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Call for Proposals for IEEE 802.15 TG6 (08-0811-03)
This document proposed an impulse radio ultra-wideband physical layer. The basic mode uses burst position modulation in order to support non-coherent energy detection. The enhanced mode uses concatenated burst modulation to achieve extremely power efficient communications, up to 27.2 Mbps with less than 10 mW.
PHY layer proposal to be considered for adoption by TG6
This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.
This document proposes an impulse radio ultra-wideband physical layer. The proposal is inspired by the IEEE 802.15.4a standard but contains modifications to reduce the complexity and/or power consumption.
Impulse radio communication uses pulses with duration in the order of nanoseconds to transfer the data. As a result, they fall under the regulations for ultra-wide band, which allow license-free operation in a large part of the spectrum world-wide. In order to satisfy those regulations, the devices will have a very low duty cycle ratio, typically well below 10%, meaning that for most of time they don’t transmit any pulses. Impulse radios support power efficient communications by switching off the analogue circuitry when nothing is transmitted.
The scarce nature of the air interface means that impulse radio is also well suited to support simultaneous multi-user communications. By employing time hopping, collisions can be avoided. Further use of spreading codes can minimize the impact of collisions. By varying the number of pulses over which a data bit is spread, data rates can easily be traded off for range. The result is a robust system that can support many nodes in an uncoordinated fashion.
The low spectral emissions (-41.3 dBm/MHz) specified by the regulatory bodies ensures that interference from UWB devices to other apparatus is avoided. Particularly in the context of BAN, this also means that the user is exposed to very little RF energy. Thanks to the wide bandwidth available, UWB devices themselves can easily avoid interference from other devices.
The first impulse radio UWB systems were based on isolated pulses. Since every transmission requires a small start-up time to allow the circuits to settle before the transmission of the actual pulse, there is an overhead that degrades the duty cycling performance. In order to minimize this overhead, the IEEE 802.15.4a standard groups the pulses corresponding to a bit in a continuous burst.
The burst phase shift keying - burst position modulation specified in 15.4a supports a wide range of receiver architectures. At low data rates, duty cycling enables power efficient communications, while the sparse transmissions support multi-user operation. However, at higher data rates, the overhead start-up and shut-down times of the circuits start to dominate and power consumption rapidly escalates. At the same time, the guard periods become too small to avoid inter-symbol interference. In order to use the burst phase modulation mandated in 15.4a, extremely stable phase references are required.
In order to avoid these difficulties, two variations are included in this proposal. The first resembles 15.4a most closely but uses burst position modulation without the burst phase shift keying. This basic mode is intended for lower data rates and supports non-coherent energy detector receivers. A second mode uses concatenated burst modulation. By concatenating the bursts in strings, the duty cycling overhead remains constant irrespective of the data rate. Moreover, the inter-symbol interference which also affects the high data rates of 15.4a can efficiently be mitigated using frequency domain equalization. The resulting enhanced mode leads to extremely power efficient data transmission.
The PHY proposal outlined in this document is a part of IMEC’s UWB PHY/MAC proposal. The complete proposal is made of this PHY used in combination with the UWB MAC presented in doc: 15-09-0332-00-0006.
Basic packet structure
In this document, the data passed from the higher layers to the PHY for transmission is referred to as the payload. The payload can be transmitted using either burst position modulation or concatenated burst modulation.
To assist receivers with timing recovery, transmission starts with a preamble which is modulated using isolated pulses drawn from a ternary alphabet. The preamble is followed by a burst position modulated PHY header that informs the receiver of the structure of the following payload.