Wireline,wirelessOFDMisusedinvariouswirelineandwirelesstechnologiesincludingADSL,802.11a/g/n,digitalaudiobroadcasting,DTVandUWB.Itisalsousedinmobilebroadcasttechnologiessuchasforwardlinkonly(FLO),whichoffersmobileTVservicesviatheMediaFLOsystem.Manydesigntrade-offsmustbeconsideredwhendevelopinganOFDM-basedsystem.Themostfundamentaltrade-offisthenumberofsubcarriers(transformsize)andtheguardintervaldu
Wireline, wireless
OFDM is used in various wireline and wireless technologies including ADSL, 802.11a/g/n, digital audio broadc
asting, DTV and UWB. It is also used 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 an OFDM-based system. The most fundamental trade-off is the number of subcarriers (transform size) and the guard interval duration.
The FLO PHY layer uses a transform size of 4,096 subcarriers (4K mode) and a guard interval defined as one-eighth of the nominal FLO OFDM symbol duration. The 4K mode provides improved mobile
performance compared with an 8K mode, while retaining a suf- ficiently long guard interval that is useful in fairly large SFN cells. Robust performance can then be maintained to greater than
200kph, with graceful degradation beyond. This is supported by the FLO pilot structure (used for channel estimation), which enables receivers to handle delay spreads greater than the guard interval.
In OFDM, information is impressed on a tone by phase and amplitude modulation. Each subcarrier is typically modulated with QPSK or QAM. The FLO air interface supports the use of QPSK, 16QAM and layered modulation techniques. It also incorporates error correction and coding techniques, including turbo inner and Reed-Solomon outer codes.
Rapid TV channel change is achieved through an optimized pilot and interleaver structure design, which also assures time diversity. The pilot structure and interleaver designs optimize channel utilization while ensuring fast channel change.
FLO-transmitted signals are organized into super frames (Figure 2, above). Each super frame consists of four frames of data, including the TDM pilots, overhead information symbols (OIS), and frames containing wide- and local-area data.
The TDM pilots are provided to allow acquisition of the OIS, which describes the location of the data for each media service carried in the super frame.
Frequency diversity
Each super frame consists of 200 OFDM symbols per megahertz of allocated bandwidth (1,200 symbols for 6MHz). Each symbol contains seven interlaces of data-bearing subcarriers.
Each interlace is uniformly distributed in frequency to achieve the full frequency diversity within the available bandwidth. These interlaces are assigned to logical channels that vary in duration and number of interlaces used.
This provides flexibility in the time diversity achieved by a given data source. Lower data rate channels can be assigned fewer interlaces to improve time diversity, while higher data rate channels may use more interlaces to minimize the radios on-time. The acquisition time for both low and high data rate channels is the same. Frequency and time diversity are maintained without compromising acquisition time.
Multicast logical channels (
MLCs) are used to carry realtime content at variable rates to obtain statistical multiplexing gains that are possible with variable rate codecs. Each MLC has a specific, independent ?
