Submitted by:
Arjun Bhatia
Girish Anand
Rajesh Gupta
Shivani Bhatia
Sunanda Mukundan
Table of Contents
 Introduction
 Heterogeneous Networks
 Relays
 Implementation of Relays
 Types of Relays
 Implementation Strategies
 Handover
 Deployment Challenges
 Interference Management
 Economic Benefits
 References
Path towards LTE Advanced
The very high data rates and large coverage area envisioned for 4G wireless systems in
reasonably large areas do not appear to be feasible with the conventional cellular
architecture as :
• They do not achieve the expected throughput to ensure seamless mobile broadband in
the uplink as users move far from the base station
• Increase in the inter-cell interference, as well as constraints on the transmit power of the
mobile devices
• Poor indoor penetration and the presence of dead-spots, which results in drastically
reduced indoor coverage
Solution
: To address these issues, there has been an increasing interest to deploy
relays, distributed antennas and small cellular access points in residential homes, subways
and offices.
PS: Deployment of more number of BTS requires cost and planning
Heterogeneous Networks
Network architectures with relays, picocells and femtocells overlaying the macrocell
network are commonly referred as heterogeneous networks
• All these nodes have different characteristics of RF power and coverage area
• Nodes with large RF coverage area are deployed in a planned way to provide coverage in
urban, suburban and rural areas
• For small coverage area, nodes provide coverage extension or throughput enhancement.
• By shrinking coverage to smaller blocks, the capacity can be shared with a fewer number
of people, resulting in higher capacity and faster data speeds
Relay
A solution for radio range extension in mobile
and wireless broadband cellular networks
•Time-division multiple access (TDMA)-based systems This scheme of relaying,
allows for easy allocation of resources to the mobile-to-relay and relay-to-mobile links.
The first system based on time-division multiplex (TDM) and relays connecting mobiles to
the fixed network was proposed in 1985
• Code-division multiple access (CDMA)-based systems Relaying in cellular
CDMA systems has been investigated by Zadeh et al
• FDD based relay systems Uplink and Downlink are separated using frequency
division duplex (FDD), as is done in IS-95 and Universal Mobile Telecommunications
System (UMTS) terrestrial radio access (UTRA) FDD
• Fixed relays (called wireless BSs) for cordless systems The European
Telecommunications Standards Institute/Digital Enhanced Cordless Telephony ( E T S I
/DECT ) standard in 1998 was the first specifying fixed relays (called wireless BSs) for
cordless systems using TDM channels for voice and data communications
RELAY NODES
• Relays are expected to be a cost efficient way to fulfill requirements on high data
rate coverage in next generation cellular networks, like LTE-Advanced.
• Low-cost low transmit power base stations using the same spectrum as backhaul
and access. And have similar power as Picos.
• Enhance performance at the cell edge for a comparatively low cost.
• Include peak data rates of 1 Gbps in downlink (DL) and 500 Mbps in uplink
(UL), bandwidth scalability up to 100 MHz
Why Relays are Required ???
•Increased broadband Traffic : Increased traffic due to new applications
• Increased capacity : Number of mobile users with high-end smart phones accessing
social networking, video and VoIP are increasing
• Provide High data rate : Path towards LTE- advanced, common convergence network
• Easy Installation : Het-Nets are being deployed initially in high data traffic areas using
street-level plug-and-play nodes
• Low Cost : Relays can be even more cost-effective than a WLAN access point, as the
former does not require a wired backhaul connection
• Low Opex cost : Reducing operating expenses such as tower leasing and maintenance
costs for the service provider
• Work well in Non Line-of-sight operation : The spectrum that will be released for 4G
systems will almost certainly be located well above the 2 GHz band. Therefore the radio
propogation in these bands is significantly more vulnerable to non LOS conditions.
Advantages of Relays
•Transmission power of user equipment and base stations can be reduced
• More radio resources can be allocated to a single user – Hence improved capacity
• Spectrum efficiency per meter square would be improved
• Higher mobility and Quality of service to the end users
• Relaying is presented as a means to reduce infrastructure deployment costs
• Exploitation of spatial diversity, multihop relaying can enhance cellular capacity –
possible to connect BTS and relay at the same time
• Networks applying relaying via fixed infrastructure do not need complicated
distributed routing algorithms, while retaining the flexibility of being able to move the
relays as the traffic patterns change over time
• Solutions to combat shadowing at high radio frequencies
Relays: Network Improvement
 Increase network density : LTE relay nodes can be deployed very easily in
situations where the aim is to increase network capacity by increasing the
number of eNBs to ensure good signal levels are received by all users. LTE
relays are easy to install as they require no separate backhaul and they are
small enabling them to be installed in many convenient areas, e.g. on street
lamps, on walls, etc.
Relays: Network Improvement
 Network coverage extension : LTE relays can be used as a convenient
method of filling small holes in coverage. With no need to install a complete
base station, the relay can be quickly installed so that it fills in the coverage
blackspot.
Relays: Network Improvement
 LTE relay nodes may be sued to increase the coverage outside main area.
With suitable high gain antennas and also if antenna for the link to the donor
eNB is placed in a suitable location it will be able to maintain good
communications and provide the required coverage extension.
Relays: Network Improvement
 Rapid network roll-out: Without the need to install backhaul, or possibly
install large masts, LTE relays can provide a very easy method of extending
coverage during the early roll-out of a network. More traditional eNBs may be
installed later as the traffic volumes increase.
Relay Types
Relays are of two types :
• Conventional Amplifiers
• Decode and Forward
Conventional Amplifiers :
Used as gap fillers. Also known as repeaters
Benefits:
•Amplify the desired signals, like repeaters
•Perform better than DF relays near the receiver
Limitations:
• Analog repeaters increase the noise level and suffer from the danger of instability
due to their fixed gain.
• Also amplify both the noise and interference
• Suffer from Loop Interference, i.e. leakage of transmit signals to receive antenna
• Concurrent transmission and reception at the same frequency band requires two
separated antennas in the relay
Decode and forward
• Remove the noise by decoding the received signals and then regenerating
and re-encoding the signal to be forwarded to the destination
• Adaptive Decode and Forward Relay: The relay forwards the signal to the
destination only if it is able to decode the signal correctly
• More attractive in both cell middle and edge deployments
• Rely on full decoding at the relay, hence cannot achieve full diversity
Implementation Strategies
• One-way relays
• Two-way relays
• Shared relays
Two way relay
Shared Relaying
TDD and FDD Relaying
 f , t3 
 f , t4 
 f , t2 
 f , t1 
eNB
 f 2 , t1 
 f1 , t2 
 f1 , t1 
FDD Relaying
R-UE
DL
TDD Relaying
 f 2 , t2 
eNB-R
UE-R
R-eNB
UL
eNB-R
R-UE
DL
R-eNB
UE-R
UL
eNB
f
t
•
•
TDD relaying suffers from link budget limitation because only a single node
transmits at a given time.
In FDD relaying, two nodes can transmit simultaneously but the scheme does
not allow power and resource sharing between eNB-Relay and Relay-UE links.
Fixed versus Mobile/Movable
(Terminal) Relaying
Relaying through Mobile Station
In a cellular network with mobile relay station, the nonactive MS (i.e. in idle state) are
potential candidate to relay the traffic of the active MS to the BS.
PICTURE
Benefits:
• Low deployment/maintenance cost, since other user’s terminals can potentially act
as relays
• The MS are able to organize themselves in order to cover some unknown dead
spots, which are difficult to predict with the operators planning tool
• They can also be used where it is not cost effective to install FRS such as
mountainous environment or subway train platform.
• They can also be used at unpredictable places like accidents or at infrequently
occurring events like demonstrations or sports event
• If there are a large number of idles MS, there are more choice to select a MRS that
can optimize the system performance. For example, a relayed user can choose a MRS
with whom it experiences a LOS link.
Limitations:
• Relaying opportunity depends strongly on the user’s density
• Relaying through other users terminals can considerably decrease the battery life of the
mobile relay station
• The relaying system performance is highly dependent of the RS selection scheme
• The signaling (new channel MS-RS, inter-relay handover, RS selection) required for the
relaying system might increase considerably
• The cost of the terminal to support relaying (hardware and software) will increase
- the implementations at the hardware become even more difficult if a MRS needs to relay
more than one MS’s traffic at the same time.
• Due to the mobility of the relayed MS and the MRS, frequent inter-relay handoffs might
occurs, which might increase the signaling and affect the relaying system performance.
• Some others issues such as fast fading, power control or security need more investigations
in order to make possible the communication between MSs.
Fixed Station Relay :
Fixed station relay (FRS) are the low cost station placed at specific location. FRS are part
of the network infrastructure, therefore their deployment will be an integral part of the
network planning, design and deployment process.
Benefits :
• Operators will have better control of coverage and capacity expected in a specific area
• The FRS can be deployed at strategic locations in order to maintain a LOS with the BS.
They can also use directional antenna to improve the propagation link with the BS
• The FRSs are less constrained by energy consumption. They may potentially be
equipped with more advanced hardware, which enable them to operate in any frequency
band as well as allow them to relay several MS at a time.
• The use of FRS eases the problem of RS selection since fewer number of FRS are
deployed compared to MRS. Therefore the signaling overhead required for the RS
selection will not be a major issue when relaying with FRS.
• The inter-relay handoffs will occur only when the relayed MS will move from one FRS
to another FRS.
• Due to their sophisticated hardware, it is more secure to relay through a FRS than MRS.
The data are always transferred through a known (fixed) RS.
Limitations :
• Infrastructure’s cost The dimensioning, planning, optimization and maintenance of
the FRS can be expensive and cost inefficient
• Furthermore it might be cost ineffective to install FRS if there are many sparse
coverage holes, in which only a few mobile terminals are located
Which one is a better approach ????
Which one is a better approach ?
Although, it has been observed that under certain conditions the uplink capacity
gain of 35% is readily achievable for both form of relay stations. But relaying with
MRS is a more challenging network than FRS. Thus before a relaying system with
MRS can be realized, many issues need to be investigated like :
• allowing the MRS’s battery to be used for someone else’s call
• increase of the signaling overhead
• additional hardware and software will increase the terminal’s cost
• frequent inter-relay handoffs.
Centralized V/s Ditributed Relays
Centralized relay pairing
Distributed relay pairing
 In this scheme the BS will act as
 In this scheme, RS collects the
a control node and collects the
channel and location
information from all the RSs and
SSs and then make the pairing
decision. This information must
be formed as a service set and
periodically updated in the local
BS to capture dynamic changes
of SSs This scheme requires
more signaling over head, and
can achieve better performance
gains.
channel and location information
from all the nearby SSs and then
makes the pairing decision. First
each RS identify its service set of
neighborhood SSs and also the
channel conditions between its BS
as well as its SS, those RS with
single service set each randomly
selects a time slot from the Nslots in the pairing scheme. If
multiple RS choose the same time
slot then collusion occurs and
those RS will be trying again in
the next pairing scheme.
Handover in Relay Enhanced LTE Network
Centralized Relaying Handover:
 Source Relay Node and Target Relay Node added to the existing network elements
 Handover initiated by the source eNBr. The source eNBr controls the source RN and the
target eNBr controls the target RN
 The UE is connected to the source eNBr via the source RN
 Measurement reports are sent to the source eNBr to make the decision. Decision depends
upon the UE’s measurements whether a handover must be done or not
 Handover request is then sent over to the target eNBr, which checks whether an admission
of the UE is made with the target RN
 An acknowledgement is sent to the source eNBr and subsequently to the source RN
 Any buffered downlink packets must be allocated to the target RN via the target eNBr by
the source eNBr
 Hence, synchronization between the UE and the target RN is established and information
regarding Timing Advance for the UE is sent on the uplink
 The handover is confirmed by the UE to the target RN and the target RN confirms the
handover to target eNBr which in turn passes the information to the Gateway via the MME
 The centralized relaying suggests that the overall process is controlled by the eNBrs (both
source and target)
Centralized Handover
Handover in Relay Enhanced LTE Network
Distributed/Decentralized Relaying
 Source RN initiates the handover. Both the RN and eNBr’s work in collaboration to
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successfully conduct the handover
Target eNBr performs the admission control on the backhaul link. The target RN
receives the handover request from the target eNBr which then performs the
admission control for the relay link
Handover request is acknowledged and the handover command is sent to the UE. The
downlink data is sent to the target RN through the target eNBr
All downlink packets are buffered at the target RN and synchronization is established
with the UE. Timing Advance for the UE is performed at the uplink and the handover
confirmation message is given to the target RN by the UE
The target RN sends the confirmation message to the target eNBr which then sends
the handover complete message to the Gateway via the MME
The target eNBr receives an acknowledgement from the Serving Gateway. The target
eNBr asks the source eNBr to release resources which in turn sends a command to the
source RN, which initiated the handover to release resources of the UE
The data transfer now takes place between the target RN and the UE after the
handover has taken place
Decentralized/Distributed Handover
Relays: Deployment Challenges
 Throughput gains due to relay deployments
Uncoordinated transmission by relays lead to increase of the overall
interference levels in the cell and could be counter-productive by reducing
the signal-to-interference plus noise (SINR) levels of users in the system
 Relay placement
Uncoordinated transmission by relays may cause near line-of-sight
interference to an edge user of the neighboring cell. optimal relay placement
depends on the transmission and scheduling strategies, transmit power ,
height of the relays etc
Relays : Deployment Challenges
• Lack of good models for relaying in cellular systems
• Fairness : Designing distributed scheduling schemes to ensure fairness.
• Co Channel Interference Uncertainty : Due to the dynamic and
synchronous nature of the network-level distributed resource allocation process
in multicarrier cellular networks, the lack of interference predictability
represents a challenging problem
• Implications of relaying on routing and radio resource management
• Additional set of signaling protocols are needed in case of Layer 2 and Layer 3
relays
• Relays introduce extra delay and overhead. Increasing number of hops
introduce more complexity and overhead in the system
Advanced Interference Management
Advanced Interference Management techniques such as resource coordination are needed
to realize full benefits of heterogeneous deployments.
 Inter-cell Interference Coordination (ICIC)
Interfering base stations can coordinate on transmission powers and/or spatial beams with
each other in order to enable control and data transmissions to their corresponding user
terminals.
 Slowly-Adaptive Interference
The goal of the slowly-adaptive resource coordination algorithm is to find a combination
of transmit powers for all the transmitting base stations and user terminals — and over all
the time and/or frequency resources that maximizes the total utility of the network. ce
Management
Economic Benefits
 The brute force solution is to significantly increase the density of
base stations, resulting in considerably higher deployment costs
that would only be feasible if the number of subscribers also
increased at the same rate. This seems unlikely, with the
penetration of cellular phones already high in developed countries.
On the other hand, the same number of subscribers will have a
much higher demand in transmission rates, making the
aggregating throughput rate the bottleneck in future wireless
systems. Under the working assumption that subscribers will not
be willing to pay the same amount per data bit as for voice bits, a
drastic increase in the number of base stations does not seem
economically justifiable.
 While conventional cellular networks are assumed to have cells of
diameter 2-5 km, relays will only be expected to cover a region of
diameter 200-500 m. This means, the transmit power
requirements for such relays are significantly reduced compared to
a base station (BS). This in turn permits economical design of the
amplifier used in the relays
Thus, the costs of the backplane that serves as the interface between the BS
and the wired backhaul network can be eliminated for a relay
The relay-to-user links could use a different (unlicensed) spectrum (e.g.,
IEEE 802.11x) than the BS-to-user links ( the licensed spectrum), yielding
significant gains from load balancing through relay
Lower Capex and Opex
 Wireless backhaul.
 Lower site acquisition cost
 Less costly antenna structure
 Low cost and complexity of relay
 Faster deployment
Improve ROI
 Higher ARPU
 High grade of service
 Low incremental cost
 Compatible with GSM, WiMAX and CDMA
technologies.
 Support current and future services(LTE, IMS)
 Since consumer is always “on” therefore better service
and lower churn rate.
References:
 www.qualcomm.com/.../lte-advanced
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heterogeneous-networks.pdf
LTE Advanced - Stanford Networking Seminar
Heterogeneous Networks - Prof. Robert W. Heath Jr.
www.radio-electronics.com › Cellular telecoms
www.hindawi.com/journals/wcn/2009/618787/
202.194.20.8/proc/VTC09Spring/DATA/09-07-03.PDF
Thanks