Evaluating OFDM as a mobile TV technology driver - Embedded.com

Evaluating OFDM as a mobile TV technology driver


Orthogonal frequency-division multiplexity (OFDM)has its roots in the military communication systems from the late1950s.Patented by Bell Labs in 1970,OFDM is based on the Fast Fourier Transform (FFT),which is a mathematical concept. Joseph Fourierdeveloped the basis for the FFT, while James Cooley and John Tukeydeveloped the FFT algorithm.

FFTs are used in a wide variety of applications. In broadcast andcommunication systems, FFT allows individual channels to maintain theirorthogonality – or distance – to adjacent channels. These techniquesallow data symbols to be reliably extracted and multiple subchannels tooverlap in the frequency domain for increased spectral efficiency.

While cellular systems such as time division multiple access (TDMA)use time to segment a communication channel and CDMAsegments according to spreading codes, OFDM segments according tofrequency.

It divides the spectrum into a number of equally spaced carriers andallocates a portion of a system's information on each tone orsubcarrier. OFDM can be viewed as a form of frequencydivision multiplexing (FDM). It allows the bundling of data overnarrowband carriers transmitted in parallel over different frequencies.

High bandwidth is achieved by using these parallel subchannels thatare spaced apart at precise frequencies while being as close aspossible without overlapping or interfering. Thus, they're orthogonal,as made possible by the FFT.

To be relatively immune to effects like selective channel-fading,OFDM systems incorporate error correction. The design of coding for anOFDM system is central to making OFDM an effective method. However,building OFDM-based systems has multiple considerations, which dependon the operating environment and application requirements.

Figure1: Each subcarrier is modulated at a low data rate, making symbolduration longer. A guard interval or cyclic prefix is appended to eachsymbol.

Mobile systems typically operate under challenging and unpredictablechannel conditions. The wireless channel is variable due to factorssuch as multipath andshadow fading,time dispersion, and Doppler or delay spread.These factors are all related to variability, which is introduced bythe user's mobility and the various environments that may beencountered by a receiver.

Signal reflections
Multipath occurs as a transmitted signal, which is reflected by objectsin the environment between the transmitter and receiver. These objectsinclude buildings, trees, hills or even automobiles. The transmittedsignal arrives at the receiver in various paths of different length.

Time dispersion represents distortion to the signal. This ismanifested by the spreading in time of the modulation symbols. Itoccurs when the coherence bandwidth of the channel is smaller than themodulation bandwidth. Time dispersion leads to intersymbolinterference, where the energy from one symbol spills over into anothersymbol, thus increasing the BER.

Doppler spread represents the random changes in the channelintroduced as a result of a user's mobility and a relative motion ofobjects in the channel. Doppler has the effect of shifting or spreadingthe frequency components of a signal.

The coherence time of the channel is the inverse of the Dopplerspread and is a measure of the speed at which the channelcharacteristics change. In effect, this determines the rate at whichfading occurs. When the channel's rate of change is higher than themodulated symbol rate, fast fading occurs. Slow fading, on the otherhand, occurs when the channel changes are slower than the symbol rate.

OFDM can be used to facilitate singlefrequency networks (SFN). In this configuration, the channelbandwidth is divided into many narrow subcarriers that are transmittedconcurrently. Each subcarrier is modulated at a low data rate, makingsymbol duration longer. A guard interval or cyclic prefix is appendedto each symbol (Figure 1, above ).

This period of repeated data allows the multiple arrivals (due tomultipath) to combine constructively while allowing the orthogonalityof the subcarriers to be maintained. The guard interval's duration istypically set to capture the most important arrivals. The performancelimits are partially determined by interference from signals with longdelays that exceed the maximum delay difference for constructivecombining outside the guard interval window.

Figure2: FLO-transmitted signals are organized into super frames.

Wireline, wireless
OFDM is used in various wireline and wireless technologies including ADSL,802.11a/g/n, digitalaudio broadcasting, DTV and UWB. It is alsoused in mobile broadcast technologies such as forward link only (FLO),which offers mobile TV services via the MediaFLO system.

Many design trade-offs must be considered when developing anOFDM-based system. The most fundamental trade-off is the number ofsubcarriers (transform size) and the guard interval duration.

The FLO PHY layer uses a transform size of 4,096 subcarriers (4Kmode) and a guard interval defined as one-eighth of the nominal FLOOFDM symbol duration. The 4K mode provides improved mobile performancecompared with an 8K mode, while retaining a suf- ficiently long guardinterval that is useful in fairly large SFN cells. Robust performancecan then be maintained to greater than 200kph, with gracefuldegradation beyond. This is supported by the FLO pilot structure (usedfor channel estimation), which enables receivers to handle delayspreads greater than the guard interval.

In OFDM, information is impressed on a tone by phase and amplitudemodulation. Each subcarrier is typically modulated with QPSK or QAM.The FLO air interface supports the use of QPSK, 16QAM and layeredmodulation techniques. It also incorporates error correction and codingtechniques, including turbo inner and Reed-Solomon outer codes.

Rapid TV channel change is achieved through an optimized pilot andinterleaver structure design, which also assures time diversity. Thepilot structure and interleaver designs optimize channel utilizationwhile ensuring fast channel change.

FLO-transmitted signals are organized into super frames (Figure 2, above ). Each super frameconsists of four frames of data, including the TDM pilots, overheadinformation symbols (OIS), and frames containing wide- and local-areadata.

The TDM pilots are provided to allow acquisition of the OIS, whichdescribes the location of the data for each media service carried inthe super frame.

Frequency diversity
Each super frame consists of 200 OFDM symbols per megahertz ofallocated bandwidth (1,200 symbols for 6MHz). Each symbol containsseven interlaces of data-bearing subcarriers.

Each interlace is uniformly distributed in frequency to achieve thefull frequency diversity within the available bandwidth. Theseinterlaces are assigned to logical channels that vary in duration andnumber of interlaces used.

This provides flexibility in the time diversity achieved by a givendata source. Lower data rate channels can be assigned fewer interlacesto improve time diversity, while higher data rate channels may use moreinterlaces to minimize the radio's on-time. The acquisition time forboth low and high data rate channels is the same. Frequency and timediversity are maintained without compromising acquisition time.

Multicast logical channels (MLCs) are used to carry realtime contentat variable rates to obtain statistical multiplexing gains that arepossible with variable rate codecs. Each MLC has a specific,independent coding rate and modulation, which provides support forvarious reliability and QoS depending on theapplication requirements.

To minimize power consumption, the FLO multiplexing scheme enablesdevice receivers to just demodulate the content of one or more logicalchannels that it is interested in.

The principal driving force of OFDM's increased popularity is thedesire for faster wireless technologies and increase in multimediaapplications, which require higher speeds and spectral efficiency. Thisis particularly illustrated in the use of OFDM in a modern technologysuch as FLO, which was designed from OFDM's fundamental principles tosupport mobile TV. This enabled it to deliver optimal channel changetime and performance for mobile devices without compromising powerconsumption.

Souheil Gallouzi is SeniorDirector, Product Management at MediaFLO Technologies, Qualcomm Inc. To read a PDF version of this story, goto “ Dealwith OFDM, a new old technology driver.”

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