U.S. patent application number 13/620617 was filed with the patent office on 2013-04-18 for mobile relay communication system.
The applicant listed for this patent is Gamini Senarath, David G. Steer, Wen Tong, Guo-Qiang Q. Wang, Derek Yu, Hang Zhang, Peiying Zhu. Invention is credited to Gamini Senarath, David G. Steer, Wen Tong, Guo-Qiang Q. Wang, Derek Yu, Hang Zhang, Peiying Zhu.
Application Number | 20130095750 13/620617 |
Document ID | / |
Family ID | 39609114 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130095750 |
Kind Code |
A1 |
Senarath; Gamini ; et
al. |
April 18, 2013 |
MOBILE RELAY COMMUNICATION SYSTEM
Abstract
A mobile relay system for supporting communications between a
fixed station and mobile terminals comprising a plurality of mobile
relay stations that are associated with each other and provided on
a moving platform, wherein each of the plurality of mobile relay
stations is capable of facilitating communications with mobile
terminals within communication range and at least one of the
plurality of mobile relay stations is further capable of
facilitating communications with a fixed station within
communication g range of the at least one of the plurality of
mobile relay stations.
Inventors: |
Senarath; Gamini; (Nepean,
CA) ; Zhang; Hang; (Nepean, CA) ; Zhu;
Peiying; (Kanata, CA) ; Tong; Wen; (Ottawa,
CA) ; Yu; Derek; (Kanata, CA) ; Steer; David
G.; (Nepean, CA) ; Wang; Guo-Qiang Q.;
(Nepean, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Senarath; Gamini
Zhang; Hang
Zhu; Peiying
Tong; Wen
Yu; Derek
Steer; David G.
Wang; Guo-Qiang Q. |
Nepean
Nepean
Kanata
Ottawa
Kanata
Nepean
Nepean |
|
CA
CA
CA
CA
CA
CA
CA |
|
|
Family ID: |
39609114 |
Appl. No.: |
13/620617 |
Filed: |
September 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12522870 |
Jul 10, 2009 |
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PCT/IB2008/000067 |
Jan 11, 2008 |
|
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13620617 |
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60884808 |
Jan 12, 2007 |
|
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Current U.S.
Class: |
455/11.1 |
Current CPC
Class: |
H04W 84/005 20130101;
H04B 7/15507 20130101; H04W 16/26 20130101; H04B 7/2606 20130101;
H04W 56/0015 20130101; H04W 60/00 20130101; H04W 36/08 20130101;
H04W 88/04 20130101 |
Class at
Publication: |
455/11.1 |
International
Class: |
H04W 88/04 20060101
H04W088/04 |
Claims
1. A mobile relay system for supporting communications between a
fixed station and mobile terminals comprising a plurality of mobile
relay stations that are associated with each other and provided on
a moving platform, wherein each of the plurality of mobile relay
stations is capable of facilitating communications with mobile
terminals within communication range and at least one of the
plurality of mobile relay stations is further capable of
facilitating communications with a fixed station within
communication range of the at least one of the plurality of mobile
relay stations.
2. The mobile relay system of claim 1 wherein the fixed station is
a fixed relay station.
3. The mobile relay system of claim 1 wherein the fixed station is
a fixed base station.
4. The mobile relay system of claim 1 wherein the plurality of
mobile relay stations are affixed to the moving platform and have a
fixed relationship with respect to one another.
5. The mobile relay system of claim 1 wherein the plurality of
mobile relay stations define a cluster.
6. The mobile relay system of claim 5 wherein the cluster appears
as a single relay station to the mobile terminals regardless of
which of the plurality of mobile relay stations are serving the
mobile terminals.
7. The mobile relay system of claim 5 wherein the cluster appears
as a single relay station to the fixed station regardless of which
of the plurality of mobile relay stations are communicating with
the fixed station.
8. The mobile relay system of claim 5 wherein the cluster is handed
off from the fixed station to a second fixed station as the moving
platform moves from a first area supported by the fixed station to
a second area supported by the second fixed station.
9. The mobile relay system of claim 8 wherein service for the
mobile terminals is maintained by the cluster throughout the
handoff.
10. The mobile relay system of claim 1 wherein certain of the
plurality of mobile relay stations are adapted to communicate with
each other to facilitate relaying communications between the fixed
station and a mobile terminal via at least two of the plurality of
mobile relay stations.
11. The mobile relay system of claim 1 wherein each of the
plurality of mobile relay stations is adapted to communicate with
the fixed station.
12. The mobile relay system of claim 1 wherein only certain of the
plurality of mobile relay stations are adapted to communicate with
the fixed station, and other ones of the plurality of mobile relay
stations are adapted to communicate with at least one of the
certain of the plurality of mobile relay stations to facilitate
communications with the fixed station.
Description
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 60/884,808 filed Jan. 12, 2007, the disclosure
of which is incorporated herein by reference in its entirety.
[0002] A relay station (RS) is a device that is used to
interconnect a base station (BS) and a mobile station (MS). By use
of a RS, a BS may cover areas, such as the interior of a shopping
center, building or parking deck, which would otherwise be
inaccessible by the BS.
[0003] A mobile relay station (MRS) is an RS, which is capable of
and intended to move while operating as a RS. An MRS is often
affixed to a mobile platform, such as a train, bus, or airplane. In
such an environment, there is a relatively defined relationship
between the MRS and the MSs on the mobile platform. However, there
may be a changing relationship between the MRS and various BSs as
the mobile platform passes through coverage areas for provided by
different BSs. In essence, the MRSs may be handed off from one BS
or another as the MRSs move from one location to another. Depending
on location or communication parameters, MRSs are capable of
interacting with multiple BSs at the same or different times to
facilitate communications between MSs supported by the MRS and a
servicing BS or BSs.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0004] FIG. 1 is an illustration of mobile relay stations on
vehicles in wireless communication with a base station and a fixed
relay station.
[0005] FIG. 2 is an illustration of an example mobile relay station
that is in direct communication with a fixed relay station.
[0006] FIG. 3 is an illustration of an example mobile relay station
that is in communication with a base station.
[0007] FIG. 4 is a schematic illustration of a mobile relay station
cluster.
[0008] FIG. 5 is an illustration of an example vehicle having two
lead relay stations, one of which is in communication with a fixed
relay station.
[0009] FIG. 6 is an illustration of an example vehicle having two
lead relay stations, each of which is in communication with a
respective base station.
[0010] The following provides an overview of the special issues
related to MRSs and highlights key technologies that can support
the smooth operation of MRSs. These issues include synchronization,
neighborhood scanning, data delivery and path management,
interference management, handoff related issues, and reliability
(See FIG. 1).
[0011] There are generally two types of MRSs. The first type of MRS
does not need to support child RSs where the MRS does not relay
signals between a BS and one or more other RSs (See FIG. 2). For
example, the MRS may only relay signals between a BS and MSs. The
second type of MRS does support child RSs where the MRS may relay
signals between a BS and one or more other RSs (See FIG. 3). For
example, the MRS may relay signals between a BS and other RSs,
including MRSs, as well as MSs. The first type is generally
sufficient for a bus whereas the second type is more applicable to
trains or ships. The second type may be employed in a moving
network.
[0012] Ideally, MRSs would have one or more of the following
characteristics. MRSs should be able to monitor external RSs and
quickly synchronize and connect or reconnect to them with minimum
interruption to child RS and MS communications. Some MRSs need to
support other child MRSs in the same mobile platform. These MRSs
may need to synchronize using its parent MRS and also obtain help
in forwarding data to/from MRs-BS. When an MRS is moving towards a
fixed RS or MR-BS, there should be a means to quickly handover all
the MSs to that RS. On the other hand, if they experience a good
signal only for a short period of time, they should not connect to
these fixed RSs, such as when a bus moves pass a bus station. When
a mobile platform has multiple RSs, all of RSs may need to be
handed over. This process should be done with relatively minimal
disturbance and overhead.
[0013] When an MRS moves toward another MRS, if the MRSs carry the
same PN code one needs to change its PN code and handoff all the
MSs. This should be done with relatively minimal interruption.
Handoffs between two MRSs in the same mobile platform should be
done with minimum impact to MSs or even transparent to the MSs
while minimizing the number of handoffs. When an MRS moves to
another MRS region, the MSs connected to each other may try to
cross connect. There should be a method to prevent that from
happening. In addition, if the MRSs schedule in the same time, the
interference among them would be large, accordingly, such
scheduling should be avoided.
[0014] The MRSs in a mobile platform should be able to connect via
a better positioned MRS in the same vehicle if necessary. The
system should support the case where MRSs connect to the fixed
network via another MRS in the same vehicle. This would reduce the
complexity and the number of handoffs and improve the connection
reliability and coverage performance.
[0015] MRSs which do not need to act as a parent to other RSs are
generally not required to transmit an RS preamble. Therefore, they
can monitor RS preambles of fixed RSs/BSs for synchronization. When
an MRS gets closer to a fixed RS, the MRS can first identify the
fixed RS and then inform the fixed RS/MR-BS to facilitate any
necessary actions, such as handoffs or coordinated interference
avoidance. However, when an MRS moves into another MRS's coverage
area, the above described detection process may not be adequate.
Special means may be employed only for the MRSs, as described
further below.
[0016] Having a separate operation scheme exclusively for MRSs
makes the synchronization and neighborhood scheme of the overall
system simple because fixed RSs do not require a fast
neighborhood-monitoring scheme. For example, fixed RSs can have
continuously synchronization using alternate RS hops by sending an
RS preamble in alternate frames. The fixed RSs may not need to
continuously monitor other RSs or may only need a very slow
monitoring process because topology would usually be changed only
with a new entry of a relay, a removal of a relay, or network based
topology change.
[0017] As mentioned, when an MRS moves to another MRS area, the MRs
cannot detect each other using the techniques described above and
one of the following methods may be employed. For MS assisted
discovery, the MSs connected to an MRS can detect the new MRS using
the frame start preamble and report the MRS in their neighborhood
profile. The MR-BS or the RS, which receives these measurements can
observe these and organize means to address the resulting issues,
such as interference and coordinated scheduling. If both MRSs use
the same preamble, the BS can determine the locations of the MRSs
using location detection techniques and use that to inform the
MRSs. Once MRS determines that another MRS is using the same
preamble, one of the MRSs may change its preamble and a'' the
mobiles connected to that MRS need to be handed over to the MRS
that maintains the preamble.
[0018] If the above detection scheme is not sufficient, the MRS can
use the following technique. If the system uses a random preamble
transmission and monitoring scheme for fixed RSs, the MRS can
participate in the same process with a higher monitoring frequency
than the fixed RS. If the system uses an alternate frame preamble
mechanism, an MRS may occasionally transmit a preamble without
impacting its own monitoring frequency so that other MRSs coming
into contact can be detected.
[0019] The following technique are beneficial where a parent MRS
supports one or more child MRSs, as may be the case for trains or
ships. RS links within the mobile platform can operate either as
in-band or out-of-band relays. The out-band relays would simplify
the system design considerably as it would make overall
synchronization and monitoring process simple and cause less
interference problems. In this case, the MRSs that are allowed to
have a connection with fixed RSs or MR-BSs would not transmit any
preamble and do monitoring in all the frames. Thus, the MRSs can
follow the same procedure as an MRS without a child RS, as
described above. Detection of the case of an MRS moving into
another MRS area can also use a similar method. The in-band relay
should consider the interference it causes to the fixed structure
transmissions which might need some extra scheduling coordination
with the BSs. This may include, strict adherence to the RS and
access/common zones in the frame structure as well as controlling
transmit power within the mobile platform and coordinated
scheduling with the BS.
[0020] In a vehicle only some RSs, referred to as lead RS (LRS),
may be allowed outside access to the fixed RS/BS. The LRSs acquire
synchronization from the external fixed RS/BS. This could be done
by listening to alternate frame RS symbols or based on the random
monitoring scheme (RMS), if the required minimum monitoring time
for synchronization can be achieved using RMS. The other MRSs may
have to acquire synchronization using an LRS as explained below.
Supporting the Synchronization of Child MRSs may be done by (a)
using the same synchronization method used by the fixed RSs or (b)
having a separate preamble called an MRS preamble for the MRSs.
[0021] If a fixed RS uses the random preamble transmission and
monitoring scheme (RPTMS) for both synchronization and neighborhood
monitoring, all the MRSs can use the same scheme with additional
identification processes to avoid connecting the MRS to fixed RSs.
If a fixed RS uses alternate frame preamble transmission scheme
(AFPTS), which transmits the preamble in alternate frames for
successive hops, the secondary MRSs can also use the same method to
synchronize to the LRSs. However, when the RS connected to the
fixed RS provides a handoff to another fixed RS with a different
preamble transmission cycle, all of the internal hops within the
mobile platform has to switch to alternative slots. In addition, if
fixed RSs do not use a separate system for neighborhood scanning,
LRSs may need to send a separate preamble in different frames (e.g.
use RPTMS scheme).
[0022] Thus, each MRS can listen to its parent MRS preamble and
synchronize with it. By having a separate preamble only for MRS,
which is only a small portion of the whole network system,
complexity can be reduced. If the fixed RSs use the AFPTS RS
preamble method for synchronization, it is generally better not to
transmit an RS preamble using the MRSs because the MRSs can
synchronize and listen for neighborhood scanning using the preamble
transmitted by the fixed RSs and BS and use a separate preamble,
referred to as an MRS Preamble. This is because MRS network is only
a small portion of the whole network system and complexity can be
reduced by this modification in the MRS. This would allow the MRS
to identify the approach of another MRS as well.
[0023] An MRS can transmit the MRS preamble in two different cycles
as per the AFPTS method. In order to avoid collisions between MRSs
that have the same preamble and are moving towards each other, a
frame number is generated using the PN code of the MRS preamble.
This means if two RSs have the same preamble, they would be
allocated two different slots for MRS preamble transmissions to
provide another dimension where MRS-to-MRS collisions can be
prevented or identified. This may be done using a centralized means
to allocate the preambles and MRS preamble slots to transportation
vehicles. Another way of identifying the case where two leading
MRSs with the same preamble getting closer to each other is by
sharing information within a group of MRSs when they are connected.
If multiple MRSs are in the same vehicle, although one may have the
same preamble with an incoming MRS others could identify it and
inform to the one having the same preamble. Basically, this is
important when two lead RSs of a cluster have the same preamble and
moves toward each other. The other cluster members can identify it
and inform the lead RS in that case. Using this, an MRS can
identify when another MRS is moving toward it, and if the preambles
are same, change the preambles.
[0024] The following addresses accessing base stations and
organizing MRS-to-MRS links within a transportation system. In the
most generic case, one can set up the system as multiple, disjoint
clusters in a single vehicle serving different MSs. A MRS cluster
is defined as a group of MRSs (one or more) having a wireless
connection to each other either directly or through one of the MRSs
in the same cluster (See FIG. 4).
[0025] In the most simple case, there can be one cluster, and only
one MRS (lead MRS) of this cluster is allowed the connection to the
external fixed wireless system (RS or BS). In another case, each
cluster has only one MRS where each MRS has a connection to an
external fixed RS and only servers its own MSs. In yet another
case, more than one MRS in a cluster is allowed direct access to
the external fixed RS/BS. Each MRS in the vehicle separately
connects to the outside infrastructure. This means the MRSs can be
synchronized with the fixed RSs without having to transmit another
preamble. When an MS moves from one place in the vehicle to
another, the handover would be time consuming as it involves a
fixed-RS or MR-BS in the process. Only a single selected MRS (LRS)
within the vehicle may be allowed direct access to the fixed
infrastructure.
[0026] Other secondary RSs (SRS) would connect through LRSs. This
would reduce the complexity and the number of handoffs, but
performance may not be as good as the first example because all the
other RSs rely on one RS. Multiple MRSs in a vehicle can act as
LRSs. This would reduce the number of handoffs and improve the
connection reliability and coverage performance. If at least one
MRS can connect to the external fixed RS, the whole vehicle area
can be covered. In FIG. 3, two LRSs are located at the front and
rear of a train and various SRSs are located between the LRSs to
improve connection reliability and performance. The MRSs in a
moving vehicle are allowed to connect to the better positioned MRS
in the same vehicle, if necessary, in a dynamic manner.
[0027] The following addresses techniques to reducing handoffs
while MSs are moving within the vehicle or when the vehicle moves
across different fixed access points (RS or BS). For this purpose,
the same preamble transmissions may be used within the clusters.
Several different schemes are possible. First, all of the MRSs in
the vehicle may send the same frame start preamble (Transparent
Relay group). This options makes all the RSs looks alike to the MS
and from the MS point of view only a single RS can be seen. This
reduces the handoffs among the MRSs within the vehicle. In
addition, when an RS is handed off to a different fixed access
point, the procedure is less complex. When multiple LRSs are used
per cluster, all of the LRSs may be connected to the access points
that belong to the same MR-BS.
[0028] Second, MRSs within a cluster may use the same frame-start
preamble. In this case, the idea is to keep the topology within a
cluster simple by each cluster having one LRS. Third, all of the
MRSs in the vehicle use different frame start preambles. An LRS in
the group may hand over to another fixed RS/BS when moving. Two
LRSs belong to the same cluster mayor may not be connected to a
different MR-BS. If they do not belong to the same MR-BS special
provisions are necessary. The following description assumes a
single cluster operation within a vehicle. However, the same scheme
applies even when multiple clusters are in operation.
[0029] The following relates to managing paths and data delivery
within the transparent clusters. There are several architectures
under which a cluster can operate. For locally centralized or
locally distributed controlling, even when multiple LRSs are in a
cluster to facilitate multiple connection points to external fixed
RSs/BSs, a central RS node can do all the control functions such as
scheduling within the cluster. In the distributed case,
interference control may be an issue as MRS nodes may interfere
with the external fixed RSs and coordination with the external
fixed RS is difficult due to multiple controlling points.
[0030] To control interference within the cluster, transmissions to
two or more MSs at the same time may be allowed if they are in
different regions without causing interference to each other. For
this purpose, neighborhood measurements can be used. When
transmitting to two mobiles at the same time, an RS can schedule
them in the same time slot under a decentralized scheme or the BS
can assign the same time slot assignment for both knowing the
interference situation. A similar method can be used for the uplink
transmissions.
[0031] For locally centralized or locally distributed forwarding,
when the access points change dynamically, in the locally
centralized scheme, all the data to and from the external fixed RS
are received first by a particular MRS node and then forwarded to
the destination(s). In the distributed scheme, all of the RS nodes
are updated. All of the MSs connected to each MRS are connected to
the external nodes. These paths are used to forward data from node
to node. In most of the cases, the MRSs within the vehicle can be
prearranged and be known so that each other could act in a more
organized manner. In that case, the RSs inside the vehicle act like
a cluster, which has locally planned paths to each other. MRSs may
possess the path information about each other and the locally
distributed scheme.
[0032] Either in the locally centralized or distributed case, there
can be one or more RSs which send data simultaneously, if the MS
received good signals from multiple RSs. Therefore, more than one
RS in the cluster can keep a given MS in its data base as a
forwarding node. That means some nodes may forward the same data to
other RSs for forwarding while they are also transmitting to the
MS. This accomplishes two types of diversity. One is where there
are multiple LRSs that can send two packets to the MS via two RSs
or combine them at one node and send them by one node. The other is
where multiple RS-MS links are provided for diversity. Duplication
can be avoided using sequence numbers. In the uplink, multiple RSs
may receive packets from the MS and send them to the MR-BS after
combining at a local point or separately send them to the BS for
combination at the BS. Those RSs are identified as a supporting RS
to the MSs for data forwarding.
[0033] The following relates to managing paths and data delivery
within the transparent clusters. For an MS handoff from RS to RS,
although this is transparent from the point of view of the MS, the
MRSs update their tables after identifying the handoff. Notably, a
handoff in this case means the update of the association of that MS
to the RS. When an LRS disconnects from the external fixed RS, the
information is fed to all the nodes quickly informing them of the
disconnection. Information about a new connection to the external
fixed RSs provided in a similar way. Therefore, at any time, all
the MRSs know the best path(s) to the external network. This could
be done dynamically without the knowledge of the MR-BS.
[0034] In a fully centralized scheme, the MR-BS may keep track of
which MS belongs to which LRS. When an MS moves from one LRS area
to another (including its SRSs), the change is noted by the MR-BS,
but no handover is initiated. When transmitting data to the MSs, a
different LRS may send data to its mobiles simultaneously if they
do not interfere with each other. This can be accomplished by using
a broadcast ID so that two MRSs send different data to two
different MSs.
[0035] The following addresses issues related to another MRS
Cluster moving close to an MRS Cluster. Since an MRS cluster may
have a fixed topology within the cluster, each MRS knows to what
MRS will receive a transmitted preamble. Therefore, by informing
ahead of time, an MRS can change the RS preamble transmission frame
according to an agreed upon. This may be done in random intervals
without impacting minimum synchronization time and giving
sufficient synchronization time for the newly entered RSs to
synchronize to the MRS preamble. This way, the MRSs can monitor
differently timed MRS preambles and identify the other MRSs that
are getting closer to a given MRS or MRS cluster. Note that in this
case, the lead MRS has to send one preamble, if the alternate frame
preamble transmission scheme is used.
[0036] When an MRS observes the same preamble in a different cycle,
the MRS can identify that it has meet another moving MRS with the
same preamble. As such, the MRS can identify an approaching MRS
that is using the same preamble. The same scheme can be applied to
the fixed RS systems if the idle and sleep mode is not used in the
fixed RS systems, in which case special messaging may be
required.
[0037] In FIGS. 5 and 6, a train is covered by two LRSs located at
the front and last compartments. From time to time, depending on
the link from the LRS to an outside fixed RSs/BS, SRSs may use one
of those LRSs to communicate with the fixed RSs/BSs. A snap shot
when both LRSs have good connections to external fixed RSs is shown
where the solid lines are now active links. Dotted links may become
active if LRS1 or LRS2 fails to connect to an external fixed access
point. All LRSs and SRSs do not transmit RS preamble but transmit
the frame start preamble. All RSs belong to same MRS cluster. All
have the same PN code and the same preamble (transparent RS). So
when an MS moves from one RS coverage area to other it is
transparent to the MS. All the MRSs update the information so that
data can be forwarded to the new RS. This may also be transparent
to the MR-BS in a decentralized scheme.
[0038] All MRSs send an MRS preamble that is in a different frame
from the RS preamble frame used for fixed RSs. MRSs listen to each
other's preamble and establish synchronization using an alternate
frame preamble structure similar to the one used by fixed RSs. For
example, SRS1 and SRS3 can use odd frames and SRS2 can use even
frames. An MRS preamble has a different pattern and preferably a
relatively low power, which is just sufficient to reach the
neighbor MRS such that it would not be confused by the other fixed
RSs as an RS preamble. Neighboring fixed RSs or BSs may use the
same slot for data transmission but since the in-vehicle relays may
be located close by, one may assume that the interference would not
impact the preamble and the synchronization process. Otherwise, the
neighboring BSs/RSs need to be advised not to use that slot when an
MRS is in the close vicinity.
[0039] The following relates to handoff procedures and network
entry. Initially an MS will choose an anchor RS during network
entry. Upon network entry identifies the cluster as one group and
tries to connect to it. Since all MRSs may send the same preamble,
the signal may appear as a large MRS to an MS. However, the RSs can
report the MSs received signal strengths (during UL ranging) to a
pre-selected RS within the cluster (locally centralized case), or
the MR-BS (centralized case) and depending on these signal
strengths a decision as to where the MS should be anchored can be
made and send via the UL frame alignment adjustment to the MS.
[0040] After network entry, the cluster identifies that the MS is
attached to one of its MRSs. This can either be transparent to the
MR-BS (locally-centralized) or informed to BS (centralized). In the
locally centralized case, all the handoffs among the cluster are
transparent to both BS and MS, which is a significant benefit to
the network. In this case, the MR-BS can send data to the cluster
using MSID or RS Cluster ID and the cluster can facilitate data
forwarding. However, the path tables in the cluster may have to be
changed during a handoff within the cluster. In addition, the MS
may need to do uplink frame alignment with the new RS for which a
ranging request may need to be issued. When a moving cluster moves
within the same MR-BS area as another, only the lead RS, which is
the one connected with the BS, needs to handover and nothing else
need to be changed within the cluster or among the MSs.
[0041] This document describes some technologies to be used for the
operation of moving relays and more specifically to the case where
a group of moving RSs move in a common platform such as a train or
a ship. The high level issues and solutions for different scenarios
that can be experienced by moving RSs have been discussed. Some of
these techniques may be applied to fixed relay operation as
well.
* * * * *