Frequency Planning and Reuse Engr. Mehran Mamonai

Frequency planning Neighborlistplanning Neighbor list planning Frequency Hopping Planning Frequency Hopping Planning Interferencerequirement Interference requirement

Spectrum Allocation

ARFCN Channel bandwidth is 200kHz

Neighbor list planning Neighbours are “neighbouring cells” that the serving cell is required to monitor and measure These neighbours are potential handover candidates Maximum of thirty two (32) can be defined – In practice, much less (10-12)

Neighbor list planning Theoretically, up to six neighbours In practice, it varies with cell shape and level of overlap Two-way relationship

Neighbor list planning

Interference requirement

Regular cell structures and re-use Conceptually hexagons

Regular cell structures and re-use In reality hexagons are not achievable: – Unequal antenna heights – Cannot acquire sites on grid – Requirement for different cell sizes (traffic/coverage) – Terrain and clutter effects

Automatic planning: predicted or measured

Manual Planning Manual planning is possible when: – Cells are on a reasonably regular grid – Cells are approximately equal in size – About the same number of TRX are in each cell – There are not too many cells After each plan, create a C/I plot – Then iterate the plan – Swap frequencies to remove interference – Adjust antenna tilts etc to remove interference – Very tedious!

Manual Planning Problem when cell sizes are different

Realistic cell structures

Frequency Hopping GSM implementation Each frequency has different fading characteristics

Frequency Hopping Used for frequency planning and fading mitigation Frequency planning (later section) Fading mitigation – Baseband hopping – Synthesized/RF hopping

Baseband Hopping 28 µs was too short for early technology to change frequencies Instead: keep TRXs at constant frequency and change inputs each frame

Baseband Hopping You can only Hop as many frequency are there as TRXs

Synthesized or RF hopping Each TRX can re-tune to any frequency.

Limitation The timeslot carrying the BCCH cannot hop. – BCCH is a “beacon” and must remain at the same frequency for Mobiles to find the network and neighbor cells. Solution: separate hopping lists for BCCH Carrier and non-BCCH carriers or separate lists for TS 0 and other timeslots

Advantages Frequency hopping may give a few dB Improvement by combating multipath fading The BCCH timeslot has no improvement Coverage must still be dimensioned for the worst case

Frequency Planning (Interference) Assign frequencies to a network using as few frequencies as possible. The quality and availability of the radio-link path is minimally affected by interference. Frequency planning is often based on the “3 dB threshold degradation approach”

Assignment Of Radio-relay Frequencies Prevention of mutual interference such as the interference between the radio frequency channels in the actual path, interference to/from. Other radio paths, interference to/from satellite communication system, etc. Aim at frequency economy of the available radio frequency spectrum. Proper and reasonable selection of frequency band that conforms to the required transmission capacity. Frequency band suitable to both path characteristics (path length, site location, terrain topography) and atmospheric effects.

Interference In Frequency Planning Far interference Near interference

Far Interference In Frequency Planning Typical far interference scenario with two “real” paths (AB and CD) and two “virtual” paths (AD and BC). The “virtual” paths are the “Interfering” Paths. Far interference is often the primary factor that limits the number of paths that can be set up within a given geographical area.

Far Interference In Frequency Planning Planning a network that is “free” from the effects of far interference requires the following issues: – Knowledge of the geographic locations of the sites, the layout, and dimensioning of the radio-link paths. – Equipment data – Existing network frequency assignment – Reasonable radio-wave propagation models far interference also affects the possibility of realizing a variety of network solutions

Far Interference In Frequency Planning

Near Interference In Frequency Planning Near interference refers to receiver disturbances that are generated by transmitters that are grouped at the same site, i.e. co-located stations.

Near Interference In Frequency Planning Disturbances may appear in the form of inter- modulation effects, i.e., the mixture of two or more transmitter frequencies that may arise close to a particular receiver frequency

Near Interference In Frequency Planning Another important characteristic, that should be considered when calculating the effect of near interference, is the coupling loss between two antennas located at the same site

Near Interference In Frequency Planning The following isolation values may be used when performing rough estimations (the values are, in fact, dependent on the distance and angles between the two antennas): – Approximately 40 dB between two antennas made up of dipoles – Approximately 80 dB between two parabolic antennas. The selection of proper duplex-bands for transmitter and receiver equipment, during the frequency-allocation process, is essential if one is to control the risk of disturbances that arise as a result of insufficient transmitter-receiver frequency separation

How may interference be avoided The following conditions shall be met: – Sufficiently weak interference signals. – No frequency overlap (receiver frequencies are sufficiently separated from interfering signals). The first condition may be very difficult to meet, often as the result of frequent occurrence of co- located radio systems (occasionally forced co- location). while the second condition may be attained but requires careful frequency planning.

ETSI Requirements Co-channel channel interference: z = 9 dB 200 kHz adjacent channel: z = 9 dB 400 kHz adjacent channel: z = 41 dB 600 kHz adjacent channel: z = 49 dB

Frequency Planning Tips Frequency planning is not carried out with the purpose of avoiding interference, rather to accomplish the quality and availability objectives of each portion of the network! Reuse frequencies, i.e., repeat frequencies as often as is possible!, Good frequency economy is always encouraged! Use antennas having high front-to-back ratios and large side-lobe suppression. These result in both good frequency economy and, in the final analysis, good overall network- economy. High performance antennas may be a suitable alternative. Do not use higher radio-link output power than necessary! Start always frequency planning with the lowest available output power

Frequency Planning Tips When assigning specific channels to the individual links in a network it is strongly recommended to start with high capacity links (the links demanding wider bandwidth). Generally, interference problems in radio-relay networks are greatest for links connected to a Interfering signals are not always in line of sight!

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