Discontinuous Transmission (DTX) is a mechanism which allows the radio transmitter to be switched off most of the time during speech pauses.
DTX may be applied independently to each direction, so that the control of DTX must take into account two components:
· The uplink mode
· The downlink mode.
DTX can be enabled or disabled for the uplink and/or downlink mode on a per-cell basis.
Reasons for DTX When DTX is applied, actual transmission on the radio path is reduced. This will cause a decrease of the interference level in co-channel cells (using the same frequency). Another advantage will appear when using DTX in the uplink mode: it saves battery power for the mobile station. However, a disadvantage of the DTX mode is that it slightly deteriorates the quality of transmission. Note that transmitting in DTX mode does not save timeslots on the air-interface.
Discontinuous reception is method used to conserve power at the MS. The paging channel, used by the BTS to signal an incoming call, is structured into subchannels.
Each MS is assigned one of these sub-channels and needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into “sleep mode”, when almost no power is used.
3. What is Erlangs? What is meant by GoS?
Traffic refers to the usage of channels and is usually thought of as the holding time per time unit (or the number of “call hours” per hour) for one or several circuits (trunks or channels). Traffic is measured in Erlangs (E), for example, if one subscriber is continuously on the telephone, this would generate one call hour per hour or 1 E of traffic. The traffic one cell can carry depends on the number of traffic channels available and the amount of congestion that is acceptable (to both the customer and the provider), the so-called Grade of Service (GoS).
4. We use two different bands for GSM/DCS communications; GSM900 and DCS-1800. Which one is the better of the two in terms of coverage?
G900 is better. Due to path loss formula as frequency is increased, the losses which the signal will encounter will be more.
5. What is TA? Why do we need TA?
Time advance (alignment) process The RF communication experiences a propagation delay over the distance between the BTS and the MS. In order to synchronize the MS to the BTS, a timing advance is used to align the time slots arriving at the BTS receiver:
1. The BTS measures the reception time of the incoming MS burst
2. The BTS requests the MS to advance its transmission to compensate for the delay over the distance. A 6-bit number indicates how many bits the MS must advance its transmission.
3. The time advance value TA can have a value between 0 and 63 bit lengths, which corresponds to a delay of between 0 and 233 ms.
4. This leads to a maximum mobile range of 35 km, which is rather determined by the TA than by the signal strength.
6. What is meant by Location Area?
7. What is location update? Why do we need location update?
A Location Area (LA) is defined as a group of cells. Within the network, a subscriber’s location is known by the LA which they are in. The identity of the LA in which an MS is currently located is stored in the VLR. When an MS crosses a boundary from a cell belonging to one LA into a cell belonging to another LA, it must report its new location to the network1. When an MS crosses a cell boundary within an LA, it does need to report its new location to the network. When there is call for an MS, a paging message is broadcast within all cells belonging to an LA.
8. What is meant by IMSI, TMSI, IMEI and MS-ISDN? Why they are needed?
IMSI = International Mobile Subscriber Identity
TMSI = Temporary Mobile Subscriber Identity
IMEI = International Mobile Equipment Identity
MS-ISDN =Mobile Station ISDN Number
A. The MSISDN is the directory number allocated to the mobile subscriber. It is dialed to make a telephone call to the mobile subscriber.
B. A MS is identified by its IMSI. The IMSI is embodied in the SIM of the mobile equipment. It is provided by the MS anytime it accesses the network.
C. The TMSI is an identity alias which is used instead of the IMSI when possible. The use of a TMSI ensures that the true identity of the mobile subscriber remains confidential by eliminating the need to transfer an IMSI code unciphered over a radio link.
A VLR allocates a unique TMSI code to each mobile subscriber that is operating in its area. This code, which is only valid within the area supervised by the VLR, is used to identify the subscriber in messages to and from the MS. When a change of location area also involves a change of VLR area, a new TMSI code is allocated and communicated to the MS. The MS stores the TMSI on its SIM.
D. IMEI codes that identify the mobile equipment deployed in the GSM system.
9. What is ARFCN?
Absolute Radio-Frequency Channel Number (ARFCN) is a code that specifies a pair of physical radio carriers and channels used for transmission and reception on the Um interface, one for the uplink signal and one for the downlink signal.
10. Explain Power Control?
Power Control enables the mobile station and/or the BTS to increase or decrease the transmission power on a per-radio link basis.
Power Control is separately performed for the uplink and downlink. In both cases the BSC is responsible for initiating Power Control; the mobile station and the BTS adopt transmit power according to the BSC Power Control commands.
Measurements While a mobile station is active on a call, it has the responsibility of providing measurement data about the performance of the air-interface to its serving BTS so that the serving BSC can decide if a power control should be performed. Also the serving BTS measures the performance of the air-interface. Whereas the mobile station measures the performance of the downlink, the BTS measures the performance of the uplink.
Downlink measurements The mobile station measures and reports the following measurements to the BSC regarding the performance of the downlink:
· Strength of the signal being received from its serving BTS (in dBm)
· Quality of the signal being received from its serving BTS (in bit error rate).
Uplink measurements The BTS measures and reports the following measurements to the BSC regarding the performance of the uplink:
· Strength of the signal being received from the mobile station
· Quality of the signal being received from the mobile station.
Periodically measuring The mobile station measures periodically the performance of the downlink, and sends the measurements in the SACCH (Slow Associated Control Channel) via the serving BTS to the BSC every SACCH multi-frame. This corresponds to the transmission of data every 104 TDMA frames or 480 ms. The base
station measures the quality of the uplink. Also, it transfers the measurements in the SACCH to the BSC every 480 ms.
Signal strength When the BSC notices that the signal strength of a particular radio link measured on the uplink becomes below the lower pre-defined threshold because the mobile station moves away from the BTS, it sends a Power Control command to the mobile station to increase its transmit power (MS_TXPWR) by a pre-defined step (typically 2 dB).
11. What is the difference between FDD and TDD?
FDD = frequency division duplexing (transmitter and receiver operates on different frequencies)
TDD = Time division Duplexing (transmitter and receiver operates on same frequency).
12. What is an extended cell? How does it impact the system? Channels and TDMA structure?
The current limitation on the range of a GSM cell site to 35km is mandated by the duration of the standard timeslots defined in the GSM specification. The maximum distance is given by the maximum time that the signal from the mobile/BTS needs to reach the receiver of the mobile/BTS on time to be successfully heard. At the air interface the delay between the transmission of the downlink (BTS) and the uplink (mobile) has an offset of 3 timeslots. Until now the mobile station has used a timing advance to compensate for the propagation delay as the distance to the BTS changes. This timing advance is defined in the GSM specification as 64 bits, which gives the theoretical maximum BTS/mobile separation as 35km.
With Extended Range Cell Feature, the BTS is able to receive the uplink signal in two adjacent timeslots instead of one. When the mobile station reaches its maximum timing advance, i.e. maximum range, the BTS expands its hearing window with an internal timing advance that gives the necessary time for the mobile to be heard by the BTS even from the extended distance. This extra advance is the duration of a single timeslot, a 156 bit period.
13. Why do we use Multiple Access Schemes? What is the difference between FDMA, TDMA and CDMA?
Multiple Access schemes allow for many users to access the network.
FDMA= Users access the network through frequency division (separation between users is made through frequency). (1G cellular systems)
TDMA= Users access the network through time division (separation between users is made through time). (2G cellular systems)
CDMA= Users access the network through code division (separation between users is made through code). (3G cellular systems)
14. Which channel(s) is used for SMS?
Either SDCCH or SACCH.
15. Which channel is used by MS to request access to the network?
RACH = random access channel.
16. What is AGCH?
AGCH (Access Grant Channel) assigns a signaling channel (SDCCH) to the MS.
17. Why do we need SDCCH?
The BTS switches to the assigned SDCCH. The call set-up procedure is performed in idle mode. The BSC assigns a TCH. (SDCCH is also used to transmit text messages).
18. What is a physical channel? How do we differentiate between physical and logical channels?
Each timeslot on a TDMA frame is called a physical channel. Therefore, there are 8 physical channels per carrier frequency in GSM.
Physical channels can be used to transmit speech, data or signaling information. A physical channel may carry different messages, depending on the information which is to be sent. These messages are called logical channels. For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic Channel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message.
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