Cellular networks divide coverage areas into hexagonal cells served by base stations to allow frequency reuse. Adjacent cells use different channels to reduce interference. As demand increases, more base stations are added and transmission power is decreased. Handoff allows calls to be transferred between base stations as users move. Different handoff strategies prioritize maintaining connections during mobility. Dynamic channel assignment allocates channels when needed to improve spectrum utilization and reduce call blocking.
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Lecture 6 t
1. Cellular Concept
• Can be reused as many times as needed as long as
co-channel interference is kept below acceptable
level.
• If demand for service increasenumber of BSs
increases + decrease of transmission power.
2.
3. Frequency Reuse
• Coverage area is called a cell
• Adjacent cells have different channels
• The design process is called Frequency Reuse or
Frequency Planning
• Cell’s Footprint : radio coverage area of the cell
• Hexagon are better than square or equilateral
triangle.
4. Frequency Reuse
• Center excited cell: BS in the middle.
• Corner Excited Cell: BS on the vertices
• Practical consideration usually do no allow BS to be
placed exactly as they appear in hexagon.
• Most system design permit BS to be positioned
upto one fourth of the cell radius away from ideal
location.
6. Channel Assignment Strategies
Fixed
• Fix number of channels are assigned to the cell
• Call can be blocked
• One variation is channel borrowing strategy.
• MSC supervises borrowing strategy.
7. Channel Assignment Strategies
Dynamic
• Channels are not allocated permanently
• When a call is made, serving BS request a channel
from MSC
• The channel is allocated following the algorithm
that takes into account parameters like likelihood of
future blocking, reuse distance distance of the
channel etc.
8. Handoff / Handover
• Connection of a MS from one BS to another BS
during a call is called HO
• As infrequent as possible
• Different types: Hard HO, Soft HO, Softer HO
9. Handoff Strategies
• Different Strategies e.g. NCHO, MCHO, MAHO and
NAHO
• In 1G MSC was responsible for HO
• Locator Receiver were used at BS for measuring
signal strength and reporting it to MSC.
• NCHO
10. Handoff Strategies
• In 2G MAHO strategy was used.
• Less burden on MSC and improved HO
• Intersystem HO: from one MSC to another MSC
• HO should be given more priority over originating
call.
• HO should be as lossless as possible.
11. Prioritizing Handover
• Guard channel capacity: some channels are
reserved for HO.
• Disadv: reducing the total carried traffic
• Efficient spectrum utilization in case of dynamic
channel allocation strategy.
• Queuing of HO request is another strategy.
12. Practical HO Considerations
• User mobility considerations
• High speed vs low speed users
• Umbrella cell approach
• Cell dragging problem in microcells
• HO thresholds and radio coverage parameters must be
adjusted carefully.
13. Practical HO Considerations
• 1G required 10 sec for HO
• Value of Δ was about 6dB to 12 dB
• 2G require 1 to 2 sec
• Value of Δ was about 0 to 6dB
• Newer cellular system consider more matrices for
HO decision making the process complex
14. Mobile Radio Propagation Models
• Okumura Model that is refined my Hata.
• Original details analysis of the Tokyo area
• For Urban environment, predicted path loss is
LdB = 69.55 + 26.16 log fc – 13.82 log ht – A(hr) + (44.9
– 6.55loght) log d
