Cellular Networks

There is a limited amount of usable frequency spectrum available for cellular communications. There are two primary reasons for this. The first reason has to do with physics. There is a limited range of frequencies that are suitable for providing a communication link between a cellular tower and the handset device. The second reason is that there are many forms of wireless communications systems already in existence, making it challenging to find spectrum that can be allocated for such a purpose. Some examples include: television broadcast; AM and FM radio broadcast; private two-way radio systems; air traffic control; and marine (ship-to-shore) communications.

Cellular networks were designed to reuse the spectrum that was allocated to a wireless communications carrier many times, spread out over a large geographic area. The concept provides a clever way of conserving frequency spectrum. The diagram above shows a four-cell architecture. This means that 4 cells are used in a repeating pattern for spectrum allocation. Notice that the cell on the top of the diagram is labeled "Cell 1". The one at the bottom left is also labeled the same. There is a dashed line around each "Cell 1" areas, which demonstrates the approximate radio coverage area. Note the fact that the radio coverage areas for both areas labeled "Cell 1" don't overlap.

The areas referred to as "Cell 2" and "Cell 3" in the previous diagram do overlap "Cell 1" (lower-left). This is done intentionally to allow time for the cellular handset, which is in a state of mobility, to establish a radio link to either of these adjacent cell sites, depending on the direction the user is traveling.

The concept of changing connections between cell sites and the mobile device has two popular terms: handoff and handover. The term handover will be used exclusively throughout this paper to simplify the jargon. Frequency spectrum licensed to a carrier can then be divided up into four bands of frequencies. In this case, each cell would be allocated 25% of the spectrum, in a non-overlapping fashion.

In the illustration above there is an arrow showing the overlapping area between Cell 1 and Cell 2, which is a geometric shape called a prolate spheroid (football shape). Since this geographic area has overlapping coverage between two cells, it allows the ability to connect to either cell site for a brief period of time.

Various cellular standards use different methods of conducting the handover between a cellular handset and the cell site. The basic idea is to periodically measure the radio signal strength, identifying when the link becomes too weak to support the connection. The next step is getting both cellular sites and the cellular handset involved in a coordinated effort to accomplish the task.

Again, using the illustration to conclude the concept of frequency reuse, when the cellular handset travels towards the upper-right of the diagram, it will reach another overlapping area between Cell 2 and Cell 1 at the top of the diagram. When another handoff is performed, the network is now using the same spectrum that was used in a different geographic area, in which active connections pose no interference problem to the current cell used for the handover connection.

First Generation Cellular Standards

The first cellular networks came into service in the early 1980s. There were several different standards developed by different groups. The one thing all these standards had in common was that they were analog systems. At that time, there were no technologies developed that supported digital communications for wireless technology.

Analog systems typically used a well-known form of modulation (the method used to convert the user's voice conversation to a frequency that was part of the carrier's licensed spectrum for the cellular network) called Frequency Modulation (FM). This was a technique that varied an analog signal by changes in the frequency pattern. This allowed the voice conversation to be converted to a certain radio frequency carrier and then transmitted. When the signal was received at the other end of the link, it was converted back to the voice conversation.

There were two key problems with analog networks. First, they are very inefficient in carving up the available frequency spectrum into individual connections between users. The second problem is that the voice conversation (converted to a Radio Frequency and carried over the airwaves) was subject to distortion by interference. Therefore, a bad connection would render background noise, making it difficult to carry on the conversation.

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