<font color=black>[Cover Story] OFDM Spreads to Broadband Communication
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This is the major reason why NTT DoCoMo has decided to switch from single-carrier to OFDM in the development of fourth-generation cell phone service. With OFDM, they say, individual carrier waves can provide stable low-rate data transmission, eliminating problems caused by higher speeds. The firm has developed a new method combining OFDM with CDMA, and plans to begin connection trials from the summer of 2002.
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A key communication technology, orthogonal frequency division multiplex (OFDM) modulation, which can utilize many carriers effectively, is expanding its application field to a variety of broadband communication schemes, including wireless local area network (LAN), asymmetric digital subscriber line (ADSL) and "fourth-generation" cellular telephony.
This modulation technology is being eyed by more than just communication carriers and broadcasting companies, though. High-priority research and development (R&D) projects are under way at a host of audio-visual (AV) and information equipment manufacturers. The reason for this is that OFDM offers unsurpassed advantages in areas like high-speed Internet access for a user who is driving, or the smooth transfer of large data streams like high-definition television (HDTV) -- rather than simply in the area of voice and still pictures.
The strength of OFDM comes from its ability to split the data among multiple carrier waves. Each individual carrier is low-speed and very stable, and together they provide the system with high-speed performance.
Divide, Recombine
In OFDM modulation, data to be sent is first divided, and each portion sent via a separate carrier wave. Adjacent carriers are then sent orthogonally -- perpendicular to each other -- to eliminate interference even if the carrier frequencies are so close they overlap.
The divided data may reduce the per-carrier symbol rate (a symbol is a data unit of one or more bits, sent in a single modulation). If the data is divided between ten carriers, the per-carrier symbol rate can be dropped to a tenth. This provides a wider safety margin, and strongly resists multipath interference caused when reflection or other factors cause it to arrive after the rest of the data signal.
If the number of carriers is simply increased in parallel, the frequency utilization efficiency might begin to drop off sharply, because the total bandwidth might require an increase, and the frequency space provided between adjacent carriers to prevent interference might also be wasted.
One way of solving these problems is to use orthogonal transfer, which is similar to the relationship between cosine and sine waves. When the cosine wave peaks, the sine wave is at zero, and vice-versa. If the center frequency of each wave is adjusted so that the carriers are orthogonal with respect to each other, then there may be no interference even if the carriers are so close that they partially overlap (Fig 1). As a result, the required bandwidth shrinks dramatically.
Alliance with Digital TV
OFDM technology first entered practical use in terrestrial digital broadcasting, beginning in 1998 in the United Kingdom. OFDM is also used in the Digital Video Broadcasting (DVB) European specification as the modulation scheme for terrestrial digital broadcasting. In Japan, OFDM terrestrial digital broadcasting will be launched in Tokyo, Osaka and Nagoya in 2003, and plans call for it to cover the nation by 2006. Implementation in broadcasting is one factor that has led to the adoption of OFDM in wireless LAN application as well.
In the wireless LAN field, Ricoh Corp of the US (the US subsidiary of Ricoh Co, Ltd of Japan) held a demonstration of a transceiver module using OFDM for wireless access with a digital camera, in March 2002. The system was capable of taking a photograph and sending it to a remote personal computer (PC) over the Internet, and also featured a World Wide Web (WWW) browser displayed in the digital camera screen. The maximum data transfer rate was a high 3 Mbits/s.
Sharp Corp, Sony Corp and Matsushita Electric Industrial Co, Ltd have held demonstrations of HDTV-quality Moving Picture Coding Experts Group (MPEG) image transmission over a 5GHz LAN, using OFDM. One target application is, for example, wireless connection between a set-top box and a display. This would mean users wouldn't have to run wires, and as long as the reception range isn't exceeded, they would be able to view an LCD television at any location.
When wireless LAN entered commercial service it enormously expanded the range of application fields for OFDM (Table 1). One such new field includes an electronic toll collection (ETC) system, which collects usage fees such as highway tolls using wireless communication. A different system has already entered use in Japan, but in the US plans are well under way toward using the IEEE802.11a 5GHz LAN specification with OFDM as the physical layer.
OFDM is also likely to show up in personal area networks (PAN), where closely located equipment (within a few meters) is linked via wireless communication. A new working group for the next-generation wireless LAN standard has been established jointly by the IEEE802.11 (wireless LAN) and IEEE802.15 (Bluetooth) standardization groups. The new working group is also considering an integrated wireless LAN/PAN specification using OFDM.
CDMA /OFDM Combo
In the area of cellular phones, engineers are working on developing a version of code division multiple access (CDMA) based on OFDM for use in fourth-generation cell phones, slated for around 2010. Most of them are adopting CDMA to make it possible to use OFDM within the cellular network system, under the name "multicarrier CDMA."
CDMA uses different spread spectrum codes for each user, thereby allowing multiple users to be handled on the same frequency band. This approach increases the number of users that can be handled within a single service cell of the mobile telephone network. Adding the concept of spread spectrum to OFDM, the multicarrier CDMA achieves the best combination of both approaches (Fig 2).
A number of differing technical proposals have been made for multicarrier CDMA, and the final decision has not yet been made for fourth-generation technology (Table 2). Most engineers in the field, however, have narrowed the candidate down to only two possibilities: multicarrier (MC) CDMA, which transfers a given symbol over multiple orthogonal subcarriers; and multicarrier/direct spread (MC/DS) CDMA, which uses multiple subcarriers, which have been directly spread processed (Fig 3). There are reports in the literature that MC-CDMA is better suited to large-capacity data transfer, and as a result it seems increasingly likely that MC-CDMA will be used for the downlink from the basestation to the terminal, and MC/DS-CDMA for the uplink.
Another field in which OFDM has been quite active recently is cable, an area where ADSL has finally won out over carrierless amplitude/phase (CAP), establishing itself as the "standard" with discrete multitone (DMT) modulation. The multiple tones that gave DMT its name are the carriers for the data, and they are orthogonal to each other. In other words, DMT is, in fact, OFDM itself. The application of OFDM in power line modems has also spread like wildfire, with high-speed products offering several dozen megabits per second instead of the more common models running at several hundred kilobits per second. Most of these high-speed designs use OFDM modulation.
Limits to Single Carrier
Ever more demanding requirements are forcing OFDM into the position of lead technology in communications. The reason is simple: conventional methods cannot handle the demands imposed by broadband and ubiquitous networks.
Until the appearance of OFDM, digital communication was almost entirely handled with single-carrier transmission. Unfortunately, this approach faces a major technical difficulty, namely multipath interference, which occurs when waves are reflected off buildings, mountains and such like.
This type of interference can seriously deform the received waveform, making equalizers essential to shape the waveform at the receiver. This is relatively simple if both the transmitter and receiver are immobile, but the situation becomes significantly more complex when mobile devices are involved. This is because adaptive equalizers, capable of responding to the timely changing reflection state in realtime, must be designed. As the data rate increases, the computational load on the adaptive equalizer rises exponentially, and this is what has made it difficult to implement the whole system in silicon.
Spread-spectrum technology, now a trend in the industry, was introduced to solve this problem using a technique called rake reception. Rake reception extracts the receiving signal components that have spatial and temporal differences generated by reflections, and makes a separation into individual signals, which are then synthesized in a uniform phase. This minimizes the affects of multipath interference, and can even make up for the reduced reception sensitivity per symbol at higher speeds, by adding delayed waves. Third-generation cell phones, which rolled out in 2001, use this technique, as do systems like satellite radio broadcasting aimed at fast-moving targets like automobiles.
Even with spread spectrum technology, however, the limits of single-carrier technology are beginning to become apparent. This is because rake reception has a ceiling on speed. NTT DoCoMo Inc of Japan, which is investigating the issue, used simulations to determine reception quality for rake reception with a 100MHz bandwidth. Results showed that problems, such as picking up erroneous symbol signals, increase with speed.
This is the major reason why NTT DoCoMo has decided to switch from single-carrier to OFDM in the development of fourth-generation cell phone service. With OFDM, they say, individual carrier waves can provide stable low-rate data transmission, eliminating problems caused by higher speeds. The firm has developed a new method combining OFDM with CDMA, and plans to begin connection trials from the summer of 2002.
Problems Without OFDM
There have been cases where systems not using OFDM have encountered major problems. The electronic toll collection (ETC) system implemented on expressways in Japan in March 2001 did not use OFDM for reasons of roll-out timing, and instead adopted the amplitude shift keying (ASK) modulation scheme widely used in non-contact IC cards. When the system was put into actual service, a number of unexpected reflections turned up, and engineers were kept busy finding ways to control multipath interference. One university professor recognized as an expert in wireless technology explained, "ASK is an excellent modulation scheme when used in non-contact IC cards and similar applications without reflection. With ETC and in similar applications, though, it is clear that reflection would be a problem. Most of the trouble could have been avoided by using OFDM instead."
The DTV terrestrial digital television (TV) broadcasting used in the US is another example where problems have arisen. Service began in November 1998, using eight-level vestigial sideband (VSB) modulation instead of OFDM. Reception was difficult with indoor antennas or portable equipment. This was because an increase in multipath interference prevented normal reception. In the US, the proportion of cable TV viewers is quite high, and as a result DTV did not spread as much as had been expected. The industry had hoped that they would be able to promote the use of portable receivers and indoor antennas to stimulate market growth; however, their plans went awry.
Answering Today's Needs
The situation has changed considerably. Just why has it now become possible to use OFDM? The key reason is that advances in IC process technology have made it possible to single-chip baseband signal processors (Fig 4). This is a major difference compared to the situation that existed a few years ago.
In 1998, when terrestrial digital TV broadcasting began in the UK, it seemed impossible to manufacture an IC that would fulfill the specifications. The standard called for simultaneous processing of about 8,000 carriers, and the highest performance achievable at the time was only about 2,000. With OFDM, a fast Fourier transform (FFT) circuit is used to batch-process multiple carriers, but it proved surprisingly difficult to implement this circuit in a single chip. This is the reason why in the UK it was decided not to use the single-frequency network (SFN), which is one advantage of OFDM, in the country's terrestrial digital broadcasting system. In Japan, which is gearing up for service in 2003, work is proceeding on the assumption that an SFN system will be used, because it is no longer impossible to make ICs capable of processing 8,000 carriers.
Wireless LAN, which requires a transmit circuit in addition to a receive circuit, can also now employ IC implementations. "It was barely possible with the processes we used one generation ago, and impossible with the processes before that," said Masakiyo Takeuchi, team leader, Development Group 2 , Advanced LSI Technology Development Center, Corporate Development Division, Semiconductor Company, Matsushita Electric Industrial Co, Ltd.
The new baseband processors, made with 0.18micron rule complementary metal-oxide semiconductor (CMOS) technology, even have power to spare, leaving room for innovation. Takeuchi added, "We can make the 10-bit quantization ADC and DAC, but we decided to make it 12 bits of the resolution to achieve even higher-quality communication."
No Proprietary Knowledge
Developing a baseband processor IC is, in fact, even easier than developing one for a single carrier. The reason is that the equalizer circuit does not need OFDM, eliminating any need for proprietary knowledge of the circuit by the developer.
The OFDM scheme itself includes counter-measures against multipath interference, which is the major problem wireless communication faces. A baseband processor merely executes preset processing precisely. Naturally, even OFDM has some circuits for demanding and relatively complex processing, such as establishing synchronization, and compensating for phase rotation occurring on the transmission path. IC chip manufacturers, which have excellent skills in the area of digital circuits, may develop this kind of technology. "An OFDM baseband processor IC has few internal circuits that rely on proprietary manufacturer knowledge," explained Masahiro Morikura, senior research engineer and supervisor, Wireless Access Systems Project, NTT Access Network Service Systems Laboratories, NTT Corp of Japan. "This means many IC manufacturers will enter the market, and competition will drive prices down -- a key factor when it comes to choosing a standard for wireless communication."
Allocating Carriers
Now that the requisite ICs are available, the industry has entered the stage of defining all the details for communication schemes using OFDM technology. Since this assumes that multiple carrier waves will be used, a number of tasks which were difficult or impossible with single-carrier systems can now be implemented easily (Fig 5). For example, it is relatively simple to use different modulation schemes (QPSK, QAM, etc) for different carriers, allocating each a different purpose.
Some examples of allocating carriers by purpose are already on the verge of commercial roll-out. For example, plans call for the several thousand carriers used in Japanese terrestrial digital broadcasting to be grouped into 13 segments, with one or three of them allocated to cell phones, automobiles and other mobile terminals (Fig 6). The people working on formulating a PAN specification based on the OFDM wireless LAN standard are planning to allocate a portion of the wireless LAN carriers to PAN. Focused investigation is scheduled to begin shortly.
As long as the orthogonal relationship is maintained, it should be possible to implement adaptive modulation, setting the modulation individually for each carrier. In fact, tests are under way now. In adaptive modulation, noise, interference and other factors are monitored for each carrier in realtime, and the modulation providing maximum throughput is selected.
Viewing the same situation the other way round, it is also possible to limit the usage for some of the carriers, and merely assure a minimum transfer rate for those. For example, terrestrial digital TV broadcasting in Japan can use quadrature amplitude modulation (QAM) with up to 64 levels, but one of the reception segments is modulated with quadrature phase shift keying (QPSK), instead of all QAM modulation. Compared to 64-level QAM, QPSK can get by with a lower number of quantization bits in the analog-digital converter (ADC) used in the baseband receiver processor.
64-level QAM requires about 10 bits, but reception is possible with about 6 bits using QPSK. In addition, the chip size of the partial-reception IC can be made considerably smaller.
Skipping Frequencies
Another feature of OFDM is that users can freely allocate information among carriers, flexibly responding to the changing environment by avoiding channels adversely affected by interference or noise (Fig 5b).
Power line modems make full use of this feature. The biggest problem in communication over power lines is the reflected waves generated at branches. These cause interference and can prevent communication at specific frequencies. In an OFDM system using multiple carrier frequencies, this problem inherent in power line networks can be overcome. This is because even if specific frequencies cannot be used for reasons of interference or noise, the data can simply be assigned to different frequencies. A source at Sumitomo Electric Industries, Ltd of Japan, which announced a power line modem in February 2002 that operates at up to 45 Mbits/s, commented, "We're still investigating, but it looks like this technology will let us minimize the affects of external sources like ham radio, too."
Sumitomo Electric's power line modem allocates a frequency band of 3.8 to 6.4MHz for the uplink, and of 8 to 12MHz for the downlink. Ham radio interference is severe around 10MHz, and so Sumitomo is considering skipping carriers in the 10MHz region.
The same advantage can probably be utilized in very high bit-rate digital subscriber line (VDSL), a high-speed version of asymmetric digital subscriber line (ADSL). With VDSL, there is competition between discrete multi-tone (DMT), which is the OFDM-equivalent, and CAP for the role of standard specification. On this point, a source at NEC, which has developed a VDSL model running at 51 Mbits/s with DMT, commented, "On telephone lines, noise rises with frequency. If the noise is too high, that frequency should not be used. It is possible [for DMT] to allocate modulation schemes for different frequencies, depending on the noise environment. For this type of application, DMT is superior to CAP."
by Masaharu Tanaka and Hiroki Yomogita
Websites:
Communications Research Laboratory: crl.go.jp
Matsushita Electric Industrial: panasonic.co.jp
NEC: nec.com
NTT: ntt.co.jp
NTT DoCoMo: nttdocomo.co.jp
Ricoh-USA:http:// www.ricoh-usa.com
Sharp: sharp-world.com
Sony:http:// www.sony.co.jp/en/GlobalSites/index.html
Sumitomo Electric Industries: sei.co.jp
Tokyo Broadcasting system:http:// www.tbs.co.jp/index.html
(June 2002 Issue, Nikkei Electronics Asia)
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