U.S. patent application number 10/855898 was filed with the patent office on 2005-01-20 for method and system for wireless communication networks using relaying.
This patent application is currently assigned to Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Larsson, Peter.
Application Number | 20050014464 10/855898 |
Document ID | / |
Family ID | 33490605 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050014464 |
Kind Code |
A1 |
Larsson, Peter |
January 20, 2005 |
Method and system for wireless communication networks using
relaying
Abstract
The present invention relates to wireless networks using
relaying. In the method according to the present invention of
performing communication in a two-hop wireless communication
network, a transmitter 210, a receiver 220 and at least one relay
station 215 are engaged in a communication session. The relay
station 215 forwards signals from a first link between the
transmitter 210 and the relay station 215 to a second link between
the relay stations 215 and the receiver 220. The forwarding
performed by the at least one relay station 215 is adapted as a
response to estimated radio channel characteristics of at least the
first link. Preferably the forwarding is adapted as a response to
estimated radio channel characteristics of both the first and
second link.
Inventors: |
Larsson, Peter; (Solna,
SE) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ)
Stockholm
SE
|
Family ID: |
33490605 |
Appl. No.: |
10/855898 |
Filed: |
May 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60473450 |
May 28, 2003 |
|
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Current U.S.
Class: |
455/11.1 ;
455/502 |
Current CPC
Class: |
H04L 25/0224 20130101;
H04W 52/46 20130101; H04L 2001/0097 20130101; H04L 25/20 20130101;
H04B 7/15528 20130101; H04L 5/0035 20130101; H04L 25/0206 20130101;
H04W 88/02 20130101; H04B 7/026 20130101; H04L 27/2602 20130101;
H04B 7/2606 20130101; H04W 52/40 20130101; H04L 1/0618 20130101;
H04L 1/0021 20130101; H04B 7/022 20130101; H04L 5/0048 20130101;
H04L 1/0001 20130101; H04L 27/2655 20130101; H04B 7/15592 20130101;
H04L 5/0007 20130101 |
Class at
Publication: |
455/011.1 ;
455/502 |
International
Class: |
H04B 015/00 |
Claims
1. A method of performing communication in a two-hop wireless
communication network, wherein a transmitter, a receiver and at
least one relay station are engaged in a communication session, and
the relay station forwards signals from a first link between the
transmitter and the relay station to a second link between the
relay stations and the receiver, wherein the forwarding performed
by the at least one relay station is adapted as a response to
estimated radio channel characteristics of at least the first
link.
2. The method according to claim 1, wherein the forwarding
performed by the at least one relay station is adapted as a
response to estimated radio channel characteristics of both the
first and second link.
3. The method according to claim 1, wherein the communication
session involves a plurality of relay stations and their respective
forwarding is adapted based on a relative transmission parameter
which is specific for each relay station and a common transmission
parameter which is common to all relay stations.
4. The method according to claim 1, wherein the method comprises
the steps of: characterizing the radio paths of the first and
second link by the use of pilots; determine at least one relative
transmission parameter at least partly based on both of the channel
estimates of each relay stations paths of the first and second
link; determine at least one common transmission parameter based;
distributing at least said common transmission parameter to all
relay station; forwarding the signal from the first link on the
second link, wherein the forwarded signal is adapted based on each
relay stations relative transmission parameter and the common
transmission parameter.
5. The method according to claim 1, wherein the adaptation of the
transmitted signal comprises an adjustment of phase.
6. The method according to claim 1, wherein the adaptation of the
transmitted signal comprises an adjustment of transmission
power.
7. The method according to claim 1, wherein the adaptation of the
transmitted signal comprises an adjustment of transmission power
and phase.
8. The method according to claim 1, wherein the adaptation of the
transmitted signal comprises an adjustment of parameters relating
to diversity.
9. The method according to of claims 8, wherein the adaptation of
the transmitted signal comprises an adjustment of parameters
relating to delay diversity.
10. The method according to of claims 8, wherein the adaptation of
the transmitted signal comprises an adjustment of parameters
relating to space time coded diversity.
11. The method according to claim 1, wherein the step of using the
relay station's respective relative transmission parameter and the
common transmission parameter(s) to adapt the subsequent
transmissions on link 2, comprises to, on the reception of signal
y.sub.k, transmit the signal: 34 z k = y k 1 P RS RS , k MS , k RS
, k ( RS , k + MS , k + 1 ) - j ( arg ( h 1 , k ) + arg ( h 2 , k )
) wherein the parameters .GAMMA..sub.RS,k and .GAMMA..sub.MS,k are
the locally determined relative transmission parameters based on
the channel estimates h.sub.1,k and h.sub.2,k, P.sub.BS is the
transmit power of the transmitter, .sigma..sup.2.sub.RS is the
noise and interference level at the relay station, P.sub.RS is the
aggregated transmit power from all relay stations,
.sigma..sup.2.sub.MS is the noise level at each receiver, and
wherein the normalizing factor .phi. is a common parameter based on
the total communication quality experienced by the receiver.
12. A relay station adapted for use in a two-hop wireless
communication network, wherein the network comprises a transmitter,
a receiver and at least one relay station, wherein the relay
station is adapted to forwarding signals from a first link between
the transmitter and the relay station to a second link between the
relay stations and the receiver wherein the relay station is
provided with means for adapting the forwarding based on a
characterization of at least the first link.
13. The relay station according to claim 14, wherein the relay
station adapt the forwarding as a response to estimated radio
channel characteristics of both the first and second link.
14. The relay station according to claim 14, wherein the relay
station is further provided with means for performing channel
characterization and means for determining relative transmission
parameters based on the channel characterization, and the
forwarding is at least partly based on said relative transmission
parameters.
15. The relay station according to claim 15, wherein the relay
station is further provided with means for receiving a common
transmission parameter, and the forwarding is at least partly based
on said relative transmission parameters and said common
transmission parameter
16. A system adapted for communication in a two-hop wireless
communication network, wherein the network comprises a transmitter,
a receiver and at least one relay station, wherein the relay
station is adapted to forwarding signals from a first link between
the transmitter and the relay station to a second link between the
relay stations and the receiver wherein the relay station uses
characterization of at least the first link for the forwarding on
the second link.
17. The system according to claim 16, wherein the relay station
adapt the forwarding as a response to estimated radio channel
characteristics of both the first and second link.
18. The system according to claim 18, wherein the relay station is
further provided with means for performing channel characterization
and means for determining relative transmission parameters based on
channel characterization and the forwarding is at least partly
based on said relative transmission parameters.
19. The system according to claim 17, wherein the system is
provided with means for determining a common transmission parameter
which is based on the total communication quality between the
transmitter and the receiver, and the relay station is further
provided with means for receiving the common transmission parameter
and the forwarding on the second link is at least partly based on
said relative transmission parameters and said common transmission
parameter.
20. A receiver adapted for use in a two-hop wireless communication
network, wherein the network comprises a transmitter, the receiver
and at least one relay station, wherein the relay station is
adapted to forwarding signals from a first link between the
transmitter and the relay station to a second link between the
relay stations and the receiver, wherein the receiver is provided
with means for determining at least one relative transmission
parameter which is based on a characterization of at least the
first link, and means for distributing said relative transmission
parameter to the relay station.
21. The receiver according to claim 20, wherein the determining
means are adapted to determine a plurality of relative transmission
parameter, one for each relay station which are engaged in the
communication session.
22. The receiver according to claim 20, wherein the relative
transmission parameter is based on characterisations of both the
first and second link.
23. The receiver according to claim 20, wherein the receiver i
further provided with means for determining a common transmission
parameter which is based on the total communication quality between
the transmitter and the receiver.
24. A base station adapted for use in a two-hop wireless
communication network, comprising a receiver according to claim
20.
25. A mobile station adapted for use in a two-hop wireless
communication network, comprising a receiver according to claim 20.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to relay supported wireless
communication to enhance communication performance. In particular
the invention relates to a method and a system for performing
communication in a two-hop wireless communication network.
BACKGROUND OF THE INVENTION
[0002] A main striving force in the development of
wireless/cellular communication networks and systems is to provide,
apart from many other aspects, increased coverage or support of
higher data rate, or a combination of both. At the same time, the
cost aspect of building and maintaining the system is of great
importance and is expected to become even more so in the future. As
data rates and/or communication distances are increased, the
problem of increased battery consumption is another area of
concern.
[0003] Until recently the main topology of wireless networks has
been fairly unchanged, including the three existing generations of
cellular networks. The topology characterized by the cellular
architecture with the fixed radio base stations and the mobile
stations as the transmitting and receiving entities in the
networks, wherein a communication typically only involves these two
entities. An alternative approach to networks are exemplified by
the well-known multihop networks, wherein typically, in a wireless
scenario, a communication involves a plurality of transmitting and
receiving entities in a relaying configuration. Such systems offer
possibilities of significantly reduced path loss between
communication (relay) entities, which may benefit the end-to-end
(ETE) users.
[0004] Attention has recently been given to another type of
topology that has many features and advantages in common with the
multihop networks but is limited to only two (or a few) hop
relaying. In contrast to multihop networks, aforementioned topology
exploits aspects of parallelism and also adopts themes from
advanced antenna systems. These networks, utilizing the new type of
topology, have cooperation among multiple stations as a common
denominator. In recent research literature, it goes under several
names, such as cooperative relaying, cooperative diversity,
cooperative coding, virtual antenna arrays, etc. In the present
application the terms "cooperative relaying" and "cooperative
schemes/methods" is meant to encompass all systems and networks
utilizing cooperation among multiple stations and the
schemes/methods used in these systems, respectively. A
comprehensive overview of cooperative communication schemes are
given in [1]. Various formats of a relayed signal may be deployed.
A signal may be decoded, re-modulated and forwarded, or
alternatively simply amplified and forwarded. The former is known
as decode-and-forward or regenerative relaying, whereas the latter
is known as amplify-and-forward, or non-regenerative relaying. Both
regenerative and non-regenerative relaying is well known, e.g. by
traditional multihopping and repeater solutions respectively.
Various aspects of the two approaches are addressed in [2].
[0005] The general benefits of cooperative relaying in wireless
communication can be summarized as higher data rates, reduced
outage (due to different forms of diversity), increased battery
life, extended coverage (e.g. for cellular).
[0006] Various schemes and topologies utilizing cooperative
relaying has been suggested, as theoretical models within the area
of information theory, as suggestions for actual networks and in a
few cases as laboratory test systems, for example. Examples are
found in [1] pages 37-39, 41-44. The various cooperation schemes
may be divided based on which entities have data to send, to whom
and who cooperates. In FIGS. 1a-f (prior art) different topologies
are schematically illustrated, showing where traffic is generated,
who is the receiver and the path for radio transmissions.
[0007] The classical relay channel, illustrated in FIG. 1a,
consists of a source that wishes to communicate with a destination
through the use of relays. The relay receives the signal
transmitted by the source through a noisy channel, processes it and
forwards it to the destination. The destination observes a
superposition of the source and the relay transmission. The relay
does not have any information to send; hence the goal of the relay
is to maximize the total rate of information flow from the source
to the destination. The classical relay channel has been studied in
[1], [7] and in [3] where receiver diversity was incorporated in
the latter. The classical relay channel, in its three-station form,
does not exploit multiple relay stations at all, and hence does not
provide the advantages stated above.
[0008] A more promising approach, parallel relay channel, is
schematically illustrated in FIG. 1b, wherein a wireless systems
employing repeaters (such as cellular basestation with supporting
repeaters) with overlapping coverage, a receiver may benefit of
using super-positioned signals received from multiple repeaters.
This is something that happens automatically in systems when
repeaters are located closely. Recently, information theoretical
studies have addressed this case. A particular case of interest is
by Schein, [4] and [5]. Schein has performed information
theoretical study on a cooperation-oriented network with four
nodes, i.e. with one transmitter, one receiver and only two
intermediately relays. A real valued channel with propagation loss
equal to one is investigated. Each relay employs non-regenerative
relaying, i.e. pure amplification. Thanks to the simplistic
assumption of real valued propagation loss, the signals add
coherently at the receiver antenna. Under individual relay power
constraints, Schein also indicates that amplification factors can
be selected to maximize receiver SNR, though does not derive the
explicit expression for the amplification factors. One of the
stations sends with its maximum power, whereas the other sends with
some other but smaller power. The shortcoming of Schein's schemes
is that it is; only an information theoretical analysis, limited to
only two relay stations, derived in a real valued channel with gain
one (hence neglecting fundamental and realistic propagation
assumptions), lacks the means and mechanisms to make the method
practically feasible. For example, protocols, power control and RRM
mechanisms, complexity and overhead issues are not addressed at
all. With respect to only addressing only two relay stations, the
significantly higher antenna gains and diversity benefits, as would
result for larger number of relays, are neither considered nor
exploited.
[0009] The concept of Multiple-access Channel with Relaying (a.k.a.
as Multiple access channels with generalized feedback) has been
investigated by several researchers lately and is schematically
illustrated in FIG. 1c. The concept involves that two users
cooperate, i.e. Exchange the information each wants to transmit,
and subsequently each user sends not just its own information but
also the other users information to one receiver. The benefit in
doing so is that cooperation provides diversity gain. There are
essentially two schemes that have been investigated; cooperative
diversity and coded cooperative diversity. Studies are reported in
[1], for example. With respect to diversity, various forms has bee
suggested, such as Alamouti diversity, receiver diversity, coherent
combining based diversity. Typically the investigated schemes and
topologies rely on decoding data prior to transmission. This
further means that stations has to be closely located to cooperate,
and therefore exclude cooperation with more distant relays, as well
as the large number of potential relays if a large scale group
could be formed. An additional shortcoming of those schemes is that
is fairly unlikely having closely located and concurrently
transmitting stations. These shortcomings indicates that the
investigated topology are of less practical interest. The broadcast
channel with relaying, illustrated in FIG. 1d, is essentially the
reverse of the topology depicted in FIG. 1c, and therefore shares
the same severe shortcomings.
[0010] A further extension of the topology depicted in FIG. 1c is
the so-called interference channel with relaying, which is
illustrated in FIG. 1e, wherein two receivers are considered. This
has e.g. been studied in [8] and [1] but without cooperation
between the receivers, and hence not exploiting the possibilities
possibly afforded by cooperative relaying.
[0011] Another reported topology, schematically illustrated in FIG.
1f, is sometimes referred to as Virtual Antenna Array Channel, and
described in for example [9]. In this concept, (significant)
bandwidth expansion between a communicating station and adjacent
relay nodes is assumed, and hence non-interfering signals can be
transferred over orthogonal resources that allows for phase and
amplitude information to be retained. With this architecture, MIMO
(Multiple Input Multiple Output) communication (but also other
space-time coding methods) is enabled with a single antenna
receiver. The topology may equivalently be used for transmission. A
general assumption is that relay stations are close to the receiver
(or transmitter). This limits the probability to find a relay as
well as the total number of possible relays that may be used. A
significant practical limitation is that very large bandwidth
expansion is needed to relay signals over non-interfering channels
to the receiver for processing.
[0012] Cooperative relaying has some superficial similarities to
the Transmit diversity concept in (a.k.a. Transmit diversity with
Rich Feedback, TDRF), as described in [10] and is schematically
illustrated in FIG. 1g. Essential to the concept is that a
transmitter with fixed located antennas, e.g. at a basestation in a
cellular system, finds out the channel parameters (allowing for
fading effects and random phase) from each antenna element to the
receiver antenna and uses this information to ensure that a (noise
free) signal, after weighting and phase adjustment in the
transmitter, is sent and adds coherently at the receiver antenna
thereby maximizing the signal to noise ratio. While transmit
diversity, with perfectly known channel and implemented in a fixed
basestation, provides significant performance benefits, it also
exist practical limitations in terms of the number of antenna
elements that can be implemented in one device or at one antenna
site. Hence, There is a limit in the degree of performance gain
that can be attained. A disadvantage for basestation oriented
transmit diversity is also that large objects between transmitter
and receiver incur high path loss.
[0013] Thus, it is in the art demonstrated that cooperative
relaying have great potentials in providing high capacity and
flexibility, for example. Still, the in the art proposed topologies
and methods do not take full advantage of the anticipated
advantages of a network with cooperative relaying.
SUMMARY OF THE INVENTION
[0014] In the state of the art methods, the quality of the first
link, the second link or a combination thereof is not considered in
adapting any transmission parameters. This has the consequence that
performance may degrade and resources are inefficiently
utilized.
[0015] Hence, a significant shortcoming of the above discussed
prior art is that they do not adapt transmit parameters of the
relays in response of the quality of a link or combination of links
(first and second) involved in the forwarding procedure. Whereby,
the prior art has not been able to fully take advantage of the
anticipated advantages of a cooperative relaying network.
[0016] Obviously an improved method and system for a cooperative
relaying network is needed, which consider the quality of the first
link, the second link or a combination thereof in adapting
transmission parameters is needed, to whereby have the ability to
better take advantage of the anticipated advantages of a
cooperative relaying network.
[0017] The object of the invention is to provide a method, a relay
station and a system that overcomes the drawbacks of the prior art
techniques. This is achieved by the method as defined in claim 1,
the relay station as defined in claim 12 and the system as defined
in claim 16.
[0018] The problem is solved by that the present invention provides
a method, a relay station and a system that makes is possible to
use estimated radio channel characteristics of both the first and
second link for adapting the forwarding of signals from a first
link to a second link performed by the relay station.
[0019] In the method, according to the present invention of
performing communication in a two-hop wireless communication
network, a transmitter, a receiver and at least one relay station
are engaged in a communication session. The relay station forwards
signals from a first link between the transmitter and the relay
station to a second link between the relay stations and the
receiver. The forwarding performed by the at least one relay
station is adapted as a response to estimated radio channel
characteristics of at least the first link. Preferably the
forwarding is adapted as a response to estimated radio channel
characteristics of both the first and second link.
[0020] The relay station according to the present invention is
adapted for use in a two-hop wireless communication network,
wherein the network comprises a transmitter, a receiver and at
least one relay station. The relay station is adapted to forward
signals from a first link between the transmitter and the relay
station to a second link between the relay stations and the
receiver. The relay station is provided with means for adapting the
forwarding based on characterization of both the first and second
link.
[0021] Thanks to the invention it is possible to better adjust the
forwarding on the second link to the actual conditions present
during a communication session. In addition the forwarding can be
better adjusted to changes in the conditions.
[0022] One advantage afforded by the present invention is that the
more precise and reliable characterization of the individual radio
paths may be used to determine and optimize different transmission
parameters. Whereby, the capabilities of a cooperative relaying
network, for example, may be more fully exploited.
[0023] A further advantage is that characterisation of the first
and second link advantageously is performed in the relay stations.
Hence, the method according to the invention fascilitates a
distribution of functionalities in the network allowing an increase
in the number of relay stations in a communication session without
any significant increase in the amount of protocol overhead that is
needed for the transmission of data from the transmitter to the
receiver.
[0024] A yet further advantage further advantage of the method and
system according to the present invention is that the improved
characterization of the first and second link facilitate to take
full advantage of the anticipated advantages of a network with
cooperative relaying that comprises a larger number of relaying
stations. With the invention used in a coherent combining setting,
the directivity gain and diversity gain increases with increasing
number of relay stations. The directivity gain itself offers
increased SNR that can be used for range extension and/or data rate
enhancement. The diversity gain, increases the robustness of the
communication, providing a more uniform communication quality over
time. While directivity and diversity gain can be provided by
various traditional advanced antenna solutions, where the antennas
are placed either at the transmitter or the receiver, the proposed
solution is generally not limited to the physical space constraints
as is seen in basestations or mobile terminals. Hence, there is
indeed a potential to use a larger number of relays, than the
number of antennas at a basestation or a mobile station, and hence
offer even greater directivity and diversity gains.
[0025] Embodiments of the invention are defined in the dependent
claims. Other objects, advantages and novel features of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE FIGURES
[0026] The features and advantages of the present invention
outlined above are described more fully below in the detailed
description in conjunction with the drawings where like reference
numerals refer to like elements throughout, in which:
[0027] FIG. 1a-g are schematic illustrations of the topologies of
some prior art utilizing cooperative relaying;
[0028] FIG. 2 schematically illustrates a cellular system using
cooperative relaying according to the present invention;
[0029] FIG. 3 is a schematic model used to describe the parameters
and terms used in the present invention;
[0030] FIG. 4 is a flowchart over the method according to the
invention;
[0031] FIGS. 5a and 5b are a schematic illustrations of two
alternative logical architectures for the cooperative relaying
network according to the present invention;
[0032] FIG. 6 is a flowchart over one embodiment of the method
according to the invention;
[0033] FIG. 7 is a schematic illustration of an alternative
embodiment of the invention utilizing relay stations with multiple
antennas;
[0034] FIG. 8 is a schematic illustration of an alternative
embodiment of the invention utilizing direct transmission between
the transmitter and the receiver;
DETAILED DESCRIPTION OF THE INVENTION
[0035] Embodiments of the invention will now be described with
reference to the figures.
[0036] The network outlined in FIG. 2 is an example of a
cooperative relaying network wherein the present invention
advantageously is implemented. The figure shows one cell 205 of the
wireless network comprising a basestation 210 (BS), a plurality of
relay stations 215 (RS) and a plurality of mobile stations (MS)
220-223. As shown in the figure, the relay stations 215 are mounted
on masts, but may also be mounted on buildings, for example. Fixed
relays may be used as line of sight conditions can be arranged,
directional antennas towards the basestation may be used in order
to improve SNR (Signal-to-Noise Ratio) or interference suppression
and the fixed relay may not be severely limited in transmit power
as the electricity supply network typically may be utilized.
However, mobile relays, such as users mobile terminals, may also be
used, either as a complement to fixed relays or independently. The
mobile stations 221 and 222 are examples of mobile relays, i.e.
mobile stations that temporarily functions also as relays. The
mobile station 220 is in active communication with the base station
210. The signalling, as indicated with arrows, is essentially
simultaneously using a plurality of paths, characterized by two
hops, i.e. via a relay station 215 or a mobile station acting as a
mobile relay 221, 222. The transmission will experience
interference from for example adjacent cells, and the effect of the
interference will vary over the different paths.
[0037] It should be noted that although relay based communication
is used to enhance communication, direct BS to MS communication may
still be used. In fact, some basic low rate signalling between BS
and MS may be required for setting up a relay supported
communication channel. For example, a cellular system function such
as paging may not use coherent combining based relaying as the
relay to MS channels are not a priori known, instead preferably, a
direct BS to MS communication is used during call setup and similar
procedures. The real world cellular system outlined in FIG. 2 is
modeled by system model shown in FIG. 3, here with focus on a
single pair of transmitter and receiver, with an artibtrary number
K of relay stations. The notation is adapted to a basestation 210
as a transmitter and a mobile station 220 as a receiver, but not
limited thereto. The communication between the basestation 210 and
the mobile station 220 can be described as comprising two main
parts: the transmissions from the base station 210 to the relay
stations 215:k referred to as Link 1, and the transmissions from
the relay stations 215:k to the mobile station 220 referred to as
Link 2.
[0038] The transmitter, i.e. BS 210 transmits with a power
P.sub.BS. Each relay station 215:k, wherein k.epsilon.{1, 2, . . .
, K} and K is the total number of relay stations, receive the
signal and re-transmits with a total power P.sub.k. The aggregate
transmit power of all relay stations 215:k is denoted P.sub.RS.
h.sub.1,k is the complex path gain from the basestation 210 to
relay station k 215:k, and h.sub.2,k is the complex path gain from
the relay station k to the mobile station, i.e. h.sub.1,k and
h.sub.2,k characterizes the individual signal paths. The receiver,
i.e. MS 220, receives a total signal denoted C.sub.r and experience
the total noise N.sub.r.
[0039] Typically, in a realistic scenario a BS in a cell is
simultaneously engaged in communication with a plurality of mobile
stations. This can be envisaged by considering each communication
as modeled in accordance to FIG. 3. For clarity only a
communication session involving one BS, one MS and a plurality of
relay station will be considered in the present application.
However, as will be apparent for the skilled in the art the
inventive architecture and method/scheme is easily applied also in
the case with a plurality of simultaneous communications between
the base station and mobile stations.
[0040] As realized by the skilled in the art, in a network
according to the above model, a large number of parameters need to
be set and preferably optimized in order to fully take advantage of
the possibilities and capacity offered by such a network. This is
also, as previously discussed, there the prior art systems display
their shortcomings as multi-relay systems, due to their presumed
complexity, are not discussed. Parameter that needs to be
considered and preferably optimized include, but is not limited to,
transmit power of the basestation 210 and each relay station 215:k,
which relay stations that should be used in the communication,
phase control (if coherent combining is used), coding, delay (in
the case of delay diversity), antenna parameters (beamforming,
spatial multiplexing), etc. The parameters needed to control and
optimize the transmission will be referred to as transmission
parameters (TP). A preferred optimization includes, but is not
limited to, optimizing the transmit powers of the base station 210
and the relay stations 215:k in order to obtain a specific SNR at
the receiving mobile station, which in turn correspond to a certain
quality of service or capacity, for example, with regards to power
consumption of the different entities and the interference level in
the cell and adjacent cells, for example.
[0041] Fundamental to all optimization and necessary for an
efficient use of the radio recourses is an accurate
characterization of the radio paths in the first and second link,
and control over how any changes in any transmission parameter will
affect the overall performance. The method according to the present
invention provides a method wherein a relay station 215:k uses
channel characteristics of both the first and second link to
determine transmission parameters for the forwarding on the second
link. In addition, according to the method, each relay station
215:k may optionally adapt its forwarding on the second link to a
quality measure on the communication in full as perceived by the
receiver 220, for example. The quality measure on the communication
in full will be referred to as the common transmission
parameter.
[0042] In the method according to the present invention of
performing communication in a two-hop wireless communication
network, a transmitter 210, a receiver 220 and at least one relay
station 215 are engaged in a communication session The relay
station 215 forwards signals from a first link between the
transmitter 210 and the relay station 215 to a second link between
the relay stations 215 and the receiver 220. The forwarding
performed by the at least one relay station 215 is adapted as a
response to estimated radio channel characteristics of at least the
first link. Preferably the forwarding is adapted as a response to
estimated radio channel characteristics of both the first and
second link.
[0043] The method according to the invention will be described with
reference to the flowchart of FIG. 4 The method comprises the main
steps of:
[0044] 400: Send pilots on the k paths of link 1;
[0045] 410: Characterize the k paths of link 1.
[0046] 420: Send pilots on the k paths of link 2;
[0047] 430: Characterize the k paths of link 2.
[0048] 440: Determine relative transmission parameters for each
relay station 215, wherein each relative parameter is based on the
characterization of the respective paths of link 1 or a combination
of link 1 and link 2.
[0049] 450: Each relay station 215:k adapts the forwarding on link
2 to the receiver 220 using its respective relative transmission
parameter.
[0050] Optionally the method comprises the step of:
[0051] 445: Determining a common transmission parameter reflecting
the quality of the communication in full.
[0052] 447: Distribute the common transmission parameter to the
relay stations (215).
[0053] and step 450 is subsequently replaced with:
[0054] 450': Each relay station 215:k adapts the forwarding on the
second link to the receiver 220 using its respective relative
transmission parameter and the common transmission parameter.
[0055] "Pilots" and "sending pilots" should be interpreted as
sending any kind of channel estimation symbols. "Hello messages"
may also be used for this purpose.
[0056] It should be noted that the sending of pilots does not have
to occur in the above order and may also be simultaneous on link 1
and 2.
[0057] The characterization of the radio paths in steps 410 and 430
is preferably adapted to the transmission technique used, and
possibly also to the type of optimization which should utilize the
characterization. The characterization may comprises of, but is not
limited to: estimating complex path gains h.sub.1,k and h.sub.2,k
characterizing each path of the first and second link,
respectively.
[0058] As there are two links, transmitter to relay and relay to
receiver, there are four possibilities of which station(s) transmit
and which station(s) estimate the channel(s). The four
possibilities are summarized in Table 1. The purpose is to
illustrate that several different implementation approaches of the
invention may be taken.
1 TABLE 1 Link 1 Link 2 Case Transmitter Relay Relay Receiver 1
Send pilot Estimate ch. Estimate ch. Send pilot 2 Send pilot
Estimate ch. Send pilot Estimate ch. 3 Estimate ch. Send pilot
Estimate ch. Send pilot 4 Estimate ch. Send pilot Send pilot
Estimate ch.
[0059] Given that channel estimation has been performed in some
station, it is also an issue who perform processing of the collated
information, i.e. determine the relative transmission parameters.
Essentially, there are three choices, the transmitter BS 210, the
receiver MS 220 or a set of relay stations RS 215. Since it is the
relay stations that must perform the adjustments of the forwarding
on link 2, this is the preferred place to determine the relative
transmission parameters. If a relay station sends a pilot signal, a
representation of the channel characterization needs to be reported
back to the relay. If a relay station instead receives a pilot, the
representation of the channel characterization does not need to be
reported anywhere (corresponding to case 1). Case one is in many
situations the preferred alternative, since it minimizes the
overhead signalling. On the other hand, one may want to keep the
relay stations as simple as possible and perform all calculations
in the receiver and/or transmitter, or in entities in connection
with the receiver or transmitter. If, so case 4 of table 1 may be
preferred, and all estimation and calculation is performed in other
entities than the relay stations. The information needed for the
relay stations to adjust their respective forwarding is sent to
each relay station. As illustrated, many possible combinations
exist and the invention is not limited to a specific one.
[0060] A preferred system according to the invention, adapted to be
able to effectuate the above-described case 1, will be described
with reference to FIG. 5a. Each relay station 215:k has means for
performing channel characterization 216 and means for determining
relative transmission parameters 217 based on the channel
characterization and means for adjusting 218 the forwarding based
on relative transmission parameters and optionally on a common
transmission parameter. The receiver 220 has means for performing a
quality measure of the collective signal 221 and optionally means
for determining a common transmission parameter 222. The common
transmission parameter is distributed from the receiver 220 to the
relay stations 215:k either as a direct broadcast to the relay
stations 215:k or via the transmitter 210. The relay stations 215:k
receive the common transmission parameter and in combination with
their relative transmission parameters adjust their forwarding of
the signal. This can be seen as comprising a logical control loop
between the receiver 220 and the relay stations 215:k. Typically
another logical control loop exists between the receiver 220 and
the transmitter 210, regulating the transmitter's transmission
parameters such as output power, modulation mode etc. Hence, the
preferred embodiment of the present invention comprises two logical
control loops: a first control loop 505 between the receiver 220
and the relay stations 215:k, providing the relay stations with the
common transmission parameter, and a second control loop 510
feed-backing transmission information from the receiver 220 to the
transmitter 210.
[0061] In an alternative embodiment, adapted to be able to
effectuate the above described cases 3-4. and described with
reference to FIG. 5b, the means for performing channel
characterization 216 and means for determining both the relative
transmission parameters 217 and the common transmission parameters
222 is centralized located in the receiver 220, for example. The
receiver receives the unprocessed results of the pilot from the
relay station 215 and/or transmitter 210. The receiver performs the
necessary estimations and sends information on the relative
transmission parameters and the common transmission parameter to
the relay stations 215, either as a broadcasted message including
all relative transmission parameters or as dedicated messages to
each relay station. Alternatively may the transmitter perform the
estimation of the radio paths of the first link (case 2), and
hence, have the means therefore. A further alternative is that the
characterization and the determination of transmission parameters
is performed. However, preferably the receiver and transmitter
communicate to present a collected message, or messages, with all
transmission parameter information to the relay stations, either as
a broadcasted message to all relay stations or as dedicated
messages to each relay station. A further alternative is that the
characterization and the determination of transmission parameters
is performed elsewhere in the network, for example in a radio
network controller (RNC) or an entity with similar
functionality.
[0062] As described the present invention makes it possible to more
precise and reliable determine and optimize different transmission
parameters. This is turn makes it possible to fully take advantage
of the capabilities of a relaying network, in particular the
capabilities of a cooperative relaying network.
[0063] The method according to the invention facilitates a
distribution of functionalities in the network allowing an increase
in the number of relay stations in a communication session without
any significant increase in the amount of protocol overhead that is
needed for the transmission of data from the transmitter to the
receiver.
[0064] To efficiently implement the method according to the above,
a procedure of taking the characterization of the radio paths of
both the links, and possibly common quality measures, into account
in determining the forwarding parameters is desirable. An efficient
procedure is outlined below and a full derivation of included
expressions "derivation of analytic expressions" is given at the
end of the detailed description. How the procedure can be adapted
to control and optimize transmitted power, phase and relay station
activation, representing different embodiments, is also given
below.
[0065] Each relay station k transmits with a total power defined by
1 P k = P kS a k 2 k = 1 K a k 2 ( 1 )
[0066] , where P.sub.RS is the aggregate transmit power of all
relay stations, a.sub.k is a un-normalized complex gain factor for
relay station k.epsilon.{1, 2, . . . , K} and K is the total number
of relay stations.
[0067] In "derivation of analytic expressions" it is shown that the
maximum receiver SNR is attained (provided received signal is
normalized to unit power) if 2 a k = RS , k MS , k RS , k + 1 RS ,
k + MS , k + 1 ( 2 )
[0068] , and if
arg{a.sub.k}=-arg{h.sub.1,k}-arg{h.sub.2,k} (3)
[0069] where 3 RS , k = h 1 , k 2 P BS RS , k 2
[0070] , and 4 MS , k = h 2 , k 2 P RS MS 2
[0071] , and P.sub.RS is the transmit power of the basestation,
.sigma..sup.2.sub.RS,k is the noise plus interference level at any
relay station, .sigma..sup.2.sub.MS is the noise level at the
mobile station, h.sub.1,k is complex path gain from the basestation
to relay station k, and finally h.sub.2,k is complex path gain from
the relay station k to the mobile station.
[0072] It is can be shown (see the detailed derivation) that a
relay station k that receives a signal y.sub.k shall transmit the
following signal 5 z k = y k 1 k = 1 K a k 2 P RS RS , k MS k RS ,
k ( RS , k + MS , k + 1 ) - j ( arg ( h 1 , k ) 1 arg ( h 2 , k ) )
( 4 )
[0073] It should be noted that .GAMMA..sub.RS,k refers to the radio
paths of the first link and .GAMMA..sub.MS,k refers to the radio
paths of the second link. Hence, the radio characteristics of both
links are taken into account in each relay stations forwarding,
.GAMMA..sub.RS,k and .GAMMA..sub.MS,k are preferably, but not
necessarily calculated at each relay station.
[0074] The .SIGMA..vertline.a.sub.k.vertline..sup.2 term act as a
power normalization factor, denoted .phi., and it is observed that
it cannot be determined individually by each relay. Instead it is
hinted here that .phi. must be determined at some other suitable
station and distributed to the relays. l/.phi. corresponds to the
common transmission parameter, and 6 P RS RS , k MS , k RS , k 2 (
RS , k + MS , k + 1 ) - j ( arg ( h 1 , k ) + arg ( h 2 , k ) )
[0075] to the relative transmission parameter for relay station
k.
[0076] The maximum attainable receiver SNR under aggregate relay
transmit power constraint can be determined to 7 Eff ( max ) = k =
1 K RS , k MS , k RS , k + MS , k + 1 ( 5 )
[0077] At closer inspection, it is noted that the SNR contribution
from each individual relay to .GAMMA..sup.(max).sub.Eff is
equivalent to that if each relay station would transmit with all
relay transmit power P.sub.RS themselves.
[0078] Moreover, "derivation of analytic expressions", expressions
for a combination of regenerative and non-regenerative coherent
combining is also presented. When studying regenerative and
non-regenerative coherent combining an interesting observation is
that a regenerative approach is generally inferior to
non-regenerative case, because regenerative relaying by necessity
is constrained to a region around the transmitter and cannot
exploit all available relays in an optical manner. With other
words, even though a signal may not be decoded, it may still
contribute when coherent combining is employed. In any case, a
combination of non-regenerative and regenerative scheme will
perform slightly better than if only the non-regenerative method is
considered. The mechanisms for power and phase control that are
discussed in the following are is independent and generic to
whether regenerative relaying is employed as well.
[0079] Phase Control
[0080] As the first implementation example the logical architecture
and the method according to the present invention is adapted for
the use of facilitating coherent combining. A prerequisite for
coherent combining is that signals are phase-aligned at the
receiver. This is enabled by compensating for the complex phase
from the transmitter 210 to the relay station 215 as well as the
complex phase form the relay station 215 to the receiver 220.
Practically, in each relay station the received signal, y.sub.k, is
multiplied with the phase factor e.sup.-j.arg(a.sup..sub.k.su- p.)
where arg{a.sub.k}=-arg{h.sub.1,k}-arg{h.sub.2,k}.
[0081] Therefore, explicit or implicit channel phase information
must be made available at each individual relay station. There are
essential two basic schemes that can be used in deriving phase
information, one based on closed loop control and one on open loop
control. The closed loop control is necessary to use when channel
reciprocity cannot be exploited, such as in FDD (used over a single
link), or when high control accuracy is required. The open loop
control scheme instead exploits channel reciprocity, e.g. enabled
by TDD (used over a single link) with channel sounding that
operates within channel coherence time. Open loop control is
generally less accurate than closed loop control, due to
asymmetries in the transmit/receive chains for a station. The
differences boils down to the effort put into hardward design, and
can always be compensated by improved design. Also, incorporating
occasional closed loop control cycles may compensate for static
open loop errors. However, in the present invention the phase error
can in principle be up to .+-.90 degrees and still combine
coherently (but not very efficiently) with other relayed signals.
Hence, absolute phase accuracy is not a must, but certainly
preferred. A closed control scheme generally relies on explicit
signalling, reporting the result of measurements and therefore
consumes more communication resources and incurs latency relative
an open loop scheme. Note that this discussion on TDD vs. FDD
considers duplexing technique over a single link at a time, e.g.
the relay station to receiver link, whereas it is also possible to
characterize the overall communication in the network on basis of
time and frequency division. For example, link one and link two may
share a frequency band or use different ones. From point of view of
the invention, however, any combination of duplexing and multiple
access schemes may be used, as long as channel phase information
can be determined and used for phase compensation in the relay
stations.
[0082] Tightly connected with closed loop and open loop control is
the issue which station sends the pilots, which has been discussed
previously in reference to table 1. Since it is the relay stations
that must perform phase adjustment, this is the natural place to
determine arg{a.sub.k}. If a relay station sends a pilot signal,
the phase (or channel) parameters need to be reported back to the
relay. This corresponds to the closed loop case. If a relay station
instead receives a pilot, the phase (or channel) parameter does not
need to be reported anywhere. This corresponds to the open loop
case. It is clear that depending whether phase (i.e. channel)
information need to be sent away in a control packet or can be kept
in the same station, this has an impact on radio resource
efficiency, power consumption as well implementation complexity. In
any case, as seen form above, a myriad of possibilities exist and
we select the most promising. A preferred combination of duplexing
and multiple access will be further discussed. However, as
appreciated by the skilled in the art a very large number of
possibilities exist and the invention is not limited to the below
exemplified.
[0083] Case one (see table 1), which is of open loop type and
suitable for TDD with "sufficient" coherence time, offers the
lowest signalling complexity as only two transmissions are
necessary and the processing is distributed on all relay stations.
Here, the transmitter as well as the intended receiver issue
channel estimation symbols often enough or whenever needed such
that each relay can track both channels. The relay station
subsequently estimates the channel phases that determine the phase
factor of a.sub.k.
[0084] Power Control
[0085] A second important aspect for resource efficient
communication, apart from phase control, is power control, since it
provides means to ensure satisfactory communication quality. The
logical architecture and the method according to the present
invention is readily adapted to be used for an effective power
control. The power control method is based on that the effective
SNR at the receiver is controlled towards a target SNR,
.GAMMA..sub.0, which assert the desired link quality. The target
SNR may of course change with time depending on how link mode or
QoS requirement changes with time. According to the logical
architecture and the method according to the present invention
power may be adjusted at the transmitter and individually at each
relay. The relay power control has common as well as individual
relay component. In the objective of minimizing the aggregate power
addresses the issue of multiple access interference minimizations
as well as minimizing relay power consumption. However, when a MS
act as a transmitter, the power control may also be use as a method
for significantly minimizing power consumption and radiated power
for the MS, which among other advantages prolongs the batter life
of the MS.
[0086] On the highest level, the power control problem may be
defined as: 8 Find { P RS , P k } , k { 1 , 2 , , K } ; such that
eff ( max ) = 0
[0087] This is preferably accomplished under some constraints, such
as minimization of P.sub.RS=.SIGMA.P.sub.k and with fixed P.sub.BS,
but other constraints may also be considered, e.g. minimization of
the total transmit power P.sub.RS+P.sub.BS or by taking
localization of relay induced interference generation into account.
In the following, we assume minimization of P.sub.RS=.SIGMA.P.sub.k
with fixed (or relatively slow) adaptation of P.sub.BS. This is a
reasonable design objective in downlink, but for uplink it may be
of greater interest to minimize the transmitter power. However, if
the relays are mobile and relay on battery power, the sum power of
relays and transmitter may be minimized.
[0088] This is the basic function of power control. From practical
viewpoint, the overall task of controlling power in a cooperative
relay network in general, and with coherent combining in
particular, is to use previous knowledge of used power P.sub.BS and
P.sub.k and update those parameters to meet desired communication
quality.
[0089] Power control share much of its traits with the phase
control as the gain of the links may be estimated in several ways,
depending on close/open loop, TDD/FDD, distribution of control
aspects. Hence, also here can a range of alternative
implementations be envisioned. In the following, similar to the
phase control discussion, it is assumed that the transmitter and
receiver issue channel estimation signals and that channel gain
reciprocity can be assumed, but the invention is not limited
hereto.
[0090] The power control being proposed here has both a distributed
component for each relay station, the relative transmission
parameter, and a component common to all relays, the common
transmission parameter. The scheme operates as follows: Through
channel estimation, and with knowledge of the power used to send
the pilot, each relay station may determine its respective path
gain towards the transmitter and receiver respectively, but also
interference and noise levels may be estimated at the same time.
Based on path gain measurement, and information about P.sub.RS and
.sigma..sup.2.sub.MS, it is possible to determine .GAMMA..sub.MS,k.
Possibly also based on path gain, noise with interference
estimation and P.sub.BS awareness, or simply direct SNR
measurements on any received signal, the SNR at the relay station,
.GAMMA..sub.RS,k, can be determined. Based on this, the relative
transmit power levels can be determined at each relay station in a
fully distributed manner. However, each relative transmit power
level need to be sealed with normalization factor .phi. to ensure
that aggregate transmit power is identical, or at least close, to
the aggregate transmit power P.sub.RS. This is the common power
control part. If .phi. is too small, then more power than optimum
P.sub.RS is sert, and hence a more optimal relative power
allocation exist for the invested transmit power. The same is valid
when .phi. is too large. Hence, it is important for optimal
resource investment to control .phi. such that the intended power
P.sub.RS is the aggregate transmit power level by the relays. N.B.,
it is not a significant problem from performance point of view if
.phi. is somewhat to small as that only improves the effective SNR,
since the relative impact of receiver internal noise is
reduced.
[0091] Referring now to the logical architecture illustrated in
FIG. 5 the normalization factor, being a common transmission
parameter, is preferably determined, as well as distributed from,
the receiver. This should be seen as a logical architecture, since
it is also possible to forward all control information to the
transmitter, which then redistribute it to the relay stations, fore
example. The first control loop 505 between the receiver 220 and
the relay stations 215:k, provides the relay stations with the
P.sub.RS, whereas the second control loop 510 from the receiver 220
to the transmitter 210, provides the transmitter with P.sub.BS.
Optionally, if the transmitter has a better view of the whole radio
system including many groups of cooperative TS-RS-RX links, similar
to what a backbone connected basestation in a cellular system would
have, then it may incorporate additional aspects that strive to
optimize the whole system.
[0092] One method to implement the control loop at the receiver is
now given, then assuming that P.sub.BS is fixed (or controlled
slowly). From a transmission, occurring at time denoted by n, the
receiver measure the power of the coherently combined signal of
interest, C.sub.r, the relay induced noise measured at the
receiver, N.sub.r, and the internal noise in the receiver N.sub.t.
Based on this, and conditioned .GAMMA..sub.0, the receiver
determines 9 P RS ( n + 1 )
[0093] and an update of a normalization factor, .phi..sup.(n+1).
This can be written as a mapping through an objective function
.function. as 10 f ( C r , N r , N i ) { P RS ( n + 1 ) , ( n + 1 )
} ; such that eff ( max ) = 0 ( 6 )
[0094] The receiver then distributed the updates,
P.sup.(n+1).sub.RS and .phi..sup.(n+1), to all relays through a
multicast control message. To illustrate the idea, assume that
P.sub.RS is kept fixed from previous transmission, but the
normalization factor is to be adapted. In the section "Derivation
of analytic expression" it is shown that optimum normalization
requires a balance between received signal, C.sub.r, and the total
received noise, interference and receiver internal noise
N.sub.r+N.sub.i according to
C.sub.r=(N.sub.r+N.sub.i).sup.3 (7)
[0095] Hence, by including the previous normalization factor
.phi..sup.(n), which is known by the receiver, and the update
needed .phi..sup.(n+1) to balance the equation, the relation
becomes 11 C r ( n ) ( n + 1 ) = ( N r ( n ) ( n + 1 ) + N i ) 2 (
8 )
[0096] , which yields .phi..sup.(n+1) by solving a simple second
order equation.
[0097] If both P.sub.RS and .phi. need to be updated, the balance
equation above, the relation for the receiver SNR, .GAMMA., can be
used together with measured signal levels and solve for P.sub.RS
and .phi.. Linearization techniques, such as Taylor expansion and
differentials, may preferably be used for this purpose and solving
for .DELTA.P.sub.RS and .DELTA..phi..
[0098] It is noted that for the first transmission, the
normalization factor is not given a priori. Different strategies
may be taken to quickly adapt the power. For instance, an upper
transmit power limit may initially be determined by each relay as
they can be made aware of .GAMMA..sub.0 and also can determine
their (coherent combining) SNR contribution. If each relay stays
well below this upper limit with some factor, power can be ramped
up successively by the control loop so ongoing communications are
not suddenly interfered with. This allows control loops, for other
communication stations, to adapt to the new interference sources in
a distributed and controlled manner.
[0099] Also note that even though transmit power limitations occur
in any relays, the power control loop ensures that SNR is maximized
under all conditions.
[0100] Another, possibly more precise, method to determine the
normalization factor is to determine the
.vertline.a.sub.k.vertline. term in each relay and then send it to
the receiver where .SIGMA..vertline.a.sub.k.vertline..sup.2, is
calculated and hence yielding the normalization factor .phi..
Subsequently .phi. is distributed to all relays, similar to
previous embodiment. Note that the amount of signalling may be
reduced and kept on an acceptable level by sampling only a subset
of all relays, i.e. some of the most important relays, in order to
produce a sufficient good estimate of
.SIGMA..vertline.a.sub.k.vertline..sup.2 cm. This is further
motivated that the .SIGMA..vertline.a.sub.k.vertline..sup.2 term
will generally not change much over short time, even in fading
channels, due to large diversity gains inherent in the
invention.
[0101] Although power control has been described in the context of
coherent combining, the framework is also applicable for power
control in other types of relay cooperation schemes, such as
various relay induced transmit diversity, such as Alamouti
diversity. The framework is similar in that the power control
considers combinations of transmitter power, individual relay power
and aggregate relay power. Another example of relay induced
transmit diversity is (cyclic/linear) delay diversity. Each relay
imposes a random or controlled linear (or cyclic) delay on the
relayed signals, and hence causes artificial frequency selectivity.
Delay diversity is a well known transmit diversity from CDMA and
OFDM based communication.
[0102] To summarize this section, this invention suggests using
power control as a concept to ensure performance optimization for
coherent combining based cooperative relaying in a realistic
channel and in particular to optimize signal to noise ratio under
aggregate relay transmit power constraints. This power control
concept is not limited to coherent combining based cooperative
relaying networks, but also other cooperative relaying oriented
networks may use the same concept, though then with optimization
objectives most suitable to the scheme being used. In addition, the
basic features for a protocol based on channel sounding and
estimation of gain parameters over both link one and link two are
suggested. A reasonable design choice for protocol design (with
commonalities with the phase control) has also been outlined, based
on low complexity, low signalling overhead and low total power
consumption. In particular, it is shown that combination of power
control loops including relay and transmitter power control may be
used. Lastly, it has been demonstrated that the control loop for
the relays may be build on distributed power control decisions in
each relay as well as a common power control part, where the whole
set of relays are jointly controlled.
[0103] The main steps of the embodiment using the inventive method
and architecture for efficient power control and phase control are
illustrated in the flowchart of FIG. 6. The method comprises the
steps of:
[0104] 600: Send pilots on the k paths of link 1, from transmitter
210' to relay stations 215:k;
[0105] 610: Each relay station 215:k estimates the k channel of
link 1, h.sub.1,k; Also interference and noise levels are estimated
in order to calculate P.sub.RS,k.
[0106] 620: Send pilots on the k paths of link 2, from receiver
220' to relay stations 215:k;
[0107] 630: Each relay station 215:k estimates its respective
channel out of the k channel of 2, h.sub.2,k;
[0108] 610: Each relay station 215:k determines relative
transmission parameters based on the channel estimates.
[0109] 650: The receiver 220' determines a normalization factor
.phi..
[0110] 660: The receiver 220' broadcast the normalization factor
.phi., P.sub.RS, and .phi..sup.2.sub.RS to the relay stations
215:k.
[0111] 670: Each relay station 215:k uses the broadcasted .phi.,
P.sub.RS, and the locally determined .GAMMA..sub.MS,k and
.GAMMA..sub.RS,k, and the phase of channel estimates h.sub.1,k,
h.sub.2,k to, on the reception of signal y.sub.k, transmit the
following signal: 12 z k = y k 1 P RS RS , k MS , k RS , k ( RS , k
+ MS , k + 1 ) - j ( arg ( h 1 , k ) + arg ( h 2 , k ) )
[0112] wherein the parameters .GAMMA..sub.RS,k is calculated based
on the channel estimate, P.sub.BS, and .sigma..sup.2.sub.RS, and
.GAMMA..sub.MS,k based on P.sub.RS, and .sigma..sup.2.sub.MS.
[0113] If the first transmission to the receiver is considered,
(then the power loop is unaware of the forthcoming link quality),
by way of example the relay may modify and upper limit the received
normalization factor .phi. such that
.phi..sub.k=c.multidot..vertline.a.sub.k.vertline..sup.2, where
c.ltoreq.1 being sent from the receiver or is a prior known.
[0114] 675: The receiver 220' feedbacks control information to the
transmitter 210' (P.sub.BS).
[0115] The first control loop, indicated in step 660 may further
comprise the substeps of:
[0116] 660:1 The receiver measure at time n, the quality of the
received signal, or more specifically the power of the coherently
combined signal, C.sub.r, the relay induced noise measured at the
receiver, N.sub.r, and the internal noise in the receiver
N.sub.1.
[0117] 660:2 The receiver determines based on the measurement of
step 675:1, and conditioned a desired .GAMMA..sub.0 target, an
update of at least one of the normalization factor, .phi..sup.(n+1)
and the aggregate relay power 13 P RS ( n + 1 ) .
[0118] 660:3 The receiver distributed the updates, 14 P RS ( n + 1
)
[0119] and .phi..sup.(n+1), to all relays through a multicast
control message.
[0120] Similarly, the second control loop, indicated in step 675,
may optionally comprise:
[0121] 675:1 The receiver update the transmitter (BS) power 15 P BS
( n + 1 ) .
[0122] Alternatively, if no estimations and calculations are to be
done by the relay stations, unprocessed results of the pilots are
forwarded to a centralized functionality, in the receiver for
example, and relevant transmission parameters transmitted to each
relay station.
[0123] Relay Stations Activation Control
[0124] The method and architecture of the present invention may
advantageously be used for deciding which relay stations 215:k to
include in a communication, either at the establishment of the
communication or during the communication session. As some relays
experiencing poor SNR conditions on either link (transmitter-relay
and relay-receiver) or both, they may contribute very little to the
overall SNR improvements. Yet, those relays may still consume
significant power due to receiver, transmitter and signal
processing functions. It may also be of interest to have some
control means to localize relay interference generation to fewer
relays. Hence, it may therefore be considered to be wasteful to use
some of the relay stations. Consequently, one desirable function is
to activate relays based on predetermined criteria. Such criteria
may be a preset lower threshold of acceptable SNR on either link,
both links or the contribution to the effective SNR. The limit may
also be adaptable and controlled by some entity, preferably the
receiving station as it has information on momentary effective SNR.
The relay may hence, e.g. together with power control information
and cannel estimation symbols, receive a relay activation SNR
threshold .GAMMA..sub.Active from the receiver to which the
expected SNR contribution is compared against, and if exceeding the
threshold, transmission is allowed, else not. The relay activation
SNR threshold .GAMMA..sub.Active corresponds to a common
transmission parameter, preferably determined by the receiver 220'
and distributed to the relay station 215. The actual decision
process, in which each relay station uses local parameters
(corresponding to the relative transmission parameters) is
distributed to the relay stations in the manner provided by the
inventive method and architecture. This test, preferably performed
in each relay prior to transmission, may e.g. be formulated
according to: 16 RS , k ( RS , k + 1 ) MS , k 2 ( n + 1 ) ( RS , k
+ MS , k + 1 ) 2 { > Active Transmit Active Silent ( 9 )
[0125] , but other conditions, depending on relay methods including
alternative relay diversity techniques, can also be used. For
instance, the relay activation condition may more generally be
characterized as an objective function .function..sub.2 according
to .function..sub.2 (.GAMMA..sub.RS,k,.GAMMA..sub.MS,k).
[0126] Moreover, The broadcasted message containing the
.GAMMA..sub.Active could further comprise fields that may be used
to pinpoint specific relays (through assigned relay addresses) that
should be incorporated, or is only allowed to be used, or must
excluded or any combination thereof. Other methods to address
certain relays may e.g. be based on address ranges. This enables
one to limit the number of involved relays as desired.
[0127] From the above discussion and expression (9) it can be noted
that the receiver 220' may, upon experiencing weakening SNR, for
example due to the movement of the MS, choose to order a increased
transmission power and/or to include more relay stations 215 by
lowering the threshold .GAMMA..sub.Active. Other communication
quality conditions, such as packet or bit error rate, may also be
used by the receiver to trigger changes in the common parameters,
such as a joint transmit power sealing of all relay powers.
[0128] Relay activation control may be incorporated in the power
and phase control algorithm described with reference to FIG. 6, by
modifying the steps 650-670, so that:
[0129] in 650: the receiver 220' also determines an activation SNR
threshold .GAMMA..sub.Active
[0130] in 660: the receiver 220' also broadcast .GAMMA..sub.Active
to the relay stations 215:k.
[0131] in 670: each relay station 215:k firstly determines if to
broadcast using the activation SNR threshold .GAMMA..sub.Active,
for example according to expression (9)
[0132] The method and architecture according the present invention
may be adapted to other topologies than the above exemplified. The
topology in FIG. 5 may, for example, be modified to include
multiple antennas in each relay station as shown in FIG. 7. The
benefit in doing that is that the number of relay stations can be
reduced while still getting similar total antenna directivity gain.
If each antenna element is separated more than the coherence
distance, diversity gain is also provided. In all, this can reduce
the cost, while providing near identical performance. However,
reducing the number of relays may have a detrimental impact due to
shadowing (i.e. log normal fading) and must be carefully applied.
From signal, processing and protocol point of view, each antenna
can be treated as a separate relay station. Another benefit of this
approach is however that internal and other resources and may be
shared. Moreover, relaying may potentially be internally
coordinated among the antennas, thereby mitigate interference
generation towards unintended receivers.
[0133] The communication quality may be further improved by also
incorporate the direct signal from the transmitter 210 to the
receiver 220. There are at least two conceivable main methods to
incorporate the signal from the transmitter. FIG. 8, depicts the
topology when direct transmission from the transmitter is also
considered.
[0134] In the first method, two communication phases are required.
The receiver combines the signal received directly from the
transmitter, in the first phase, with the relay transmission, from
the second phase. This is somewhat similar to receiver based
combining in the classical relay channel, but with coherent
combining based relaying. Maximum ratio or interference rejection
combining may be employed.
[0135] In the second method, Transmit-relay oriented Coherent
Combining, only one communication phase is used, and used for
coherent combining of the direct signal from the transmitter to the
receiver with the relay signals. This can be made possible if
relays can transmit and receive concurrently, e.g. over separated
antennas. The phase of a.sub.k must then ensure alignment of
relayed signal with the direct signal as
arg{a.sub.k}=-arg{h.sub.1,k}-arg{h.sub.2,k}-arg{h.sub.BS,MS}+c.sub.1
[0136] , where h.sub.BS,MS is the complex channel from the
basestation to the mobile station. A consequence of incorporating
the direct signal for coherent combining is that the relays must
adaptively adjust their phase relative the direct signal. A closed
loop can be used for this. Similar to the normalization factor
power control, the receiver issues phase control messages to the
whole group of relay stations, but with a delta phase .theta. to
subtract from the calculated phase compensation
(-arg{h.sub.1,k}-arg{h.sub.2,k}).
[0137] As the basestation does not induce any noise through its
transmission, its transmit power does not need to be adjusted for
optimal performance as was needed for the relays. Instead,
performance increases monotonically with increasing basestation
transmit power. One option is however to try to minimize the
overall transmit power, aggregate relay power and basestation
power. The parameter setting for this is similar to what has been
derived in the discussion on regenerative relaying, assuming that
the basestation is considered as a relay. In addition to above,
multiple antenna elements at the transmitter may also be used,
similar to the discussions on relays with multiple antennas.
[0138] The derivation of the relative and common transmission
parameters is also directly applicable to multi carrier
transmission, such as OFDM by handling each subcarrier
independently. This will then include a common amplitude
normalization, phase and distributed relay amplitude compensation
per subcarrier. For doing this, the path over FFT-processing-IFFT
is taken, or possible through time domain filtering. The power
control may send a normalization factor .phi. and relay power
indication P.sub.RS in vector form to optimize performance per
subcarrier. A more practical solution, is to send .phi. and
P.sub.RS as scalars, acting on all subcarriers. In case of
subcarrier optimization, the power control may then try to minimize
power the total transmit power over all subcarriers to meet desired
communication quality. This then provides some diversity gain in
the frequency domain.
[0139] Another OFDM aspect is that it is a preferred choice for the
transmit-relay oriented Coherent Combining described above. The
reason is that the cyclic prefix allow for some short relay
transfer latency, where phase and amplitude is modified through a
time domain filter enabling immediate transmission.
[0140] For single carrier transmissions, such as CDMA, and with
frequency selective channels, a frequency domain operation similar
to OFDM may be employed or optionally the phase alignment can be
performed on the strongest signal path, or with a time domain
filter as discussed for OFDM.
[0141] For coherent combining to work, it is important to
synchronize relay station frequency to a common source. In a
cellular system, the BS is a natural source as since the clock
accuracy is generally better at the basestation than in any mobile
station. This function can exploit the regular frequency offset
compensation as performed in traditional OFDM receiver
implementations, that mitigates inter channel interference.
[0142] However, the relays may optionally exploit GPS for frequency
synchronization, if available.
[0143] While the invention has primarily been described in a
context of coherent combining, the invention is not limited hereto.
The invention may be applied on various types of existing and
forseen methods for 2-hop (cooperative) relaying. In the most
general case, the transmit parameters of the relays are functions
of communication characteristics of the first link, communication
characteristics of the second link, or a combination thereof. The
communication quality has been described outgoing from complex
channel gain (suitable for coherent combining), however when other
schemes are considered (offering diversity and/or spatial multiplex
gains), other link characteristic metrics may be of more relevance.
As an example, for Alamouti diversity it may be more preferable to
use average path gain metric, G, instead of complex channel gains,
h.
[0144] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiments, it is to be understood that the invention is not to be
limited to the disclosed embodiments, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included with the spirit and scope of the appended claims.
Detailed Derivation
[0145] In the analysis we assume that there are K relay stations
arbitrarily located. Each relay station k.epsilon.{1, 2, . . . , K}
receives a signal composed of an attenuated version of the desired
signal, e.g. modeled as complex Gaussian x.about.N(0,1), as well as
a noise plus interference term, n.sub.RS,k, according to
y.sub.k=h.sub.1,k-{square root}{square root over
(P.sub.BS)}.multidot.x+n.- sub.RS,k
[0146] , where h.sub.1,k is the complex path gain from the
basestation to relay station k and P.sub.BS is the transmit power
of the basestation.
[0147] In the relay, y.sub.k is (for analytical tractability)
normalized to unit power, and multiplied with a complex factor that
generates output z.sub.k. subsequently z.sub.k is sent over link
two, towards the receiver and is on its way attenuated with complex
path gain h.sub.2,k, where it is super-positioned with signals from
other relays and noise and interference is added.
[0148] As it is assumed that each relay normalize the received
power plus noise to unit power prior amplification and phase
adjustment, the relay transmit power constraint can be incorporated
in the analysis by letting each station k use transmit power 17 P k
= P RS a k 2 k = 1 K a k 2
[0149] , where P.sub.RS is the total transmit power of all relay
stations, and a.sub.k is a un-normalized complex gain factor for
relay station k.
[0150] For aggregate power constrained relay transmission, the SNR
at the receiver (Mobile Station, MS, assumed here) may then be
written 18 = k = 1 k P RS a k q = 1 K a q 2 h 1 , k P BS h 1 , k 2
P BS + RS , k 2 h 2 , k 2 k = 1 K P RS a k 2 q = 1 K a q 2 RS , k 2
h 1 , k 2 P BS + RS , k 2 h 2 , k 2 + MS 2
[0151] , where .sigma..sup.2.sub.MS is the noise plus interference
level at the mobile station.
[0152] A condition for coherent combining is phase alignment of
signals, which can be achieved by ensuring
arg{a.sub.k}=-arg{h.sub.1,k}-arg{h.sub.2,k}+c.sub.1
[0153] , where c.sub.1 is an arbitrary constant
[0154] The expression for the effective SNR resulting from coherent
combining may then be rewritten as 19 Eff = k = 1 K a k RS , k MS ,
k RS , k + 1 2 k = 1 K a k 2 MS , k + RS , k + 1 RS , k + 1
[0155] , where 20 RS , k = h 1 , k 2 P BS RS , k 2
[0156] , and 21 MS , k = h 2 , k 2 P RS MS , k 2
[0157] Note that .GAMMA..sub.MS,k is a "virtual SNR" in the sense
that it is the SNR if relay station.sup.k would use all aggregate
relay stations transmit power by itself.
[0158] It is noticed that the SNR expression has the form 22 Eff =
k = 1 K a k c 1 , k 2 k = 1 K a k 2 c 2 , k
[0159] which can be transformed by using
.vertline.b.sub.k.vertline..sup.2=.vertline.a.sub.k.vertline..sup.2.multid-
ot.c.sub.2,k
[0160] , which yields 23 Eff = k = 1 K b k c 1 , k c 2 , k 2 k = 1
K b k 2
[0161] Now, the nominator is upper limited by Cauchy-Schwarz's
inequality 24 k = 1 K b k c 1 , k c 2 , k 2 k = 1 K b k 2 k = 1 K c
1 , k c 2 , k 2
[0162] , hence for an optimal b.sub.k equality can be attained and
the resulting SNR is then 25 Eff ( max ) = k = 1 K b k c 1 , k c 2
, k 2 k = 1 K b k 2 = k = 1 K b k 2 k = 1 K c 1 , k c 2 , k 2 k = 1
K b k 2
[0163] This may be conveniently expressed in SNRs as 26 Eff ( max )
= k = 1 K RS , k MS , k RS , k + MS , k + 1
[0164] Through identification, it is seen that the maximum SNR can
be attained if 27 b 2 = Const c 1 , k c 2 , k
[0165] , where Const is an arbitrary constant that can be set to
one for convenience.
[0166] From power control perspective, it is interesting to note
that the nominator is exactly the square of the denominator for
optimum SNR. This knowledge can therefore be used as a power
control objective.
[0167] Using the reverse transformation, one yields 28 a k = c 1 ,
k c 2 , k
[0168] , or expressed in SNRs 29 a k = RS , k MS , k RS , k + 1 RS
, k + MS , k + 1
[0169] Hence a relay receiving a signal y.sub.k can determine
z.sub.k by determining 30 z k = P RS k = 1 K a k 2 - j ( arg ( h 1
, k ) + arg ( h 2 , k ) ) RS , k MS , k RS , k + 1 RS , k + MS , k
+ 1 y k h 1 , k 2 P BS + RS , k 2 = y k 1 k = 1 K a k 2 P BS RS , k
MS , k RS , k ( BS , k + MS , k + 1 ) - j ( arg ( h 1 , k ) + arg (
h 2 , k ) )
[0170] Regenerative Relaying Add-on
[0171] If the SNR at a relay station is high enough, the received
signal may be decoded prior relaying the signal. To model this
behavior, let's say that larger than a minimum SNR,
.GAMMA..sub.Decode, is sufficient for decoding. The benefit in
doing this, is that forwarding of detrimental noise (and
interference) can be avoided all together, and hence result in a
further enhanced SNR at the receiver. In this case however, the
decoded signal should be phase compensated only for the second hop,
i.e.
arg{a.sub.k}=-arg{h.sub.2,k}
[0172] By setting .sigma..sup.2.sub.KS,k=0 for those stations in
the previous expressions, one can derive the magnitude of the
multiplicative factor .vertline.a.sub.k.vertline. as well as the
contribution to the SNR improvement. The combination of both
noise-free (regenerative) and noisy (non-regenerative) transmission
then takes the form 31 Eff ( max ) = k = 1 K { RS , k MS , k RS , k
+ MS , k + 1 , if RS , k < Decode MS , k , if RS , k Decode
[0173] , and 32 a k = { V RS , k MS , k RS , k + 1 RS , k + MS , k
+ 1 , if RS , k < Decode MS , k , if V RS , k Decode ,
[0174] , and 33 arg { a k } = { - arg { h l , k } - arg { h 2 , k }
, if RS , k M < Decode - arg { h l , k } , if RS , k Decode
[0175] Note that .GAMMA..sub.RS,k<.GAMMA..sub.Decode is only a
model useful to assess performance in a mixed non-regenerative and
regenerative relaying scenario. In practice, the upper expressions,
i.e. corresponding to .GAMMA..sub.RS,k<.GAMMA..sub.Decode, are
used when the signal is not forwarded in a non-regenerative manner,
and the lower expressions, i.e. corresponding to
.GAMMA..sub.RS,k>.GAMMA..sub.Decode, are used when the signal is
not forwarded in a regenerative manner.
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* * * * *