I have received a lot of questions on MIMO antenna design and how it applies to cellular systems. I hope this (long overdue) blog post helps motivate the need for antenna design in cellular systems and answer some common questions regarding the challenges faced by antenna designers. The details of antenna design, especially for base stations, will be discussed in a future blog post.
So here goes ….

‘Antenna design for cellular systems’ is a very broad and generic term. I would like to point out here that antenna design requirements and capabilities for base stations and mobile terminals are very different. Let us start with the requirements of a base station antenna. First of all, it needs to be directional, i.e. the antenna should have a high gain in the direction facing cellular sector being illuminated, as shown in Fig. 1(a). Secondly, the major beam of the antenna should not be parallel to the ground. It should be tilted down slightly to ensure that the users on the ground get coverage (see Fig. 1(b)). The third requirement deals with the space occupied by a base station antenna. It is widely assumed that we can afford to have more antenna elements, larger inter-element spacing between the elements, and broadly speaking, larger antenna elements as compared to the receiver (mobile terminal). However, this is not completely true. As cellular service providers and base station antenna manufacturers will tell you, it is very expensive to mount antennas on towers, rooftops, etc. The cost can range from anything between $500 - $3000 a month. Sometimes, the charge is on the basis of the sq. footage or antenna units mounted. Hence, the all-so-common assumption of having any number of antenna elements on the base station side, separated by large inter-element spacings is actually quite impractical. This is where antenna design comes into picture. Our aim is to try and design base station antennas that satisfy all the above mentioned requirements, while ensuring that performance is not affected. When we say performance, we mean the average data rates obtained in the cell and the outage probabilities that a base station antenna design will yield.

fig1
Fig 1: Directional base station antennas.

Now we come to the part where MIMO fits into all this. Current (single antenna) cellular technologies like CDMA, GSM, EDGE etc. cannot support the growing number of cellular service users, coupled with the large demand for high data rate applications. Cellular service providers are now looking to MIMO as a solution to meet this demand for high data rates. The inclusion of MIMO in standards like WiMAX and 3GPP-LTE has increased pressure on the cellular service providers to make the shift (and make it soon). The only problem that the providers have with making this shift is that it might require that the transceivers at the base stations be upgraded to support MIMO. It is not clear if the base station antennas should be changed or if the existing setup will suffice for MIMO as well. We in WSIL are currently investigating if the existing base station antenna designs will work well for MIMO. This will make the transition for cellular service providers a whole lot easier.

Coming now to the mobile terminals. These devices, as we well know, are placed and held at random orientations. This implies that the antenna used should be such that it can receive (and transmit) equally well in all directions. This is the first requirement for antennas for a mobile terminal. Note that omnidirectional antennas generally do not perform as well as directional antennas, making this a challenge for the antenna designer. The second requirement for the mobile terminal antenna is that it should occupy as little space as possible. This is where low profile microstrip (patch) antennas, planar inverted-F antennas (PIFAs), etc. come into picture. Note that these are embedded antennas, whereas the earlier models of cell phones used whip, retractables, etc. The third requirement is that the cell phone antenna should be able to support a large number of functions apart from just supporting phone calls, like FM or bluetooth, for example. This requires that the antenna used should be a multi-band antenna supporting all the frequencies needed or that we use different antennas for each frequency. It is obvious that the single antenna is a much better option, both in terms of cost and space occupied. Designing multi-band antennas is a challenge for the designer if the number of bands to be supported is greater than two. The larger the number of frequencies required, the greater the difficulty in design.

I will conclude with a brief statement on why cellular antenna design is important. An antenna designer spends a lot of time and effort trying to optimize antenna parameters like cross-pol pattern discrimination, the port-to-port isolation, etc. These parameters need to be optimized by (in some cases) minutely tweaking the physical structure of the antenna. This process can at times take days to accomplish. By using effective antenna design techniques, we can optimize these parameters using simulations and can give a conservative estimate beyond which the performance will not be affected. This in turn, saves time and effort (and money!).

That’s it for now. I will post another blog on some details of antenna design and, in particular, the work we do in WSIL on this topic.