0021] FIG. 10 is a block diagram of an evolved WCDMA architecture
in which radio protocol layer 2 is terminated in NodeB.
[0022] FIG. 11 is a chart of signals sent during the present
invention radio link addition as performed in an architecture
according to FIG. 10.
[0023] FIG. 12 is a chart of signals sent during the present
invention radio link removal as performed in an architecture
according to FIG. 10.
[0024] FIG. 13 is a chart of signals sent during the present
invention combined radio link addition and removal as performed in
an architecture according to FIG. 10.
[0025] FIGS. 14a and 14b is one chart, split over two pages, of
signals sent during the present invention smooth hard handover as
performed in an architecture according to FIG. 10.
[0026] FIG. 15 is an alternative system architecture in which radio
protocol layer 2 is terminated in NodeB.
[0027] FIG. 16 is a chart of signals sent during the present
invention radio link addition as performed in an architecture
according to FIG. 15.
[0028] FIG. 17 is a chart of signals sent during the present
invention radio link removal as performed in an architecture
according to FIG. 15.
[0029] FIGS. 18a and 18b is one chart, split over two pages, of
signals sent during the inventive smooth hard handover.
[0030] FIG. 19 is a block diagram of parts of a mobile station
structure relevant for the invention.
[0031] FIG. 20 is a block diagram of some base station structures
relevant for the invention
[0032] FIGS. 21a and 21b is a table of modified messages and their
content.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] FIG. 1 is a schematic view of base stations BS1-BS5
supporting respective cells C1-C5 with radio communication. In real
radio communication system the cell boarders are not distinct and
coverage of the various cells overlap. A mobile station MS is also
disclosed in FIG. 1 with an active radio link set up with a first
radio base station BS1. A group of the cells C1-C3 that are hatched
marked in FIG. 1 will be described further in connection with the
method of FIG. 2.
[0034] FIG. 2 discloses an inventive method of smooth hard
handover. In a first step S21, a first radio link is set up between
a base station and a mobile station and transmission is started
over the radio link, for example the first base station BS1 and the
mobile station MS of FIG. 1. The radio link used for the
transmission is referred to as the active radio link.
[0035] While transceiving on the active radio link the mobile
station MS makes measurements on base stations BS2-BS5 neighbouring
the first base station BS1. If the signal strength from one or more
of the radio base stations BS2-BSS5 measured are strong enough, a
radio link is prepared between the mobile station MS and the base
station BS2-BS5, see the second step S22 of FIG. 2. In the prepared
state the radio link is established, synchronization is obtained
but it is not activated, which means that it is not used for actual
data transmission. The base stations supporting a radio link,
active or prepared, with the mobile station MS are referred to as a
preparation-set. In the example, the first, second and third base
stations BS1-BS3 are assumed to form a preparation set and hence
their respective cells C1-C3 are hatched marked in FIG. 1.
[0036] The preparation of the radio link, in the second step S22,
is made similarly to the set up of a diversity soft handover link,
whereas no transmission or reception of user data is made in the
mobile station MS or in the second and third base stations BS2, BS3
supporting the prepared radio links.
[0037] In a third step, S23, the radio quality of the active radio
link is evaluated and compared to the estimated radio link quality
of the one or more prepared radio links. Handover is decided
according to pre-determined thresholds of the qualities.
[0038] Should the evaluation in the third step S23 result in
handover being decided to another of the preparation set base
stations BS2, BS3, for example the second base station BS2,
transceiving is started on the new active radio link supported by
the second base station BS2 and transceiving on the radio link
supported by the first base station BS1 is stopped, according to a
fourth step S24. The handover involves a switch in the fixed
network transmission links for transmission via a second radio base
station when the communication is handed over from the first base
station, as is the case for hard handovers. Unlike the conventional
hard handover, transmission in the direction to the mobile station
MS may start from the target base station before it has ceased from
the old base station. This optional of two base stations
transmitting in parallel during the handover differs from the
conventional soft handover in that the two base stations transmit
different user data. The old base station transmits user data that
it has received for transmission and buffered until radio
transmission recourses are available, while the target radio base
station starts transmission as soon as it receives user data.
[0039] After the evaluation of preparation-set base stations
BS1-BS3 for handover in the third step S23 and the possible
handover of the fourth step S24, follows the fifth step, S25, of
evaluating and selecting base stations for the preparation set. The
evaluation and selection are preferably made in the same way as
when active-set base station are selected in the case of soft
handover. Candidates for being added to the preparation set are
neighbour base stations BS4-BS5 to the preparation set base
stations BS1-BS3.
[0040] If a change of base station BS2-BS5 in the preparation-set
is decided in the fifth step, S25, step six S26 follows and in case
of a base station BS4, BS5 being added, a radio link with it is
prepared or in case base station BS2, BS3 is omitted its prepared
radio link is released.
[0041] After the sixth step S26, or in case of no change of
preparation-set being decided in the fifth step S25, the third step
S23 of evaluating and deciding on a possible handover is repeated.
The third to sixth steps S23-S26 are then repeated in a loop as
long as the communication with the mobile station MS continues.
[0042] The process of selecting, adding or removing base stations
BS1-BS3 to and from the preparation-set as disclosed in the second,
fifth and sixth step S22, S25, S26 is similar to that of selecting,
adding or removing base station to and from an active set in the
prior art soft handover case with the exception of adding a base
station to the active set implies soft handover while adding of a
base station to the preparation-set does not involve any handover
just a radio channel being prepared. The smooth hard handover of
the third and forth steps S23, S24 is a separate process from that
of controlling the preparation set, however, only base stations
BS1-BS3 within the preparation set are candidates for handing over
the active radio link.
[0043] In the process of evaluating the preparation-set base
stations BS1-BS3 for a possible handover, e.g. the third step S23,
measurements are frequently made in the mobile station MS on the
prepared radio links with the preparation-set base stations BS1,BS2
or alternatively on the pilot channels of the preparation-set base
stations BS1-BS3 and in the preparation-set base stations BS1-BS3
on the uplink radio channel from the mobile station MS. The
measurements are accurate and occupy resources in the
preparation-set base station BS1-BS3 as well as in the mobile
station MS. The number of preparation-set base stations BS1-BS3
should be limited for example to three. Hence, selection of base
station also involves release of base stations BS1-BS3 from the
preparation set.
[0044] In communications system bases on CDMA technology, such as
WCDMA a physical channel is characterised by the spreading code
used by the transmitter. When the radio link is handed over to a
new base station, the physical channel used in the direction from
the base station to the mobile station change. The mobile station
MS may, however, continue to transmit on the same physical channel
when the radio link is served by the new base station as it did
before the handover.
[0045] Of course, different parts of the procedure described by the
flowchart in FIG. 2 can be implemented either in the mobile station
or in network elements, such as the base station or central
node.
[0046] The smooth hard handover can advantageously be made more
frequently that the ordinary hard handover because it is quick and
safe. In order to avoid too frequent hard handovers the target base
station generally need to provide a radio quality a certain level
degree better than that of the serving base station. This causes a
risk that the mobile station adversely interferes with the traffic
in the target cell until the prior art handover is finalised. One
reason for this is that only the base station supporting the active
radio link can regulate the transmit power of the mobile station
MS. With the smooth hard handover of the present invention,
handover can be made more frequent and rapidly and hence uplink
interference reduced in base stations providing a prepared radio
link with the mobile station MS.
[0047] The smooth hard handover procedure of FIG. 2 can be
implemented in various radio communications systems, cellular
system as well indoor systems.
[0048] When the method is implemented in a WCDMA system, a radio
link in the prepared state, i.e. prior to being in active state,
have a dedicated physical control channel set up but not a
dedicated physical data channel. Control information sent on the
dedicated physical control channel is used to keep the prepared
radio link synchronized and keep track of correct power levels. The
dedicated physical channel need be set up only in the downlink
direction i.e. from the base station to the mobile station. When
the radio link enters the active state, the dedicated physical
control channel is maintained and the dedicated physical data
channel is set up.
[0049] When applying the WCDMA terminology, the term transmitting,
receiving or transceiving on a radio link always refers to the
transmission, reception of user data on a dedicated physical data
channel and hence implies an active state radio link. However,
transmission, reception of physical layer control information may
be also ongoing in case of prepared state radio links on the
dedicated physical control channel.
[0050] Prior Art WCDMA Architecture
[0051] Further detailed embodiments on the signalling between the
network nodes and the base station will be given with the invention
as employed in a WCDMA (Wideband Code Division Multiple Access)
system standardised by 3GPP. Because the 3GPP standard uses other
words for several nodes than what is used when speaking of cellular
radio systems in general, the 3GPP specific wordings or
abbreviations will be used in the disclosure of the WCDMA
implementation of the present invention. This will be made also
with reference to FIG. 3 that is a schematic view of some basic
nodes of the WCDMA system well known before the priority date of
the present patent/application. It should be noted that the nodes
and interfaces are described for the purpose of ease the
understanding of the present invention and may not be complete.
FIG. 3 discloses a mobile station MS which is named User Equipment
and abbreviated UE in 3GPP standard. In the further description UE
will be used for the WCDMA mobile station and MS is the reference
sign to the FIG. 3. Base stations BS1-BS3 are named NodeB in the
WCDMA standard and thus will be named so also in the further
description while the reference signs BS1-BS3 are used with
reference to FIG. 3. Two radio network controller 31, 32
abbreviated RNC 31, 32 controls the radio link set up of a
respective plurality of connected NodeBs BS1-BS3 over respective
lub interface and makes connections to other RNCs 31, 32 over the
lur interface. The RNCs 31, 32 have an lu interface for set up of
communication links to the core network 33 for further links to
other radio networks or to other types of networks such as a Public
Switched Telephone Network PSTN 34. A radio link is set up between
the UE MS and one or more NodeB over the Uu interface.
[0052] Should the UE MS have started the communication over a NodeB
BS4 under the control of a first of the RNCs 31 and then during
communication being handed over to a NodeB BS1-BS3 under the
control of a second of the RNCs 32, both RNCs will remain some
control of the communication. The first of the RNCs 31 will act as
a SRNC (Serving Radio Network Controller) for the communication
link with the UE MS and be responsible for its connectivity with
the core network 33. The second RNC 32 acts as a DRNC (Drift RNC)
for the communication with the UE MS and support the SRNC 31 with
communication link via any of the base stations BS1-BS3 under its
control. Should no handover be made from the NodeBs under the
control of the SRNC 31 it also takes the responsibilities as a
DRNC.
[0053] Prior Art Signalling Procedures in WCDMA
[0054] The relevant signalling charts of the standardised
procedures will initially be described for understanding of the
difference of the signalling schemes of the present invention when
it is described further down.
[0055] Prior Art Soft Handover Signalling
[0056] FIG. 4 is chart of the messages sent between the RNC 31, the
NodeBs and the UE during the soft handover procedure of adding a
new NodeB to the active set and that includes setting up an active
radio link between the added NodeB and the UE. The performance of
the UE, the NodeB and the RNC 32 are under the control of software
programs and that is organised in protocol stacks with different
layers of the stacks handling corresponding functions in the
communication. Accordingly, the messages sent between the three
nodes are handled by different protocol layers in the nodes. The
protocol layers receiving or initiating a message is indicated in
the message chart of FIG. 4. These protocol layers are L1 e.g.
layer 1 or the physical layer in the UE, the NodeB and the RNC and
the RRC (Radio Resource Control layer) relevant for the UE and the
RNC. Moreover, should separate RNCs act as DRNC and SRNC, L1
functions and signalling are handled by the DRNC while the
RRC-layer functions and signalling are handled by the SRNC.
[0057] The transmission of messages proceeds from top downwards in
FIG. 4, and is initiated by the RNC determining that a new NodeB
shall be added to the active set NodeBs. The RNC determination is
based upon measurements made by the UE on NodeBs neighbouring the
active set NodeBs and that are reported to the RNC.
[0058] The initial determination 41 of soft handover is indicated
in FIG. 4. The signalling procedure then starts by the SRNC-RRC
sending the new NodeB L1 a command 42 to start transceiving on a
specified radio channel by the message CPHY_RL-Setup-Req. The NodeB
immediately starts transceiving 43 and confirms this by sending 44
a CPHY-RL-Setup-CNF message to the RNC-RRC. Then the RNC-RRC
instructs 45 its own L1 to open a user data connection for the
channel with the new NodeB by sending the command CPHY_RL-Setup-Req
to RNC-L1. Next the RNC-RRC commands 46 the UE-RRC to add the new
NodeB to the active set by an Active Set Update command. Upon
receipt of this command the RRC-layer of the UE commands 47 the
UE-L1 to start receiving 48 on the radio link from the new NodeB by
a message CPHY-RL-Setup-REQ. The UE confirms 49 to the RNC-RRC that
the reception from the new NodeB is started by sending an Active
Set Update Complete message. The UE-L1 also confirms 411 reception
on the channel from the new NodeB by sending to UE-RRC a message
CPHY-RL-Setup-CNF.
[0059] Adding the new NodeB includes start of both transmission and
reception in the NodeB, while only reception is started in the UE
MS. The reason is that the uplink channel from the mobile station
is characterised by one spreading code that is decoded by all the
active set NodeBs. In the downlink, the active set NodeBs uses
separate scrambling codes characterising respective downlink
channels. The UE receives the respective downlink channels
separately.
[0060] The delays that are associated with certain parts of the
soft handover procedure are also indicated in the FIG. 4 signalling
chart. The first part of the procedure when the new radio link is
setup in the NodeB BS and in the RNC 32 takes roughly 20-30 ms,
which is not significant. The overall delay of the procedure from
the point when the decision is made in the RNC until the active set
update is acknowledged by the UE is roughly 200-400 ms.
[0061] Prior Art Hard Handover
[0062] The prior art hard handover message scheme will now be
described with reference to FIGS. 5a and 5b. The signalling nodes
and protocols of FIGS. 5a and 5b are the same as in FIG. 4 and that
are described above. The exchange of messages starts in FIG. 5a by
an inter-frequency handover being determined 501 by the SRNC.
[0063] Initially transmission and reception in the new NodeB is
started by the RNC-RRC commanding 502 the new NodeB by a message
CPHY-RL-Setup-REQ. The transmission and reception is started 503a
immediately and the new NodeB confirms 503b this by sending the
RNC-RRC a message CPHY-RL-Setup-CNF. Next the RNC-RRC requests 504
the RNC-L1 to open a radio link with the new NodeB with the command
CPHY-RL-Setup-REQ. Then the RNC-RRC command 505 the UE to perform
the hard handover by sending a Physical Channel Reconfiguration
command to the UE-RRC. The UE-RRC commands 506 the UE-L1 to stop
transceiving 507 on the radio link supported by the previous NodeB
by the message CPHY-RL-release-REQ and next commands 508 the UE-L1
to start transceiving 509 on the radio channel supported by the new
NodeB. The further process is continued in FIG. 5b. Before
communication can be started over the new radio link the UE needs
to perform 510 L1 synchronisation and reestablishment 512 of the L2
connection. This involves control that packets are not lost during
the handover. When the L1 synchronisation and L2 reestablishment is
completed the UE-RRC informs the RNC-RRC on this by sending 513 a
message Physical Channel Reconfiguration Complete to the layer 2
signalling protocol of the RNC. The RNC-RRC then starts release of
the radio link supported by the previous NodeB by sending 514 a
CPHY-RL-Release-REQ. The previous supporting NodeB then stops
transceiving on the radio link and confirms 516 this to the RNC-RRC
by the command CPHY-RL-Release-CNF. Last the RNC-RRC commands 517
the RNC-L1 to release the link over lub to the previous NodeB.
[0064] To the right of the signalling scheme in FIGS. 5a and 5b are
indicated the estimated times for setting up the new radio link in
the new NodeB and in the UE and that, to the knowledge of the
inventors, are not made public known before the priority date of
this patent/application. Notably the time for setting up the radio
link in the UE is expected to be considerably longer than 200-400
ms while the time for setting up the link in the NodeB is
approximately 20-30 ms. The reasons for the long time of setting up
the UE radio link are the L1 synchronisation and the L2 connection
reestablishment. The delay inversely effects the end users
perception of the communication link.
[0065] Embodiments of the Present Invention as Implemented in
Standardised WCDMA Architecture Having RNC Terminated Radio
Protocols
[0066] According to WCDMA standard at the priority date of this
patent/application radio protocols are terminated in the RNC. In
the embodiments following it is assumed that the radio protocols
are terminated in the RNC as is standardised for WCDMA at the
priority date of the present patent/application. The architecture
assumed is that sketch in FIG. 3.
[0067] Control of the Preparation Set
[0068] FIG. 6 is chart of the messages sent during the process of
adding a new radio link to the preparation set, and that
corresponds to the preparation of a radio link in the second and
sixth step S22, S26 in the method described with reference to FIG.
2. The order in which messages are sent is basically the same as
for the prior art link addition in the soft handover case,
described above with reference to FIG. 4. The main difference is
that the radio link setup should not imply the start of reception
and transmission in the NodeB or in the UE. Therefore, the messages
are modified, and "Prepared" is added to the names of the modified
messages 62, 64, 64, 67a, 67b sent between the RNC and the NodeB
and internally in the UE and RNC between the different protocol
layers. In the direction from the RNC to the UE the modified
messages is Preparation Set Update 66 that indicates to the UE what
radio channels that shall be added to the preparation set and in
the opposite direction the Preparation Set Update Complete 69 that
indicates that the UE has set up the new radio link without active
reception on it.
[0069] In the message charts the activity of "Setup radio link" and
"Release radio link" refer to setting up and releasing a prepared
radio link, that is, setting up and releasing a control channel in
the direction from the NodeB to the UE. The "Start rx/tx" and "Stop
rx/tx" refer to the activation and deactivation of a data channel
corresponding to the control channel that has been set up during
preparation, that is, to start transceiving actual user data. The
start and stop rx/tx have this meaning all signalling diagrams of
the application.
[0070] The delays associated with the link addition are expected to
be roughly the same as in case of soft handover (.about.200-400
ms).
[0071] The synchronization of radio links in the preparation set
can be different for each NodeB, which means that different NodeBs
do not need to be synchronized. Both the UE and the corresponding
NodeB are responsible for keeping the synchronization of prepared
radio links via the control channel. When the active radio link is
handed over to one of the prepared radio links the UE has to send
its uplink transmissions and receive its downlink receptions
according to the synchronization of the new radio link.
[0072] The UE is power controlled by the active NodeB that the UE
is actually communicating with. However, the other NodeBs may also
listen to the transmission of the UE and send back power control
commands to the UE on the control channel. These power control
commands are not followed by the UE, they are used only to keep an
estimate of the proper power level to each NodeB, which can be used
to quickly adjust to the right power when a handover to a new Node
B occurs. However, the use of this power estimate is optional.
[0073] The message chart of removing a radio link from the
preparation set is shown in FIG. 7. The process is the same as used
in the case of the soft handover, with the exception of the
messages being modified for indicating that a radio link in
prepared state is to be removed.
[0074] In more detail, the signalling is triggered by the SRNC
decision 70 of removing one of the preparation set radio links. The
SRNC-RRC sends 71 a Preparation Set Update message to the UE-RRC.
The UE-RRC commands 72a the UE-L1 to release 73 its receiver of the
link to be released and the receiver confirms 72b the release. Then
the UE-RRC confirms 74 the release by the message Preparation Set
Update Complete. Next, the SRNC-RRC commands 75 the NodeB to
deactivate 76 the data channel of the radio link by a command
CPHY-RL_-Release-REQ, and receives a confirmation 77
CPHY-RL_-Release-CNF from the NodeB. Last, the RNC-RRC commands 78
the RNC-L1 to release the data channel.
[0075] Finally, the process of combined radio link addition and
removal is shown in FIGS. 8a and 8b. This is obtained as a
combination of the radio link addition of FIG. 6 and removal
processes of FIG. 7. In the combined process of FIGS. 8a and 8b,
the Preparation Set Update message from the SRNC-RRC to the UE-RRC
is a command 86 of both preparing a radio link to a new NodeB and
of releasing a radio link to an old NodeB.
[0076] The Inventive Smooth Hard Handover
[0077] The message chart of the smooth hard handover process is
shown in FIGS. 9a and 9b. The first part of the process, disclosed
in FIG. 9a, which is associated with the radio link setup 91-94 at
the new NodeB, is roughly the same as the hard handover scheme of
FIGS. 5a and 5b. A difference is though that according to the
Smooth Hard Handover of the present invention the actual radio link
has been prepared when the new NodeB was added to the preparation
set. Therefore, at the actual handover the radio link only needs to
be activated, which will result in the start of reception and
transmission of user data at the new NodeB. This will save some
delay compared to hard handover, but it is not expected to be
significant, since the delay of the radio link establishment
process itself is not significant either (.about.20-30 ms) and it
is determined by the signalling delay between the RNC and NodeB.
The expected delays are indicated to the right of the signalling
chart in FIGS. 9a and 9b.
[0078] The rest of the inventive smooth hard handover process that
is associated with the actual link change is significantly
different compared to the prior art hard handover scheme. For the
smooth hard handover a new message, the LINK ACTIVATE message 95,
is introduced and sent by the RNC to the UE to trigger the handover
at the UE. The UE activates 96a, 97 the radio link to the new
NodeB, which means that the UE starts receiving 97 on the new link
and it immediately sends a LINK ACTIVATE COMPLETE 98 back to the
RNC. At this point the UE has parallel radio links with both NodeBs
and the L2 handover process can be considered finished, since user
data traffic may start to flow on the new link. The old radio link
can be kept until pending packets in buffers are transmitted. When
the old radio link can be deactivated the RNC sends a LINK
DEACTIVATE message 99 to the UE, which triggers the UE to stop
receiving 910, 911 on the old link. The UE confirm the deactivation
by sending a LINK DEACTIVATE COMPLETE message 912 to the RNC. Then,
the RNC instructs 913 the old NodeB to deactivate the radio link,
which, in turn, stops transceiving 914 and confirms 915 this to the
RNC. Finally, the RNC deactivates 916 its own link to the old
NodeB.
[0079] The deactivation of the old link can be initiated also by
the UE. It should be noted that the old link is not released during
the smooth hard handover process, it is only deactivated but it
still remains in the preparation set until a separate decision on
releasing it is taken.
[0080] During the handover transition, i.e. the period when radio
links to both the old and new NodeB are active, the UE is power
controlled by both NodeBs. If the UE is able to set its transmit
power levels differently toward the two NodeBs it sets the power to
each link individually according to received power control
commands. If the UE is not able to set power levels individually
then it uses the following strategy to set its power. It increases
power if any of the NodeBs command for an increase and decreases
power if all NodeBs instruct for a decrease. Regarding downlink
transmit powers, the UE may control the transmit power of each
NodeB separately or it may send only one common power control
command to all NodeBs, in which case a power increase is commanded
if the received power level from any of the NodeBs was too weak and
a power decrease is commanded if the received power level from all
NodeBs were high enough. After the handover is finished the UE will
be power controlled only by the active NodeB.
[0081] The L2 connection can be maintained continuously during the
handover in the case when the L2 protocols are located in the RNC.
Note also in the figure that the UE does not perform a L2 link
reestablishment with the RNC as it was the case with the original
hard handover scheme.
[0082] When comparing the LINK ACTIVATE process of the smooth hard
handover chart of FIGS. 9a and 9b and the PHYSICAL CHANNEL
RECONFIGURATION process of the original hard handover scheme, the
inventive smooth hard handover process will be found to be simpler
than that of the original hard handover. This suggests that a
significant gain in handover delay can be achieved with smooth hard
handover. The main advantage of smooth hard handover process is
that the radio link is basically continuous during the handover due
to the make-before-break type of BS change. There is no need for
radio link setup, radio link synchronization and L2 link
reestablishment during the handover. Therefore, the delay
associated with the link change in case of SHH is expected to be
significantly smaller even than the active set update process
(<<200-400 ms).
[0083] Because of the make before brake in the smooth hard handover
process it could also be compared to the soft handover. A
significant difference is though that for the soft handover case
there need to be a network node splitting stream of data in the
direction to the UE MS to be received by several NodeBs and in the
direction from the mobile station data received in parallel over
several nodes combining the data into one stream. For the smooth
hard handover, no such splitting and combining is needed.
[0084] Embodiments of Present Invention as Implemented in an
Evolved WCDMA Architecture Having NodeB Terminated Radio
Protocols
[0085] Further signalling charts of the inventions will be
disclosed as implemented in an evolved WCDMA architecture having
the current RNC functions split into two nodes. FIG. 10 discloses
the nodes of the evolved radio access network. The RS node 132
(Radio Server node) performs the radio resource control functions.
FIG. 10 further discloses NodeBs BS1-BS3 and an UE MS. In the
example the NodeBs BS1-BS3 are included in the preparation set 100
for the UE MS and the first of the NodeBs BS1 supports an active
radio link with the UE MS. The user data transmission link is
indicated in FIG. 10 with a continuous line from the UE MS to the
first NodeB BS1 over the active radio link and further to user data
plane node 133. The prepared radio links between the UE MS and the
second and third NodeBs BS1-BS3 of the reparation set are indicated
with dashed lines. The RS node 132 controls links to the
preparations set NodeBs BS1-BS3 and the active radio link to the UE
MS which all are indicated with dotted lines.
[0086] In the evolved architecture of FIG. 10 the user plane L2
protocols are moved to the NodeBs BS1-BS3.
[0087] Control of the Preparation Set
[0088] The message chart for adding a new radio link to the
preparation set, assuming NodeB terminated radio protocols, is
shown in FIG. 11. The message chart is basically the same as that
of the RNC terminated case. A difference is that in the NodeB
terminated case the L1 radio link setup in the RNC is eliminated,
which could result in some minimal delay savings. However, the
overall delay of preparation set update in the NodeB terminated and
RNC terminated cases are expected to be roughly the same.
[0089] The message charts for radio link removal and combined radio
link addition and removal are shown in FIG. 12 and in FIG. 13,
respectively.
[0090] The Inventive Smooth Hard Handover
[0091] The message chart of the smooth hard handover process in
case of NodeB terminated radio protocols is shown in FIGS. 14a and
14b. The difference compared to the RNC terminated case is that
there need to be a L2 link reestablishment between the UE and the
new NodeB during the handover process and the L2 buffers may also
have to be moved. However, the moving of L2 buffers is typically
not needed since pending packets at the old NodeB can be
transmitted on the old radio link, which remains active in parallel
with the new link during the transition. Otherwise the smooth hard
handover process is the same as when implemented in WCDMA with RNC
terminated radio protocols. The L2 link setup adds some additional
delay to the overall handover process compared to the RNC
terminated case. However, the overall delay of the handover process
is expected to be in the same order in both cases.
[0092] Alternative Architecture for NodeB Terminated Radio
Protocols without Radio Server Node
[0093] In case of NodeB terminated radio protocols the architecture
can be further simplified by removing the Radio Server node. The
simplified architecture is disclosed in FIG. 15. The NodeBs can
communicate directly with each other. When the present invention is
implemented in the architecture of FIG. 15, decisions about the
addition/removal of NodeBs to/from the preparation set and the
decision about the actual handover are made by the UE MS. The UE MS
commands the NodeB BS1-BS3 to prepare, activate or deactivate its
corresponding radio link. Recall that these commands were sent by
the RNC or Radio Server node in the previous two scenarios. The RRC
protocol is also moved from the Radio Server to the NodeB
BS1-BS3.
[0094] Control of Preparation Set
[0095] The message chart of adding a new radio link to the
preparation set is shown in FIG. 16. The radio link addition is
triggered in the UE and the corresponding PREPARATION SET UPDATE
message is sent to the NodeB that the UE MS has the active radio
link with. The active NodeB forwards the request to the candidate
NodeB via the fixed network. The candidate NodeB prepares the radio
link without start of active transceiving and confirms this to the
UE via the active NodeB. Last the UE prepares the radio link with
the candidate NodeB.
[0096] The message chart for radio link removal is shown in FIG.
17. The UE initiates the removal by sending a command via the
active radio link and the active NodeB forwards the command via the
fixed network to the NodeB that shall be released. Confirmation on
the release is sent back to the UE from the release NodeB the same
way. Last the UE releases the prepared radio link.
Smooth Hard Handover
[0097] In FIGS. 18a and 18b the smooth hard handover process is
disclosed. It is initiated by the UE triggering the handover, in
FIG. 18a. The candidate NodeB is commanded by the UE to activate
its prepared radio link via the already established active radio
link. Not until the new active radio link is established the old
active radio link is released.
[0098] Alternatively, to the signalling charts of FIGS. 16, 17 and
18a&b it is possible that the UE sends the request directly to
the candidate NodeB using the random access channel.
[0099] General Remarks to the Methods
[0100] In the signalling charts of the smooth hard handover the new
active radio link is taken into use before the old active radio
link is released. The make-before-brake transition is preferred
because it being smooth and safe. As an alternative, a
break-before-make solution may be implemented, meaning the old
active radio link is released before a new radio link is
activated.
[0101] Conventional cellular radio networks mobility management
functions involves a network switching for changing the streams of
data between the established links via an old and new base station
at handover. For implementation of the present invention such a
mobility management function is assumed to present in the network
and accordingly the data stream in direction to the mobile station
is switched from one base station to another during the handover. A
difference is though that at the make-before-brake transition the
links are established before the data streams are switched.
Mobile Station
[0102] FIG. 19 is a block diagram of some structures in mobile
station MS that is relevant for the present invention. The mobile
station comprises a receiver chain 1901-1905, a transmitter chain
1908-1512 and a handover part 1906, 1907 adapted for the smooth
hard handover operation. First in the receiver chain a receiver
unit 1901 receives the physical signal, keeps synchronization of
the physical channel, measures signal strength and interference and
performs any other physical layer functions that are necessary for
the correct reception of the signal. The output from the receiver
unit 1901 is coupled to a descrambling and demultiplexing unit 1902
that removes the scrambling code added by the various transmitting
base stations BS and also demultiplexes traffic channels according
to the different base stations BS1-BS3. According to the smooth
hard handover scheme the mobile station normally communicates with
only one base station BS1 at a time. Only during the handover
transition may it have parallel communication with two base
stations BS1, BS2. The figure shows the more generic case when
there is communication with two base stations BS1,BS2.
[0103] Next in the receiver chain 1901-1905 a demodulation,
despreading, and channel decoding unit 1903 performs the
demodulation, separates the different traffic and control channels
by removing the respective spreading codes and it performs channel
decoding. The output is the individual data traffic channels and
control channels from each base station BS1, BS2, respectively. The
data streams received over the traffic channels are fed into the
RLC/MAC protocol unit 1905 as well as higher layer control
information on logical control channels, while the while layer 1
control information are processed by the control channel processing
unit 1904. The control channel process unit 1904 extracts the power
control command sent by the base stations BS1, BS2 and feed it to
the transmitter unit 1912 last in the transmitter chain 1908-1912.
The transmitter unit 1912 is responsible for formulating the
correct physical signal, setting the transmit power level according
to the received power control commands and sending out the signal
over the air interface. The RLC/MAC protocol unit 1905 forwards
control commands on preparation of a radio link or the release of
it to the handover part (1906-1907).
[0104] In the transmitter chain 1908-1912 the blocks have basically
the inverse functions of those of the receiver side. Accordingly,
data is processed in the RLC/MAC processor unit 1908 and control
information for respective receiving base station is handled by
control channel generator 1909 and data and control info from both
units 1908, 1909 are fed to a channel coding, spreading and
modulator unit 1910. A scrambling unit 1911 is next in the
transmitter chain and last is the transmitter 1912.
[0105] In the handover part 1906,1907 of the mobile station MS a
handover control unit 1906 is responsible for managing handovers,
preparing radio links and releasing them. The handover control unit
1906 has an output to a Radio Link Setup/Release Mgmt unit 1907, an
input from the radio receiver 1901, an input from the RLC/MAC
processor unit 1905 in the receiver chain and output to the
corresponding node 1908 in the transmitted for receiving and
transmitting control data from the network. Hence, the handover
control unit 1906 may have a control communication with the RNC 32
via the RLC/MAC protocols and it may receive commands from the RNC
32, on update of the preparation set or to perform a handover.
[0106] Alternatively, when there is neither RNC 31 nor Radio Server
node 132 in the network, the Handover Control unit in the mobile
station MS will decide about the preparation set base stations and
any handovers itself based on measurements received from the
receiver unit 1901 and optionally measurements received from the
network over the active radio link.
[0107] When the handover control unit 1906 has to add or release a
channel in the prepared state it instructs the Radio Link
Setup/Release Mgmt. unit 1907 to prepare or release the radio
channel in all other units on the receiver and transmitter sides.
Accordingly, when a handover is to be performed the handover
control unit 1906 first instructs the Radio Link Setup/Release
Mgmt. unit 1907 to activate the prepared radio channel and secondly
to deactivate the old active radio link, and the Radio Link
Setup/Release Mgmt. unit 1907 control all other units on the
receiver and transmitter chains to do so.
[0108] With respect to the signalling diagrams disclosed above with
connection to the different system architectures, the handover
control unit handles the messages on the UE-RRC protocol layer
while the Radio Link Setup/Release Mgmt. unit 1907 handles the
UE-L1 functions.
[0109] The physical structure of the base station may be
implemented as the structure disclosed in FIG. 19. The functions
performed by the different entities of FIG. 19 may be implemented
in signalling processor units that may be shared by several of the
functional entities. The physical transmitter 1912 and receiver
1901 units should be separate entities and the transmitter and
receiver chains 1901-1905, 1908-1912 should also be separate.
[0110] Base Station
[0111] FIG. 16 is a block diagram of some base station BS
structures relevant for the present invention. The base station
structure is similar to those in the mobile station. The main
difference compared to the mobile station is that the BS includes
several pairs of receiver chain 2001-2005 and transmitter chain
2009-2012 each for supporting a radio link with a respective mobile
station. In other respects the receiver and transmitter chains
2001-2005, 2009-2012 in the base station BS assigned to one mobile
station are very similar to the corresponding chains in the mobile
station MS.
[0112] Depending on the scenario a Handover control unit 2006 in
the base station BS may receive commands from the RNC 32 or Radio
Server node 132 on prepare of a radio link, on release of a
prepared radio link or on activation of a prepared link or
deactivation of an active link The handover control unit 2006
controls the operation according to the commands via a radio link
setup/release mgmt. unit 2007 that controls the operation in the
various entities in the receiver and transmitter chains 2001-2005,
2008-2012.
[0113] The handover control unit 2006 may also receive measurement
reports from its own receiver unit 2001, which can be either
forwarded to the RNC 32 or, in the scenario when there is no RNC 32
or Radio Server node 132 in the network the measurement reports are
used by the handover control unit 2006 for handover decisions. In
the later scenario the handover control unit 2006 has control
communication with the same function in the mobile station or in
other base stations.
* * * * *