Radiowave Propagation at Millimetre-wave Frequencies

Millimetre-wave (mm-wave) wireless communication systems are emerging as a technologically feasible and efficient means of transmitting and propagating ultra high-speed data and, thus, providing the capacity to support a wide range of digital wireless applications over short and medium range point-to-point and point-to-multipoint links. This is mainly due to the width range of the available spectrum, which can accommodate high-speed information throughput – albeit over short or medium distances, and the limited impact from external interfering sources. Communication links can be established within indoor, indoor-outdoor and outdoor wireless channels that provide the access medium for applications. These networks are known as Local Multipoint Distribution Systems (LMDS) or as Broadband Fixed Wireless Access (BFWA). Examples are high-speed wireless interfaces between portable devices, point-to-point high speed data transmission (within a room or between rooms or buildings) or from hubs to multiple subscribers in WLAN or WPAN pervasive networks of digital video broadcast, audio & video conferencing, wireless internet and multimedia services.

Due to the spectrum congestions at the 2.4 and 5GHz bands for WLAN & WPAN applications and the continuous demands for multi-Gbps communication systems to support today’s wireless multimedia loads, the Federal Communications Commission (FCC) has recently allocated the 57–64 GHz mm-wave band (known as the 60 GHz frequency band) for unlicensed use. This is the largest contiguous radio spectrum ever allocated which presents great solutions in terms of capacity and flexibility.

At millimetre wavelength and corresponding high frequency bands, the wireless channel in a communication system differs with that in GSM/UMTS mobile, WiFi or WiMAX technologies which operate at the lower GHz frequencies (less than 10 GHz) mainly because of the smaller nature of the wavelength. For example at 40 GHz the wavelength is about 7.15 mm only, while at UMTS and WiFi operational frequencies, it is 16.7 cm and 12.5 cm respectively and, hence, these differences are likely to be significant in radiowave propagation terms.

Transmitted signals in radio channels at millimetre wavelengths do not easily penetrate through building surfaces and material, and therefore can only reach the receiver, mainly, through unobstructed LOS paths. Accordingly, the radio channel is often described as being either a LOS link from a direct and unobstructed path between the BTS and the receiver’s antenna, or a NLOS in which the attenuated signal in this case, reaches the receiver through reflections, scattering and diffractions of electromagnetic waves from and around building surfaces and edges. However at these high frequencies (i.e. as the wavelength decreases) diffraction path contributions decrease significantly and building surfaces tend to generate many incoherent reflections accompanied by high propagation losses. Building scatter becomes a significant issue in determining dominant and interfering signal levels. These levels, in conjunction with the radiation characteristics of the hub and subscriber antennas, can influence system planning and deployment of interference limited networks. Click here to see our developed RF models.