When a mobile user moves in a different cell while a conversation is in progress, the received signal strength from Base Station becomes weak. BS is not able to detect the signal transmitted by the MS and vice versa. Thus, the call may be dropped (discontinued). Therefore, to maintain the continuity of a call, it is necessary to allocate new frequency channel from the cell where the MS is currently located. In cellular system it is performed by the process of Hand off/ Handover. The process of transferring a Mobile Station’s call from one frequency channel or Base Station to another is called as Handoff/Handover. When a mobile moves in a different cell while a conversation is in progress, the call gets automatically transferred to a new channel which belongs to the new Base Station.

Handoff should be performed successfully and as infrequently as possible to minimize load on the system and must be unknown to the user. Hence, system designers have to decide on optimum power levels at which Hand-off should be initiated. Also, Hand-off must be complete at the right time in order to minimize call drops. To achieve all this, a complete information of the minimum detectable signal level, handoff threshold is required which is explained further.

The minimum usable signal level for acceptable voice quality is usually seen to be -90 dBm to -100 dBm. If the received signal falls below this level, it cannot be detected by the receiver. A slightly stronger signal level is used as threshold at which the Hand-off is initiated.

Figure 14(a) demonstrates the case where a handoff is not made and the signal drops below the minimum acceptable level required to keep the channel active. This dropped call event can happen when there is a large delay by the MSC in assigning a handoff or when the threshold is set too small for the handoff time in the system. Excessive delays may occur during high traffic conditions due to computational loading at the MSC or due to the fact that no channels are available on any of the nearby Base Stations (thus forcing the MSC to wait until a channel in a nearby cell becomes free).

Figure 14(b) demonstrates a proper hand off scenario in which the handoff threshold was correctly defined and the signal did not fall beyond the threshold.

This handoff threshold level should not be very high to avoid unnecessary handoff scenarios.It should also not be very low as there will be insufficient time given to complete the process handoff which may cause call drops. Hence the delta value i.e. difference between the minimum detectable signal level and Handoff threshold is usually kept in the range of 0-6 dBs in modern cellular communication systems. Practical value taken by most of the service providers is 2 dBs.

Types of Hand offs

The Handoffs can be classified based on purpose of Hand off, nature of Handoffs and based on algorithms of Hand off.

Classification Based on Purpose of Handoff: This classification is based on network view. They are intra cell, inter cellular and intersystem hand offs.

Handoff Within a Cell (Intra Cellular Handoff): Sometimes, even if the user is moving within a cell, the received signal starts dropping which initiates a Hand-off. Thus there is a change of frequency channels during a call, although the mobile station has not changed its cell. This is called Intra cellular handoff.

The time over which a call may be maintained within a cell, without handoff, is called the dwell time. The dwell time of a particular user is governed by a number of factors, including propagation, interference, distance between the subscriber and the base station, and other time varying effects. Even when a mobile user is stationary, ambient motion in the vicinity of the Base Station and the mobile can produce fading; thus, even a stationary subscriber may have a random and finite dwell time. The statistics of dwell time vary greatly, depending on the speed of the user and the type of radio coverage. The statistics of dwell time are important in the practical design of handoff algorithms.

Inter Cell or Inter Base Station Handoffs: During the course of a call , if a subscriber moves from one cell boundary to another cell, the signal strength becomes weak or channel quality degrades or traffic load balancing problem occurs, the call is transferred to new cell or base station. The hand off is known as Inter cell hand off. Thus inter cell hand off shifts a call in progress from one cell to another, while the intra cell hand off shifts the call in progress from one logical channel on one cell to another logical channel of the same cell.

Inter System Handoffs:

During the course of a call, if a mobile moves from one cellular system to a different cellular system controlled by a different MSC, an intersystem handoff becomes necessary. An MSC engages in an intersystem handoff when a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress. There are many issues that must be addressed when implementing an intersystem handoff. For instance, a local call may become a long-distance call as the mobile moves out of its home system and becomes a roamer in a neighboring system. Also, compatibility between the two MSCs must be found out before implementing an Intersystem handoff.

Handoff Mechanism in 1G

In first generation analog cellular systems, signal strength measurements were made by the base stations and supervised by the MSC. Each base station constantly monitored the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the base station tower. In addition to measuring the RSSI of calls in progress within the cell, a spare receiver in each base station, called the locator receiver, was used to scan and determine signal strengths of mobile users which are in neighboring cells. The locator receiver was controlled by the MSC and was used to monitor the signal strength of users in neighboring cells which appeared to be in need of handoff and reported all RSSI values to the MSC. Based on the locator receiver signal strength information from each base station, the MSC decided if a handoff is necessary or not.

Handoff Mechanism in 2G

In second generation systems, handoff decisions are mobile assisted. In mobile assisted handoff (MAHO), every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time. The MAHO method enables the call to be handed over between base stations at a much faster rate than in first generation analog systems since the handoff measurements are made by each mobile, and the MSC no longer constantly monitors signal strengths. MAHO is particularly suited for micro cellular environments where handoffs are more frequent. The Hand-Off mechanisms followed in 3G and 4G will be discussed later.

Classification based on nature of Handoff : Handoffs can be classified based on sequence of releasing the connection of the voice channels. Accordingly, there are two types of Handoffs which are, Hard and Soft.

Hard Handoff: In Hard Handoff, the existing connections (voice channels)are released and then the new connections are formed. This type is also called as Break before Make Hand off.

Pros: Once the voice channels are released, they can be used to serve some other user. Thus in this hand off frequency resources are efficiently utilized.

Cons: Channels are released before getting new channels, If there may be slight delay in completion of Hand off, there are high chances of call drops.

Soft Handoff: In Soft handoff, the new voice channels are allocated before releasing the existing voice channels. It is also called a Make before Break Hand off.

Pros: For a very small instant, the mobile station is tuned (or using) two pairs of channels. Hence, this will affect the frequency resource management.

Cons: Minimized chances of call Drops.

2) Prioritizing Handoffs

It is more important to all service providers to serve an existing call efficiently than giving service to a new call request. Hence there are different mechanisms to prioritize handoffs.

Guard channel requests: Fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell. This method has the disadvantage of reducing the total carried traffic, as fewer channels are allocated to originating calls. Guard channels, however, offer satisfactory spectrum utilization with dynamic channel assignment strategies.

3) Outcomes of Handoff consideration

In practical cellular systems, several problems arise when attempting to design for a wide range of mobile velocities. High speed vehicles pass through the coverage region of a cell within a matter of seconds, whereas pedestrian users may never need a handoff during a call. Also, as congestion increases, mobile operators tend to reduce the cell size. The MSC can quickly become burdened if high speed users are constantly being passed between very small cells and continuously request for handoffs. Several schemes have been devised to handle the simultaneous traffic of high speed and low speed users while minimizing the handoff intervention from the MSC. One of them is Umbrella Cell approach.

Umbrella Cell Concept

It is used for high speed users to ensure that number of handoffs are minimized.

It provides large area coverage to high speed users while providing small area coverage to users traveling at low speeds. It is an approach in which by using different antenna heights(often on the same building or tower) and different power levels, it is possible to provide “large”and “small” cells which are co-located at a single location. The bigger cell is called Umbrella cell.

As shown in Figure 15, if a mobile has started a voice call in cell A, it will be first served by its own Base Station. The MSC continuously monitors it’s RSSI level and predicts about the speed of the user. If it is found to be a high speed user, then immediately the control is transferred to the bigger Base Station. The bigger Base station now assigns its own channels to the user and serves the call. This process reduces the need of many handoff requests.

The channels allotted to the umbrella cell site is such that it will cause negligible interference to the channels of the smaller cells.

All high speed users are served by the bigger Base Station so that hand offs is not required when the mobile moves from cell to cell. The speed is determined at the Base Station by evaluating how rapidly the RSSI is changing.

A practical handoff problem in microcell systems is known as cell dragging.

Cell Dragging

Cell dragging results from pedestrian users that provide a very strong signal to the Base Station. Such a situation occurs in an urban environment when there is a line-of-sight (LOS) radio path between the subscriber and the BS. As the user travels away from the Base Station at a very slow speed, the average signal strength does not decay rapidly. Even when the user has travelled well beyond the designed range of the cell, the received signal at the base station may be above the hand off threshold, thus a handoff may not be made. This creates a potential interference and traffic management problem, since the user has mean while travelled deep within a neighboring cell. To solve the cell dragging problem, handoff thresholds and radio coverage parameters must be adjusted carefully.