1. Relays in Heterogeneous Networks
Submitted by:
Arjun Bhatia
Girish Anand
Rajesh Gupta
Shivani Bhatia
Sunanda Mukundan

2. Table of Contents Introduction Heterogeneous Networks Relays
Implementation of Relays
Types of Relays
Implementation Strategies
Handover
Deployment Challenges
Interference Management
Economic Benefits
References

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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.

13. Relay Types Conventional Amplifiers : Relays are of two types :
Decode and Forward
Conventional Amplifiers :
Used as gap fillers. Also known as repeaters

14. Benefits: Limitations: 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

15. 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

16. Implementation Strategies
One-way relays
Two-way relays
Shared relays

17. Two way relay

18. Shared Relaying

19. TDD and FDD Relaying
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.

20. Fixed versus Mobile/Movable (Terminal) Relaying
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

21. 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.

22. 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.

23. 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.

24. 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.

25. Limitations : Which one is a better approach ????
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 ????

26. 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.

27. Centralized V/s Ditributed Relays
Centralized relay pairing
Distributed relay pairing
In this scheme the BS will act as 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.
In this scheme, RS collects the 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 N- slots 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.

28. 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)

29. Centralized Handover

30. Handover in Relay Enhanced LTE Network
Distributed/Decentralized Relaying
Source RN initiates the handover. Both the RN and eNBr’s work in collaboration to 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

31. Decentralized/Distributed Handover

32. 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

33. 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

34. 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

35. 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 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

36. 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 x) than the BS-to-user links ( the licensed spectrum), yielding significant gains from load balancing through relay

37. Lower Capex and Opex Wireless backhaul. Lower site acquisition cost
Less costly antenna structure
Low cost and complexity of relay
Faster deployment

38. 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.

39. References:
LTE Advanced - Stanford Networking Seminar
Heterogeneous Networks - Prof. Robert W. Heath Jr.
› Cellular telecoms
/proc/VTC09Spring/DATA/ PDF

40. Thanks