October 25, 2005
Technology in Focus features analysis of recent technology news articles, by the consultants in Technology Group, IDA. This is the top pick of the month from a list of 10-20 news analysis compiled monthly.
UWB Competitor Squeezes More Bits Through Limited Spectrum, 16 Sep 05
by Yeap Yean Wei, Associate Consultant, Network Technologies
Background - Shortage of Spectrum
The major problem currently faced by the world is the increasing shortage of the usable frequency spectrum. The rule maker provisions the spectrum usage in the nation and usually takes the approach of fixed spectrum allocation for wireless technologies. As more and more wireless technologies emerge, the rules makers are now confronting with spectrum scarcity in the air. Due to this reason, the Federal Communications Commission (FCC) adopted cognitive or "smart" radio systems to facilitate new wireless technologies and to permit more efficient spectrum usage. This technology enables radio equipment (i.e. base station) or device (i.e. handheld) and its antenna to adapt its use of spectrum based on the real-time conditions of its operating environment. The development is currently spearheaded by the Software Define Radio (SDR) Forum.
Practical limit of Existing Broadband Wireless Technologies
The main challenge, on the other hand, for equipment vendors is how to squeeze more bits into limited radio spectrum using higher modulation and coding scheme, enhanced with the advance techniques such as Multiple Input Multiple Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM). Conventionally data is modulated onto the fixed narrowband carrier. If more data is squeezed, high order of modulation scheme (i.e. 256-QAM) will be used and the required signal to noise ratio will be many times higher. To date, 256-QAM is the practical limit that a narrowband carrier can take. To further increase the data rate, a technology known as OFDM can be used to squeeze even more bits by multiplexing multiple narrowband sub-carriers. This technique is already used in the UWB, WLAN, WiMax technologies.
Although OFDM allows more bits to be squeezed into the spectrum, but each sub-carrier has its practical limit of an amount of bits that can be squeezed into. If extremely large amount of bits are to be transmitted simultaneously, the bandwidth required will be proportionally large. For instances, NTT DoCoMo demonstrated wireless transmission of 1 Gbps using OFDM technology in 100MHz of bandwidth. This implies that huge data modulated onto OFDM technology is still requiring considerably wide bandwidth.
xMax Solution
To address the dilemma of lack spectrum bandwidth, a company called xG Technology has developed an UWB-like xMax that squeezes broadband into narrowband channels. Conceptually, xMax is a conventional narrowband radio frequency technology with low-powered wideband Pulse Position Modulation (PPM) that permit spectrum reuse. The narrowband carrier is not carrying any information as other wireless technology does. On the contrary, it is used to coordinate and synchronise the transmitter and receiver only. The data is transmitted in the side bands, which the company called it "Flash Signal", at levels typically 60dB to 100dB below the narrowband carrier. The information-bearing signal is spread over maximum 100MHz sideband, giving it the appearance of an UWB system. The UWB encodes one bit across hundreds or thousands of pulses whereas xMax encodes one bit per pulse, giving the system a greater data density per MHz of bandwidth used. The novelty of this technology is its capabilities to reject the noise produced by other transmitter. It is a disputable fact that high power amplifier in every transmitter amplifies the signal as well as the noise. The key to xGs claimed lies in their receiver technology, which will reject the noise produced by other transmitters. This enhancement can give their system as much as 49dB advantage, which significantly improve the signal detection of tens of thousand times weaker.
Performance of xMax Trial
Assuming this advantage is real, the transmitter basically does not need to transmit at very high power. In the tests over the air in Florida, xG said that a 900MHz transmitter running at 150mW achieving a range of around 50km from an antenna at 260m, outperform a standard system running at 50W. This may also implies that the network employing xMax required far fewer base stations than existing technologies require. Since high power transmission is not required, the battery operated device can be hence prolonged. A bonus to xMax is that it does not interfere with other radio systems and yet it is able to rejects interference from the others. In the prototype, xG used portions of the published WiMax specifications, making this a part-WiMax, pre-xMax network.
xG envisaged that fixed radio networks would be the first candidate to use xMax technology. The company claimed that xMax could be a serious contender for 4G technologies, too. According to xG, the reason is that xMax technology can provide high rate, battery friendly and lower-cost network by increasing range. xG promised to have public demonstration in the year end, delivering share bandwidth of 40 Mbps over 15 miles, using less than 1W of power at sub-GHz frequencies. xG will be formally launched in November and more testing will be carried out. The public will soon able to see if xMax is more than a pipe dream or beyond reality.
Some words about the writer
Yeap Yean Wei is an associate consultant with the Technology Group, tasked with the responsibility of identifying and driving the adoption of emerging wireless telecommunication technologies in Singapore. He specializes in a wide range of wireless and radio frequency technologies, including ultra-wideband (UWB), fixed and mobile broadband wireless access.
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