For implementing a mobile network, a handoff mechanism must be defined to maintain uninterrupted user communication session during his/her movement from one location to another. Handoff mechanism handles subscriber station (SS) switching from one Base Station (BS) to another. Different handoff techniques have been developed. In general, they can be divided into soft handoff and hard handoff.





Figure. Soft Handoff
A SS maintains multiple connections. Delay is very minimal

Soft handoff is used in voice-centric cellular networks such as GSM or CDMA. It uses a make-before-break approach whereas a connection to the next BS is established before a SS leaves an ongoing connection to a BS. This technique is suitable to handle voice and other latency-sensitive services such as Internet multiplayer game and video conference. When used for delivering data traffic (such as web browsing and e-mail), soft handoff will result in lower spectral efficiency because this type of traffic is bursty and does not require continues handover from one BS to another.

Hard handoff in Mobile WiMAX


Figure. Hard Handoff
A SS maintains a connection to a single BS at any given time.

Mobile WiMAX has been designed from the outset as a broadband technology capable of delivering triple play services (voice, data, video). However, a typical Mobile WiMAX network is supposedly dominated by delay-tolerant data traffic. Voice in Mobile WiMAX is packetized (what is called VoIP) and treated as other types of IP packets except it is prioritized. Hard handoff (HHO) is therefore used in Mobile WiMAX. In hard handoff, a connection with a BS is ended first before a SS switches to another BS. This is known as a break-before-make approach. Hard handoff is more bandwidth-efficient than soft handoff, but it causes longer delay. A network-optimized hard handoff mechanism was developed for Mobile WiMAX to keep a handoff delay under 50 ms.

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1 Cell

A Cell is the basic geographical unit of a cellular system. Commonly represented as a hexagon.

The term cellular comes from the hexagonal or honeycomb shape of the coverage area. Each cell has a BS transmitting over a cell. Because of constraints imposed by natural terrain and man made structure . The true shapes of cell are not hexagonal. The coverage area of cell is called the footprint. The BS simultaneously communicates with many mobile using one channel (pair of frequencies) per mobile. One frequency is for the forward link (Base station to the mobile) and other frequency is for the reverse link (Mobile to the Base station). Each cell size varies depending on landscape, subscriber density and demand within particular region. Cells can be added to accommodate growth e.g.; creating new cells by overlaying, splitting, or sectoring existing cells. This technique increases the capacity of the system. Sectoring existing cells and then using directional antenna can also increase capacity.

2 Cluster

A cluster is a group of cells. No frequencies are reused within a cluster. Frequencies used in one cell cluster can be reused in another cluster of cells. A large number of cells per cluster arrangement reduces interference to the system.

3 Frequency Reuse

Frequency reuse is a technique of allocating channels to the cellular system. Because of the unavailability of the spectrum at the cellular band, channels frequencies must be reused. Cells are assigned group of channels that are completely different from those of neighboring cells. Cells with the same number have the same set of frequencies, if the number of available frequency is 7, the frequency reuse factor is 1/7, which implies that each cell is using 1/7 of available frequencies . Frequency reuse introduces interference into the system.

In Cellular Communication there are two types Interference when we want to do Frequency Re-use. They are Known as: Adjacent Channel Interference (ACI), Co-Channel Interference (CCI).

1.Adjacent Channel Interference(ACI):

ACI is the interference between signal's having frequencies close together.

For example suppose channel 1 has frequencies 825.030 MHz (Mobile) and 870.030 MHz (BS) and channel 2 has frequencies 825.060 and 870.060 MHz. Channels 1 and 2 have frequencies close to one another, which will results in ACI.

2.Co-Channel Interference (CCI):
CCI is the interference between signal's having the same frequency(i.e; re-use frequency).
For example suppose channel 1 has frequencies 825.030 MHz (Mobile) and 870.030 MHz (BS).

Any othe signals having the frequencies of channel 1 825.030 (Mobile) and 870.030MHz (BS) are co-channel signals and will suffer from co-channel interference (CCI).

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