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17 June 2009

 

Fujitsu develops impulse-radio-based mm-band transmission equipment exceeding 10Gb/s

At last week’s 2009 IEEE MTT-S International Microwave Symposium (IMS 2009) in Boston (7-12 June), Tokyo-based Fujitsu Limited and its subsidiary Fujitsu Laboratories Ltd of Kawasaki, Japan presented the development of what is claimed to be the first impulse-radio-based high-capacity wireless transmission equipment using millimeter-band transmissions in the 70-100GHz band, resulting in throughput exceeding 10Gb/s. This development work was conducted as part of the 'Research and Development Project for Expansion of Radio Spectrum Resources', sponsored by Japan's Ministry of Internal Affairs and Communications.

In an impulse radio transceiver (Figure 1), there is an RF transmitter (consisting of three components: a short-pulse modulator, a filter, and a power amplifier) and an RF receiver (consisting of a low-noise amplifier, detector, and limiting amplifier). Last year, Fujitsu developed the first RF transmitter (excluding the power amplifier) and subsequently began developing the transmission equipment (including a new RF receiver intended for transmission testing). This has culminated in the new equipment (incorporating the power amplifier in the transmitter and the low-noise amplifier in the receiver).

Figure 1 (above): Impulse-radio millimeter-band transceiver.

To accommodate the growing appetite for bandwidth on the Internet and on wireless networks, a global effort is underway to lay high-capacity fiber-optic trunk lines. For those regions where it is difficult to lay fiber-optic cable, wireless equipment that operates at 10Gb/s - on a par with fiber-optic cable - has been considered.

For wireless transmissions at speeds exceeding 10Gb/s, it is best to use the 70-100GHz millimeter band, as it is relatively easy to secure wide swaths of bandwidth and is thus suitable for long-distance transmissions. However, equipment that operates at these high frequencies requires the use of multiple, single-purpose electronic components, resulting in a high parts count (there has been minimal progress on miniaturizing the equipment and reducing its cost).

An attractive option for development is impulse radio, which involves generating a broadband pulse that varies over extremely short periods of time, and using filters to extract only the usable frequency component for transmission. This dispenses with the bulky oscillators and other components (such as mixers) required in conventional wireless transmission technologies, resulting in more compact and less costly millimeter-band transmission equipment. Fujitsu says that the new technology is suitable as an alternative to fiber-optic trunk lines, and can also be used for a wide range of other potential applications, including indoor ultra-fast wireless local-area networks (WLAN) and high-resolution radar.

However, to send and receive millimeter-band pulse signals using an impulse radio, the following issues - which do not appear in conventional transmission methods - had to be overcome:

• On the receiving side, waveform distortions - introduced into the signal by the wiring path from the antenna to the amplifier - need to be reduced, and the weak millimeter-band signals need to be amplified faithfully.

• On the sending side, the millimeter-band pulse signal sent from the transmitter has a propensity towards time variations. At the receiving end, this can lead to slippage on the determination of a millimeter-band pulse being a 0 or 1, resulting in faulty reception. The time variation hence needs to be minimized.

To address these issues, Fujitsu Laboratories developed the following technologies based on its indium phosphide high-electron-mobility transistors (InP HEMTs):

• Broadband and high-sensitivity receiving technology (receiver). Using InP HEMTs, which offers higher speeds and lower noise than gallium arsenide HEMTs, a broadband, high-gain, low-noise amplifier was developed. This is connected to the receiving antenna by an interconnect that has the reverse transmission characteristics of the low-noise amplifier, so that waveform distortions introduced by the interconnect cancel each other out, resulting in a signal waveform that is close to how it was received.

• High-stability short-pulse modulation technology (transmitter). The time variation in the sent signal is attributable to jitter in the timing of short-pulse generation in the short-pulse modulator. In particular, generating a short pulse from a 10Gb/s data signal with significant timing jitter will make the time variation very conspicuous. Fujitsu Laboratories used a new circuit for the InP HEMT short-pulse generator that generates a short pulse based on a 10GHz clock signal, which has minimal jitter compared to the 10Gb/s data signal, while referencing the 10Gb/s data signal.

Using these technologies, Fujitsu Laboratories developed impulse-radio millimeter-band transmission equipment, including a baseband unit with a fiber-optic interface. The receiver had a sensitivity of 0.25 microwatts (0.25µW) and, along with the sensitivity needed for kilometer-class wireless transmissions, was demonstrated to achieve good received waveforms (Figure 2). The transmitter held jitter on a 10Gb/s millimeter-band pulse signal to 0.3ps (a more than fivefold improvement over the stability achieved in 2008). Indoor transmission testing with a paired transmitter and receiver resulted in the world's first wireless transmissions to exceed 10Gb/s using impulse radio in the millimeter band.

Figure 2 (above): New impulse-radio RF receiver (left) and measurement results of 10Gb/s-reception waveform (right) (y-axis: output voltage; x-axis: time).

Fujitsu says that it will begin field testing the new technology with the aim of developing commercial systems by 2012.

See related item:

Fujitsu develops first impulse radio transmitting at over 10Gb/s in millimeter band

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