Interference Testing with Handheld Spectrum Analyzers Manual

Interference Testing with Handheld Spectrum Analyzers Manual
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Interference Testing with Handheld Spectrum Analyzers Manual

  • 6Knowing that the radio spectrum has a limited number of channels and the number of users continues to grow, many radios system are designed to share a single frequency channel by dividing the transmission time among several users. This is called duplexing. For example, a cellular GSM mobile subscriber (MS) will transmit signals to the base transceiver station (BTS) using an assigned time slot along with the assigned frequency channel. The North American (NA) GSM 850 system divides a 4.615 millisecond length of time into 8 time slots for shar-ing the frequency channel between multiple users in order to increase system capacity. This technique is referred to as time division multiple access (TDMA). In regards to interference, an in-band interferer could now affect multiple users that are time sharing this TDMA frequency channel. Half-duplexWireless communication radios typically contain a transmitter and a receiver but in some systems, only one is active at a time. These types of radios are referred to as “half-duplex” and allow for simple, low-cost radio configurations. An example of a half duplex configuration is a push-to-talk (PTT) radio used by emergency service personnel and available as an option on many cellular networks. Data communication devices such as WLAN also use a half-duplex configuration. If the half-duplex radio uses the same frequency channel for both its transmitter and receiver, then an interfering signal would corrupt both links of the communication system.Full-duplexWireless systems that allow the simultaneous transmission and reception of signals, such as found in traditional cellular and military intelligence, surveillance, and reconnaissance (ISR) point-to-point radios, are referred to as “full-duplex” radios. These full-duplex radios typically use separate frequency channels for the transmitter and the receiver. Using the example of a cellular system, communication from the mobile transmitter to the BTS, referred as the uplink or forward link, operates at a different frequency channel than the com-munication from the BTS to the mobile receiver, referred to as the downlink or return link. The reason to separate the uplink and downlink signals is to prevent the mobile’s own transmit signal from leaking into the mobile’s receiver and appearing as an interference that could not be filtered if the two operated on the same channel. A specialized filter, called the duplexing filter, can separate two frequency channels and, when placed between the mobile’s transmitter, the receiver and antenna, allows two communications links to occur at the same time. A full-duplex radio system is effectively just two separate communication links occurring in independent frequency channels, transmitting at the same time. In regards to in-band or co-channel interference, the frequency separation between the two communication links would result in an interfering signal affecting only one side of the communication, either as an uplink or a downlink interference. This information is useful when troubleshooting performance issues in a wireless network. For example, the NA GSM 850 has uplink channels in the frequency band covering 824.2 to 848.8 MHz, and downlink channels from 869.2 to 893.8 MHz. If the system is experiencing problems only on the downlink, the first place to look for interference is over the range of downlink frequencies. Radio Duplexing