Wireless MAC protocols[无线MAC协议](-41).ppt

* Channel plan with a reuse factor N=7 1 2 4 6 5 3 7 1 7 6 2 * Service provider A Has a total of 832/2 = 416 channels Set aside 21 for control Therefore 395 traffic channels Per cell, we can have 56 and 3/7 channels (N=7) Four cells are given 56 channels and three cells are given 57 channels The 395 channels fit over 25Mhz Therefore, the spectrum efficiency of an AMPS system is conversations/cell/Mhz * Question So how many control channels are present in each cell? What is the implication of this question? * Typical parameters Average delay spread refers to the multipath delay spread (the same signal takes different paths, and there is a difference in delay incurred by the different paths) * Increasing Capacity Add new channels Dynamic channel allocation – frequencies can be taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels (typical: 3) Microcells – antennas move to buildings, hills, and lamp posts * Radio channel handoff Intra-cell handoff is when the channel needs to be changed when the user is still within a cell Reason: user moves into a sector served by a different directional antenna More commonly understood meaning for handoff: base-station to base-station (cell site) Neighboring cells use different channels (for interference reasons) As a user moves from cell to cell, channels are changed * When to handoff? Relative signal strength handoff from basestation A to basestation B if signal strength at B exceeds that at A Relative signal strength with threshold signal at A is lower than the threshold the signal at B is stronger Relative signal strength with hysteresis if signal at B is stronger by a hystersis amount H hand back to A only if signal at A is higher by H * When to handoff? Relative signal strength with hysteresis and threshold current signal drops below a threshold target