1) What is OFDM?
Ans: OFDM (Orthogonal Frequency Division Multiplexing) is a broadband multicarrier modulation method that offers superior performance and benefits over older, more traditional single-carrier modulation methods because it is a better fit with today’s high-speed data requirements and operation in the UHF and microwave spectrum.
2) How does OFDM work?
Ans: OFDM is based on the concept of frequency-division multiplexing (FDD), the method of transmitting multiple data streams over a common broadband medium. That medium could be radio spectrum, coax cable, twisted pair, or fiber-optic cable. Each data stream is modulated onto multiple adjacent carriers within the bandwidth of the medium, and all are transmitted simultaneously. A good example of such a system is cable TV, which transmits many parallel channels of video and audio over a single fiber-optic cable and coax cable.
3) Why has there been all the interest in OFDM in the past few years?
Ans: OFDM has been adopted as the modulation method of choice for practically all the new wireless technologies being used and developed today. It is perhaps the most spectrally efficient method discovered so far, and it mitigates the severe problem of multipath propagation that causes massive data errors and loss of signal in the microwave and UHF spectrum.
4) Name some of the wireless technologies that use OFDM?
Ans: The list is long and impressive. First, it is used for digital radio broadcasting. It is used in TV broadcasting.You will also find it in wireless local-area networks (LANs) like Wi-Fi. The wideband wireless metro-area network (MAN) technology WiMAX uses OFDM. And, the almost completed 4G cellular technology standard Long-Term Evolution (LTE) uses OFDM. The high-speed short-range technology known as Ultra-Wideband (UWB) uses an OFDM standard set by the WiMedia Alliance. OFDM is also used in wired communications like power-line networking technology. One of the first successful and most widespread uses of OFDM was in data modems connected to telephone lines. ADSL and VDSL used for Internet access use a form of OFDM known as discrete multi-tone (DMT). And, there are other less well known examples in the military and satellite worlds.
5) How is OFDM implemented in the real world?
Ans: OFDM is accomplished with digital signal processing (DSP). We can program the IFFT and FFT math functions on any fast PC, but it is usually done with a DSP IC or an appropriately programmed FPGA or some hardwired digital logic. With today’s super-fast chips, even complex math routines like FFT are relatively easy to implement. In brief, we can put it all on a single chip.
6) What are the benefits of using OFDM?
Ans: The first reason is spectral efficiency, also called bandwidth efficiency. What that term really means is that you can transmit more data faster in a given bandwidth in the presence of noise. The measure of spectral efficiency is bits per second per Hertz, or bps/Hz. For a given chunk of spectrum space, different modulation methods will give you widely varying maximum data rates for a given bit error rate (BER) and noise level. Simple digital modulation methods like amplitude shift keying (ASK) and frequency shift keying (FSK) are only fair but simple. BPSK and QPSK are much better. QAM is very good but more subject to noise and low signal levels. Code division multiple access (CDMA) methods are even better. But none is better than OFDM when it comes to getting the maximum data capacity out of a given channel. It comes close to the so called Shannon limit that defines channel capacity C in bits per second (bps) as
C = B * log2(1 + S/N)Here, B is the bandwidth of the channel in hertz, and S/N is the power signal-to-noise ratio. With spectrum scarce or just plain expensive, spectral efficiency has become the holy grail in wireless.
7) What else makes OFDM so good?
Ans: OFDM is highly resistant to the multipath problem in high-frequency wireless. Very short-wavelength signals normally travel in a straight line (line of sight, or LOS) from the transmit antenna to the receive antenna. Yet trees, buildings, cars, planes, hills, water towers, and even people will reflect some of the radiated signal. These reflections are copies of the original signal that also go to the receive antenna. If the time delays of the reflections are in the same range as the bit or symbol periods of the data signal, then the reflected signals will add to the direct signal and create cancellations or other anomalies. The result is what we usually call Raleigh fading.
8) What are the downsides to OFDM?
Ans: Like anything else, OFDM is not perfect. It is very complex, making it more expensive to implement. However, modern semiconductor technology makes it pretty easy. OFDM is also sensitive to carrier frequency variations. To overcome this problem, OFDM systems transmit pilot carriers along with the subcarriers for synchronization at the receiver. Another disadvantage is that an OFDM signal has a high peak to average power ratio. As a result, the complex OFDM signal requires linear amplification. That means greater inefficiency in the RF power amplifiers and more power consumption.
9) What is OFMDA?
Ans: The A stands for access. It means that OFDM is not only a great modulation method, it also can provide multiple access to a common bandwidth or channel to multiple users. You are probably familiar with multiple access methods like frequency-division multiplexing (FDM) and time division multiplexing (TDM). CDMA, the widely used cellular technology, digitally codes each digital signal to be transmitted and then transmits them all in the same spectrum. Because of their random nature, they just appear as low-level noise to one another. The digital coding lets the receiver sort the individual signal out later. OFDMA permits multiple users to share a common bandwidth with essentially the same benefits.
10) Is there anything better than OFDM?
Ans: Not right now. What makes OFDM even better is MIMO, the multiple-input multiple-output antenna technology.