卫星无线蜂窝网络-homework1

1、How a cellular phone call is made？Cellular telephone, sometimes called mobile telephone, is a type of short-wave analog or digital telecommunication in which a subscriber has a wireless connection from a mobile phone to a relatively nearby transmitter. The transmitter’s span of coverage is called a cell. As the cellular telephone user moves from one cell or area of coverage to another, the telephone is effectively passed on to the local cell transmitter.A cell phone is - put simply - a kind of two-way radio that acts in the same way as a telephone. However, naturally a cellular phone is much more complex than a simple walkietalkie, as cell phones permit two people to speak at the same time, making calls and receiving calls, three-way calls, call holding, voice mail, text messaging, etc.However, similar to walkie talkies, all mobile phone calls are entirely unprotected and can be intercepted by other devices. Cell phones aren't at all as "secure" as wired telephones. They have the same security level as a radio - which, after all, they are.When you place a cellular phone call, you dial the number and press the send button. A number of steps then follow:●Your cell phone scans for the nearest base station in order to provide it with thestrongest signal and, in turn, the best possible connection. It checks 21 different control channels to determine the strongest available signal.●Your cell phone then selects the strongest signal for its use.●An origination message (a very short message of about ¼ second in length) isthen sent by the cellular phone, which includes its MIN (Mobile Identification Number, that is, your cellular phone number), as well as the ESN (Electronic Serial Number), and the number that has been dialed.●Once the cellular service provider verifies that you are among its customers -based on the sent-out MIN and ESN - the base station sends a channel assignment message to the cellular phone (another ¼ of a second in length), telling the phone where the conversation will be.●The cell phone then tunes into that assigned channel and the call begins.All of this has happened by the time you hear the ringing or busy signal on the other end of the phone.2、Why the cellular pattern is hexagon?In a cellular System a land area is divided into regular shaped cells, which can be hexagonal, square, circular or some other irregular shapes, although hexagonal cells are conventional. This is because there are some criteria for the cell shape, which are ●Geometric shape●Area without overlap●Area of the cellAnd the eligible shapes for these criteria are Square, circle, equilateral triangle&hexagon.The Geometric shape & Area without overlap is satisfied by a hexagon,square, equilateral triangle as they can be fitted in a manner where there is no area of overlap. The circle on the other hand would overlap (which implies interference of signals) or leave gaps (which means loss of coverage in those areas) when not overlapping.When the area factor is considered a circle has the highest area however it does not satisfy the second criteria of overlap. Therefore we have to consider a shape which fits correctly and also has maximum area.For this purpose we shall compare the area of the remaining shapes to the area of circle to see which has the maximum area.The area of an equilateral triangle to a circle approx = 17.77%The area of a square to a circle approx = 63.7%The area of a hexagon to a circle approx = 83%Which means hexagon has the highest coverage area after a circle from the lot.Thus of the lot hexagon satisfies all the conditions which is why the shape of a cell is hexagonal in cellular network.More than likely because the antennas used have a radiation pattern of 60 degrees. Thus 6 are required for full 360 degree coverage.If you look at the top of a cell phone tower, you will see the vertical arrays that constitute the antennas, usually with the form of long vertical rectangles or boxes, sometimes arranged in parallel groups. A more detailed inspection would reveal that the vertical arrays are separated 120ºfrom each other. This means that each array is pointing to the center of its hexagonal cell. Hence, each cell is covered by six arrays from six separate towers, one at the apex of each of its six angles.Each provider can use a fixed set of frequencies, which forces them to assign a limited number of frequencies to each adjacent cell; the frequencies start repeating on the non-adjacent cells. Similarly, each antenna array can serve a limited number of subscribers at any given time. All this means that when traffic increases within a cell (more users), the only way of coping with the extra traffic is to reduce the area of the cell so it serves less subscribers. This is done by adding more towers to form smaller cells, hence the constant demand for cell tower sites. When a cell is fractioned in smaller cells, the antenna arrays that serve the new cells are lowered and their power is reduced so their emitted energy does not go beyond the adjacent cells to avoid interfering with those other cells, farther cells that use the same sets of frequencies.Having the cell phone towers closer to each other means that the signal, although purposely weakened, can now reach inside elevators and concrete buildings. This also means that cell phones need to transmit in their high power setting much less time while in use, which allows for smaller handsets whose batteries last longer. Hint,Farthest for interference and largest area of cellFewest number of cellClose to circular pattern , applying omni-direction antenna。