Wireless Personal Area Network (WPAN) devices transmit over small distances. Hence, the loss in signal propagation is small. Greater capacity can be achieved through greater bandwidth. The use of ultra wide bandwidth (UWB) under FCC guidelines offers tremendous capacity potential over short ranges (<10m). For UWB, the power radiated must be low. However the mean effective isotropic radiated power (EIRP) should be -41.3dBm/MHz.

The FCC defines UWB signals as having a fractional bandwidth of greater than 0.2 (i.e. bandwidth greater than 500 MHz). UWB bandwidth is defined as frequency band bounded by the points that are 10dB below highest radiated emission. The FCC ruling allows UWB communication devices to operate in an unlicensed spectrum from 3.1 to 10.6 GHz.

The IEEE 802.15.3a has set out to develop a flexible standard which enables high data WPAN (110mbps at 10m, 200mbps at 4m, 480 mbps at 2m) using a cost effective architecture. The standard enables a broad range of applications including wireless transmission of images and video.

The IEEE 802.15.a focuses on physical layer for UWB technology and uses some MAC sub layer as IEEE 802.15.3.

There are two options for UWB channel utilization: ‘Orthogonal frequency division multiplexing (OFDM)’ based multiband approach and a Dual band impulse radio (IR) spread spectrum approach.

1. OFDM based multiband approach:

• The UWB band from 3.1 to 10.6GHz is divided into several 528 MHz channels. The lower three channels are mandatory and upper four channels are optional.
• The mandatory three bands are group A and upper four are group C. Group C is used in areas where simulation operating piconets are in close proximity. The others are group B and C which are reserved for future use.
• Frequency hopping is used along with OFDM. This increases transceiver complexity but it reduces the effects of ‘inter symbol interference (ISI)’
• This scheme has better coexistence characteristics with systems such as IEEE 802.11 a due to adaptive selection of bands.
• The signaling rate required to achieve same data rate is less than that required by the other scheme. This reduces complexity, cost and integration concerns for most electronics.
• This scheme is promoted by WiMedia alliance, which is not for profit open industry association. WiMedia UWB is optimized for WPAN devices delivering high speeds (480mbps and beyond)

2. IR based spread spectrum approach:

• It uses dual band: high band (above 5.2 -5.8 GHz unlicensed band) and lower band (from 3.1GHz to just below 5.2 – 5.8 GHz unlicensed band). It exploits all of UWB spectrum.
• Very short duration pulses (sub-nano sec) transmit over each band with bandwidth (>1GHz)
• The root raised cosine pulse shaping provides the required roll off.
• The phase of the pulse is inverted to give simple BPSK modulation.
• The DSSS based code sequences are used to transmit the data with 24 chips per symbol.
• This scheme is supported by ‘UWB forum’. It has greater precision for position location and realizes better spectrum efficiency compared to multiband UWB.
• However, it fails if government choose to limit the UWB spectral allocation to smaller range; as it utilities full spectrum.