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Multichannel basebands meet challenge of cognitive radio

Posted: 16 Nov 2004 ?? ?Print Version ?Bookmark and Share

Keywords:ieee? 802.11? wlan? chipset? wideband?

There are real challenges for cognitive radio to overcome, but when it comes to ensuring on-going channel and radio sensing and minimum network disruption when an incumbent system is detected, today's IEEE 802.11 WLAN chipset providers are already ahead of the game. These providers rely on two specific methods: utilization of a wideband receiver or a multichannel baseband. Each method has its relative advantages and disadvantages.

CR itself is a methodology for opportunistic utilization of fallow spectrum and can be categorized into two broad classes: Unlicensed CRs operating in the unlicensed bands and unlicensed CRs operating in the licensed bands. Each class has unique challenges to ensure successful operation.

The second category is particularly challenging since there are parts of the radio spectrum that are used by passive receivers such as radio astronomy where weak distant objects are being observed. A typical signal power in radio astronomy is less than a trillionth of a watt. To avoid interference with such systems, CRs must accurately determine its location and avoid utilizing that part of the spectrum. One solution is to focus on systems operating in the unlicensed radio or other bands where non-passive transceivers operate are being under utilized by their rightful owner. These bands will be available to be used as if they were unlicensed. However, devices that operate in these bands will need to adhere to certain operating rules. In particular, this requires meeting today's WLAN requirements for systems operating in the newly allocated 5.47GHz to 5.725GHz band and the 3.65GHz to 3.7GHz band currently under consideration.

The incumbent owner has the right of first use of these bands, as such a CR must sense the presence of the incumbent spectrum owner and vacate within a short period with a minimum number of transmissions. It must also perform communications with a minimum amount of transmit power. In the 5.47GHz to 5.725GHz band, the incumbent users are radio location, radio navigation and meteorological radars.

In the 3.650GHz to 3.700GHz band, the incumbent user is the C-band fixed-space to earth-satellite service. The transmission characteristics can vary significantly even within the same band. According to ITU-R M.1461, pulse repetition rates for radars operating in the 5GHz band can vary from 20 to 100,000 pulses per second and the 3dB bandwidth of the transmitter varies from approximately 500kHz to approximately 150MHz. This is a particularly challenging identification problem.

Detection must be successful irrespective of the varied transmission characteristics of the incumbent device and irrespective of the instantaneous RF propagating conditions. To ensure that minimum power is transmitted, the device must determine the minimum power that will be necessary to maintain communications in a dynamic RF environment. Furthermore, to avoid unnecessarily declaring a channel as occupied and causing significant disruption to the network, the designer must trade-off the stringent requirements of the probability of detection against an unnecessarily high probability of false alarm.

Usually 802.11 system designers perform this trade-off by combining an energy detect threshold that can be triggered by noise with a correlator that detects packet preambles. The initial energy detect can be used to adapt the gain in the front-end of the receiver while the correlator can be used to establish if a valid packet is truly present. Using a static energy detect threshold is problematic as the duration and bandwidth of the incumbent signals is so variable.

Designers must build a spectrum analysis capability into their chipset to effectively contend with this situation. Fortunately, there is a spectral analysis engine already present in OFDM systems; this is the FFT that is used to demodulate the received data. To use this same FFT to analyze the environment a modification is required to the design of a standard 802.11 receiver.

Another challenging problem that needs to be addressed is that of moving the network and resuming communications with minimum disruption to the network. In the European Telecommunications Standards Institute regulatory domain, a radio LAN must vacate a channel in 10s after the first pulse is detected with a channel closing time, i.e., maximum transmission time during a channel move, of 260ms. Furthermore, before a channel can be utilized it must be sensed for a minimum of 60s. This presents some significant challenges. One of the most important is detecting the interferer while maintaining communications and simultaneously being prepared to move to a new channel without ceasing operations and disruption communications for at least 60s.

To ensure ongoing channel and radio sensing and minimum network disruption when an incumbent system is detected, there are two methods: utilization of a wideband receiver or multichannel baseband. Below we describe the benefits and disadvantages of each approach.

The wideband receiver is a system that senses a larger part of the spectrum than the current channel that is used to maintain communications. This approach has the advantage of being able to provide an accurate assessment of a large part of the spectrum at any one instant in time. Furthermore, because a large number of samples are being used, it can also detect objects with a faster time transients. This is a simple application of the uncertainty principal.

The major disadvantage of this type of design is that it requires fast and highly accurate ADCs. Also, large dynamic range is required in the ADC since energy from the transmitter will be observed at the transmitter while one is simultaneously trying to establish if other parts of the spectrum are idle. Typically a signal of 20dBm is being transmitted while the received signals can be in the order of -90dBm. Although DSP techniques and intelligent design can reduce the dynamic range requirements, this presents a significant challenge. For example, to sense the 2.4GHz to 2.483GHz band with 70dB dynamic range, 12bit ADCs operating at greater than 160MSps are needed. While converters with this speed and resolution are possible with today's technologies, extending this approach to the 4.9GHz to 6GHz band is not practical. Covering this complete part of spectrum is essential for worldwide compliance.

An alternative CR approach is that of a multichannel baseband. In this system, a chipset for the access point, or station for that matter, has another independent channel operating constantly. The aim of this second channel is to continuously characterize the RF environment detecting any incumbent devices. Furthermore, since this "secondary or monitoring" baseband is independent of the primary base band, it has the ability to independently sense any part of the spectrum. Since the monitoring baseband needs only to characterize a small part of the spectrum at any one time, it is possible using today's technologies to build a system that scans the 4.9GHz to 6.0GHz region for an open channel or the 2.4GHz to 2.483GHz or even 3.65GHz to 3.7GHz bands (when this spectrum becomes available).

This system can provide a list of open channels to the network allowing the network to quickly communicate to the nodes the availability of another channel without causing a 60s delay in network operations. This device can also detect rogue access points over the whole spectrum and reports the information to the network manager for appropriate action.

- Efstratios Skafidas

Chief Technology Officer

Bandspeed Inc.

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