A handoff refers to the process of transferring an active call or data session from one cell in a cellular network to another or from one channel in a cell to another. A well-implemented handoff is important for delivering uninterrupted service to a caller or data session user.
Handoffs may be classified into two types:
- Hard Handoff: Characterized by an actual break in the connection while switching from one cell or base station to another. The switch takes place so quickly that it can hardly be noticed by the user. Because only one channel is needed to serve a system designed for hard handoffs, it is the more affordable option. It is also sufficient for services that can allow slight delays, such as mobile broadband Internet.
- Soft Handoff: Entails two connections to the cell phone from two different base stations. This ensures that no break ensues during the handoff. Naturally, it is more costly than a hard handoff.
Inter System Handoff:
- If during ongoing call mobile unit moves from one cellular system to a different cellular system which is controlled by different MTSO, a handoff procedure which is used to avoid dropping of call is referred as Inter System Handoff.
- An MTSO engages in this handoff system. When a mobile signal becomes weak in a given cell and MTSO cannot find other cell within its system to which it can transfer the call then it uses Inter system handoff.
- Before implementation of Inter System Handoff MTSO compatibility must be checked and in Inter System Handoff local call may become long distance call.
Intra System Handoff:
- If during ongoing call mobile unit moves from one cellular system to adjacent cellular system which is controlled by same MTSO, a handoff procedure which is used to avoid dropping of call is referred as Intra System Handoff.
- An MTSO engages in this handoff system. When a mobile signal becomes weak in a given cell and MTSO finds other cell within its system to which it can transfer the call then it uses Intra system handoff.
- In Intra System Handoff local calls always remain local call only since after handoff also the call is handled by same MTSO.
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like other Multiple Access systems, coordinates access between multiple users. Alternatives include TDMA, CDMA, or SDMA. These protocols are utilized differently, at different levels of the theoretical OSI model.
Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission.
- In FDMA all users share thesatellite transponderor frequency channel simultaneously but each user transmits at single frequency.
- FDMA can be used with both Analog and digital signal.
- FDMA requires high-performing filters in the radio hardware, in contrast toTDMAand CDMA.
- FDMA is not vulnerable to the timing problems thatTDMA Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
- Due to the frequency filtering, FDMA is not sensitive tonear-far problemwhich is pronounced for CDMA.
- Each user transmits and receives at different frequencies as each user gets a unique frequency slots.
FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between the uplink and downlink(for instance, the traffic going back and forth between a mobile-phone and a mobile phone base station). Frequency-division multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer.
FDMA also supports demand assignment in addition to fixed assignment. Demand assignment allows all users apparently continuous access of the radio spectrum by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT for use on the Intelsat series IVA and V satellites.
There are two main techniques:
- Multi-channel per-carrier (MCPC)
- Single-channel per-carrier (SCPC)
Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and I DEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and PON networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below.
TDMA frame structure showing a data stream divided into frames and those frames divided into time slots.
TDMA is a type of time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its pees
Characteristic of HANDOFF:
- Shares single carrier frequency with multiple users
- Non-continuous transmission makes handoff simpler
- Slots can be assigned on demand in dynamic TDMA
- Less stringent power control thanCDMAdue to reduced intra cell interference
- Higher synchronization overhead than CDMA
- Advancedequalizationmay be necessary for high data rates if the channel is “frequency selective” and creates Intersymbol interference
- Cell breathing(borrowing resources from adjacent cells) is more complicated than in CDMA
- Frequency/slot allocation complexity
- Pulsating power envelope:interferencewith other devices