Comparison of LTE and WiMAX
by Rajesh S. Pazhyannur
Abstract
This article provides a high-level comparison between LTE and WiMAX. The focus
of paper is on two primary areas: System
Architecture and Physical Layer. The System 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.
Introduction
LTE (Long Term Evolution) and WiMAX
(Worldwide Interoperability for Microwave
Access) are expected to be primary technologies for mobile broadband wireless
for the next 10 years. As with most emerging and competing technologies, there is
considerable effort by the corresponding 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 differences across various technology/technical
factors. This is motivated by the fact that 1)
technological factors only partially contribute to determining winners, and in some
cases play a small role and 2) technical differences are not universally advantageous.
The goal of the paper is to primarily focus
on technical/technology aspects as compared 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-architecture and protocol stacks for the control
and user traffic is provided and the air
interfaces for LTE and WiMAX described.
Figure 16 - Evolution of LTE
Figure 17 - Evolution of Mobile WiMax
LTE Evolution
The 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 cellular standards that made use of digital
modulation and signal processing. The
second generation also led to a technology 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 3GPP and
3GPP2 branches. (3GPP and 3GPP2 are
the standardization bodies responsible for
technical specifications.) These branches
remained separate as they migrated to 3G
systems focusing on more efficient voice
transport as well providing data-services.
LTE originated in the 3GPP standards organization, and a competing specification
(EV-DO Rev C) started in the 3GPP2 body
as the next evolutionary step. However, the
IP NGN ARCHITECTURE THOUGHT LEADERSHIP JOURNAL - Q1 FY2010
support for EV-DO Rev C has waned and
it has now become clear that the 3GPP2
radio interface evolution has effectively
ceased, allowing a single cellular technology —LTE.
As shown in Figure 16, the 3GPP and
3GPP2 cellular technology offerings have
evolved and 3GPP2 operators are now
switching camps and backing a single
specification based on LTE.
WiMAX Evolution
WiMAX evolved almost independently
(and in parallel) to the cellular standards
mentioned earlier. In the late 90s, IEEE
started a working group to create an airinterface for point to multipoint broadband
wireless standard. The working group leveraged 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
Technology
Highlights
UMTS (aka WCDMA)
CDMA, Spread Spectrum, 5 MHz spectrum
Circuit Voice and Packet Data (up to 384 Kbps)
Deployed since 2003
HSDPA (High Speed
Downlink Packet
Access)
CDMA, Spread Spectrum, 5 MHz
Downlink Only; Data Only
Multiple Codes per Subscriber
Up to 16 QAM, Peak Rates of 14.4 Mbps
Deployed since 2005
HSUPA (High Speed
Uplink Packet Access)
CDMA, Spread Spectrum, 5 MHz
Uplink Only; Data Only
Multiple Codes per Subscriber
Up to 16 QAM, Peak Rates of 4.5 Mbps
Deployed since 2007
HSPA+(Evolved High
Speed Packet Access)
CDMA, Spread Spectrum, 5 MHz
Up to 64 QAM, MIMO. Peak Rates (DL,UL): 42, 11
Mbps
Likely to be deployed in 2009-2010
LTE
Scaleable OFDM on downlink, Single Carrier
FDMA on uplink
Variable Spectrum Width from 3 to 20 MHz
Up to 64 QAM, MIMO, Spatial Multiplexing(SM),
Beamforming
Likely to be deployed between 2010-2012
WiMAX
Scaleable OFDM on downlink and uplink
Variable Spectrum Width from 1.25 to 10 MHz
Up to 64 QAM, MIMO, Spatial Multiplexing, Beamforming
Mobile WiMAX deployed since 2008
•
•
•
•
Table 1: Technology Summary
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.)
Looking forward, the 802.16e standard is
evolving to 802.16m which focuses on enhancements to air-interface specifications.
This evolution is shown in Figure 17.
Technology Summary
As seen from Table 1, the main differences
between the 3G 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 WiMAX
The 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 3G
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, improving 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 techniques (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 networks 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-definition video can be transmitted over
broadband wireless links. Specifically,
the goal is to increase the peak rates
from range of (3-10) Mbps to (50-100)
Mbps.
Lower Infrastructure Costs: Traditional
cellular networks comprise a combination of TDM and packet infrastructure
partly because of the need to carry
circuit voice. LTE and WiMAX networks
simplify the network considerably, migrating towards an all-IP infrastructure
relying on IP network for transporting
data and control messages. Additionally, both networks embody a design
principle of “flattening” the architecture
wherein the system eliminates a centralized base station controller (or Radio
Network Controller (RNC)) in favor of
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distributing the functionality to Base
Stations and Access Gateways.
System-Level
Comparison
Architecture
Figure 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 element 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 performing the function of Radio Resource
Management on the control plane,
in terms of authentication, setting up
connections, allocating resources and
performing functions like packet transmissions, MAC, H-ARQ and link-adaptation on the user-plane. In addition, the
base stations provide an interface into
Figure 18 - LTE and WiMAX System Architecture
the packet network. Both systems use
an IP tunnel to route user plane traffic to
an access gateway. There are significant differences in the air interface
standards that are described next.
IP NGN ARCHITECTURE THOUGHT LEADERSHIP JOURNAL - Q1 FY2010
•
MME/S-GW and ASN-GW: Functionally 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 functions, 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 ASNGW. 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 tunneling protocol and UDP for control plane
transport. The specific control messages transferred differ as well and are
defined by corresponding specifications: S1 for LTE and R6 for WiMAX. A
function unique to MME and S-GW is
to interface with legacy 3G networks
(omitted from Figure 3). 3GPP has
defined interfaces from the MME and
S-GW to connect to WCDMA systems
as well as CDMA-1X and EV-DO systems. The WiMAX forum is expected
to define corresponding interfaces
between WiMAX and 3G systems in
future releases.
•
PDN-GW and HA: Functionally speaking, 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 R3 interface is Mobile IPv4 (MIPv4), and in most
instances, the ASN-GW performs Proxy
MIP (PMIP). LTE defines two alternatives 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.
Figure 19 - LTE and WiMAX User Plane Protocol Stacks
Other Architectural Considerations
All IP (Packet-only) Systems: As shown
in Figure 18, LTE and WiMAX are packetonly systems. There are no defined interfaces to circuit switched systems. Moreover, 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 eliminating 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 Gateway (R8 or R6 relay in WiMAX), across BS’
connected to different Access Gateway
(R4 in WiMAX).
Figure 20 - LTE and WiMAX Control Plane Protocol Stacks
Protocol Stacks
The user and control plane stacks further
illustrate the similarities and differences
between LTE and WiMAX and are given in
Figure 19 and Figure 20 respectively. As
shown in Figure 19 the key difference is
that the interface between base-site and
access-gateway uses GTP and S5 uses either 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 station (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).
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Remarks
Remarks
Scalable Bandwidth
LTE: 1.4,3, 5, 10, 15, 20 MHz
WiMAX: 1.25, 5, 10 MHz
Duplexing Mode
LTE is primarily for FDD (though TDD is defined).
WiMAX is primarily for TDD (though FDD is being considered)
Downlink
Transmission
OFDMA
Frequency Bands
MIMO
2x2 (STBC and SM)
LTE: 700, 1700, 1900, 2100, 2500, 2600
WiMAX: 2300, 2500 and 3500
Uplink Transmission
LTE: SC-FDMA
WiMAX: Uplink Transmission is OFDMA
Table 2: Air Interface Similarities
Frame Duration and
LTE: 1 msec frame; subcarrier frequency :15KHz
SubCarrier Frequncy WiMAX: 5 msec frame; subcarrier frequency : 10KHz
Air Interface
Similarities
Table 2 provides the key similarities between LTE and WiMAX air Interface.
•
•
•
Scalable Bandwidth: 3G technologies were designed to operate in a
fixed bandwidth. For example, WCDMA
bandwidth is 5 MHz. Unlike 3G, 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 Multiplexing). 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.
Differences
Table 3 provides the key similarities between LTE and WiMAX air Interface. A little
more detail is provided on these below
Table 3: 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 contiguous spectrum. Cellular/3G 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 technical 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 lowend 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 nonavailability of GPS signals becomes an
additional issue.
IP NGN ARCHITECTURE THOUGHT LEADERSHIP JOURNAL - Q1 FY2010
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 expected to be deployed are quite different. 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 directions. LTE deploys OFDMA on the
downlink but SC-FDMA (Single CarrierFrequency 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 transmissions 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 receiver complexity at the BS compared
to OFDMA receiver). This improve-
these elements are considerably
different (motivated partly by the
existing protocols in 3G systems
and to facilitate backward compatibility with already deployed 3G
systems).
•
Figure 21 - Frequency Bands for LTE and WiMAX
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.
Summary
In this paper, we outlined the key similarities and differences between the technologies. 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 technology and business factors. Given the
relatively similar technology (for example,
OFDM) and design choices, the business
factors will play a bigger role in determining 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 development. 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 implementations: LTE and WiMAX have deployed 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 decomposition (such as separating radio resource
management from IP management and
locating RRM in the BS and IP management in an access gateway). However,
the specific protocols used between
•
Air Interface Efficiency: This is
often a highly debated and contentious 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 [3]
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 3G
(GSM) deployments. WiMAX appears to be the clear choice for operators with TDD spectrum as well
operators with frequency in the 2.5
GHz and 3.5 GHz band and operators with little or no legacy cellular
deployments (mostly in emerging
markets).
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Cisco Systems, Inc.
San Jose, CA
Asia Pacific Headquarters
Cisco Systems (USA) Pte. Ltd.
Singapore
Europe Headquarters
Cisco Systems International BV
Amsterdam, The Netherlands
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
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