Biomedical Engineering Reference
In-Depth Information
complexity. Due to the short pulse width and low power level of the transmitted
signal, the front-end circuitry of an UWB receiver is complex in design and has
high power consumption [
1
]. Synchronization of the IR-UWB pulses at receive
stage using low power front end circuitry is one of the major problems that
restricts the use of IR-UWB receivers for implant applications.
MAC protocols for UWB systems govern the multiple access of the UWB
channel. The MAC protocols for UWB systems have to be designed in a way such
that they enhance the advantages provided by the UWB signals and overcome the
drawbacks such as high receiver complexity [
2
]. In general, MAC protocols based
on carrier sensing and Clear Channel Assessment (CCA) are not appropriate for
UWB based MAC protocols, because it is extremely difficult to assess the channel
condition of a wideband UWB channel that uses narrow pulses to transmit data.
CCA for IR-UWB cannot be implemented using a peak detector, matched filter or
correlation method [
3
]. A frequency domain method to implement CCA for
IR-UWB signal is proposed in [
3
]. This method requires a large number of narrow-
band filters and energy detectors. The proposed circuit is designed for the detection
of IR-UWB signals spread across the entire 7.5 GHz band. It is not suitable for
channelized UWB systems where only a sub-band of UWB is utilised. In a
channelized UWB system, the typical transmission bandwidth is between
500 MHz to 1 GHz. Most of the energy detectors in the CCA circuit will register a
false reading when a strong narrowband interferer is present.
The MAC protocols for UWB systems may preferably use a random medium
access method or a transmit-only MAC protocol for the multiple access of the
UWB channel. This chapter intends to give a critical analysis of the recently
published work on UWB MAC protocols that has the potential usage for WBAN
applications.
2.2 The IEEE 802.15.6 Standard
The IEEE 802.15.6 standard [
4
] is the first standard that defines the MAC archi-
tecture for in-body and on-body wireless communications. The standard defines
the physical layer communication using UWB and other narrow band technolo-
gies. The standard recommends the star topology to form a network for the
wireless nodes in a WBAN. Multiple access is achieved in the time domain with
the aid of a super frame structure. The super frame is divided into equal length
time slots, which are allocated to the contending sensor nodes by a central coor-
dinator, which controls the shared access to the wireless medium.
The IEEE 802.15.6 standard supports three communication modes [
4
]:
1. Beacon mode with super frame boundaries:
The super frame structure is divided by beacons transmitted in the downlink by
the coordinator in this communication mode. Several medium access mechanisms
