I. J. Communications, Network and System Sciences. 2008; 1: 1-103
Published Online February 2008 in SciRes (http://www.SRPublishing.org/journal/ijcns/).
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
Multiband Scheduler for Future Communication
Radio Communications CTC, Nokia Research Center, Helsinki, Finland
P.O. Box 407, FI-00045 NOKIA GROUP, Finland
E-mail: {Klaus.Doppler, Carl.Wijting, Tero.Henttonen, ext-Kimmo.Valkealahti}@nokia.com
Operation in multiple frequency bands simultaneously is an important enabler for future wireless communication
systems. This article presents a new concept for scheduling transmissions in a wireless radio system operating in
multiple frequency bands: the Multiband Scheduler (MBS). The MBS ensures that the operation in multiple bands is
transparent to higher network layers. Special attention is paid to achieving low delay and latency when operating the
system in the multiband mode. In particular, we propose additions to the ARQ procedures in order to achieve this.
Deployment details and assessment results are presented for two multiband deployment scenarios. The first scenario is
operation in a spectrum sharing context where multiple bands are used: one dedicated band for basic service and one
shared extension band for extended services. In the second scenario we consider multiband operation in a relay
environment, where the two bands have different propagation properties and relays provide extra coverage and capacity
in the whole cell.
Keywords: Multiband Operation, Scheduling, Multiband Scheduler, IMT-Advanced, Relaying, Dynamic Spectrum
Use, Flexible Spectrum Use, Spectrum Sharing
1. Introduction
Recently the World Radio Communications
Conference (WRC-07) has allocated new spectrum for
future radio communication systems in different
frequency bands (including new allocations in the UHF
and C band). Furthermore, spectrum that is currently
allocated to second and third generation wireless
communication systems may be reused. Consequently, it
would be advantageous for such systems to be able to
operate in multiple bands. They can use these multiple
bands for balancing the load of the networks or for
providing required quality of service levels. It is also
predicted that some of these bands, here referred to as
basic (B) bands, might be dedicated to specific services or
operators, and that other bands, the extension (E) bands,
might be shared between different operators and/or
different services (e.g. mobile communications and fixed
satellite services (FSS)). Sharing the spectrum with other
radio technologies is seen as a promising technique for
increasing the spectrum utilization. However, flexible and
fast mechanisms for band transfers are required when the
extension bands are (temporarily) unavailable. IMT-
Advanced systems are mobile systems that include new
capabilities that go beyond those of IMT-2000 (UMTS,
WiMAX, etc.). Such systems provide access to a wide
range of telecommunications services, including
advanced mobile services, supported by mobile and fixed
networks, which are increasingly packet-based. IMT-
Advanced systems support low to high mobility
applications and a wide range of data rates, in accordance
with service demands in multiple user environments (100
Mbit/s for high mobility and 1 Gbit/s for low mobility
were established as research objectives) [1]. Key
innovation areas for these future wireless systems include
new concepts such as spectrum sharing [2] and network
relays [3]. Several research projects have already
addressed these needs; for example the European
WINNER project has developed a flexible and scalable
radio interface, which covers different domains (local
area, metropolitan area, and wide area) with the same
radio interface [4].
In this article we introduce the multiband scheduler
(MBS), which enables simultaneously high flexibility in
terms of spectrum use and high spectral efficiency—two
goals that are difficult to combine. The MBS is located in
the medium access control (MAC) system layer, which
controls the physical layer, including the radio resource
allocation, the spatial processing and the packet
scheduling [5]. Multiband operation is made possible by
adding an MBS to single-band MACs. It schedules
protocol data units (PDU) to the correct band, enables fast
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
switching to other bands and provides functions for load
balancing between the bands. After introducing the
concept of the MBS, we illustrate how to apply the MBS
to a communication system that shares one band with
another wireless system and to relay networks that
operate in multiple bands.
Bands with lower center frequency have better
propagation properties (larger ranges) than higher
frequency bands, and relay nodes (RN) are a cost efficient
way to extend the coverage area of the higher bands. RNs
can be used in multiband operation for the purpose of
balancing the coverage area of different frequency bands.
Some of the RNs may operate in only one spectrum band,
while the rest operate in several bands. In the latter case
an MBS is needed at the base station (BS). In addition, a
RN with an MBS can receive from the BS PDUs
transmitted in the higher-capacity band and forward them
to the user terminal (UT) using any band. However,
relays add additional delay to the network, and thus a
multiband operation with fast retransmissions and low
delays is important.
The importance of delay can be seen from the
following simplified delay budget calculation. Assuming
a two-hop scenario where we want to achieve an end-to-
end delay of up to 20 ms between two peer IP entities
(this is the maximum delay for highly interactive services
[6]) and assuming that a delay of maximum 10 ms is
required over the air interface itself (as in 3GPP-LTE [7]).
Allowing one retransmission over two hops and assuming
an air interface transmission delay of 1 ms, the delay
budget can then be determined as follows: one
transmission on each link, plus one feedback message and
a retransmission adds up to 4 ms1 plus additional
processing delay, leaving a margin of only 6 ms [8].
The remainder of this paper is organized as follows.
Section 2 provides an introduction to the multiband
scheduling concept. Section 3 focuses on the concept of
hybrid ARQ context transfers. Sections 4 and 5 present
the multiband scheduler in a spectrum-sharing context
and in a relay context, respectively. Both sections provide
numerical results in order to demonstrate the benefits of
the multiband scheduler. Finally, section 6 presents the
conclusions of the study.
2. Multiband Scheduling
The general framework in which the MBS operates is
presented in Figure 1. Higher-layer protocol data units
(PDU) arriving at the IP convergence layer (IPCL) are
converted into Radio Link Control (RLC) service data
units (SDU) after header compression and ciphering. In
accordance with the decision of the scheduler, a certain
amount of data is selected from the RLC SDU buffer and
1 In [8] it was found that in a relaying scenario 95% of the users
experience a delay of up to 3ms.
segmented and/or concatenated, depending on the size of
the SDU. A user terminal (UT) identification code, a
transmission sequence number, and optionally a CRC
code are added. If RLC acknowledged operational mode
is used, then an outer end-to-end ARQ is performed at the
RLC level. The multiband scheduler schedules the RLC
PDUs for transmission in the correct band. The resource
scheduler (RS) of each band fetches the RLC PDUs from
the buffer and constructs from them transport blocks (TB),
which are scheduled for transmission. Hybrid ARQ
(HARQ) can be used for improving the transmission of
the TBs. The MAC adds a retransmission sequence
number to TBs that use HARQ. The RS of each band
operates independently and the coordination of the
different bands is done exclusively by the MBS. The
operation of the different ARQ schemes is depicted in
Figure 2.
Higher layer PDU
Multi-band Scheduler
Band A
Band B
Higher layer PDU
Higher layer PDU
Segmentation &
Header compression &
Mu l t i p lex i ng
Figure 1. Illustration of the Multi Band Scheduler and the
data unit it is working on.
Different Hybrid ARQ protocols exist and two
possible approaches can be used: chase combining
(retransmission of the whole TB and combining these at
the receiver), or incremental redundancy (retransmission
of additional redundancy bits, providing the receiver with
more information about the TB).
In the case that a certain band is no longer available,
the MBS provides a mechanism for transferring the user
context from that band to any other band that is still being
serviced (Context Transfer Unit). The user context
comprises all the information necessary to continue the
active services to the user in the other band. This transfer
occurs in real time, and is transparent to the end user.
During the context transfer, control parameters essential
for the transmission are transferred. Adaptation and
prioritization of the user data flows may be needed, since
the new band might not be able to accommodate all traffic
from the band that is no longer available. The timing of
the transfer is an important issue; rapid changes when a
band becomes unavailable are supported, as well as
preparation of context transfers when information is
obtained that the availability of a band will change in the
(near) future. This is done by the so-called Band
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
Monitoring functionality in the MBS.
In the multiband architecture considered here, many
common functions for the operation in the different bands
can be identified. These functions may be shared between
the different bands. For example, the same flow ID, UT
ID, etc. can be used in both bands, which simplifies the
context transfer between the bands. However, some
problems related to ARQ and synchronization remain,
and need to be solved in order to allow fast switching.
These will be addressed in the following sections. In the
conceptual discussion above we spoke of switching
between two spectrum bands, but a more generalized
approach can also be used in the case of a spectrum that is
more fragmented.
2.1. Handovers for MBS and Non-MBS Cases
We assume that the network supports mobility via
normal inter- and intra-frequency handovers, and that the
handover mechanism is hard handover (meaning that
there is always a short break in the connection when a
handover is done). Further, we assume that handover
decisions are done by the network and that only UTs
perform measurements and that the UTs keep the serving
BS informed of the measured values and of triggered
events (such when the signal of a neighbor BS has
become stronger than that of the serving BS).
A UT performs periodical measurements, both for
identifying neighboring BSs and for measuring the signal
level of the identified BSs. A measurement report,
containing filtered measurement results, is then sent to the
serving BS, either periodically or when a network-
configured event occurs. Based on the measurement
reports, the serving BS decides whether a handover
should be performed.
After a handover from the serving BS to a target BS is
triggered, the serving BS sends a request to the target BS
to confirm that the UT is allowed to do the handover. If
the target BS allows the handover, the serving BS sends a
handover command to the UT, identifying when and to
which BS the handover should be done. Finally, the UT
responds to the handover command by sending an
acknowledgement to the serving BS, and then breaks the
connection to the serving BS at the agreed time and
connects to the target BS.
In the non-MBS case a UT needs to perform a
handover when switching between the B and E bands.
This means that there is always a break in the connection
when moving from the B band to the E band, which
causes the user throughput to drop to zero for a while.
The overall delay caused by the handover is of the order
of tens of milliseconds. In our simulations the delay has
been assumed to be 20 ms.
In contrast, when an MBS is used the biggest delay is
scheduling delay, which is only of the order or few
milliseconds. A small additional delay arises from the
time needed by the UT for switching bands. This allows
for more seamless switching between resources and thus
for better load balancing and higher user throughput.
3. Hybrid ARQ Context Transfer
The design of the automatic repeat request (ARQ)
mechanism is very important with regard to the speed of
context transfers between bands. If ARQ retransmissions
have to be finished before switching to another band is
allowed, then, depending on the ARQ design, switching
delays of up to 20 ms are possible. In order to make fast
context transfer possible, we propose the use of an ARQ
mechanism that consists of an outer ARQ for RLC SDUs
and an inner HARQ for retransmissions of transport
blocks [8].
3.1. E2E ARQ (Outer ARQ)
The E2E ARQ is situated on a higher protocol plane
than the MBS, and thus context transfers from one band
to another do not affect the E2E ARQ process (Figure 2
illustrates this). After a context transfer the new band is
briefly not in use, and the multiband scheduler takes this
into account in its scheduling decisions.
3.2. HARQ (Inner ARQ)
Each band has an independent HARQ process, and
thus a fast context transfer is required for these processes.
In order to allow the HARQ process to continue
uninterrupted, we propose a mechanism in which the
HARQ buffer is transferred between the two bands, as
illustrated in Figure 2. The Context transfer Unit of the
MBS coordinates the exchange of the HARQ data and
B- Qu eu e
E-Queu e
Qu eu e
Qu eu e
Logical channel:
Flow address + terminal class
Multi-band scheduler
Scheduler state,
e.g. current band
of a flow etc
Scheduler state,
e.g. current band
of a flow etc
swi t ch
Traffic SchedulerTraffic Scheduler
Co nt e x t
Transfer Unit
Co nt e x t
Transfer Unit
Band MonitoringBand Monitoring
Figure 2. Main functions and operation of the Multband
Scheduler. Fast context transfer during a switch to another
band during an ongoing HARQ process.
When a UT switches for example from the extension
(E) band to the basic (B) band, also the HARQ buffer is
transferred, and as a result, the HARQ retransmissions
can be continued on the B band. This requires the
1) The HARQ processes in the B queue and the E
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
queue use a common numbering scheme.
2) After the switch, the arriving data is directed to
the scheduling queue of the new band and any
data remaining in a buffer is transferred to the
queue of the other band.
Next to the HARQ buffer, the number of performed
retransmissions and information specific to the utilized
HARQ scheme is exchanged.
3.3. Preparation for Band Switch
In many situations the timing of the band switch is
known beforehand and preparations can be made before
the switch. For example, if the BS knows that the E band
will not be available any more after 5 ms, then it can
command the UT to synchronize with the B band. The BS
can assist the synchronization by sending, for example,
the time shift to the beginning of the next frame on the B
band, the frequency shift between the two bands, and
other system information, such as the position of the
resource allocation table. Furthermore, the UT can
estimate the pathloss or an initial channel quality
indicator and report it to the BS well before switching to
the B band. When the UT switches bands, this
information is forwarded from the E band to the B band
scheduler. The anticipation of the band switch allows for
a seamless handover, because contexts can be exchanged
before the connection on the band is actually lost.
4. Use of MBS for Flexible Spectrum Sharing
The ITU-R studies [10][11] show that a considerable
amount of new spectrum will be needed to provide the
total capacity that is needed for delivering the predicted
services and traffic in the future. However, spectrum for
wireless networks is already a scarce resource and will
become even scarcer in the future. Therefore sharing the
spectrum with other radio systems is a possibility to
access additional spectrum. It is based on the assumption
that when one network operator or radio system is in
demand of spectrum, another network operator might
have spectrum available. Thereby the exploitation of
available unused spectrum or sharing of spectrum
between technologies leads to a better utilization of
spectrum throughout a multi-operator or a multi-radio-
network environment.
In the preparation phase towards WRC-07 several IMT
candidate bands have been identified [12] and parts of
these bands have been allocated as IMT bands for
communication systems at WRC07. The newly identified
spectrum comprises the following spectrum bands: 450-
470 MHz was identified globally, 698-802 MHz was
identified in the Americas and some Asian countries, 790-
862 MHz was identified in Europe, Africa and most
Asian countries, 2300 - 2400 MHz has been identified
globally, and 3400 - 3600 MHz was identified in most
countries in Europe and Africa, and in several countries
in Asia.
Fixed Satellite Service (FSS) is the primary service
deployed in large portions of the candidate bands. In the
allocated C-band (3.4 to 3.6 GHz) dedicated spectrum
will be available for IMT systems, but not world wide. To
enable further deployment in the C-band sharing with
FSS will be required. When bands are available on a
sharing basis their availability cannot always be
guaranteed. For example transmission exclusion zones
around technologies with which the spectum is shared,
might be defined. On the one hand the shared new bands
should be accessible to guarantee high capacity, but on
the other hand, dedicated and guaranteed bands are
required to offer guaranteed network access. Therefore,
the UTs have to operate in a multi band environment. A
possible spectrum allocation for a system deploying one
dedicated band between 3.4GHz and 3.6GHz and a
shared band at higher frequency in the same band is
illustrated in Figure 3.
Figure 3. Possible spectrum allocation for IMT-Advanced in
a multi band deployment.
Flexible Spectrum Use (FSU) between operators using
the same technology is another possibility of dynamic
spectrum use, enabling flexible deployments with a
limited amount of available spectrum. Also in this case a
shared band for enhanced capacity and a dedicated band
with guaranteed access can be defined [13].
A fast context transfer is required when a user terminal
served on the shared extension band moves into an area
where it would interfere with the other system’s
transmissions, e.g. when it enters the transmission
exclusion zone around a satellite earth station. The fast
transfer to the basic band can be provided by the MBS,
assuming that one BS handles both B and E band.
4.1. Case Study
Figure 4 depicts a simulation scenario for which we
study a band transfer of a UT with and without MBS at
the BS. The circle in the center marks a transmission
exclusion zone, where UTs are not allowed to use the E
band and have to switch to the B band.
In our simulations we use an event driven dynamic
system simulator that simulates UL and DL directions
simultaneously with OFDMA symbol resolution and uses
an Exponential Effective SINR Mapping (EESM) link to
system mapping [13].
We use the handover procedures described in section
Basic band with limited
bandwidth in 3.4-3.6 GHz band
(guaranteed access)
Extension band with larger
bandwidth at alternative location in
the C-band (shared access)
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
2.1 to model BSs that are/are not equipped with an MBS.
Figure 5 illustrates the instantaneous throughput of a UT
when entering and leaving the exclusion zone and
changing to the B band for the MBS and the non-MBS
case. It clearly shows that the MBS and the anticipation
of the band switch avoids periods with zero throughput
when entering the exclusion zone. Nevertheless, in both
cases the UT will experience lower throughput because of
the lower capacity of the B band.
Figure 4. Macro-cellular simulation scenario with 27 sectors.
Results are presented for the 6 sectors in the center. The
circle in the center marks an exclusion zone. UTs in this zone
are not allowed to use the E band.
0100 200 300 400500 600 700 800 900
2x 10
Time in ms
Throughput in bits/sec
UT enters
UT is in
zo ne
UT leaves
zo ne
Figure 5. Instantaneous User Throughput averaged over
5ms with and without MBS when entering an exclusion zone.
If the B band is congested the MBS can relocate traffic
from the B band to the E band or decide to drop or reduce
the bandwidth of less important flows in the B band.
When leaving the exclusion zone, the UT can
immediately receive data on the E band and benefit from
the increased capacity. However, without an MBS the UT
has to go through a handover procedure, which typically
involves some time to trigger the handover, pending
HARQ transmissions are lost and the initial throughput is
lower because the serving RAP does not have CQI
information from the UT.
5. Application of MBS to Relaying
In a multiband operation, Relay nodes (RN) can be
used to extend the coverage of the band with worse
propagation characteristics. For the example spectrum
allocation in Figure 6 a radio access point (RAP) is able
to provide wide area coverage on the basic (B) band at
860MHz. However it cannot cover the same area with the
shared extension (E) band at 3.4GHz because of the
differences in the propagation loss due to the different
carrier frequencies.
Figure 6. Possible spectrum allocation for IMT-Advanced in
a multi band deployment.
In such a scenario RNs can extend the E band
coverage to the areas of interest. The RNs do not require
a backhaul connection and an E band radio interface is
sufficient. Thus the RNs are less complex and cheaper
than adding additional BSs. The RNs in the E band do not
have to provide ubiquitous coverage but they should
cover most of the area to make the high capacity E band
available for most of the UT.
The BS uses the MBS for load balancing between the
5.1. Case Study
We study the performance difference with and without
RN in the E band for the scenario presented in Figure 4.
The BSs are equipped with both B band and E band radio
interface. The Inter-Site Distance is 3km, and the BS can
provide the basic coverage for the B band at 860MHz
using the pathloss model in [15].
Each BS sector has 6 RN to extend the coverage area
of the E band at 3.4GHz, two of them are evenly
distributed on a circle around the BS with a radius of
500m and the other 4 are on a circle with a radius of
1000m as illustrated in Figure 7. We assume a line-of-
sight (LOS) link between RN and BS and for the BS-UT
and RN-UT links we assume a non-LOS link. The
corresponding channel and pathloss models can be found
in [16]. The BS transmit power is 43dBm per sector and
the RN transmit power is 37dBm. 2000 UTs move in the
area at a speed of 3km/h and the scenario contains no
exclusion zone.
Table 1 compares the average cell throughput of the
two center cells. Adding the E band to the B band at the
BS increases the cell throughput seven fold. However, the
high capacity E band is not available for most of the users
Extension band with 90MHz
bandwidth at 3.4GHz in the C-band
(shared access)
Basic band with 10MHz
bandwidth at 860MHz
(guaranteed access)
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
in the cell and the increase does not fully scale with the
increase in bandwidth. When using RNs to extend the E
band coverage the throughput almost doubles. Further,
the high capacity E band is available to most of the users
in the cell.
Table 1. Cell throughput comparison with/without E band
and RN.
Scenario Average cell throughput
Only B band BS 22
BS with both bands 150
BS with both bands
and RN with E band
For quality-of-service support the BS has to take
additional criteria into account in the presence of relays,
when deciding which data packets should be sent on the
basic band and which packets on the extension band:
Services with low delay requirements should be
scheduled on the band/link that requires fewer hops
(this will mainly be the B band as it has the better
propagation conditions and therefore a wider
coverage area).
High speed users with an E band served by RNs
should be transferred to the basic band (RN will
have smaller coverage area and this policy will
reduce the number of missed packets, because the
UT has left the coverage area of the RN).
Figure 7. Each sector of Figure 4 is augmented with 6 RN.
The BSs transmit on both B band and E band, whereas the
RNs only transmit on the E band.
5.2. Relays with MBS
So far we have presented the multi-band operation in
RECs for RNs that operate only in the E band. However
some of the RNs might be equipped with both a B band
and an E band radio interface.
Figure 8 illustrates the case where the RN closest to
the BS is equipped with an MBS and therefore packets
that should be transmitted on the B band to the UT by the
RN can be received on the E band. Typically the E band
offers higher capacity than the B band. Thus, it is
beneficial to use the E band on the BS-RN link, even if
the RN serves the UT on the B band. Here the MBS
allows balancing the load of the two bands on the link
between the BS and the RN.
To investigate the potential benefits of RNs equipped
with an MBS and a B band radio interface for the
spectrum allocation in Figure 6 we study the coverage for
indoor users in the scenario presented in Figure 9. Each
BS is equipped with two sectors and they form together
with 3 RN in the same street a REC. The sectors to the
right and down have 2 RNs wheras the RN closer to the
BS is equipped with an MBS. The other RNs have only
an E band radio interface.
Figure 8. Relay enhanced cell with B band at lower
frequency and E band at higher frequency. BS/RN with
MBS can decide on which band to transmit packets. RN
without MBS receive and forward packets only on extension
Figure 9. Relay based deployment in the Manhattan grid.
The closest RN right of the BS in horizontal streets and
down from the BS in vertical streets is equipped with a B
and E band radio interface and an MBS.
Table 2 compares the coverage area of the different
bands for this scenario. The coverage area has been
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
calculated using the pathloss models in [16]. In particular
we applied the B1 LOS model for points in the same
street than the RAP (BS or RN) and the B1 NLOS model
for points in different streets. Inside the building blocks
we use the B4 outdoor-to-indoor model, whereas we
assume an indoor pathloss of 0.5dB/m for the E band (as
specified by the model) and 0.3dB/m for the B band to
take into account the lower indoor propagation losses at
860MHz than at 3.4GHz. In both cases the BS and RN
use a transmit power of 30dBm and the BS is equipped
with a 120 degree sector antenna having 11dBi antenna
gain, whereas the RN is equipped with an omni-
directional antenna and an antenna gain of 7dBi,
following the assumptions in [17].
Table 2. Area with a spectral efficiency higher than 1bps/Hz.
E-band 60%
B band without RN 68%
B band with RN 83%
Figure 10. Coverage of B band with BS and part of the
RN having an B band interface.
Figure 11. Coverage of the B band with only BS having a B
band interface.
The E band can only provide a spectral efficiency of
more than 1b/s/Hz in 60% of the area even though every
RAP is equipped with an E band interface. In contrary,
the BS alone can already provide this spectral efficiency
for the same area on the B band. The coverage area can
be further increased to 83% by equipping one third of the
RNs with a B band interface.
The coverage for the B band in the center area of the
scenario in Figure 9 is illustrated in Figure 10 and Figure
11 for the case when only BS have a B band radio
interface and for the case where additionally one third of
the RNs are equipped with a B band radio interface,
respectively. This comparison clearly shows that the B
band should be available at the RN as well to provide
coverage. However, due to the lower bandwidth of the B
band, the B band should only be used to serve UT that
cannot be served on the E band but not for forwarding
data to the RN. Therefore, the RN should be equipped
with an MBS to be able to receive data on the E band and
forward it on the B band to UTs that it serves on the B
On the other hand, for relay deployments with more
than two hops the BS might be able to reach RNs via one
hop on the B band and via multiple hops on the E band.
In this case, it will be beneficial for delay sensitive traffic
to send data on the B band to the RN, which forwards it
then to the UT.
5.3. ARQ in relay network with MBS
Another important aspect of a REC is the handling of
the outer ARQ (E2E ARQ) between BS and UT,
sometimes also referred to as relay ARQ [8]. Without an
outer ARQ between BS and UT, the BS does not know
whether data sent to the RN is successfully transmitted to
the UTs. Thus, in case of handovers, data that is still in
the buffer of RNs might be lost, even if the handover
destination is within the REC. Additionally, an outer
ARQ (E2E ARQ) process is used on each hop to recover
from residual inner ARQ (HARQ) errors, caused for
example by a NACK that is misinterpreted as an ACK. A
detailed description of the ARQ handling in RECs can be
found in [8].
For relay deployments with more than two hops, an
MBS offers additional degrees of freedom. Even though
the BS initiates a handover to a RN in the E band, the BS
or another RN with MBS might still be able to serve the
UT on the B Band. In this case, outer ARQ
retransmissions of delay sensitive traffic can be
performed on the B band by these nodes.
5.4. MBS and cooperative relaying
Next to single path relaying, cooperative relaying can
be integrated as an add-on to single path relaying as
proposed for example in [8]. A receiving node combines
signals from more than one transmitting node.
Cooperative relaying and intelligent deployment reduce
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences. 2008; 1:1-103
the total cost of a multi-hop network by reducing the
number of relays needed for a given performance [8].
Thus, it is important to show, that the MBS concept fits
well with cooperative relaying. In particular we discuss it
using the cooperative relaying concept in [8].
Multi-hop diversity is one example, in which a
receiving node combines the signals received from
previous nodes in the path. In a 2-hop DL path, the UT
combines the signal from a RN with the signal from the
BS that can be received on different resources. MIMO
cooperative relaying is another example, where the BS
first transmits the data to be forwarded in a cooperative
transmission to the RN. Then the BS and the RN antennas
form a virtual antenna array and perform a joint MIMO
transmission on the same resources.
In both cases the BS allocates the resources for all the
cooperative transmissions, i.e. even in a case where the
RN is equipped with an MBS, the BS also decides on
which band and resources the cooperative transmissions
will take place.
For the multi-hop diversity case, cooperative
transmissions could in principle be scheduled on two
different bands requiring the UT to be associated with
two bands at the same time which increases the UT
complexity and power consumption. Thus, cooperative
transmissions on different bands should be avoided and
the BS has to balance the gain from utilizing cooperative
relaying and from using different bands on the first and
second hop. However, as described above, by using the
HARQ context transfer, retransmissions can be performed
on another band and additional diversity gain can be
obtained without requiring the UT to operate
simultaneously on two bands.
This is not an issue in the MIMO cooperative relaying
case, where the BS can send for example data on the E
band and then perform a joint MIMO transmission on the
B band, whereas the UT only receives on the B band.
6. Conclusion
A new concept called Multi-Band Scheduler (MBS)
for future communication systems was introduced. This
scheduler allows for operation on multiple bands in a
delay constrained environment. The proposed tight
integration between multiple bands enables a fast and
seamless switch between different bands. The Multi-Band
scheduler also ensures that the PHY and MAC layer
operation is abstracted from the higher layers. This means
that the higher layers are not aware of the actual resources
used, but only of the available capacity. The fast switch
between multiple bands adds additional degrees of
freedom for optimizing the network operation. Moreover,
the MBS can be utilized to efficiently balance the load of
the bands in the network or to provide required quality of
service levels to the UTs.
The operation of the MBS was discussed in detail for
two scenarios: Spectrum sharing and relays.
The spectrum sharing case comprises a multiband
operation with a smaller band dedicated to the
communication system and an extension band that is
shared with another system but offers higher capacity. In
this scenario the MBS can be applied in two ways, on the
one hand the MBS enables simultaneous access to a
guaranteed basic band and the band shared with another
technology. On the other hand the MBS can be used for
sharing spectrum between different operators of the same
technology where a part of the band is dedicated to each
operator and the rest of the band is shared between the
operators. Our simulation results show that using the
MBS, a seamless switch can be made if the shared band is
no longer available.
The relay case illustrates a scenario with different
propagation properties and thus different coverage area of
the used bands. In this scenario RNs are used to extend
the coverage of the high capacity extension band that has
a higher propagation loss than the basic band. Our
simulation results show that RNs are an effective way to
balance the coverage of the bands. Thereby they greatly
increase the overall capacity of the network and the high
capacity band is available to most of the user terminals.
Further, the MBS at both the BS and the RN enables to
balance the network load on each hop by utilizing
different bands.
7. Acknowledgement
The authors would like to thank Kennett Aschan for
his valuable support and fruitful discussions during the
preparation of this article.
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