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an ILA which incorporates two lenses (the immersion lens and the objective lens). The design operates at the mmW frequency range of 75-79 GHz. Although it provides a low scan loss, its steering range is limited to only ±30o.

Given that the lens antenna depends on a network of switches or a phased array antenna, the disadvantages associated with switched array antennas also apply to ILAs, where a switched array antenna is used to steer the beam and similarly, the disadvantages associated with a phased array antenna also apply to ILAs, in the case where a phased array antenna is used to steer the beam. An example of a beam steerable lens can be found in Figure 2.5

Figure 2.5: Simplified geometry of a beam steerable lens.

2.3

Millimetre Wave Beamforming

It is widely accepted that antennas with highly directional radiation patterns are re- quired for use at mmW frequencies, as explained earlier [82]. MmW frequencies give the opportunity of employing directional beamformers (BF) with large array antennas, and hence improve the link margin [5]. BF can be considered to be one of the key enablers for cellular communications at mmW frequencies, since, as previously mentioned, it can be used to overcome the unfavorable path loss at these frequencies [104].

Depending on the BF architecture, BF can be achieved in the digital or analogue domain. An analogue BF consists of variable gain amplifiers (VGAs) which enable the amplitude of the applied signals to be varied, along with phase shifters, which enable adjustment of the phase of the applied signals. It is therefore possible to control both amplitude and phase of the signals applied to the antenna elements. Figure 2.6 shows the basic architecture of an analogue BF. The entire phased array antenna is driven by a single base-band (BB) processing module and only one RF chain is required to perform control the beam and steer it towards different directions. Analogue BF suffers from some hardware limitations which add constrain to the system [5] and which make the systems performance sub-optimal [32]. Furthermore, analogue BFs are unable to provide multiplexing gains since in these systems it is not possible to send multiple parallel streams of data [5] (unless multiple antennas are used). Analogue BF provides an effective method of generating high BF gains from a large number of antennas. It is cheaper to implement and operate than digital BF. However, analogue BF provides fewer degrees of freedom than digital BF thus resulting in poorer performance.

Digital BF, is achieved using digital precoding. It required multiple RF chains for controlling the beam in terms of its direction and amplitude. Figure 2.7 shows the basic architecture of a digital BF system. It involves multiplying a particular coefficient to the modulated baseband (BB) signal per RF chain. It offers the ability of sending data in parallel streams. This enables the antenna to exploit spatial diversity and multiplexing. In conventional lower frequency systems, precoding is usually done in the BB in order to have better control over the entries of the precoding matrix [5]. However, the high cost and power consumption of mixed signal components make full digital BB precoding prohibitive for mmW frequencies, with the currently available semiconductor technologies. The design of the precoding matrices in digital BF usually relies on having complete channel state information (CSI). CSI is difficult to obtain in mmW systems due to the large number of antennas [106]. Another reason that CSI is difficult to acquire, is the small Signal-to-Noise ratio (SNR) associated with the signals before BF is applied [5]. Digital BF provides higher degree of freedom in comparison to analogue BF technologies, improving the systems performance. However there are many open issues with this technology including calibration, complexity and

2.3. Millimetre Wave Beamforming 27

cost [32]. For example it is necessary for each antenna element to be equipped with a dedicated RF chain. The implementation of a large number of RF chains, on top of the existing RF chains present within a portable device (e.g. GSM, 3G, GPS, etc.) would add further to the system: cost, complexity (due to mixed signal circuits), and power consumption. For these reasons digital BF is unsuitable for use within battery based devices. There is therefore a trade-off between system performance and complexity between analogue and digital BF which drives the need for a third approach called hybrid BF.

Furthermore since the elements within an array antenna must be placed close together in order to prevent grating lobes, the analogue components, such as the LNA or PA and the down- or up- converter associated with each antenna element, must be tightly packed with the antenna element. This space limitation appears to be a great challenge for mmW frequencies[41].

Figure 2.6: Architecture of analogue beam-steering with both amplitude and phase control technology [12].

2.3.1 Hybrid Beamforming at Millimetre Wavelengths

Hybrid BF was first introduced in [131] and was re-introduced in [9] where mmW frequencies were exploited. Hybrid BF involves the combination of analogue and digital BF. It provides a trade-off between performance/flexibility and simplicity/cost [104]. In

Figure 2.7: Architecture of digital beamforming technology [54].

hybrid BF a directive beam is formed by analogue BF with the aid of phase shifters and variable gain amplifiers, whereas digital BF is used to provide the flexibility required for advanced multi-antenna techniques such as multi-beam MIMO [122].

Hybrid BF is able to overcome the hardware constrains of analogue-only BF, while ex- ploiting the performance advantages associated with digital BF. While this technology can overcome the hardware limitations of analogue BF whilst supporting the transmis- sion of multiple streams, it suffers from certain disadvantages. For example it requires multiple RF chains along with a complicated architecture [32]. Furthermore, an impor- tant issue with hybrid BF, which also exists in digital BF, is the difficulty in obtaining CSI knowledge at the BS transmitter side [5], [32]. CSI is hard to obtain due to the large number of antennas that probably future wireless communications systems will employ in this application, as mentioned earlier. Figure 2.8 illustrates the architecture of a hybrid BF system.

It it important to note that, in the case of any BF technique, a suitable array antenna and a suitable RF chain must be designed. Controlling the weights of the antenna through digital BF or through VGAs and phase shifters or using a combination of the two, efficiently, is not enough to achieve good steering capability and high multiplexing gain. A highly directive array antenna with suitable design characteristics such as high