OFDM for Mobile Data
Communications
Definition
Orthogonal frequency division multiplexing (OFDM) is a communications
technique that divides a communications channel into a number of equally
spaced frequency bands. A subcarrier carrying a portion of the user informationis transmitted in each band. Each subcarrier is orthogonal (independent of eachother) with every other subcarrier, differentiating OFDM from the commonlyused frequency division multiplexing (FDM).

Overview
This tutorial describes OFDM and its application to mobile communications.
OFDM is a modulation and multiple-access technique that has been explored formore than 20 years. Only recently has it been finding its way into commercial
communications systems, as Moore's Law has driven down the cost of the signal
processing that is needed to implement OFDM–based systems.
OFDM, or multitone modulation as it is sometimes called, is presently used in a
number of commercial wired and wireless applications. On the wired side, it isused for a variant of digital subscriber line (DSL). For wireless, OFDM is the basis
for several television and radio broadcast applications, including the European
digital broadcast television standard, as well as digital radio in North America.
OFDM is also used in several fixed wireless systems and wireless local-area
network (LAN) products. A system based on OFDM has been developed to deliver
mobile broadband data service at data rates comparable to those of wired
services, such as DSL and cable modems.
It is important that the overall system design be well matched to the service
profiles to maximize the performance of the system and balance the ultimate user
experience it provides relative to the cost to the operator. OFDM enables thecreation of a very flexible system architecture that can be used efficiently for awide range of services, including voice and data. For any mobile system to createa rich user experience, it must provide ubiquitous, fast, and user-friendly
connectivity. OFDM has several unique properties that make it especially well

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suited to handle the challenging environmental conditions experienced by mobile
wireless data applications.

Topics1. Characteristics and Evolution of Data Communications for a Mobile
Environment
2. Overview of the Wireless Environment
3. Overview of Traditional Mobile Wireless Systems
4. OFDM for Mobile Communications
5. Leading-Edge Mobile OFDM Technologies
6. Conclusion
Self-Test
Correct Answers
Glossary

1. Characteristics and Evolution of Data
Communications for a Mobile Environment
Cellular and personal communications service (PCS) communications systems
have historically been designed with voice traffic in mind. The patterns associatedwith voice communications are well known, having been observed since theinvention and widespread use of the telephone. Voice can be characterized as
relatively predictable, with each party talking about half the time in an interactive
manner. The statistics of call duration and time of day are well understood,
allowing traffic engineers to use a standard methodology to estimate the amountof capacity needed in a communications system. The wireline telephone network
has been engineered in a hierarchical fashion using large circuit switches toefficiently connect one voice user to another. The physical circuit over which acall is made is held open for the entire duration of a call, hence the term circuit
switching. Voice in wireline and mobile settings have similar characteristics.
Existing cellular telephone systems have therefore been designed in a similar way
and optimized to efficiently provide voice service.
Data traffic differs from voice in at least three important ways:
1. First, data traffic is much more unpredictable than voice traffic. Data ischaracterized as bursty, meaning that there is significant variability inwhen the traffic arrives, the rate at which it arrives, and the number of bitsin the messages.
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2. Second, data has very different requirements in terms of reliability.
Whereas voice is very robust and capable of being understood even in anoisy environment with a high bit-error rate (BER) approaching one
percent, data applications require extremely reliable delivery, with
virtually no tolerance for bit errors. Because some bit errors areunavoidable on wireless links, it is important that fast and efficient
recovery schemes are implemented to get the correct data bits to theapplication. A combination of forward error correction (FEC) and fastacknowledgements (an automatic retransmission request [ARQ]) satisfies
this need. Powerful FEC is employed to dramatically reduce the BER, andan ARQ is used to guarantee reliable delivery.
3. Third, data traffic encompasses a much different and wider range ofservices than voice. Different types of services have different requirements
along several dimensions. A data service can be characterized by its
importance or priority. This is determined by the quality of service (QoS)
that is required, which can be measured by the amount of delay that a user
is willing to tolerate, and the reliability required. A service provider may
offer differing service rates, or classes of service (CoS), accordingly.
Premium service users may be given priority over best-effort users, whose
traffic is sent if there is capacity available at the time. The data rate that isrequired and granted to a user is another dimension for a data service. A
user may have a service-level agreement (SLA), which guarantees a certain
minimum rate and allows a maximum or average rate over some period oftime. A final aspect of a data service is latency, or response time. This
determines the degree of interactivity that can be achieved, which is ameasure of how quickly channel resources can be assigned at the requestof a user.
Many feel that data services differ from voice in one other way, which is related to
the variability in capacity demanded by the end user. If data users are allowed toconsume as much bandwidth as they can, provided that there are no higherpriority users contending for resources, a system will tend to always be in a stateof high utilization. The admission control mechanism, which governs how users
access the system and are allocated resources, becomes a potential bottleneck
under such circumstances. To provide low latencies in a wireless environment,
where errors are unavoidable and packets must be retransmitted, it is necessary
for a system to employ a fast ARQ capability so that packets received in error canbe quickly retransmitted. Fast ARQ, in turn, requires that the user's access to thesystem be quick in order to transmit acknowledgements upon the receipt ofcorrect packets. Systems employing a contention-based admission control
generally exhibit ever-growing latencies as utilization increases and cannot
support a fast ARQ capability. A well-designed system employing scheduled, noncontention-based access can yield much lower latency and support rapid ARQs.

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Today's data traffic is primarily driven by wireline users and generated by a broad
range of sources and applications, including Internet use, electronic messaging,file transfer, and, to some degree, voice traffic such as voice over Internet
protocol (VoIP) (using primarily desktop and portable computers). The
transmission control protocol (TCP)/Internet protocol (IP) suite, which is thedata-transmission protocol used for the Internet, is the most widely used andgoverns the bulk of all data traffic.
In the future, the number of devices generating data traffic is expected toskyrocket by an order of magnitude. Information appliances and other datacentric wireless and consumer products will fuel this growth, leveraging theinherent value that mobility provides.
Figure 1. Wireless User versus Data-Device Growth
Sources: Cahners In-Stat, Goldman Sachs, IDC, Lehman
Brothers, Semico

The types and mix of applications and services used will also change over time as
the mix of devices changes, with more interactive applications such as voice,gaming, two-way messaging, and even video joining less-interactive applications
such as streaming audio and video and traditional Web browsing. Much of thistraffic will go over wireless data networks.
The true potential for ubiquitous wireless data communications can be unleashedwhen end-user devices efficiently support native TCP/IP connectivity, without
the need for special translators and filters. Mobile wireless communications has
traditionally posed a difficult performance challenge for TCP/IP protocols. TCP
was designed and optimized around reliable wireline links, where BERs and
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packet-error rates are substantially lower than that typically achievable via
wireless. When TCP encounters dropped or lost packets, it assumes that there iscongestion on the link, but not that the link itself is unreliable. Congestion ishandled by reducing the information rate at which the sender is allowed totransmit. By interpreting the unavoidable errors that occur in a wirelessenvironment as congestion, the effective data rate seen by the end user isreduced. This is further compounded by the fact that the initial data rate at thestart of a TCP session is low—far below the ultimate peak rate—and graduallybuilds over time as the systems figure out where the peak rate is. This slow-start
aspect of TCP can dramatically add to latencies as the link is throttled down due
to errors and then slowly ramped back up.
In summary, there is a much wider range of requirements and characteristics fordata communications than there is for voice. This variability prohibits data frombeing efficiently carried over the hierarchical networks designed for voice traffic,
whether wireline or wireless. Mobile data systems face additional challenges as aresult of the vagaries of the wireless environment.

2. Overview of the Wireless Environment
Mobile cellular wireless systems operate under harsh and challenging channel
conditions. The wireless channel is distinct and much more unpredictable than
the wireline channel because of factors such as multipath and shadow fading,
Doppler spread, and time dispersion or delay spread. These factors are all related
to variability that is introduced by the mobility of the user and the wide range ofenvironments that may be encountered as a result.
Multipath is a phenomenon that occurs as a transmitted signal is reflected by
objects in the environment between the base station and a user. These objects canbe buildings, trees, hills, or even trucks and cars. The reflected signals arrive at
the receiver with random phase offsets, because each reflection generally follows
a different path to reach the user's receiver. The result is random signal fades as
the reflections destructively (and constructively) superimpose on one another,which effectively cancels part of the signal energy for brief periods of time. The
degree of cancellation, or fading, will depend on the delay spread of the reflectedsignals, as embodied by their relative phases, and their relative power. Figure 2
shows the arrival of a signal and two multipath components.

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Figure 2. Example of Time-Delayed Multipath Signals

Figure 3. Demonstration of Multipath Reflections

Time dispersion represents distortion to the signal and is manifested by the
spreading in time of the modulation symbols. This occurs when the channel isband-limited, or, in other words, when the coherence bandwidth of the channel issmaller than the modulation bandwidth. Time dispersion leads to intersymbolinterference, or ISI, where the energy from one symbol spills over into anothersymbol, and, as a result, the BER is increased. It also leads to fading.
In many instances, the fading due to multipath will be frequency selective,
randomly affecting only a portion of the overall channel bandwidth at any given
time. Frequency selective fading occurs when the channel introduces timedispersion and when the delay spread exceeds the symbol period. When there is
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no dispersion and the delay spread is less than the symbol period, the fading will
be flat, thereby affecting all frequencies in the signal equally. Flat fading can leadto deep fades of more than 30 decibels (dB).
Figure 4. Demonstration of Time-Varying Fading

Doppler spread describes the random changes in the channel introduced as aresult of a user's mobility and the relative motion of objects in the channel.
Doppler has the effect of shifting, or spreading, the frequency components of asignal. The coherence time of the channel is the inverse of the Doppler spreadand is a measure of the speed at which the channel characteristics change. This ineffect determines the rate at which fading occurs. When the rate of change of thechannel is higher than the modulated symbol rate, fast fading occurs. Slowfading, on the other hand, occurs when the channel changes are slower than thesymbol rate.
The statistics describing the fading signal amplitude are frequently characterized
as either Rayleigh or Ricean. Rayleigh fading occurs when there is no line of sight(LOS) component present in the received signal. If there is a LOS component
present, the fading follows a Ricean distribution. There is frequently no direct
LOS path to a mobile, because the very nature of mobile communications meansthat mobiles can be in a building or behind one or other obstructions. This leads
to Rayleigh fading but also results in a shadow loss. These conditions, along with
the inherent variation in signal strength caused by changes in the distance
between a mobile and cell site, result in a large dynamic range of signals, which
can easily be as much as 70 dB.
In addition to the aforementioned channel impairments, spectrum is a scarce
resource for wireless systems, and thus is reused within cellular systems. Thismeans that the same frequencies are allocated to each cell, or to a cluster of cells,
and are shared. This increases the overall system capacity at the expense ofincreased potential for interference within a cell and between cells as each
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channel is reused throughout the system. This generally results in cellular
systems being interference-limited.

3. Overview of Traditional Mobile Wireless
Systems
All modern mobile wireless systems employ a variety of techniques to combat the
aforementioned effects. Some techniques are more effective than others, with theeffectiveness depending on the air-interface and the system-architecture
approach taken to satisfy the requirements of the services being offered. As
mobile systems evolved from analog to digital, more sophisticated signalprocessing techniques have been employed to overcome the wirelessenvironment. These techniques include equalization, channel or error-correction
coding, spread spectrum, interleaving, and diversity.
Diversity has long been used to help mitigate the multipath-induced fading that
results from users' mobility. The simplest diversity technique, spatial diversity,
involves the use of two or more receive antennae at a base station that are
separated by some distance, say on the order of five to 10 wavelengths. The signal
from the mobile will generally follow separate paths to each antenna. This
relatively low-cost approach yields significant performance improvement by
taking advantage of the statistical likelihood that the paths are not highly
correlated with each other. When one antenna is in a fade, the other one will
generally not be.
Spread spectrum systems employ a form of diversity called frequency diversity.
Here the signal is spread over a much larger bandwidth than is needed fortransmission and is typically greater than the coherence bandwidth of thechannel. A wideband signal is more resistant to the effect of frequency selective
fading than is a narrowband signal, because only a relatively small portion of theoverall bandwidth use is likely to experience a fade at any given time. There are
two forms of spread spectrum, code division multiple access (CDMA) andfrequency hopping (FH).
CDMA systems, such as those used in interim standard (IS)–95 and thirdgeneration (3G) wideband CDMA (WCDMA), mix the base-band information
stream with a much higher rate pseudorandom spreading sequence code prior totransmission. This effectively increases the signal bandwidth. As a result, CDMAsystems are termed spread spectrum. The spreading of the signal provides
processing gain, which is used to limit the effects of multipath and Doppler
fading, and allows the same frequency spectrum to be used by each user in eachcell, provided that the transmitted power of every user is limited to only what isneeded for acceptable reception. The amount of processing gain is determined by
the ratio of the spread bandwidth to the bandwidth of an individual user's data
stream.
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Each user is assigned a unique spreading code when they have information tosend. The same frequencies can be used by each user, because the receiver, which
is listening for that user's specific code, is able to effectively ignore other users'
signals as long as the codes are orthogonal. A problem that CDMA systems haveis that the code sequences are not truly orthogonal in the presence of multipath
delay spread. This results in interference between users within a cell. Called
multiple access interference, it ultimately limits the capacity of the cell.
CDMA systems also employ time diversity. The multipath signals that arereceived can be time and phase adjusted so that they can be added together aslong as the delay is more than one code symbol or chip time. This occurs in what
is called a RAKE receiver.
Spread spectrum systems provide the further performance advantage ofinterference averaging, which is achieved as the user's signal occupies the entire
system bandwidth—either through spreading or hopping. A CDMA systemspreads out any interference uniformly over the entire band, making it appear asa small increase in the overall noise floor, while a frequency hopping systemaverages interference by minimizing the time when a user and interferer occupy
the same frequency. In a cellular system, the majority of interference comes fromother users rather than from external sources. Although more than half of theinterference present in a CDMA cell is due to the users in that cell (rather thanfrom other cells), the interference is still effectively averaged over all users so thatno particular user is disproportionately disadvantaged.
Interference averaging enables a system to be radio frequency (RF) engineered
for the average interference experienced rather than the worst-case interference
experienced, which allows a higher degree of frequency reuse to be used. This
increases the overall system capacity.
Systems based on CDMA are very complex, with intensive signal processing
required to implement them. Fast and accurate power-control loops must be used
so that the signal from one user does not drown out the signals from other users,and each cell must be tightly synchronized. Errors in power control and timesynchronization will result in reduced capacity and system performance.
Equalization is a technique used to overcome the effects of ISI resulting fromtime dispersion in the channel. Implemented at the receiver, the equalizer
attempts to correct for the amplitude and phase distortions that occur in thechannel. These distortions change with time, because the channel response istime varying. The equalizer must therefore adapt to, or track, the changing
channel response to eliminate the ISI. The equalizer is fed a fixed-length training
sequence at the start of each transmission, which enables it to characterize thechannel at that time. A training sequence may also be sent periodically tomaintain the equalizer's characterization of the channel.

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Systems based on time division multiple access (TDMA), such as IS–136 andGlobal System for Mobile Communications (GSM) typically must use equalizers
because their modulation symbol rate exceeds the coherence bandwidth of thechannel (i.e., they operate in band-limited channels). The TDMA system divides
the available spectrum into a number of equal bandwidth channels, and thensubdivides each channel into timeslots according to a framing structure. The
frame structure defines the duration of each time slot and how often they are
repeated. Figure 5 shows an example of a TDMA structure with a 20-millisecond
frame consisting of three time slots.
Figure 5. TDMA Frame Structure Showing Recurring Slots

TDMA systems assign one or more time slots to a user for transmission. There istypically some guard time included between time slots to allow for time tracking
errors at the mobile station. The use of equalizers adds to the complexity andcosts of the TDMA systems, since equalization requires significant amounts ofsignal-processing power. The need to transmit a fixed sequence of training bits
also adds overhead to the communications. Unlike CDMA–based systems, TDMA
systems cannot use every frequency in every cell because of co-channelinterference, and therefore need to be frequency planned. TDMA systems also
have less inherent immunity against multipath fading than CDMA systems,
because they use a much narrower signal bandwidth. On the plus side, TDMA
users within a cell are orthogonal to each other, because they transmit at different
times. Therefore, there is no intra-cell interference.

4. OFDM for Mobile Communications
OFDM represents a different system-design approach. It can be thought of as a
combination of modulation and multiple-access schemes that segments a
communications channel in such a way that many users can share it. Whereas
TDMA segments are according to time and CDMA segments are according to
spreading codes, OFDM segments are according to frequency. It is a techniquethat divides the spectrum into a number of equally spaced tones and carries a
portion of a user's information on each tone. A tone can be thought of as afrequency, much in the same way that each key on a piano represents a uniquefrequency. OFDM can be viewed as a form of frequency division multiplexing(FDM), however, OFDM has an important special property that each tone isorthogonal with every other tone. FDM typically requires there to be frequency
guard bands between the frequencies so that they do not interfere with eachother. OFDM allows the spectrum of each tone to overlap, and because they are
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orthogonal, they do not interfere with each other. By allowing the tones tooverlap, the overall amount of spectrum required is reduced.
Figure 6. OFDM Tones

OFDM is a modulation technique in that it enables user data to be modulated
onto the tones. The information is modulated onto a tone by adjusting the tone's
phase, amplitude, or both. In the most basic form, a tone may be present or
disabled to indicate a one or zero bit of information, however, either phase shift
keying (PSK) or quadrature amplitude modulation (QAM) is typically employed.
An OFDM system takes a data stream and splits it into N parallel data streams,
each at a rate 1/N of the original rate. Each stream is then mapped to a tone at aunique frequency and combined together using the inverse fast fourier transform(IFFT) to yield the time-domain waveform to be transmitted.
For example, if a 100-tone system were used, a single data stream with a rate of 1
megabit per second (Mbps) would be converted into 100 streams of 10 kilobits
per second (kbps). By creating slower parallel data streams, the bandwidth of themodulation symbol is effectively decreased by a factor of 100, or, equivalently,
the duration of the modulation symbol is increased by a factor of 100. Proper
selection of system parameters, such as the number of tones and tone spacing,
can greatly reduce, or even eliminate, ISI, because typical multipath delay spread
represents a much smaller proportion of the lengthened symbol time. Viewed
another way, the coherence bandwidth of the channel can be much smaller,
because the symbol bandwidth has been reduced. The need for complex multi-tap
time-domain equalizers can be eliminated as a result.
Figure 7. OFDM Transmitter Chain

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OFDM can also be considered a multiple-access technique, because an individual
tone or groups of tones can be assigned to different users. Multiple users share agiven bandwidth in this manner, yielding the system called OFDMA. Each user
can be assigned a predetermined number of tones when they have information tosend, or alternatively, a user can be assigned a variable number of tones based on
the amount of information that they have to send. The assignments are controlled
by the media access control (MAC) layer, which schedules the resource
assignments based on user demand.
OFDM can be combined with frequency hopping to create a spread spectrum
system, realizing the benefits of frequency diversity and interference averaging
previously described for CDMA. In a frequency hopping spread spectrum system,
each user's set of tones is changed after each time period (usually corresponding
to a modulation symbol). By switching frequencies after each symbol time, thelosses due to frequency selective fading are minimized. Although frequency
hopping and CDMA are different forms of spread spectrum, they achieve
comparable performance in a multipath fading environment and provide similarinterference averaging benefits.
OFDM therefore provides the best of the benefits of TDMA in that users are
orthogonal to one another, and CDMA, as previously mentioned, while avoiding
the limitations of each, including the need for TDMA frequency planning andequalization, and multiple access interference in the case of CDMA.
Figure 8. Two-Dimensional Illustration of OFDM Channel
Resource

Theory of OFDM Operation
The sinusoidal waveforms making up the tones in OFDM have the very special
property of being the only Eigen-functions of a linear channel. This special
property prevents adjacent tones in OFDM systems from interfering with one
another, in much the same manner that the human ear can clearly distinguish
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between each of the tones created by the adjacent keys of a piano. This property,
and the incorporation of a small amount of guard time to each symbol, enables
the orthogonality between tones to be preserved in the presence of multipath.
This is what enables OFDM to avoid the multiple-access interference that ispresent in CDMA systems.
The frequency domain representation of a number of tones, shown in Figure 6,
highlights the orthogonal nature of the tones used in the OFDM system. Noticethat the peak of each tone corresponds to a zero level, or null, of every other tone.
The result of this is that there is no interference between tones. When thereceiver samples at the center frequency of each tone, the only energy present isthat of the desired signal, plus whatever other noise happens to be in the channel.
To maintain orthogonality between tones, it is necessary to ensure that thesymbol time contains one or multiple cycles of each sinusoidal tone waveform.
This is normally the case, because the system numerology is constructed suchthat tone frequencies are integer multiples of the symbol period, as issubsequently highlighted, where the tone spacing is 1/T. Viewed as sinusoids,
Figure 9 shows three tones over a single symbol period, where each tone has aninteger number of cycles during the symbol.
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Figure 10. Integer Number of Sinusoid Periods

In absolute terms, to generate a pure sinusoidal tone requires the signal start attime minus infinity. This is important, because tones are the only waveform thancan ensure orthogonality. Fortunately, the channel response can be treated as
finite, because multipath components decay over time and the channel iseffectively band-limited. By adding a guard time, called a cyclic prefix, thechannel can be made to behave as if the transmitted waveforms were from timeminus infinite, and thus ensure orthogonality, which essentially prevents one
subcarrier from interfering with another (called intercarrier interference, or ICI).
The cyclic prefix is actually a copy of the last portion of the data symbol appended
to the front of the symbol during the guard interval, as shown in Figures 9 and 11.
Multipath causes tones and delayed replicas of tones to arrive at the receiver withsome delay spread. This leads to misalignment between sinusoids, which need to
be aligned as in Figure 11 to be orthogonal. The cyclic prefix allows the tones tobe realigned at the receiver, thus regaining orthogonality.

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Figure 11. Cyclic Extension of Sinusoid

The cyclic prefix is sized appropriately to serve as a guard time to eliminate ISI.
This is accomplished because the amount of time dispersion from the channel issmaller than the duration of the cyclic prefix. A fundamental trade-off is that thecyclic prefix must be long enough to account for the anticipated multipath delay
spread experienced by the system. The amount of overhead increases, as thecyclic prefix gets longer. The sizing of the cyclic prefix forces a tradeoff between
the amount of delay spread that is acceptable and the amount of Doppler shiftthat is acceptable.

5. Leading-Edge Mobile OFDMTechnologies
Unlike most existing forms of wireless access, including 3G technologies,
conventional wireless systems have been designed primarily at the physical layer.
To address the unique demands posed by mobile users of high-speed dataapplications, new air interfaces must be designed and optimized across all thelayers of the protocol stack, including the networking layers. A prime example ofthis kind of optimization is found in flash-OFDM™ technology by Flarion. As its
name suggests, the system is based on OFDM, however, flash-OFDM is much
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exploits the unique physical properties of OFDM, enabling significant higherlayer advantages that contribute to very efficient packet data transmission in acellular network.

Packet Switched Air Interface
The telephone network, designed basically for voice, is an example of circuitswitched systems. Circuit-switched systems exist only at the physical layer thatuses the channel resource to create a bit pipe. They are conceptually simple as thebit pipe is a dedicated resource, and there is no control of the pipe required once
it is created (some control may be required in setting up or bringing down thepipe). Circuit-switched systems, however, are very inefficient for burst data
traffic.
Packet-switched systems, on the other hand, are very efficient for data traffic but
require control layers in addition to the physical layer that creates the bit pipe.
The MAC layer is required for the many data users to share the bit pipe. The link
layer is needed to take the error-prone pipe and create a reliable link for the
network layers to pass packet data flows over. The Internet is the best example ofa packet-switched network.
Because all conventional cellular wireless systems, including 3G, were
fundamentally designed for circuit -witched voice, they were designed andoptimized primarily at the physical layer. The choice of CDMA1 as the physicallayer multiple-access technology was also dictated by voice requirements. FlashOFDM, on the other hand, is a packet-switched designed for data and is
optimized across the physical, MAC, link, and network layers. The choice of
OFDM as the multiple-access technology is based not just on physical-layer
consideration, but also on MAC–, link-, and network-layer requirements.

Physical-Layer Advantages
As discussed earlier, most of the physical-layer advantages of OFDM are wellunderstood. Most notably, OFDM creates a robust multiple-access technology todeal with the impairments of the wireless channel, such as multipath fading,delay spread, and Doppler shifts. Advanced OFDM–based data systems typicallydivide the available spectrum into a number of equally spaced tones. For each
OFDM symbol duration, information carrying symbols (based on modulationsuch as QPSK, QAM, etc.) are loaded on each tone.
Flash-OFDM uses fast hopping across all tones in a pseudorandom
predetermined pattern, making it a spread spectrum technology. With fast
hopping, a user that is assigned one tone does not transmit every symbol on thesame tone, but uses a hopping pattern to jump to a different tone for every
symbol. Different base stations use different hopping patterns, and each uses the
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entire available spectrum (frequency reuse of 1). In a cellular deployment, thisleads to all the advantages of CDMA systems, including frequency diversity2 andout of cell (intercell) interference averaging—a spectral-efficiency benefit thatnarrowband systems such as conventional TDMA do not have.
As discussed earlier, different users within the same cell use different resources(tones) and hence do not interfere with each other. This is similar to TDMA,
where different users in a cell transmit at different time slots and do not interferewith one another. In contrast, CDMA users in a cell do interfere with each other,increasing the total interference in the system. Flash-OFDM therefore has thephysical-layer benefits of both CDMA and TDMA and is at least three times more
efficient than CDMA. In other words, at the physical layer, flash-OFDM creates
the fattest pipe of all cellular technologies. Even though the 3x advantage at thephysical layer is a huge advantage, the most significant advantage of flash-OFDM
for data is at the MAC and link layers.

MAC and Link-Layer Advantages
Flash-OFDM exploits the granular nature of resources in OFDM to come up withextremely efficient control layers. In OFDM, when designed appropriately, it ispossible to send a very small amount (as little as one bit) of information from thetransmitter to the receiver with virtually no overhead. Therefore, a transmitterthat is previously not transmitting can start transmitting, transmit as little as one
bit of information, and then stop, without causing any resource overhead. This isunlike CDMA or TDMA, in which the granularity is much coarser and to merely
initiate a transmission wastes a significant resource. Hence, in TDMA, forexample, there is a frame structure, and whenever a transmission is initiated, aminimum of one frame (a few hundred bits) of information is transmitted. The
frame structure does not cause any significant inefficiency in user datatransmission, as data traffic typically consists of a large number of bits. However,for transmission of control-layer information, the frame structure is extremely
inefficient, as the control information typically consists of one or two bits but
requires a whole frame. Not having a granular technology can therefore be very
detrimental from a MAC– and link-layer point of view.
Flash-OFDM takes advantage of the granularity of OFDM in its control-layer
design, enabling the MAC layer to perform efficient packet switching over the air
and at the same time providing all the hooks to handle QoS. It also supports alink layer that uses local (as opposed to end-to-end) feedback to create a very
reliable link from an unreliable wireless channel, with very low delays. The
network layer's traffic therefore experiences small delays and no significant delay
jitter. Hence, interactive applications such as (packet) voice can be supported.
Moreover, Internet protocols such as TCP/IP run smoothly and efficiently over a
flash-OFDM airlink. TCP/IP performance on 3G networks is very inefficientbecause the link layer introduces significant delay jitter so that channel errors are
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misinterpreted by TCP as network congestion and TCP responds by backing off tothe lowest rate.
Packet switching leads to efficient statistical multiplexing of data users and helps
the wireless operators to support a much higher number of users for a given user
experience. This, together with QoS support and a 3x fatter pipe, allows theoperators to profitably scale their wireless networks to meet the burgeoning datatraffic demand in an all-you-can-eat pricing environment.
1 3G system in Europe (WCDMA) and the United States (CDMA 2000) are based on CDMA technology.
3 Frequency diversity provides immunity in a fading environment, where a users' signal spans a wide spectrum and usually does notfade at the same time.

6. Conclusion
This tutorial highlights the unique design challenges faced by mobile datasystems that result from the vagaries of the harsh wireless channel, the wide andvaried service profiles that are enabled by data communications, and theperformance of wireline-based protocols, such as TCP/IP, with the realities ofwireless links. OFDM has been shown to address these challenges and to be a key
enabler of a system design that can provide high-performance mobile data
communications.
OFDM is well positioned to meet the unique demands of mobile packet data
traffic. Nevertheless, to seamlessly unwire all the IP applications inherent in thewired Internet and intranets (including interactive data applications and peer-topeer applications), all layers of the OFDM air interface need to be jointly
designed and optimized from the ground up for the IP data world. This means to
not rely solely on OFDM's physical layer advantages, but rather to leverage theminto all of the higher layers of the system.

Self-Test
1. What makes the wireless channel more unpredictable than the wireline
channel?
a. Multipath
b. Doppler spreadc. Shadow fadingd. All of the above
2. Rayleigh fading occurs when there is:
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a. A LOS component present
b. No LOS component present
c. Intercell interferenced. None of the above
3. Spread spectrum systems employ a form of diversity called _________,
where the signal is spread over a much larger bandwidth than is needed.
a. Frequency diversity
b. Spatial diversity
c. Bandwidth diversity
d. Radio diversity
4. The use of equalizers adds to the complexity and costs of the ______
systems, because equalization requires significant amounts of signalprocessing power.
a. CDMAb. TDMAc. OFDM
d. FDMA
5. With ______, fast and accurate power-control loops must be used so that thesignal from one user does not drown out the signals from other users, andeach cell must be tightly synchronized. Errors in power control and timesynchronization will result in reduced capacity and system performance.
a. CDMAb. TDMAc. OFDM
d. FDMA
6. Both OFDM and CDMA systems have code sequences that are truly
orthogonal in the presence of multipath delay spread.
a. True
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b. False
7. Whereas TDMA segments are according to time and CDMA segments areaccording to spreading codes, OFDM segments are according to frequency.
a. Trueb. False
8. OFDM therefore provides the best of the benefits of TDMA (in that users are
orthogonal to one another) and CDMA, thus enabling frequency diversity andout-of-cell interference.
a. Trueb. False
9. Time dispersion leads to intersymbol interference, or ISI, where the energy
from one symbol spills over into another symbol, decreasing the BER as aresult.
a. Trueb. False
10. OFDM allows the spectrum of each tone to overlap, and because they are
orthogonal, they do not interfere with each other. By allowing the tones tooverlap, the overall amount of spectrum required is reduced.
a. Trueb. False

Correct Answers
1. What makes the wireless channel more unpredictable than the wireline
channel?
a. Multipath
b. Doppler spreadc. Shadow fadingd. All of the above

See Topic 2
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2. Rayleigh fading occurs when there is:a. A LOS component present
b. No LOS component present
c. Intercell interferenced. None of the above

See Topic 3
3. Spread spectrum systems employ a form of diversity called _________,
where the signal is spread over a much larger bandwidth than is needed.
a. Frequency diversity
b. Spatial diversity
c. Bandwidth diversity
d. Radio diversity

See Topic 3
4. The use of equalizers adds to the complexity and costs of the ______
systems, because equalization requires significant amounts of signalprocessing power.
a. CDMAb. TDMAc. OFDM
d. FDMA

See Topic 3
5. With ______, fast and accurate power-control loops must be used so that thesignal from one user does not drown out the signals from other users, andeach cell must be tightly synchronized. Errors in power control and timesynchronization will result in reduced capacity and system performance.
a. CDMAb. TDMA
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c. OFDM
d. FDMA

See Topic 3
6. Both OFDM and CDMA systems have code sequences that are truly
orthogonal in the presence of multipath delay spread.
a. Trueb. False

See Topic 3
7. Whereas TDMA segments are according to time and CDMA segments areaccording to spreading codes, OFDM segments are according to frequency.
a. Trueb. False

See Topic 4
8. OFDM therefore provides the best of the benefits of TDMA (in that users are
orthogonal to one another) and CDMA, thus enabling frequency diversity andout-of-cell interference.
a. Trueb. False

See Topic 4
9. Time dispersion leads to intersymbol interference, or ISI, where the energy
from one symbol spills over into another symbol, decreasing the BER as aresult.
a. Trueb. False

See Topic 2
10. OFDM allows the spectrum of each tone to overlap, and because they are
orthogonal, they do not interfere with each other. By allowing the tones tooverlap, the overall amount of spectrum required is reduced.
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a. Trueb. False

See Topic 4

Glossary
3G
Third-Generation
ARQ
Automatic Retransmission Request
BER
Bit-Error RateCDMA
Code Division Multiple AccessdB
DecibelDSL
Digital Subscriber LineFDM
Frequency Division Multiplexing
FEC
Forward Error Correction
FH
Frequency Hopping
ICI
Intercarrier Interference
IFFT
Inverse Fast Fourier Transform
IP
Internet Protocol
ISI
Intersymbol Interference
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LOS
Line of Sight
MAC
Media Access Control
OFDM
Orthogonal Frequency Division Multiplexing
PCS
Personal Communications Service
PSK
Phase Shift Keying
QAM
Quadrature Amplitude Modulation
QoS
Quality of Service
RFRadio Frequency
SLA
Service-Level Agreement
TCP
Transport Control Protocol
TDMATime Division Multiple Access
VoIP
Voice over Internet Protocol
WCDMA
Wideband Code Division Multiple Access

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