LTE Tutorials - 3 - OFDM FAQ

Q: What is OFDM?

A: OFDM 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.

Q: Is OFDM a new technique?

A: No. Conceptually, it has been known since at least the 1960s and 1970s. Originally known as multicarrier modulation, as opposed to the traditional single-carrier modulation, OFDM was extremely difficult to implement with the electronic hardware of the time. So, it remained a research curiosity until semiconductor and computer technology made it a practical method.

Q: Why has there been all the interest in OFDM in the past few years?

A: 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.

Q: What are some of the wireless technologies that use OFDM?

A: The list is long and impressive. First, it is used for digital radio broadcasting—specifically Europe’s DAB and Digital Radio Mondial. It is used in the U.S.’s HD Radio. It is used in TV broadcasting like Europe’s DVB-T and DVB-H. You will also find it in wireless local-area networks (LANs) like Wi-Fi. The IEEE 802.11a/g/n standards are based on OFDM. 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.

Q: How does OFDM work?

A: 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.

Q: Is that how OFDM works today?

A: Sort of. The FDD technique is typically wasteful of bandwidth or spectrum because to keep the parallel modulated carriers from interfering with one another, you have to space them with some guard bands or extra space between them. Even then, very selective filters at the receiving end have to be able to separate the signals from one another. What researchers discovered is that with digital transmissions, the carriers could be more closely spaced to one another and still separate. That meant less spectrum and bandwidth waste.

Q: Given the multiple parallel channels, what is the actual modulation process?

A: The serial digital data stream to be transmitted is split into multiple slower data streams, and each is modulated onto a separate carrier in the allotted spectrum. These carriers are called subcarriers or tones. The modulation can be any form of modulation used with digital data, but the most common are binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM). The outputs of all the modulators are linearly summed, and the result is the signal to be transmitted. It could be upconverted and amplified if needed.

Q: That sounds like a straightforward approach. Is OFDM really implemented this way?

A: Not really. OFDM works best, as explained later, if hundreds or even thousands of parallel subcarriers are used. To implement that with hardware is a challenge even with modern semiconductor technology. It’s just not done. Instead, the whole process can be accomplished in computer hardware by using the fast Fourier transform (FFT) or, more specifically for the transmitter, the inverse FFT (IFFT).

Q: I don’t have time for a math lesson, so give me a quick overview of the FFT.

A: The FFT is a variation of the discrete Fourier transform (DFT). Fourier, as you may remember from your college math days, was the Frenchman who discovered that any complex signal could be represented by a series of harmonically related sine waves all added together. He also developed the math to prove it. The math is difficult, and even early computers couldn’t perform it quickly. Cooley/Tukey developed the fast Fourier transform in the 1960s as a way to greatly speed up the math to make Fourier analysis more practical. In general, you can take any analog signal, digitize it in an analog-to-digital converter (ADC), and then take the resulting samples and put them through the FFT process. The result is essentially a digital version of a spectrum analysis of the signal. The FFT sorts all the signal components out into the individual sine-wave elements of specific frequencies and amplitudes—a mathematical spectrum analyzer of a sort. That makes the FFT a good way to separate out all the carriers of an OFDM signal.

Q: Then how does the IFFT work?

A: The IFFT just reverses the FFT process. All the individual carriers with modulation are in digital form and then subjected to an IFFT mathematical process, creating a single composite signal that can be transmitted. The FFT at the receiver sorts all the signals to recreate the original data stream.

Q: Just how does the FFT process keep the individual modulated carriers from interfering with one another?

A: This is where the term “orthogonal” comes in. Orthogonal really means at a right angle to. The signals are created so they are orthogonal to one another, thereby producing little or no interference to one another despite the close spacing. In more practical terms, it means that if you space the subcarriers from one another by any amount equal to the reciprocal of the symbol period of the data signals, the resulting sinc (sin x/x) frequency response curve of the signals is such that the first nulls occur at the subcarrier frequencies on the adjacent channels. Orthogonal subcarriers all have an integer number of cycles within the symbol period. With this arrangement, the modulation on one channel won’t produce intersymbol interference (ISI) in the adjacent channels.

Q: How is OFDM implemented in the real world?

A: OFDM is accomplished with digital signal processing (DSP). You 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, you can put it all on a single chip.

Q: What are the benefits of using OFDM?

A: 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 x 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.

Q: What else makes OFDM so good?

A: 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.

Q: How does OFDM deal with this?

A: The high-speed serial data to be transmitted is divided up into many much lower-speed serial data signals. Then OFDM sends these lower-data-rate signals over multiple channels. This makes the bit or symbol periods longer, so multipath time delays have less of an effect. The more subcarriers used over a wider bandwidth, the more resistant the overall signal is to the multipath phenomenon. This means you can use the higher frequencies with fewer multipath effects to worry about. But the really good news is that you can use them in mobile situations where either the transmitter or receiver or both are moving and undergoing changing environmental conditions with good signal reliability.

Q: What are the downsides to OFDM?

A: 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.

Q: What is OFMDA?

A: 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.

Q: How is OFDMA accomplished?

A: The OFDM system assigns subgroups of subcarriers to each user. With thousands of subcarriers, each user would get a small percentage of the carriers. In a modern system like the 4G LTE cellular system, each user could be assigned from one to many subcarriers. In LTE, subcarrier spacing is 15 kHz. Using a 10-MHz band, the total possible number of subcarriers would be 666. In practice, a smaller number like 512 would be used. If each subscriber is given six subcarriers, you could put 85 users in the band. The number of subcarriers assigned will depend on the user’s bandwidth and speed needs.

Q: Is there anything better than OFDM?

A: Not right now. What makes OFDM even better is MIMO, the multiple-input multiple-output antenna technology. It is currently used in 802.11n Wi-Fi and the forthcoming LTE. Look for MIMO in another FAQ Tutorial.

OFDM or OFDMA?

IEEE 802.16d (fixed service) uses Orthogonal Frequency Division Multiplexing (OFDM). IEEE 802.16e (mobile) uses Orthogonal Frequency Division Multiple Access (OFDMA). So, what’s the difference between the two, and why is there a difference?

OFDM is sometimes referred to as discrete multi-tone modulation because, instead of a single carrier being modulated, a large number of evenly spaced subcarriers are modulated using some m-ary of QAM. This is a spread-spectrum technique that increases the efficiency of data communications by increasing data throughput because there are more carriers to modulate. In addition, problems with multi-path signal cancellation and spectral interference are greatly reduced by selectively modulating the “clear” carriers or ignoring carriers with high bit-rate errors.

The OFDM spread-spectrum scheme is used for many broadly used applications, including digital TV broadcasting in Australia, Japan and Europe; digital audio broadcasting in Europe; Asynchronous Digital Subscriber Line (ADSL) modems and wireless networking worldwide (IEEE 802.11a/g).

OFDM allows only one user on the channel at any given time. To accommodate multiple users, a strictly OFDM system must employ Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels). Neither of these techniques is time or frequency efficient: TDMA is a time hog and FDMA is a bandwidth hog.

OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers, as discussed above). WiMAX uses OFDMA, extended OFDM, to accommodate many users in the same channel at the same time.

OFDMA distributes subcarriers among users so all users can transmit and receive at the same time within a single channel on what are called subchannels. What’s more, subcarrier-group subchannels can be matched to each user to provide the best performance, meaning the least problems with fading and interference based on the location and propagation characteristics of each user.

The WiMAX forum established that, initially, OFDM-256 will be used for fixed-service 802.16d (2004). It is referred to as the OFDM 256 FFT Mode, which means there are 256 subcarriers available for use in a single channel. Multiple access on one channel is accomplished using TDMA. Alternatively, FDMA may be used.

On the other hand, OFDMA 128/512/1024/2048 FFT Modes have been proposed for IEEE 802.16e (mobile service). OFDMA 1024 FFT matches that of Korea’s WiBRO. OFDM 256 also is supported for compatibility with IEEE 802.16d (fixed, 2004). The final IEEE 802.16e standard is expected to be completed and published in December of this year.

The bottom line is that, most likely, the finalized selection for the OFDMA mode will be 1024 FFT, to be compatible with WiBRO. However, it will not be compatible with the OFDM 256 FFT Mode initially specified for WiMAX fixed service. Perhaps service providers will simply abandon 802.16d in favor of 802.16e for both fixed and mobile services.