1 GSM Architecture 1 GSM NETWORK INFRASTRUCTURE NETWORK SWITCHING SUBSYSTEM (NSS) Home Location Register Mobile Switching Center and. Cellular Mobile Systems and Services (TCOM) Cellular Mobile Systems and Services (TCOM) May GSM Radio – Part 1: Physical Channel Structure 1 2 3 4 5
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Global System for Mobi Physical Channel Structure 1 2 3 4 5 5. The most of the world except North America uses these bands. These bands are, however, are not available in North America as they were allocated to some other wireless services. An MS that supports multiple frequencies is called multiband MS. Besides the standard GSM bands as above there are many special bands exist to meet special requirements http: The table shows the extents ranges of each band and not its center frequency Note 2: There are a number of channels which are reserved and not used for traffic or control.
A number of them are used as guard band from the neighboring bands. GSM radio channel is 0. When the network assigns a channel to an MS mobile station it identifies this number. The ulink signal, which is lower in frequency, suffers less attenuation. The MS, therefore, requires less transmission power.
The TDMA scheme divides the channel into ? For a voice channel mkbile 8th time-slot belongs to the same user. That is, a continuous digitized voice stream is sent ccellular as data-burst roughly ? The voice channels are duplex channels. The above calculation is to provide the concept.
The accurate calculation is little bit complicated and will be discussed later. The following figure illustrates that feature. Such a channel can be used for a voice or mix of a variety of control and management signaling discussed later.
A GSM system identifies a time slot using 3-bit code 0 to 7. Thus the multiplexing in GSM takes the following format. The multiframe has two different sizes: The reason of such structures is to solve the following problem: Suppose a voice call is connected to Slot 4 of a frequency but Slot 4 of another frequency is set for its paging.
In that case the user can not listen to the paging for another call think call waiting service if both the frames have the identical period of repetition. With the and multiframe structure a mobile station may miss on page due to coincidence of the voice and the page time-slots but will be able to capture the next repetition of the page since there will be no overlap of those time-slots. There are too many time values to remember.
One of the easiest ways to remember all is remembering: The following table summarizes the numbers. An MS sends or receives signal or information in the form of burst that is, not continuously. A burst is put in a TDMA-timeslot. That is, a burst is carried by a time-slot. A burst in a time-slot must not overlap the bursts in the previous and next time-slots.
Cellular mobile systems and services (tcom) pdf
Therefore, the time-slot is set to The spare time minimum This consists of all 0s un-modulated carrier. It typically celllular of alternating 1s and 0s. The whole data space bits is used for unmodulated carrier pure sinusoid or carrier modulated with all zero bits. This burst format is used by SCH channel tcoj1010. This channel makes a mobile station time-synchronized with the base station clock. That is why the synchronization training sequence is very large for this burst.
Only one training sequence is defined for this burst. For that reason, the actual message is relatively cellulwr and have a long tdom1010 band GB in order to make sure that there will be no overlap with the next burst.
The length of the guard band in the access burst The GSM allows a cell radius up to of 35 km. That is, an RACH message from an MS at a distance of up to 35 km from the base station can reach to the base station antenna without overlapping the next burst.
The FACCH channel uses this burst during handover operation when the timing advance of new cell is not yet known. Few important features of the celoular is stated below. That is the flag is 0 for TCH and 1 for others. Normal Burst This is like normal burst but has no meaning of its payload bits. Every 8th slot belongs to a TDMA channel.
A consecutive 26 such that is, 8th slots of a TDMA channel forms a multiframe. That is why the length of the multiframe is ms.
The following diagram depicts the multiframe structure with an example of ombile beacon frequency. It is formatted as multiframe. The next one is SCH.
Cellular Mobile Systems and Services (TCOM1010) GSM Architecture
BCCH channel 4 slots long appears once per multiframe and it takes Slot 2 to 4 3rd, 4th, 5th and 6th slots? The beacon is a downlink channel. That uplink contains RACH channel in this example see the figure below.
The following figure depicts a beacon TDMA channel up and down link for normal capacity cell. One of those frequency channels is defined as the base-frequency beacon frequency or BCCH frequency. The first time-slot Slot0 of the base-frequency TDMA is used as the base-control channel or beacon channel.
Remaining part of the frequency channel Slot-1 to 7 can be used as any mix of traffic mobilr control channels. All other frequencies are mostly for traffic but can also be used for control channels.
Mix of traffic and control channels depends on number of frequency channels per BTS that is the capacity of a cell and the traffic patterns. GSM standard provides a number of combinations for traffic and control channels in order to suit different conditions.
A list of such combinations is given below. Typical small capacity cell with only one frequency channel May Note 1: Note that, many implementations do not reserve any physical location for AGCH. Medium Capacity Cell example: Large Capacity Cell example: These slots carry voice traffic bursts. Note that, the slots are like physical carrier, good for any type of data. A duplex pair of them becomes TCH when they are used or designated for voice traffic. The timing of uplink and downlink slots maintains a 3-slot distance in order to ensure that a cell-phone does not require transmission and reception operations simultaneously.
This helps avoid a number of complexities including the requirement of high peak power, processor speed and large memory. This also helps simplify transceiver circuit. Page 16 of