Wireless Relays for Next Generation Broadband Networks Aniruddha Chandra, Chayanika Bose, and Manas Kr. Bose
Introduction Next generation wireless networks are envisioned to provide highly mobile, ever present, broadband data access, that too over heterogeneous radio interfaces: WiMAX (worldwide inter-operability for microwave access), WiBro (wireless broadband), BGAN (broadband global area network), LTE (long term evolution) and many more. In order to meet these conflicting demands, there has recently been an upsurge in research interest, both in industry and academia, addressing issues related to relay assisted wireless systems.
Relaying has been an age old concept for radio range extension in long-distance microwave communications. In its simplest form, a relay is a simple repeater like device that receives signal from some source, boost it up and resends to the intended destination. However, depending on the complexity, relays may take part in higher level network functionalities as detailed later.
In this article we first outline the major challenges today’s broadband networks face. We also highlight the important performance and cost benefits relay stations could potentially offer, and present some scenarios where relays are likely to be deployed. Next, different modes of relaying are presented followed by a glimpse of the current wireless technologies that uses relay. Then some interesting real-life implementation issues like, whether to use two-hop or, multi-hop relay and disadvantages of relay based systems are described before drawing the concluding remarks.
Challenges for Future Broadband Networks The principal challenges for future wireless networks are - high throughput/ capacity, wide coverage, maximum mobility, scalability and interoperability. But these demands are often incompatible in nature. For a fixed cell size, increasing data rate reduces reliability, since it is well known that for a given transmit power level, the symbol energy decreases linearly with the increasing transmission rate. On the other hand, to maintain minimum quality of service (QoS), a given signal to noise ratio (SNR) at the receiver is required (depending on the type of data service) that may be difficult to achieve at the cell edge unless the transmit power is increased, causing a reduction in effective coverage area. Limited battery life of mobile stations (MS), however, dictates the upper limit on the transmit power in the uplink. Thus the signal power cannot be increased arbitrarily to maintain the same coverage area. Mobility of the moving nodes, coverage holes, dead spots due to geographical terrain etc. all accounts for further degradation of data services.
The spectrum for 4G systems will almost certainly be located at 2-3.5 GHz, which in turn, limits the range of wireless broadband access networks and necessitates use of increased number of base stations (BSs)/ access points (APs) for the same coverage area. As per the current literatures, for good quality indoor coverage in a suburban cellular environment, 4 times as many BS is to be deployed at 2 GHz than at 1 GHz, and 10 times as many for 3.5 GHz.
Increasing the BS density (and thereby reducing the cell size) will obviously increase network capacity. This is mainly due to the increased frequency reuse, the same frequency band being repeated again and again for communication, now at a shorter distance. However, this strategy is not scalable due to several reasons. The geographical area covered by a cell has been already diminished from tens of kilometres to a few meters as we moved from macrocell to microcell to picocell and further to femtocellular domain. Not only that, with each BS there will be an additional cost for hardware equipments, antenna space and the wired backhaul. Thus adding abundant BSs is only feasible if the number of subscribers is also increased at a sufficient rate. This seems unlikely, with the penetration of cellular phones already high in developed countries. One may argue that the transmit power requirement is lesser for these pico or, femto cells. But with the growth of BSs, the increased interference (both co-channel and adjacent-channel type) would soon outweigh the benefits of limiting the signal power.
Advantages of Relaying Installation of relay stations (RSs) can help in overcoming some of the major challenges that are faced by present day wireless broadband networks. The advantages are mainly threefold: first, RSs improve overall network performance by enhancing coverage, capacity and power efficiency. Second, both implementation and maintenance cost may be lowered by incorporating relays at various stages of network rollout. Lastly, RSs may play an important role in optimizing network and higher level functions like routing, load balancing etc. The next three sub-sections describe these benefits in more detail.
Network performance improvement Fig. 1 illustrates how the use of RSs affects the two key network performance metrics, coverage and capacity. Let us focus on the coverage first. A BS/ AP, compared to a conventional one-hop system, can cater much larger cells with the help of several fixed RSs. Cooperative transmission
RS-MS link BS-RS link MS 1
RS
RS
MS 2
RS
Coverage/ radio range extension
BS/ AP Traditional direct transmission
MS 3
Capacity enhancement through replacing low rate, unreliable links with multiple high rate, reliable links
Traditional service boundary
Fig. 1: Demonstration of performance advantages through relaying
As shown in Fig. 1, the mobile station in the right (MS 1), although residing outside the traditional service boundary, may still get service if an RS is placed between the BS and mobile station. When radio range is not an issue, better radio coverage inside the existing cell may be realized, especially in areas heavily shadowed from a BS/ AP (urban indoor coverage holes, or, rural coverage holes created by foliage, rough terrain).
Of course nothing comes for free and the price we have to pay in this case is reduced capacity and increased latency. Relays generally work in store and forward mode. For a two-hop system in the downlink (uplink) the signal through BS-RS (MS-RS) is stored in relay and in the next time slot is forwarded through RS-MS (RS-BS) link. The network capacity falls as MS-RS and BS-RS links operate at different times using same wireless band (in-band relaying).
The capacity advantage with relaying comes mainly from the exploitation of path or, site diversity. To exemplify the fact let us take another look to Fig. 1, where the MS 2 is served by two RSs simultaneously. Wireless links are subjected to fading (path loss and shadowing), resulting in loss of information and frequent link failure/ outage. If two or, more alternate paths are available via different RS then the MS can switch to the RS having the strongest signal. As fading occurs independently in different paths, it is very unlikely that signal will experience deep fading in all the paths at a given point of time. It is important to understand that this capacity increment is feasible only if the BS and RS employ orthogonal channels, i.e. BS-RS and RS-MS links are operating at the same time but at different frequency bands (also known as out-of-band relaying). As it is difficult to
install multiple antennas in a small mobile unit, cooperative relaying with simultaneous transmission through RSs may provide alternative to conventional MIMO (multiple input multiple output) techniques. There is yet another possibility of capacity gain through frequency reuse if the RSs form smaller cells within the cell served by main BS.
Relays, when employed in the uplink path, replace long-range high-power transmission to a BS with short-range low-power relay transmissions. This enhances the lifetime of battery-driven MSs. As far as power consumed by RSs is concerned, they require only mains supply. To save energy, relays could even rely on solar power supply.
Cost benefit The RS concept opens up the possibility of installing temporary coverage in areas where permanent connections are not needed (e.g. construction sites, conference/ meeting rooms). In case a fast initial network roll-out has to be performed, relay based solutions are economically more attractive compared to traditional BSs networked via wired backhaul. RSs enable lesser capital expenditures (CapEx) in terms of infrastructure hardware as cellular networks with relays allow low-height antennas, low-gain amplifiers and relay nodes do not need a copper/ fibre wired backhaul. Also RS cell sites are likely to be less expensive than BS cell sites, smaller in size and hence, allow greater flexibility in site selection. Further, with relays operating expenditures (OpEx) such as tower leasing and maintenance costs are trimmed.
Once a network has been set up, the upgradation required for increased number of subscribers or, time varying subscribers may be performed by replacing some of these RSs with BSs in a gradual manner. This approach provides a way out to many of the network operators to sustain their business at a time of great worldwide recession.
Other advantages Apart from the performance and cost benefits, introduction of relays offer several other advantages too. By regulating traffic from congested (hot) to non-congested (cool) cells in unlicensed frequency bands, relaying reduce call blocking probability and improve quality of service (QoS) of end users. This kind of load balancing between the neighbouring cells, cell A and cell B, has been depicted in Fig. 2. To understand the situation better, let us assume that all the MS are residing in the service area allotted to BS A and there is only a single frequency available for communication between MS and BS. Now if both MS 1 and MS 2 want to make a call but MS 1 takes hold of the common channel first then the call from MS 2 is blocked. However, the blocked call may be completed through BS B if MS 2 can forward the call through a nearby RS (controlled by BS B), provided that BS B is currently not engaged. This method is known as primary relaying. Cell A
Cell B Primary relaying
MS 2 RS MS 1
BS A
BS B
MS 3 Secondary relaying
Fig. 2: Demonstration of load balancing with relays
A second kind of relaying, known as secondary relaying, may also be realized in the following manner. Suppose MS 3 is currently communicating through BS A, MS 1 also expresses its willingness to enter the network, and MS 2 is idle. As there are no RS in the vicinity of MS 1, it is not possible to execute primary relaying. To maximize resource utilization, MS 1 may be allotted the direct line through BS A and MS 3 may establish a relayed communication link to BS B.
Some other advantages of relaying include integration of heterogeneous wireless networks, ad hoc relaying to enhance multicast throughput, and above all to provide stability to networks with an ever increasing subscriber base.
Application Scenarios There are a multitude of possible applications for relays as demonstrated in Fig. 3. Depending on the wireless environment, these applications can be classified into three major categories: fixed, nomadic, and mobile. Involved RS nodes may be either standalone devices (fixed infrastructure mode) or simply normal MSs that are idle (ad hoc mode) for the time being.
Fixed Relay Station (F-RS) Fixed-infrastructure relays, like BSs, may be deployed by the service provider/ end users to improve coverage, reliability, or per user throughput in areas not sufficiently covered (in-building, shadowed regions like the valley between high risers, tunnels, underground metro rail, subways, or, at cell edge), or in densely populated areas (railway terminus, bus stand, amusement park, stadium). In rural areas, where the traffic density is low and the population is sparsely distributed, it may not be economically viable to build traditional cellular networks with full-fledged BSs. Rather, a more efficient and flexible architecture would be a single BS served by many RS nodes.
Coverage extension at cell edge M-RS
Damaged BS Earthquake
Public Transport
N-RS
F-RS
Race Track
Disaster recovery/ Hot-spot MMR-BS
Shadow of buildings F-RS
Fixed RS (F-RS)
F-RS
Nomadic RS (N-RS) Stadium
F-RS
F-RS
Mobile RS (M-RS) F-RS
Rural Area
Fig. 3: Applications of fixed, nomadic and mobile relaying
Nomadic Relay Station (N-RS) Broadband networks are often faced with the challenge of time-varying unbalanced traffic in hot-spots. Hot-spots mostly occur due to events where a large group of people are densely packed into a small area (fair, exhibition, or downtown areas on Monday morning). Since the locations of hot spots vary from time to time, it is difficult, if not impossible, to provide the guarantee of sufficient resources in each cell in a cost-effective way. Low-power, low-complexity portable nomadic relays may be employed to enhance coverage required only for the duration of a particular event. These kind of temporary relays also can be deployed in emergency/ disaster recovery situations (natural calamities like flood, earthquake, cyclone, or for damaged BS). The idea of nomadic relays basically imitates the well known self-organizing, infrastructure-free, ad-hoc approach, where MSs establish connections in a distributed peer-to-peer fashion and dynamically adapts to the fast-changing wireless environment.
Mobile Relay Station (M-RS) A mobile RS can be mounted on a vehicle (bus, train, or, ferry) where several people are located very closely together, and the vehicle is moving quickly through cells. The RS provides a fixed access link to terminals residing on the public transport platform. However, mobility being the biggest issue, the mobile RSs should be able to mitigate velocity related effects such as Doppler fading.
Relaying Methods Depending on how a RS processes the received signal, there exist mainly three different physical layer realizations for relaying: amplify-and-forward (AF), decode-and-forward (DF), and compress-and-forward (CF).
Amplify-and-Forward (AF) AF is the simplest possible relaying scheme where the relays act as analog repeaters (layer 1 relay). Fig. 4 describes a simple AF type relay in operation. To keep the complexity of the analysis to a minimum, let us consider the downlink path only.
S
Source
RS = hSR S + nSR
RD = GRS
Relay
D = hRD RD + nRD
Destination
Fig. 4: Operation of a relay (AF type)
The transmitted signal (S) from a BS/ AP is first received by a relay node, amplified with a gain (G) which may be adaptable if channel state information (CSI) is available, and finally retransmitted to the destination (D) MS. The terms hSR and hRD accounts for the channel attenuation due to fading and nSR and nRD denotes additive noise, in the sourcerelay (SR) and relay-destination (RD) links respectively. The received signal at relay node may be expressed as RS = hSR S + n SR and at destination
(1)
D = hRD RD + n RD
(2)
Finally combining (1) and (2) through the relation RD = G.RS we get, D = hRD G (hSR S + n SR ) + n RD = hSR hRD GS + hRD Gn SR + n RD 123 144244 3 double fading
(3)
noise term
As demonstrated in (3), during the amplification process, variations due to fading and noise that has been accumulated in BS-RS transmission are also magnified. The signal S experiences double fading instead of single fading present in a simple MS-BS transmission and additional noise terms do appear. To compensate the noise and fading effect a variable gain at RS is generally proposed. If perfect CSI is available at all instants of time, this gain G may be made inversely proportional to the fading attenuation and noise. The corresponding method is known as instantaneous power scaling (IPS). However, in reality, it is not possible to assess the instantaneous channel fading all the time. Accordingly, a variable gain proportional to the inverse of average noise/ fading is found to be more suitable. This is known as the average power scaling (APS) approach.
Decode-and-Forward (DF) In DF mode, RSs act as digital regenerative repeaters (layer 2 relay), i.e. the relay demodulates, decodes, re-encode and re-modulate the received signal prior to retransmission. The forwarded signal does not contain additional degradation; rather it is affected by only bit errors resulting from the decoding process. To improve reliability the DF relay may be allowed to retransmit only if the decoding is satisfactory, otherwise an ARQ (automatic repeat request) algorithm takes care of it. Compared to AF strategy, DF
scheme provides better QoS. But an increase in cost, complexity, and power consumption is involved on the part of relay nodes.
Compress-and-Forward (CF) The CF scheme is a hybrid solution, an attempt to retain the better features of both AF and DF strategies. In this method the RS does not decode the input data, but it quantizes and compresses (via source coding) the received signal and transmits it to the destination. The possible estimation errors in quantization and coding process are the main sources of signal degradation in this case. CF is also sometimes referred as estimate-and-forward (EF) technique in literatures.
State of the Art The boost in research interest for relay based deployment can be substantiated by citing some of the activities of standardization bodies throughout the world. From a seed concept in 3GPP (third generation partnership project) under the name ODMA (opportunity driven multiple access), multi-hop relaying has now flourished to 3G LTE release 9, and probable mesh extensions in the later release IEEE 802.16m. Significant thrust to relay research came with the establishment of IEEE 802.16j task group in March 2006. The IEEE 802.16j amendment is primarily meant to incorporate mobile multi-hop relay (MMR) capabilities in the current IEEE 802.16 WiMAX standard to enhance coverage, throughput, and system capacity. The standard does not impose any
modification on the part of end user, and provides full backward compatibility with 802.16-2004 (fixed) and 802.16e-2005 (mobile) WiMAX systems. Fixed relay stations acting as wireless bridges has also been suggested as an overlay to cellular radio systems extending the coverage of MAC (media access control) frame based access protocols like IEEE 802.11, 802.15.3 and HiperLAN/2. In presence of diverse data networks, researchers even proposed two-hop relay architecture, which integrates wireless wide area network (WWAN) with wireless local area network (WLAN), and thereby enhancing the system capacity of existing WWAN as well as improving coverage of WLAN.
Multi-hop or, Dual Hop? In order to meet the coverage and throughput demand, when a single-hop link between a MS and BS/ AP is converted to a dual-hop wireless channel by inserting a RS between them, it seems natural to extend the process by putting few more RS when the demand grows further. However, one should keep in mind that, as RSs close to BS/ AP have to carry traffic originated from and destined to multiple MSs and RSs, traffic aggregation occurs and tends to grow larger with more number of hops. The congestion with multihop forwarding will be at its peak at the RS nearest to BS. When all RSs share common cellular bandwidth, exposed terminal problem will also be intensified rendering the random medium access protocols inefficient. Because of unnecessary congestion and rise in media access time, the overall end-to-end packet delay would increase. On the other
hand, due to node mobility and radio signal strength fluctuation, the routing path from the source node to the destination node could easily become invalid. Frequent route changes and the resulting route discovery procedures could cause high signaling overheads. No wonder, IEEE 802.11 has been reported not to perform well in multi-hop relay environments. Considering packet delay, signaling overhead, and system complexity, simple dual-hop relay implementation may appear better in contrast to the multi-hop case.
Disadvantages of Relaying In general relaying trades off radio range for capacity as RS works in time-sharing or, frequency-sharing basis. In time division duplexing (TDD) mode the relay receives signal from BS during the first time slot and uses the second slot for retransmission. This results in reduction of data rate, since the relayed data has to go twice over the radio channel. If an frequency division duplexing (FDD) technique is used instead, orthogonal frequency bands are used for simultaneous BS-RS and RS-MS communication which reduces the effective bandwidth, not to mention the complexity and cost incurred due to multiple radio interfaces. For relatively smaller cells, where BS-MS link is operating satisfactorily, relay deployment would unnecessarily eat up the capacity of the network. Even worse, using relay may cause unintended accumulation of noise, fading effects, delay, and interference. Therefore, it seems, relays should be adopted only if there exists direct reliable line-of-sight (LoS) BS-RS path, overcoming the deteriorations of random non-
LoS BS-MS channel. A detailed cost analysis is also necessary because implementing relay is not the only way to extend coverage, and thus it must be the most cost effective approach. Finally, it remains to be seen how wireless relays will perform against other competing technologies, such as MIMO-orthogonal frequency division multiplexing (MIMO-OFDM), femto BSs, and software defined radio (SDR).
Conclusions Wireless radio relays are one of the possible contenders to extend the capacity and reliability of broadband data channels envisaged for 4G and above. Several research and standardization groups are engaged in evaluating and evolving the range of applications with radio relays. There are both pros and cons of this technology which needs to be carefully addressed before its actual implementation. Its cost effectiveness will also be a deciding factor in a particular application scenario with respect to other competing technologies.
Read more about it •
K. J. Rayliu, A. K. Sadek, W. Su, and A. Kwasinski, Cooperative Communications
and Networking, Cambridge University Press, New York, 2009. •
J. Sydir and R. Taori, “An evolved cellular system architecture incorporating relay stations,” IEEE Commun. Magz., vol. 47, no. 6, Jun 2009, pp. 115-121.
•
IEEE 802.16's Relay Task Group; http://www.ieee802.org/16/relay/
•
S. W. Peters and R. W. Heath, “The future of WiMAX: multihop relaying with IEEE 802.16j,” IEEE Commun. Magz., vol. 47, no. 1, Jan. 2009, pp. 104-111.
•
D. Soldani, and S. Dixit, “Wireless relays for broadband access,” IEEE Commun.
Magz., vol. 46, no. 3, Mar. 2008, pp. 58-66. •
L. Le, and E. Hossain, “Multihop cellular networks: potential gains, research challenges, and a resource allocation framework,” IEEE Commun. Magz., vol. 45, no. 9, Sep. 2007, pp. 66-73.
•
K. H. Teo, Z. Tao, and J. Zhang, “The mobile broadband WiMAX standard,” IEEE
Signal Proc. Magz., vol. 24, no. 5, Sep. 2007, pp. 144-148. •
R. Pabst et al., “Relay-based deployment concepts for wireless and mobile broadband radio,” IEEE Commun. Magz., vol. 42, no. 9, Sep. 2004, pp. 80-89.
About the Authors Anirudhha Chandra (
[email protected]) is serving as a lecturer in the department of Electronics and Communication Engineering, National Institute of Technology, Durgapur, India since 2005. He received his B.E. degree in electronics and telecommunication engineering and his M.E. degree in communication engineering from Jadavpur University, Kolkata, India, in 2003 and 2005 respectively. He is currently pursuing his Ph.D. at Jadavpur University.
Chayanika Bose (
[email protected]) received her B. Tech., M. Tech. and Ph.D. degrees in radio physics and electronics from the University of Calcutta in 1981, 1983 and 1990. Presently she is a reader in the department of Electronics and Telecommunication Engineering, Jadavpur University.
Manas Kr. Bose (
[email protected]) received his B. Tech., M. Tech. and Ph.D. degrees in radio physics and electronics from the University of Calcutta in 1981, 1983 and 1993. Presently he is working as a railway signaling professional at Ansaldo-STS, New Delhi, India.
