Sistem Transmisi Radio

Sistem transmisi radio

Terminologi

  • Daya pancar. Dinyatakan dalam milliwatts atau di dBm.
  • Penguatan Antena. Antena adalah perangkat pasif yang dapat membuat efek amplifikasi berdasarkan bentuk fisik mereka (dinyatakan dengan dBi)
  • Minimum Received Signal Level (RSL), atau sensitivitas dari penerima. Minimum RSL selalu dinyatakan sebagai dBm negatif (-dBm)
  • Cable Loss. Beberapa energy sinyal akan hilang di kabel, di konektor atau pada perangkat lain, pada saat sinyal merambat dari radio ke antena.
  • Effective Isotropic Radiated Power (EIRP) adalah daya pancar total perangkat setelah diperhitungkan dengan antena dan gangguan lainya

EIRP = dBm alat + dBi antena – Losses

  • Antenna Isotropic adalah sebuah antenna ideal yang mendistribusikan daya secara merata ke segala arah
  • Free space path loss / free space loss adalah hilangnya kekuatan sinyal yang disebabkan oleh perluasan alami dari suatu gelombang, sering disebut divergensi beam. Energi sinyal RF menyebar ke area yang luas sepanjang perjalanannya menjauhi antena, dan sebagai hasilnya, terjadilah peredaman kekuatan sinyal tersebut

Free Space Loss

The power of the signal is spread over a wave front, the area of which increases as the distance from the transmitter increases. Therefore, the power density diminishes.

Free Space Loss

Satuan Daya

  • dB (Decibel). Merupakan satuan perbedaan (atau Rasio) antara kekuatan daya pancar signal.
  • dBm (dB milliWatt). Merupakan satuan kekuatan signal atau daya pancar.
  • dBi (dB isotropic). Penguatan dari sebuah antenna terhadap suatu antenna standar imaginar (isotropic antenna)

dBm adalah nilai 10 log dari sinyal untuk1 mW

dBm = 10 log P (mWatt)

dBw  adalah nilai 10 log dari sinyal untuk 1 W

dBW = 10 log P (Watt)

Power in a wireless system

Power in a wireless system

Link Budget

Link budget atau System Operating Margin merupakan sebuah cara untuk menghitung mengenai semua parameter dalam transmisi sinyal, mulai dari gain dan losses dari Tx sampai Rx melalui media transmisi.

The performance of any communication link depends on the quality of the equipment being used. Link budget is a way of quantifying the link performance.

The received power in an 802.11 link is determined by three factors : transmit power, transmitting antenna gain, and receiving antenna gain.

If that power, minus the free space loss of the link path, is greater than the minimum received signal level of the receiving radio, then a link is possible.

The difference between the minimum received signal level and the actual received power is called the link margin. The link margin must be positive, and should be maximized (should be at least 10dB or more for reliable links).

Perhitungan Link Budget

PRX = PTX + GTX LTX LLFS + GRX LRX

Receiver Power (dBm)

Transmitter output power (dBm)

Transmitter antenna power (dBm)

Transmitter antenna Gain (dBi)

Transmitter Losses (coax, connectors ….) (dB)

Free space Loss or path loss (dB)

Receiver antenna Gain (dBi)

Receiver Losses (coax, connectors ….) (dB)

Free Space Loss

L(db) = 32.44 + 20 log D + 20 log f

  • Pada saat sinyal radio berpropagasi di udara akan mengalami redaman dari udara. Besarnya redaman yang terjadi dapat dihitung secara empiris
  • Jarak antara kedua antena adalah D dan frekuensi transmisi yang digunakan adalah f
  • D dalam satuan Kilometer dan f dalam satuan Megahertz

Cable Loss

Loss kabel akan terjadi setiap 30 meter transmisi 2,4 GHz. Besar loss untuk beberapa jenis kabel adalah:

  • RG 8 losses 10 dB setiap 30 meter
  • LMR 400 losses 6,8 dB setiap 30 meter
  • LMR 600 losses 5,4 dB
  • Heliax 3/8 “ losses 5,36 dB
  • Heliax ½ “ losses 3,47 dB
  • Heliax 5/8 “ losses 2,15 dB

Example link budget calculation

Let’s estimate the feasibility of a 5 km link, with one access point and one client radio. 

The access point is connected to an antenna with 10 dBi gain,  with a transmitting power of 20 dBm and a receive sensitivity of -89 dBm

The client is connected to an antenna with 14 dBi gain, with a transmitting power of 15 dBm and a receive sensitivity of -82 dBm.

The cables in both systems are short, with a loss of 2dB at each side at the 2.4 GHz frequency of operation.

AP to Client link

Fresnel Zone

The First Fresnel Zone is an ellipsoid-shaped volume around the Line-of-Sight path between transmitter and receiver. The Fresnel Zone is important to the integrity of the RF link because it defines a volume around the LOS that must be clear of any obstacle for the the maximum power to reach the receiving antenna. Objects in the Fresnel Zone as trees, hilltops and buildings can considerably attenuate the received signal, even when there is an unobstructed line between the TX and RX.

Line of Sight and Fresnel Zones

a free line-of-sight IS NOT EQUAL TO a free Fresnel Zone

Fresnel Zone

The Fresnel zone is an imaginary football-shaped area that surrounds the path of the visual LOS between two point-topoint antennas. Theoretically, there are an infinite number of Fresnel zone’s, or concentric ellipsoids, that surround the visual LOS. The closest ellipsoid is known as the first Fresnel zone, the next one is the second Fresnel zone, and so on.

Reflected Signal

Reflected Signal

When a signal is reflected two things happen

  • The phase of the signal reverses and the signal changes phase by 180º.
  • Since the signal is being reflected and not going in a direct line, it travels slightly further to the refection point and then on to the receiver. Therefore, the signal is shifted further in phase, by the difference in path length. Over a long path, this can amount to 180º or more

Line of Sight and Fresnel Zones

r = sqrt (d1 * d2 *λ / d)

Fresnel Zone Clearance

(Distance between the ground and the actual main signal path)

r adalah radius F1 (meter)

D adalah jarak antara antena pemancar dan penerima (kilometer)

f adalah frekuensi gelombang yang dipancarkan (gigahertz)

The important component of the Fresnel Zone is the clearance between the First Fresnel zone F1 ellipse and the surface of the earth.

If the ratio of (Fresnel zone earth clearance / Fresnel zone radius) is greater than 60%, the radio path is considered “clear, line of sight” and incurs no diffraction loss. This is also referred to as Free Space Loss.

Referensi :

http://wireless.ictp.it/uneca-ictp/presentations/Link_Budget-v1.15.pdf


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