Cellular Network Optimization: Handoffs, Capacity, 4G & 5G
Handoff Strategies in Cellular Networks
Handoff strategies are essential in cellular networks to maintain call quality when a mobile device moves from one cell to another. The Mobile Switching Center (MSC) facilitates the seamless transfer of calls, ensuring minimal disruption to users.
Handoff Prioritization
Handoffs are given priority over new call requests. This is crucial for maintaining ongoing conversations without interruption. Ensuring successful and infrequent handoffs are important for user satisfaction, as frequent handoffs can burden the system and affect call quality.
Many systems prioritize handoff requests over new call attempts to minimize dropped calls. This approach enhances the user experience by ensuring ongoing calls are maintained, as users find call drops more disruptive than blocked new calls.
Guard Channel Concept
One method for prioritizing handoffs is the guard channel concept, whereby a fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls that 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 efficient spectrum utilization when dynamic channel assignment strategies, which minimize the number of required guard channels, are employed through efficient demand-based allocation.
Queuing Handoff Requests
Queuing of handoff requests is another method to decrease the probability of forced termination of a call due to lack of available channels. There is a trade-off between the decrease in probability of forced termination and total carried traffic. Queuing of handoffs is possible due to the finite time interval between the time the received signal level drops below the handoff threshold and the time the call is terminated due to insufficient signal level. The delay time and size of the queue are determined from the traffic pattern of the particular service area. It should be noted that queuing does not guarantee a zero probability of forced termination, since large delays will cause the received signal level to drop below the minimum required level to maintain communication and hence lead to forced termination.
Signal Level Thresholds
A specific signal strength threshold is set to initiate a handoff. This level is slightly above the minimum acceptable signal strength for good voice quality.
The threshold, denoted as Delta (Δ), must be balanced. If Δ is too large, it leads to unnecessary handoffs, while if it is too small, it may cause dropped calls due to insufficient time for handoff completion.
Avoiding Momentary Fading
To avoid unnecessary handoffs due to temporary signal drops, base stations monitor the signal level over a period.
This running average ensures that handoffs are based on sustained signal drops rather than transient fading, making the handoff decision more reliable.
Dwell Time
Dwell time refers to the duration a call can be maintained within a cell before a handoff is needed.
Factors affecting dwell time include user speed, propagation conditions, interference, and distance from the base station. Fast-moving users have shorter dwell times, requiring quicker handoffs.
Generational Differences in Handoffs
First-generation systems rely on base stations to measure signal strength and make handoff decisions.
Second-generation systems use Mobile Assisted Handoff (MAHO), where mobile devices measure and report signal strengths. This speeds up the handoff process and reduces the load on the MSC, especially useful in microcell environments.
Intersystem Handoff
This occurs when a mobile moves between different cellular systems controlled by different MSCs.
Key issues include ensuring compatibility between MSCs and handling changes in call types, such as local to long-distance calls. Intersystem handoffs are crucial for maintaining service continuity across different network providers.
Effective handoff strategies are vital for the seamless operation of cellular networks. By prioritizing handoffs, setting optimal signal thresholds, monitoring signal levels over time, and understanding dwell time variations, network designers can significantly enhance call quality. Technological advancements, such as MAHO in second-generation systems, have further improved the efficiency and speed of handoffs, particularly in high-mobility and microcell environments. These strategies collectively ensure that users experience minimal disruptions during their calls, maintaining the overall quality of service in cellular networks.
Cellular System Capacity Enhancement
As the demand for wireless service increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users.
At this point, cellular design techniques are needed to provide more channels per unit coverage area.
Techniques such as cell splitting, sectoring, and coverage zone approaches are used in practice to expand the capacity of cellular systems.
Cell Splitting
Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power.
Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused.
By defining new cells with a smaller radius than the original cells and by installing these smaller cells (called microcells) between the existing cells, capacity increases due to the additional number of channels per unit area.
Cell splitting allows a system to grow by replacing large cells with smaller cells, while not upsetting the channel allocation scheme required to maintain the minimum co-channel reuse ratio Q between co-channel cells.
For the new cells to be smaller in size, the transmit power of these cells must be reduced.
This is necessary to ensure that the frequency reuse plan for the new microcells behaves exactly as for the original cells.
In practice, not all cells are split at the same time, and different cell sizes may exist simultaneously.
Special care needs to be taken to keep the distance between co-channel cells at the required minimum, and handoff issues must be addressed so that high-speed and low-speed traffic can be accommodated.
Sectoring
Sectoring achieves capacity improvement by reducing the relative interference without decreasing the transmit power.
The co-channel interference in a cellular system may be decreased by replacing a single omni-directional antenna at the base station with several directional antennas, each radiating within a specified sector.
This technique, called sectoring, reduces interference and increases system capacity.
A cell is typically partitioned into three 120° sectors or six 60° sectors.
When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and used only within a specific sector.
This reduces the number of interferers in the first tier, thus decreasing interference and increasing capacity. In practice, sectoring enables planners to reduce the cluster size and provides an additional degree of freedom in assigning channels.
Channel Assignment Strategies
For efficient utilization of the radio spectrum in cellular systems, a frequency reuse scheme consistent with the objectives of increasing capacity and minimizing interference is crucial.
Various channel assignment strategies have been developed to achieve these goals, categorized as either fixed or dynamic.
The choice of strategy significantly impacts system performance, particularly regarding call management during handoffs.
Fixed Channel Assignment Strategy
In a fixed channel assignment strategy, each cell is allocated a predetermined set of voice channels.
Any call attempt within the cell can only be served by the unused channels within that specific cell.
If all channels in the cell are occupied, the call is blocked, and the subscriber does not receive service.
Variations of this strategy exist, including the borrowing strategy, where a cell can borrow channels from a neighboring cell under the supervision of the Mobile Switching Center (MSC) to avoid disrupting ongoing calls.
Dynamic Channel Assignment Strategy
Contrary to fixed assignment, dynamic channel assignment does not permanently allocate voice channels to cells.
Instead, each time a call request is made, the serving base station requests a channel from the MSC.
The MSC then allocates a channel to the requested cell based on various factors such as the likelihood of future blocking within the cell, frequency of channel use, reuse distance, and other cost functions.
Dynamic assignment reduces the likelihood of blocking, thereby increasing the trunking capacity of the system, as all available channels in a market are accessible to all cells.
Dynamic channel assignment strategies require the MSC to continuously collect real-time data on channel occupancy, traffic distribution, and radio signal strength indications (RSSI) of all channels.
While this increases the storage and computational load on the system, it provides the advantage of increased channel utilization and reduced probability of blocked calls.
Trunking and Grade of Service in Cellular Radio Systems
Trunking plays a pivotal role in cellular radio systems, efficiently managing a limited number of radio channels to accommodate a large user base within a restricted radio spectrum.
This mechanism allows multiple users to share available channels dynamically, with each user allocated a channel on a per-call basis.
Upon call termination, the channel is promptly returned to the pool for reuse, exploiting statistical user behavior to maximize channel utilization.
Trunking theory, pioneered by Erlang, facilitates the design of systems catering to varying user demands.
Erlang introduced the concept of traffic intensity, where one Erlang represents the traffic carried by a fully occupied channel.
Grade of Service (GOS) measures a user’s ability to access the system during peak hours, typically defined based on the busiest periods.
GOS benchmarks the desired performance of a trunked system, specifying the likelihood of user access given the available channels.
Calculating Traffic Intensity and GOS
Traffic intensity is calculated based on call request rates and holding times per user.
For a system with multiple users and channels, the total offered traffic intensity determines system capacity.
GOS, often expressed as the likelihood of call blocking, depends on factors like call arrival patterns and queuing times.
Trunked systems may either block calls immediately or queue them for later access.
Types of Trunked Systems
Blocked Calls Cleared (BCC)
- Users are immediately blocked if no channels are available.
- This system type uses the Erlang B formula to estimate the probability of call blocking.
Blocked Calls Delayed (BCD)
- Blocked calls are queued until channels become free.
- The Erlang C formula is applicable here and calculates the likelihood of call delays exceeding a specified time.
Grade of Service (GOS) Explained
GOS is a measure used to assess the performance and reliability of a trunked system during periods of peak demand, typically defined as the busiest hour of operation.
It represents the likelihood or probability of a user being able to access the system and obtain a channel for communication without experiencing blocking or excessive delay.
GOS is an essential benchmark used in the design and optimization of trunked systems.
It helps wireless designers determine the appropriate number of channels needed to meet specified performance objectives.
It is typically expressed as:
- The probability of call blocking, or
- The probability of call delay exceeding a certain threshold.
Achieving the desired GOS involves striking a balance between resource allocation and user demand.
It ensures users have access when needed while avoiding over-provisioning of resources.
GOS plays a crucial role in ensuring the quality of service in cellular networks.
4G (Fourth Generation) Technology
Key Technologies
4G marked a significant evolution in mobile communication, transitioning from 3G to more efficient and higher-capacity wireless systems.
It introduced LTE (Long-Term Evolution), which significantly improved spectral efficiency and offered IP-based communication for both voice and data, enabling seamless internet connectivity and multimedia support.
Data Speeds
4G provides much higher data speeds, with theoretical peak download rates ranging from 100 Mbps to over 1 Gbps, depending on the implementation and device support.
These speeds allow for smoother streaming, faster downloads/uploads, and better browsing experiences compared to earlier generations.
Features
- Ultra-fast internet browsing: Web pages and content load almost instantly, improving user satisfaction.
- HD video streaming: Supports high-definition (1080p or even 4K) video content without buffering.
- Low latency for real-time applications: Latency is significantly reduced (to around 30-50 ms), enabling smooth video calls and online gaming.
- Enhanced gaming experiences: Provides fast response times and reliable connections necessary for multiplayer online gaming and cloud gaming.
Advantages
- Substantial increase in data speeds: Offers a transformative mobile experience by supporting high-bandwidth applications.
- Enhanced network capacity: 4G can support a larger number of users and devices simultaneously without significant degradation in performance.
Limitations
- Requires compatible devices and infrastructure upgrades: Only devices that support LTE can fully benefit from 4G services.
- Limited availability in some regions: Rural or underdeveloped areas may still rely on older 3G or 2G networks due to lack of infrastructure.
5G (Fifth Generation) Technology
Key Technologies
5G is the latest advancement in mobile communication, leveraging millimeter wave (mmWave) frequencies and Massive MIMO (Multiple Input, Multiple Output) antennas to support unprecedented levels of speed and connectivity.
It also introduces beamforming, edge computing, and network slicing, enabling highly customized and efficient communication services.
Data Speeds
5G supports exponentially higher data rates, with real-world speeds ranging from 100 Mbps to over 10 Gbps under ideal conditions.
These speeds allow near-instant downloads and support data-intensive applications like 8K video and immersive virtual environments.
Features
- Ultra-low latency for mission-critical applications: With latencies as low as 1 ms, 5G can support applications where delay is not tolerable, such as remote surgery or autonomous driving.
- Massive connectivity for IoT devices: Designed to support millions of connected devices per square kilometer, making it ideal for smart cities and industrial IoT.
- Enhanced mobile broadband: Offers superior mobile internet experiences for high-definition video, AR/VR, and seamless streaming.
- Network slicing for customized services: Allows operators to create virtual networks tailored for specific use cases, such as low-latency gaming or high-reliability communication.
Advantages
- Unprecedented data speeds and network capacity: Redefines mobile capabilities by enabling real-time communication and high-speed data exchange.
- Support for emerging technologies: 5G provides the foundation for AR/VR, autonomous vehicles, telemedicine, smart homes, and more, empowering innovation across industries.
Limitations
- Initial rollout limited to select urban areas: Due to the high cost and complexity of deployment, early 5G access is mainly available in densely populated cities.
- Requires significant infrastructure investments: Upgrading networks, installing more base stations, and acquiring spectrum require massive capital investment and time.