Hence the wireless operators are looking to enhance their macro-cell coverage with the help of micro-cell coverages(indoor) by deploying small base stations such as Femtocells or with the help of Relay technology. These miniature base stations are the size of a DSL router or cable modem and provide indoor wireless coverage to mobile phones using existing broadband Internet connections.
Pointing out some key advantages of Femtocells and Relays we will then focus on their adoption in advanced wireless networks(WiMAX and LTE)
Low Cost: The Business Model would be initially by offering Femtos as a consumer purchase through mobile operators
Low Power: around 8mW- 120 mW lower than Wi-Fi APs.
Easy to Use: Plug-and-Play easily installed by consumers themselves
Compatibility & Interoperability: Compatibility with UMTS,EVDO standards and WiMAX,UMB & LTE standards
Deployment: In Wireless Operator owned licensed spectrum unlike WiFi
Broadband connected: Femtocells utilize Internet protocol (IP) and flat base station architectures, and will connect to mobile operator networks via a wired broadband Internet service such as DSL, cable, or fibre optics.
Customer’s point of view:
Increased Indoor Coverage: Coverage radius is 40m – 600m in most homes providing full signal throughout the household
Load sharing: Unlike in macro cells which supports hundreds of users, Femtos will support 5-7 users simultaneously enabling lesser contention in accessing medium delivering higher data rates/user.
Better Voice Quality: As the users will be in the coverage envelope and closer to Femtos, they will definitely be supported with a better voice and sound quality with fewer dropped calls
Better Data/Multimedia Experience: It will deliver better and higher data performance with streaming musics, downloads and web browsing with lesser interruptions and loss of connections compared to a macro-cell environment.
Wireless Operator’s point of view:
Lower CAPEX: Increased usage of femtocells will cut down huge capital costs on macro cell equipments & deployments. This includes costs savings in site acquisitions, site equipments, site connections with the switching centers.
Increased network capacity: Increased usage of femtocells will reduce stress on macro cells increasing overall capacity of mobile operators
Lower OPEX: With lesser macro cell sites it reduces the overall site maintenance, equipment maintenance and backhaul costs.
Newer Revenue Opportunities: With provision of excellent indoor coverage and superior user experience with voice and multimedia data services operators has an opportunity of raising its ARPU with more additions to family plans
Reduced Churn: Due to improved coverage, user multimedia experience and fewer dropped calls, will lead to a significant reduction in customer churn
Spectrum: Femtocells works on licensed spectrum and as the spectrum is the most expensive resource it will be a major technical hurdle for the wireless operator for frequency planning.
RF Coverage Optimization: Radio tuning and optimization for RF coverage in macro cells is manually done by technicians which is now not possible at each femtocell level, henceforth self optimization and tuning over time according to the indoor coverage map has to be done either automatically or remotely which is a technical challenge.
RF Interference: Femtocells might be prone to femto-macro interference and also femto-femto interference in highly dense macro or micro environments which might affect the user experience.
Automatic System Selection: When an authorized user of a femto cell moves in or out of the coverage of the femto cell – and is not on an active call – the handset must correctly select the system to operate on. In particular, when a user moves from the macro cell into femto cell coverage, the handset must automatically select the femto cell, and visa versa.
Handoffs: When an authorized user of a femto cell moves in or out of coverage of the femto cell – and is on an active call – the handset must correctly hand off between the macro cell and femto cell networks. Such handoffs are especially critical when a user loses the coverage of a network that is currently serving it, as in the case of a user leaving the house where a femto cell is located.
Security & Scalability: A femto cell must identify and authenticate itself to the operator’s network as being valid. With millions of femto cells deployed in a network, operators will require large scale security gateways at the edge of their core networks to handle millions of femto cell-originated IPsec tunnels.
Femto Management: Activation on purchase and plug and play by end user is an important step and with a proper access control management allowing end-user to add/delete active device connections in the household. In addition, operators must have management systems that give first-level support technicians full visibility into the operation of the femto cell and its surrounding RF environment.
Relay transmission can be seen as a kind of collaborative communications, in which a relay station (RS) helps to forward user information from neighboring user equipment (UE)/mobile station (MS) to a local eNode-B (eNB)/base station (BS). In doing this, an RS can effectively extend the signal and service coverage of an eNB and enhance the overall throughput performance of a wireless communication system. The performance of relay transmissions is greatly affected by the collaborative strategy, which includes the selection of relay types and relay partners (i.e., to decide when, how, and with whom to collaborate).
Relays that receive and retransmit the signals between base stations and mobiles can be used to effectively increase throughput extend coverage of cellular networks. Infrastucture relays do not need wired connection to network thereby offering savings in operators’ backhaul costs. Mobile relays can be used to build local area networks between mobile users under the umbrella of the wide area cellular networks
Increased Coverage: With multi-hop relays the macro cell coverage can be expanded to the places where the base station cannot reach.
Increased Capacity: It creates hotspot solutions with reduced interference to increase the overall capacity of the system.
Lower CAPEX & OPEX: Relays extending the coverage eliminates the need of additional base stations and corresponding backhaul lines saving wireless operators deployment costs and corresponding maintenance costs. The relays can be user owned relays provided by operators and can be mounted on roof tops or indoors.
Better Broadband Experience: Higher data rates are therefore now available as users are close to the mini RF access point.
Reduced Transmission power: With Relays deployed there is a considerable reduction in transmission power reducing co-channel interference and increased capacity.
Faster Network rollout: The deployment of relays is simple and quickens the network rollout process with a higher level of outdoor to indoor service and leading to use of macrodiversity increasing coverage quality with lesser fading and stronger signal levels.
As a hot research topic with great application potential, relay technologies have been actively studied and considered in the standardization process of next-generation mobile communication systems, such as 3GPP LTE-Advancedand IEEE 802.16j (multihop relays for WiMAX standards).
Pairing Schemes for Relay Selection
One of the key challenges is to select and pair nearby RSs and UE units to achieve the relay/cooperative gain. The selection of relay partners (i.e., with whom to collaborate) is a key element for the success of the overall collaborative strategy. Practically, it is very important to develop effective pairing schemes to select appropriate RSs and UE units to collaborate in relay transmissions, thus improving throughput and coverage performance for future relay-enabled mobile communication networks.
This pairing procedure can be executed in either a centralized or distributed manner. In a centralized pairing scheme, an eNB will serve as a control node to collect the required channel and location information from all the RSs and UE units in its vicinity, and then make pairing decisions for all of them. On the contrary, in a distributed pairing scheme, each RS selects an appropriate UE unit in its neighborhood by using local channel information and a contention-based medium access control (MAC) mechanism. Generally speaking, centralized schemes require more signaling overhead, but can achieve better performance
Relay Transmission Schemes
Many relay transmission schemes have been proposed to establish two-hop communication between an eNB and a UE unit through an RS
Amplify and Forward — An RS receives the signal from the eNB (or UE) at the first phase. It amplifies this received signal and forwards it to the UE (or eNB) at the second phase. This Amplify and Forward (AF) scheme is very simple and has very short delay, but it also amplifies noise.
Selective Decode and Forward — An RS decodes (channel decoding) the received signal from the eNB (UE) at the first phase. If the decoded data is correct using cyclic redundancy check (CRC), the RS will perform channel coding and forward the new signal to the UE (eNB) at the second phase. This DCF scheme can effectively avoid error propagation through the RS, but the processing delay is quite long.
Demodulation and Forward — An RS demodulates the received signal from the eNB (UE) and makes a hard decision at the first phase (without decoding the received signal). It modulates and forwards the new signal to the UE (eNB) at the second phase. This Demodulation and Forward (DMF) scheme has the advantages of simple operation and low processing delay, but it cannot avoid error propagation due to the hard decisions made at the symbol level in phase one.
Comparison between 3GPP LTE Advanced and IEEE 802.16j RSs
Below shows comparison between Type I(3GPP- LTE Advanced) and Non-Transparency(IEEE -802.16j) RSs
Practical issues of cooperative schemes like signaling between relays and different propagation delays due to different locations of relays are often overlooked. If the difference in time of arrival between the direct path from source to destination and the paths source-relay-destination is constrained then relays must locate inside the ellipsoid as depicted below. Thus, in practice, such a cooperative system shoiuld be a narrow band one, or guard interval between transmitted symbols should be used to avoid intersymbol interference due to relays.
In band relays consume radio resources and Out of band relays need multiple transceivers.