LTE and WiMAX are the key fundamental technologies of 4G wireless technology. Both technologies use the same fundamental wireless standard known as OFDM (Orthogonal Frequency Division Multiplexing). Peak data rate of currently using WiMAX 802.16e is less than the LTE release 8 but comapritively WiMAX 802.16m is similar to the LTE-Advanced (Release10). In present scenario, LTE is supported by big telecommunication companies and international standard bodies where as WiMAX is supported by IEEE and computer companies but the important thing WiMAX is waning support from the telecommunication companies. Different surveyes and view of the telecommunication operators show that most of the telecom operators are interested towards the LTE. In this paper we are suggesting some technical issues to all the WiMAX operators who are interested towards the LTE. Network transition from one technology to the another technology depends on the operator’s future plan, interest, subscriber’s needs and capacity of operator’s investment for the network but we refer towards the TD-LTE for existing WiMAX operators to compete in future 4G wireless technology.
This paper is mainly concentrated with the Telecom opetators who have already launched WiMAX technology but they are interested in LTE due to the abruptly diverted market trend towards the LTE. The surprising case is that the operators started WiMAX in the past few months are interested to start the LTE in their network and they are planning towards LTE as possible.The main issue of every telecom opetators who are planning towards LTE but they have already WiMAX net-work with them is financial issue in establishing both WiMAX and LTE network. Our paper will b e helful for those operators who are interested in LTE.WiMAX and LTE share most of the Radio Access Net-work (RAN) and core network, so the transition path toLTE will be smoother than, for instance, the transition path from HSPA to LTE. Most of the WiMAX base stations and product on the market are based on 802.16e .Duplex mode of 802.16e is TDD. So, technically network trasnsition from WiMAX to TD-LTE is seemed to be suitable because both use TDD duplexing. The most commonversion of LTE, the one deployed by most mobile operators, uses FDD paired spectrum, with one channel used for uplink transmission and one for downlink transmission. A second version of LTE, TD-LTE uses TDD spectrum in which operators are interested to implement in their network too. The transition path from WiMAX toTD-LTE is more straightforward and easily implementing than the FDD LTE. The same spectrum band can be used and infrastructure equipment upgrades are possible (al-though not necessarily available, depending on the vendor and on the version of the equipment). For operators trying network transition from WiMAX to FDD LTEwill be difficult and expensive because FDD LTE requires new spectrum and getting new FDD spectrum is more costly than the TDD spectrum. In this paper we recommend to the operators who are using WiMAX 802.16e version and older version of WiMAX to switch over their networks towards the TD-LTE if they are interested in LTE for competing in future 4G wireless market. The paper is organized as follows. Section 2 briefly discusses about the background of the WiMAX and LTE with their new features and comparision between the WiMAX and LTE in the current scenario. Section 3 presents migrating factors of WiMAX operators towards LTE. Sections 5 and 6 present the solution of network migration from WiMAX to LTE for the WiMAX operators and Conclusion.
WiMAX is a telecommunication protocol which provides fixed and full mobile internet access. It uses IEEE 802.16 standards, established by the IEEE Standards Board in1999 for the global deployment of wireless broadband networks , . From 802.16, the standard was amended many times and changed to 802.16a, 802.16c, 802.16d, and 802.16e the most popularly used standard for WiMAXtoday. Revisions of the standard have gone up the alphabet all the way to 802.16m, with the most advanced version expected to be completed by December 2009 and approved by March 2010. Each amended version of the WiMAX standard increases coverage capacity and service performance. Accroding to WiMAX Forum, 592 WiMAX Networks have been commercially deployed in 149 countries (www.wimaxforum.org). But networks are not increasing worldwide as they are expecting.
2.2 LTE 3GPP Long Term Evolution (LTE) , is the latest standard in the mobile network technology tree that produced the GSM/EDGE and UMTS/HSDPA network technologies. It is a project of the 3rd Generation Partnership Project (3GPP), operating under a name trademarked by one of the associations within the partnership, the European Telecommunications Standards Institute. It is thelatest step of 3GPP in moving forward from the cellular3G services to LTE. LTE may also be referred more formally as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio AccessNetwork (E-UTRAN). Even though 3GPP created standards for GSM/UMTS family, the LTE standards are completely new, with exceptions where it made sense.The main objectives of LTE are: 1) Increased downlink and uplink peak data rates, 2) Scalable bandwidth, 3) Improved spectral efficiency, 4) All IP network and 5) Astandard’s based interface that can support a multitude of user types. LTE networks are intended to bridge the functional dataexchange gap between very high data rate fixed wireless Local Area Networks (LAN) and very high mobility cellular networks. The first version of LTE is documented in Release 8 of the 3GPP specifications. It defines a new physical layer radio access technology based on Orthogonal Frequency Division Multiple Access(OFDMA) for the downlink, similar in concept to the PHY layer of Mobile WiMAX, and uses SC-FDMA (single Carrier Frequency Division Multiple Access) for the uplink. In September 2009, 3GPP submitted its LTE-Advanced proposal for IMT-Advanced, officially called “document 3GPP Release 10.”
2.3 LTE- WiMax Comparision
Table 1 shows the general technical specification ,  ofthe LTE and WiMAX with comparisions. Both LTE and WiMAX use orthogonal Frequency Division Multiplexing(OFDMA) in the Downlink but LTE uses Single Carrier Frequency Division Multiple Access (SC-FDMA). So, in the LTE uplink signal achieves saving power capacity without degrading system flexibility and performance. There is provision of both TDD and FDD in WiMAX802.16m (R2.0) but current market of WiMAX is based on802.16e. So, in this case we can say WiMAX uses TDD andLTE uses both TDD and FDD. User mobility and Datarate is higher in LTE than the commercial WiMAX. The latency requirement in the WiMAX and LTE specifications is small enough to support realtime applications,such as voice applications. A voice application could tolerate a delay of between 50 and 200 ms without the user perceiving a decrease in quality. Low latency is thus essential in these mobile broadband standards. The low latency is also coupled with high data rates to satisfy bandwidth-intensive applications. Both standards support mobility in that users can carry the device travelling at speeds of up to 350 km/h. So, users on a high speed train, for example, could connect to a 4G network. The main difference between WiMAX and LTE is thatWiMAX benefits from its earlier development and deployment, while LTE has the advantage of being developed by telecommunications companies who get to choose which technology to deploy. WiMAX jump started the mobile broadband market. According to the WiMAX Forum, WiMAX has about 592 deployments worldwide with more than 10 million subscribers. Also, WiMAX has spectrum allocated for it in 149 countries, and many telecommunications companies are involved in WiMAX activities. However, now that LTE’s development has picked up, some telecommunications companies have backed away from WiMAX. Recently, Cisco announced that it will discontinue offering WiMAX base stations andwill focus on radioagnostic IP core solutions. Alcatel-Lucent made a similar announcement. However, companies such as Clearwire that have invested in WiMAX don’t have to discontinue their offerings. WiMAX could coexist in the broadband arena with LTE. We expect the ITU to make its recommendations for IMT-Advanced this summer . However, this doesn’t necessarily mean that WiMAX or LTE will prevail at that time, as we’ve learned from previous ITU recommendations. The IMT-2000 (3G) recommended several independent technologies that meet the same goals. For example, in 2007, ITU added OFDM as part of 3G at the request of IEEE. Thus, ITU can include multiple standards in its recommendation, which means the real battle between WiMAX and LTE will be how successfully they’re deployed and used. LTE supports handover and roaming with the 3GGP mobile networks but with WiMAX these services are not easy to achieve. From telecom operator point of view, the roaming service generates numerous benefits for operators. It extends the coverage of the operator using the network of other carriers, it generates more benefits of visitors from other carriers and it provides to users an important service i.e. user can travel far away from his operator.
3. FACTORS FOR MIGRATING TOWARDS LTE
In fact both WiMAX and LTE can play vital roles in future wireless network. Up to certain time we can say that LTEand WiMAX play different roles as access and backhaul respectively i.e. LTE provides the access technology of choice and WiMAX provides an ideal backhaul technology for 4G networks . In future, there will not need to be categorized into different access and backhaul networks. Finally they will be 4G access networks. In current scenario, WiMAX is well matured technology but LTE is in starting phase. Talking about the outlook of WiMAX and LTE, they are quite similar for adoption of current 4G technologies but we have to consider different a spects while chosing the technology. In current scenario, more than 592 WiMAX networks are deployed worldwide in 149 countries where as only few LTE networks are deployed worldwide. But, important issue while chosing WiMAX and LTE is their current technical supports. Major telecom companies like Cisco, Alcatel-lucent  have announced that will discontinue offering WiMAX base stations and will focus on radio agnostic IP core solutions. LTE is supported by big telecommunication companies and international standard bodies where as WiMAX is supported by IEEE and computer companies but the important thing is WiMAX is waning support from the telecommunication companies. LTE provides full mobility but WiMax needs a mobile target with a speed lower than 120 km/h where as LTE still operates with a target up to 350 km/h. LTE supports handover and roaming with the 3GGP mobile networks but with WiMAX these services are not easy to achieve. Power consumption is also the important factor while chosing technology. Power consumption is less in LTE comparision with WiMAX because LTE uses SC-FDMA modulation in Uplink channel. So, due to those above metioned new features and current trends, existing WiMAX operators and other telecom operators are interested in LTE network.
4. SOLUTION OF NETWORK MIGRATION FROM WIMAX TO LTE
According to many recent surveyes and reader forums, the operators using WiMAX 802.16e standard should upgrade their existing WiMAX standard for competing with new 4G technologies. Currently WiMAX Forum[www.wimaxforum.org] is working on preparing specification of the WiMAX 802.16m. So, operators have two choices for sustaining in current competitive 4G market. Either they can upgrade current 802.16e (Release 1.0) to802.16m (Release 2.0) or they can migrate to the LTE. They have opportunity to upgrade from 802.16e to802.16m for few years and after then towards the LTE but this rapidly growing telecommunication market and rapidly changing technology might not wait for them for1or 2 years more. So, second option might not be favorable for any operators who really want to compete with the new 4G wireless technology. In our opinion, in current competitive market, it will be better to upgrade their network towards LTE. The way of migration towards LTE depends on the operator capacity of investment and revenue earning from after migrating towards LTE, number of subscribers of that operator etc. but we suggest some fundamental issues while deciding network transition from WiMAX to LTE.
4.1 Spectrum Selection
Technically operators are free to use both FDD LTE andTD-LTE but in commercial point of viewTD-LTE is more suitable for the operators who are implementing WiMAX 802.16e because both TD-LTE and 802.16e uses TDD duplexing mode so that network transition will be easy and cost effective. Using the same spectrum band, there will be easy in equipment upgrading from one technology to another technology. Moving towards FDD LTE is better in one scenario i.e. equipments are commercially available in the market but it arises issues of frequency spectrum. Getting new spectrum is expensive and difficult too in comparision to getting commercially few available types of equipment.
4.2 Base Stations
Base station architecture consists of base band, radio,transport, clock/synchronization and power modules.WiMAX and LTE use OFDMA as access technology onthe downlink. There might be problem in clock synchronization but most of the base stations can be upgraded from WiMAX to TD-LTE. Upgrading towards FDD LTE will be difficult because in FDD LTE it uses paired FDD spectrum, with one channel used for uplink transmission and one for downlink transmission.
4.3 End Devices End devices used in the current WiMAX market do not support either TD-LTE or FDD LTE. Finally all the end equipments used in the WiMAX network will be replaced but we suggest for operators to run both existing WiMAX and TD-LTE for certain period using all the possible common networks of the existing WiMAX so that in that period operators can encourage their subscribers to use multimode devices. So, using multimode devices by the subscribers, operators can move their network from Wi-MAX to LTE as well as in that period subscriber can experience real taste of WiMAX and LTE.
4.4 Network Integration
The main important part of the network transition is network integration. Both WiMAX and LTE use IP backbone for the access part. So, there is not any problem in access part, it is easily upgradable but we have to careful about the core elements. The deployment of an integrated architecture that allows users to seamlessly switch between these two types of networks would present several advantages to both users and service providers . By offering integrated LTE/WiMAX services, users would benefit from the enhanced performance and high data rate of such combined service. For the providers, this could capitalize on their investment, attract a wider user base and ultimately facilitate the ubiquitous introduction of high speed wireless data. The required LTE access network may be owned either by the WiMAX operator or by anyother party, which then requires proper rules and Service Level Agreements (SLAs) set up for smooth interworking on the basis of business and roaming agreements betweenthe LTE and mobile WiMAX operators. In , authors proposed integrating architecture of the WiMAX and LTE. In Fig.1, the Mobile WiMAX supports access to a variety of IP multimedia services via WiMAX radio access technologies which is called Access Service Network (ASN). The ASN is owned by a Network Access Provider (NAP) and comprises one or more BS and one or more ASN gateways (ASN-GW) that form the radioaccess network. Access control and traffic routing for Mobile Stations (MSs) in Mobile WiMAX is entirely handled by the Connectivity Service Network (CSN), whichis owned by a Network Service Provider (NSP), and provides IP connectivity and all the IP core network functions. The LTE network may be owned either by the NAP or by any other part in which case the interworking is enabled and governed by appropriate business and roaming agreement. 3GPP and Mobile WiMAX accesses are integrated through the Evolved Packet Core (EPC). 3GPP access connections are supported by the Serving Gateway(SGW), and Mobile WiMAX accesses are connected to the Packet Data Network Gateway (PGW). Specifically, the legacy serving GPRS support node (SGSN) is connectedto the SGW. New logical entities are also added to the system architecture. The ANDSF is an entity that facilitates the discovery of the target access. The target accesssu pported by the ANDSF can be either a 3GPP or Mobile WiMAX cell. This entity is introduced by 3GPP in order to minimize the impacts on the use of radio signals. The use of radio signals for neighbour cell discovery requires the User Equipment (UE) to utilize multiple antennas,which result in power consumption. Moreover, if the cell information is not broadcast, the UE is unable to acquire the appropriate target cell information. Optionally, the ANDSF can provide additional information about neighbour cells, such as QoS capabilities, which cannot be distributed by radio signals due to high data demand. Integration architecture proposed in Fig.1 is basically interworking between WiMAX and LTE. In network transition from WiMAX to LTE we can run parallel both network utilizing all the existing elements of the WiMAX including certains elements of the LTE so that it solves the problem of service interruption in switch over the system and subscribers get experience from both technology for certain period.
Fig.1 WiMAX-LTE Integrating Architecture
4.5 Network Management System
Network management is also the important factor to be considered while moving from one network to the other network. There might be some difficulty in handling by the same network management system after moving fromWiMAX to LTE but we can use existing network management system. Existing network management system can use after switching towards the LTE and it can be used for traffic handling from one network to the other network during switchover.
Although WiMAX and LTE are based on the same fundamental wireless standard, they have difference in performance like peak data rate, user mobility, power consumption, handover, roaming facilities etc. But the main difference between two technologies is: LTE is increasing its momentum in the current 4G wireless technology and WiMAX is losing the current market day by day. In this scenario, any of the operators wants to sustain in the market for the future. Due to competeion in the 4G wireless technology of WiMAX and LTE in the current market, WiMAX operators are in confusion of their future. We recommend current WiMAX operators to move their network towards the TD-LTE so that they can survive in the current 4G wireless technology, they can save cost in migrating their network from one technology to another technology and they can use same spectrum after migrating to the new technology.