comparison of lte and wimax

IP NGN ARCHITECTURE THOUGHT LEADERSHIP JOURNAL - Q1 FY2010

AbstractThis article provides a high-level compari-son between LTE and WiMAX. The focus of paper is on two primary areas: System Architecture and Physical Layer. The Sys-tem Architecture describes the different functional elements in LTE and WiMAX and attempts to map similar functionality (such as mobility, security, access-gateway). We also compare and contrast the various aspects (such as transmission modes, duplexing types) of the physical layer.

IntroductionLTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access) are expected to be primary tech-nologies for mobile broadband wireless for the next 10 years. As with most emerg-ing and competing technologies, there is considerable effort by the correspond-ing technology advocates to frame the discussion as LTE versus WiMAX with the end result of declaring one technology as the “winner”. We take a different approach in this paper. We frame the discussion, rather, in terms of similarities and differ-ences across various technology/technical factors. This is motivated by the fact that 1) technological factors only partially contrib-ute to determining winners, and in some cases play a small role and 2) technical dif-ferences are not universally advantageous. The goal of the paper is to primarily focus on technical/technology aspects as com-pared to business and strategic aspects.

The article is organized as follows.Firstly, we describe the evolution of LTE and WiMAX as well as provide the primary motivations. A system-level comparison of LTE and WIMAX focusing on system-archi-tecture and protocol stacks for the control and user traffic is provided and the air interfaces for LTE and WiMAX described.

Comparison of LTE and WiMAXby Rajesh S. Pazhyannur

LTE EvolutionThe first generation of cellular systems were based on analog standards and introduced in the mid-80s. These quickly led to a second generation of digital cel-lular standards that made use of digital modulation and signal processing. The second generation also led to a technol-ogy fragmentation. At one point many competing standards existed, however what remains now are two main branches: referred to as GSM and CDMA branches or alternately referred as the �GPP and �GPP2 branches. (�GPP and �GPP2 are the standardization bodies responsible for technical specifications.) These branches remained separate as they migrated to �G systems focusing on more efficient voice transport as well providing data-services. LTE originated in the �GPP standards or-ganization, and a competing specification (EV-DO Rev C) started in the �GPP2 body as the next evolutionary step. However, the

support for EV-DO Rev C has waned and it has now become clear that the �GPP2 radio interface evolution has effectively ceased, allowing a single cellular technol-ogy —LTE.

As shown in Figure 16, the �GPP and �GPP2 cellular technology offerings have evolved and �GPP2 operators are now switching camps and backing a single specification based on LTE.

WiMAX EvolutionWiMAX evolved almost independently (and in parallel) to the cellular standards mentioned earlier. In the late �0s, IEEE started a working group to create an air-interface for point to multipoint broadband wireless standard. The working group lev-eraged DOCSIS (data over cable service interface specification) standard heavily especially in the definition of the MAC layers. The original standard was modified into 802.16d in 2004 introducing OFDM as

Figure 16 - Evolution of LTE

Figure 17 - Evolution of Mobile WiMax

the transmission scheme. This standard was targeted at fixed applications and is sometimes referred to as fixed WiMAX.

In 2005, 802.16d was further enhanced to provide support for mobility as well as provide a scalable OFDM transmission system. This standard is known as 802.16e and also as mobile WiMAX. (It should be noted that products based on 802.16d and 802.16e exist in the marketplace and both are classified as WiMAX products leading to some ambiguity about which specific standard is supported—802.16d or 802.16e.)

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Technology Highlights

UMTS (aka WCDMA) CDMA, Spread Spectrum, 5 MHz spectrumCircuit Voice and Packet Data (up to �84 Kbps)Deployed since 200�

HSDPA (High Speed Downlink Packet Access)

CDMA, Spread Spectrum, 5 MHz Downlink Only; Data OnlyMultiple Codes per SubscriberUp to 16 QAM, Peak Rates of 14.4 MbpsDeployed since 2005

HSUPA (High Speed Uplink Packet Access)

CDMA, Spread Spectrum, 5 MHz Uplink Only; Data OnlyMultiple Codes per SubscriberUp to 16 QAM, Peak Rates of 4.5 MbpsDeployed since 2007

HSPA+(Evolved High Speed Packet Access)

CDMA, Spread Spectrum, 5 MHz Up to 64 QAM, MIMO. Peak Rates (DL,UL): 42, 11 MbpsLikely to be deployed in 200�-2010

LTE Scaleable OFDM on downlink, Single Carrier FDMA on uplinkVariable Spectrum Width from � to 20 MHzUp to 64 QAM, MIMO, Spatial Multiplexing(SM), BeamformingLikely to be deployed between 2010-2012

WiMAX Scaleable OFDM on downlink and uplinkVariable Spectrum Width from 1.25 to 10 MHzUp to 64 QAM, MIMO, Spatial Multiplexing, Beam-formingMobile WiMAX deployed since 2008

Table 1: Technology Summary

Looking forward, the 802.16e standard is evolving to 802.16m which focuses on en-hancements to air-interface specifications. This evolution is shown in Figure 17.

Technology SummaryAs seen from Table 1, the main differences between the �G technologies and 4G technologies such as LTE/WiMAX are the different transmission schemes (OFDM compared to CDMA) and much higher peak rates.

Motivation for LTE and WiMAXThe primary motivations for both LTE and WiMAX are similar and can be stated as::

• Mobile Data Network: The primary usage of both networks is to provide a data-centric network as compared to voice-centric network of 2G and �G systems. This aspect is highlighted by the absence of any provisions to carry any circuit-type service. The networks do support voice, but in the form of packetized VoIP service.

• Improve Spectral Efficiency: Given the scarcity of licensed spectrum, improv-ing efficiency is a major impetus for both networks. The main technologies to enable higher efficiency are to move towards higher modulation schemes (like 64 QAM), smart antenna tech-niques (MIMO, Beam Forming, etc) and OFDM.

• Spectrum Flexibility: Unlike previous networks which operated on a fixed width spectrum (5 MHz for WCDMA and 1.25 MHz for CDMA-DO), both net-works allow scaleability from 1.25 MHz up to 20 MHz.

• Higher Peak Data Rates: Both networks attempt to improve the peak data rate on the downlink and uplink so that high data rate services such as high-defi-nition video can be transmitted over broadband wireless links. Specifically, the goal is to increase the peak rates from range of (�-10) Mbps to (50-100) Mbps.

• Lower Infrastructure Costs: Traditional cellular networks comprise a combina-tion of TDM and packet infrastructure partly because of the need to carry circuit voice. LTE and WiMAX networks simplify the network considerably, mi-grating towards an all-IP infrastructure relying on IP network for transporting data and control messages. Addition-ally, both networks embody a design principle of “flattening” the architecture wherein the system eliminates a cen-tralized base station controller (or Radio Network Controller (RNC)) in favor of


distributing the functionality to Base Stations and Access Gateways.

System-Level ComparisonArchitectureFigure 18 provides a simplified view of the LTE and WiMAX architecture (not all nodes and interfaces are shown, only the main elements involved in user and control plane traffic).

We first compare the main functional ele-ment below.

• eNodeB and BS: Functionally speaking, the LTE and WiMAX BS are quite similar. Both handle the traffic to/from the subscriber device. This involves per-forming the function of Radio Resource Management on the control plane, in terms of authentication, setting up connections, allocating resources and performing functions like packet trans-missions, MAC, H-ARQ and link-adapta-tion on the user-plane. In addition, the base stations provide an interface into

the packet network. Both systems use an IP tunnel to route user plane traffic to an access gateway. There are sig-nificant differences in the air interface standards that are described next.

• MME/S-GW and ASN-GW: Function-ally speaking, the combined functions of MME and S-GW match closely to those performed by the ASN-GW. This element (in LTE and WiMAX) provides mobility between BS, security func-tions, QoS functions, idle state (paging) management. LTE defines a functional element, the MME, for handling control plane traffic and another element for handling the user plane traffic called the Serving Gateway. WiMAX (at least in Profile C) does not separate the control and user plane handling into separate elements. The control and user plane traffic both are carried by the ASN-GW. The protocols used between the gateways and the BS’ differ between LTE and WiMAX as well. LTE uses GTP (GPRS Tunneling Protocol) for the S1u and S1-AP/SCTP for S1c interface, while WiMAX uses GRE/UDP as the tunnel-ing protocol and UDP for control plane transport. The specific control mes-sages transferred differ as well and are defined by corresponding specifica-tions: S1 for LTE and R6 for WiMAX. A function unique to MME and S-GW is to interface with legacy �G networks

Figure 18 - LTE and WiMAX System Architecture

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(omitted from Figure �). �GPP has defined interfaces from the MME and S-GW to connect to WCDMA systems as well as CDMA-1X and EV-DO sys-tems. The WiMAX forum is expected to define corresponding interfaces between WiMAX and �G systems in future releases.

• PDN-GW and HA: Functionally speak-ing, the PDN-GW and HA are similar. Both provide mobility between the Access Gateways (S-GW for LTE and ASN-GW for WiMAX). In WiMAX R1.0, the defined protocol for the R� inter-face is Mobile IPv4 (MIPv4), and in most instances, the ASN-GW performs Proxy MIP (PMIP). LTE defines two alterna-tives for the S5 interface: One is based on GTP (GPRS Tunneling Protocol) and the other is based on Proxy MIPv6 (PMIPv6). PMIPv6 is being defined as an option for WiMAX R1.5.

Other Architectural ConsiderationsAll IP (Packet-only) Systems: As shown in Figure 18, LTE and WiMAX are packet-only systems. There are no defined inter-faces to circuit switched systems. More-over, all RAN and Core Network systems are IP based.

Inter BS interface: LTE and WiMAX define interfaces to optionally route traffic related to handover between BS’ directly eliminat-ing the need to go through a core network element. This is referred to as the R8 interface in WiMAX and X2 interface in LTE. This interface can improve the latency in handovers between BS as well reduce the control and user plane traffic traversing the access gateways.

Multiple forms of Mobility: LTE and WiMAX define multiple forms of mobility: across BS’ connected to the same Access Gate-way (R8 or R6 relay in WiMAX), across BS’ connected to different Access Gateway (R4 in WiMAX).

Figure 19 - LTE and WiMAX User Plane Protocol Stacks

Figure 20 - LTE and WiMAX Control Plane Protocol Stacks

Protocol StacksThe user and control plane stacks further illustrate the similarities and differences between LTE and WiMAX and are given in Figure 1� and Figure 20 respectively. As shown in Figure 1� the key difference is that the interface between base-site and access-gateway uses GTP and S5 uses ei-ther PMIPv6 or GTP in LTE, while in WiMAX the corresponding protocols are GRE and PMIPv4.

As shown in Figure 20, the key difference in the control plane is that LTE defines two

control stacks for the subscriber. One stack is for RRM messages and is between the UE and eNB. The other stack is for security, idle state management, QoS negotiation, etc and is between the UE and the MME (and known as Non-Access Stratum (NAS) layer). In comparison, the subscriber sta-tion (SS) never communicates directly with the ASN-GW. The 802.16e layer defines procedures between the SS and the BS (shown as MAC in Figure 5) while the WiMAX Forum defines the procedures between the BS and the ASN-GW (shown as R6 in Figure 20).


Air InterfaceSimilaritiesTable 2 provides the key similarities be-tween LTE and WiMAX air Interface.

• Scalable Bandwidth: �G technolo-gies were designed to operate in a fixed bandwidth. For example, WCDMA bandwidth is 5 MHz. Unlike �G, LTE and WiMAX are defined over a wide range of bandwidth ranging from 1.5 to 20 MHz. This allows the operators (service providers) deployment flexibility based on spectrum availability and capacity/coverage needs.

• Downlink Transmission: LTE and WiMAX deploy OFDM for downlink transmission. The transmission is divided into time intervals (frames) and the spectrum is divided into a number of subcarriers. Downlink Resources are managed by a scheduler at the Base Station that determines the number of subcarriers and time intervals for each user on the downlink and uplink.

• MIMO: LTE and WiMAX allow for MIMO options comprising STBC (Space Time Block Coding) or SM (Spatial Multiplex-ing). WiMAX Release 1.0 defines 2 x 2 MIMO (and higher MIMO are being developed for future release). The LTE specification allows up to 4 x 4 MIMO.

DifferencesTable � provides the key similarities be-tween LTE and WiMAX air Interface. A little more detail is provided on these below

Remarks

Duplexing Mode LTE is primarily for FDD (though TDD is defined). WiMAX is primarily for TDD (though FDD is being considered)

Frequency Bands LTE: 700, 1700, 1�00, 2100, 2500, 2600 WiMAX: 2�00, 2500 and �500

Uplink Transmission LTE: SC-FDMA WiMAX: Uplink Transmission is OFDMA

Frame Duration and SubCarrier Frequncy

LTE: 1 msec frame; subcarrier frequency :15KHz WiMAX: 5 msec frame; subcarrier frequency : 10KHz

Table �: Air Interface Differences

• Duplexing Mode: WiMAX is currently defined as a TDD system (though there are plans to define a FDD system in a future release). LTE has a defined TDD and FDD specifications, though most deployments are expected to be FDD. FDD uses “paired” spectrum (one for uplink and other for downlink). TDD on the other hand requires contigu-ous spectrum. Cellular/�G systems are FDD and cellular operators have unused (or in-use) paired spectrum that can be utilized for LTE. One of the key benefits of TDD is the reciprocal nature of the channel, facilitating the use of beamforming techniques to provide improved edge of cell performance as well as stabilizing multipath in wide area MIMO deployments. Another techni-cal aspect of TDD and FDD systems is the synchronization requirement. TDD systems have to be synchronized to ensure non-interference of uplink and downlink burst across different BS’. FDD systems do not require this form of synchronization. A typical way of implementation of achieving the synchronization is by using an accurate GPS receiver than can provide a pulse at 1 PPS (Pulse per second). In low-end base stations such as Pico Base Stations and Femto Base Stations, the additional GPS receiver cost becomes an important consideration while in indoor Femto Base stations, the non-availability of GPS signals becomes an additional issue.

FDD is a natural choice for cellular operators and partly explains the preference shown by existing cellular operators to migrate towards LTE.

• Frequency Bands: The frequency bands that LTE and WiMAX are ex-pected to be deployed are quite differ-ent. This is also related to the fact that cellular operators are expecting to use existing frequency bands for LTE usage in the future. See Figure 6 for more details.

LTE is specified over a large number of spectrum bands owned by cellular provided throughout the world.

• Uplink Transmission: WiMAX deploys OFDMA in uplink and downlink direc-tions. LTE deploys OFDMA on the downlink but SC-FDMA (Single Carrier-Frequency Division Multiple Access) on the uplink. The choice of SC-FDMA is motivated by reducing the PAPR (Peak to Average Power Ratio) on the uplink. PAPR ratio has a direct impact on the requirements of the power amplifier and resulting battery life. (OFDM trans-missions consist of multiple subcarriers leading to a relatively larger PAPR than those for a single-carrier.)

SC-FDMA provides a 1-2 dB PAPR advantage over OFDMA that in turn improves battery life of subscriber devices (SC-FDMA would increase re-ceiver complexity at the BS compared to OFDMA receiver). This improve-

Remarks

Scalable Band-width

LTE: 1.4,�, 5, 10, 15, 20 MHz WiMAX: 1.25, 5, 10 MHz

Downlink Transmission

OFDMA

MIMO 2x2 (STBC and SM)

Table 2: Air Interface Similarities

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ment is available to users at the edge of the cell, e.g., in order to increase the up-link coverage or throughput in such scenarios.

• Frame Duration: LTE uses a frame of 1 msec while WiMAX uses a frame of 5 msec. The shorter duration leads to more complex implementation in the form of larger processors, etc. However, this reduces end-end latency and can lead to improved H-ARQ (Hybrid ARQ) performance, faster channel quality feedback channel.

SummaryIn this paper, we outlined the key similari-ties and differences between the tech-nologies. The ultimate success of either technology (as measured by number of worldwide deployments, number of subscribers, total revenue, etc) will be determined by a combination of tech-nology and business factors. Given the relatively similar technology (for example, OFDM) and design choices, the business factors will play a bigger role in determin-ing the success of LTE and WiMAX. Most of the major equipment vendors (such as Cisco, Nokia-Siemens, etc) with a few notable exceptions (such as Ericsson,

and Intel) plan to provide LTE and WiMAX equipment. Recently, some of the leading vendors announced that they are scaling the investments in WiMaX. In most cases, the equipment vendors intend to leverage the commonality in their product devel-opment. This may indicate that both may successfully co-exist in a manner similar to co-existence of GSM and CDMA for the last 10-15 years.

In closing, we suggest the following key takeaways:

• Similar Technology but different imple-mentations: LTE and WiMAX have de-ployed similar air interface technology (OFDMA, MIMO) but have considerably different implementations (such as FDD versus TDD, 1 msec versus 5 msec frames, etc). These design choices have been made for a variety reasons and the relative merits of these are hotly contested by proponents of the LTE and WiMAX communities. From a systems architecture standpoint they deploy similar functional decomposi-tion (such as separating radio resource management from IP management and locating RRM in the BS and IP manage-ment in an access gateway). However, the specific protocols used between

these elements are considerably different (motivated partly by the existing protocols in �G systems and to facilitate backward compat-ibility with already deployed �G systems).

• Air Interface Efficiency: This is often a highly debated and con-tentious matter. The fact that both technologies use OFDMA and MIMO would lead to comparable spectral efficiency up to first order of approximation. However, design choices about protocol overheads, control channel overheads, would determine the resulting efficiency. Initial comparisons indicate that LTE efficiency is slightly better than WiMAX Release 1.0, (see [�] below) but author believes that this improvement would disappear with modifications in WiMaX Release 1.5 and IEEE 802.16m.

• Likely Deployments: LTE and WiMAX have unique advantages that will ultimately determine where they will be deployed. For example, LTE appears to be clear choice for operators with FDD spectrum as well as operators with existing �G (GSM) deployments. WiMAX ap-pears to be the clear choice for op-erators with TDD spectrum as well operators with frequency in the 2.5 GHz and �.5 GHz band and opera-tors with little or no legacy cellular deployments (mostly in emerging markets).

Figure 21 - Frequency Bands for LTE and WiMAX

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Cisco has more than 200 offices worldwide. Addresses, phone numbers, and fax numbers are listed on the Cisco Website at www.cisco.com/go/offices.

CCDE, CCENT, CCSI, Cisco Eos, Cisco HealthPresence, the Cisco logo, Cisco Lumin, Cisco Nexus, Cisco Nurse Connect, Cisco Stackpower, Cisco StadiumVision, Cisco TelePresence, Cisco WebEx, DCE, and Welcome to the Human Network are trademarks; Changing the Way We Work, Live, Play, and Learn and Cisco Store are service marks; and Access Registrar, Aironet, AsyncOS, Bringing the Meeting To You, Catalyst, CCDA, CCDP, CCIE, CCIP, CCNA, CCNP, CCSP, CCVP, Cisco, the Cisco Certified Internetwork Expert logo, Cisco IOS, Cisco Press, Cisco Systems, Cisco Systems Capital, the Cisco Systems logo, Cisco Unity, Collaboration Without Limitation, EtherFast, EtherSwitch, Event Center, Fast Step, Follow Me Browsing, FormShare, GigaDrive, HomeLink, Internet Quotient, IOS, iPhone, iQuick Study, IronPort, the IronPort logo, LightStream, Linksys, MediaTone, MeetingPlace, MeetingPlace Chime Sound, MGX, Networkers, Networking Academy, Network Registrar, PCNow, PIX, PowerPanels, ProConnect, ScriptShare, SenderBase, SMARTnet, Spectrum Expert, StackWise, The Fastest Way to Increase Your Internet Quotient, TransPath, WebEx, and the WebEx logo are registered trademarks of Cisco Systems, Inc. and/or its affiliates in the United States and certain other countries.

All other trademarks mentioned in this document or website are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (0903R)

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