In the North American cellular system there are 832 full duplex channels available for use including the expanded spectrum. Channel spacing is 30 kHz with 45 MHz duplex spacing. The 832 channels are divided into two bands. Band A is the non-wireline carrier and band B is the wireline carrier. At the center of these bands are 42 channels dedicated to control purposes. These channels are used for call setup purposes and carry no speech. The remaining 395 channels are voice channels which carry speech. In this document we will cover how a cellular system processes calls. These call processing events will be covered:
Each mobile contains a memory referred to as a Number Assignment Module(NAM).The NAM contains the mobile's directory number, system parameter information, options and other phone programmable information. This information is programmed into the mobile's memory at the time of sale by the dealer. The mobile's telephone number is referred to as MIN1 and MIN2. MIN1 is the 7 digit office code and unit number while MIN2 is the area code. Another piece of information programmed into the mobile is the cellular system's System Identification Number(SID). This number is assigned to the cellular carrier by the Federal Communications Commission. Electronic Serial Number
Another area of the mobile's memory contains the unit's Electronic Serial Number(ESN). This number is unique to each mobile.
Each carrier (non-wireline, wireline) has 21 channels allocated for control purposes. One of these channels is assigned to each cell site. A control channel is on the air 24 hours a day, 365 days a year. The control channel actually combines three functions into one channel:
The information that is continuously transmitted is on the Forward Control Channel (FOCC). Continuos transmission is needed here because the mobile phone monitors the level of the received FOCC to determine when they approach the edge of the cell and needs to be passed to an adjacent cell base station. There are three types of messages that are multiplexed in this channel:
|Overhead Messages||These are messages that are intended for all mobiles. Information in this message includes the cellular system's SID, combined paging and access channel information, and subscriber registration information.|
|Mobile Control Messages||These messages contain information for a specific mobile. This includes paging data, voice channel assignment, power level, or other commands that cause a mobile to respond.|
|Control Filler Messages||These are messages that are transmitted when there are no other messages to be transmitted on the FOCC. This maintains the mobile's synchronization to the Overhead Message Train. This message can also be used to tell the mobiles what power level to use when accessing the system on the Reverse Control Channel.|
Before a call can be originated or received, the mobile unit must go through an initialization
sequence. This happens when the unit is powered up, either by pushing the "Power" button or by
starting a vehicle.
Upon application of power, the mobile does a series of internal diagnostics. Memory is checked and sanity tests are done on the processor(s). A tone is injected into the audio circuitry. If the circuits are good, the tone is heard from the speaker. All indicator lights are activated and "8's" are shown in the display. If any portion of the diagnostic routine fails, the mobile powers down.
Control Channel Access
Next, the mobile scans the 21 control channels looking for an active channel. If it is programmed for the A side it scans from channel 333 down 21 channels. If it is assigned to the B side, it scans from channel 334 up 21 channels. The mobile looks for the strongest control channel and starts decoding the transmitted information. This includes system SID, combined paging and access channel information, and how many channel to scan for paging/access channels. Now the mobile scans the 21 channels a second time. This time it is looking for the strongest paging channel. (It got the number of channels to scan during the first scan.) The mobile now locks onto the strongest paging channel and goes into the "idle task". Although the mobile scanned the 21 control channels a second time looking for a "paging" channel, it actually returned to the same control channel. Control, paging, and access functions are multiplexed onto the same channel.
In the idle task, the mobile is constantly monitoring the overhead message train. It is looking for control information which will most likely be a page of its directory number. It is also scanning its keyboard looking for data entry. This would be when the user makes a call.
The control-filler message is also used to specify a control mobile attenuation code(CMAC) for use by mobiles accessing the system on the RECC. Also in the control-filler message is a wait- for-overhead-message(WFOM) bit indicating whether or not the mobile must read the overhead message train before accessing the system.
This section of Call Processing will deal with various call types. Each of the call types we'll be
covering assumes the mobile is in the idle task and in its home system. The first one discussed
discuss is the Mobile to Land call.
Before this section is covered there are two signals that the user needs to know.
They are :-
|The diagram on the left shows the SAT assignments to cell clusters.|
When a landside party calls a cellular mobile the PSTN determines that the call must be routed to
the MSC. A CDR is opened and the MSC performs validation and translation of the dialed
A page message is sent from the MSC to the system's cell sites. This page message is transmitted on all FOCC in all cell sites. Systems can also be configured so that just the last known paging area the mobile was in will transmit the page message.
When the mobile sees the page on the FOCC it rescans the control channels looking for an access channel and locks on to the strongest one. The mobile checks the busy/idle bits (just like the mobile to land Call scenario) and transmits a page responds message on the RECC.
The cell site receives the page response message and selects an idle voice channel. The selected voice channel keys its transmitter and begins transmitting SAT. The cell site transmits on the FOCC an IVCDM to the mobile that contains the voice channel assignment, SAT, and VMAC information.
When the mobile receives the IVCDM on the FOCC, it tunes to the assigned voice channel, checks and transponds SAT. The MSC updates the CDR with the IVCDM information.
Next, the cell site transmits an alert order on the FOVC to the mobile. This message tells the mobile to start ringing. When the mobile starts ringing, it transmits ST continuously until the mobile is answered. When the mobile goes off hook, ST is shut off. This loss of ST at the cell indicates that the call has been answered. The MSC now connects the landside party with the mobile. Both forward and reverse audio paths are unmuted, conversation takes place, and the CDR is updated.
Landside to mobile call termination was previously discussed under the Call Termination section.
DIGITAL CALL PROCESSING (TDMA) Equalization Equalization is a term used to describe the process that characterizes and compensates for the non-ideal frequency response characteristics of an RF channel. The equalizer used in the DRU is not a physical device, but rather a signal processing algorithm implemented on Digital Signal Processors (DSPs). The equalizer's primary job is to compensate for the effects of Doppler Shift, carrier error, and channel dispersion encountered in a digital mobile environment. To a stationary base station, the frequency of the moving mobile transmitter varies slightly with the speed of the mobile. The carrier frequency received is higher than the transmitted frequency when the mobile is moving toward the base station and lower when moving away from the base station. This difference in frequency due to motion is call Doppler Shift. This phenomenon along with allowable tolerances in the mobile carrier frequency may cause the base station to not to demodulate the received signal exactly. The residual frequency caused by carrier tolerance and Doppler Shift combine to form a frequency error estimate. The estimate is done on each received time slot. The equalizer calculates this frequency estimate and then compensates for this error resulting in reduced error rates. Channel dispersion is caused by multiple signal paths in the mobile environment. This dispersion causes errors in the received signal. The equalizer will characterize the channel dispersion and calculate a filter that will recover the original transmitted symbols. The equalizer is able to adapt quickly to the rapidly varying channel by decoding adjustment, or training, information from the synchronization data field on a per slot basis.
This section will discuss the process the mobile goes through when placing a digital call. There
are many areas in the processing of a digital call that is similar to an analog call. This section will
concentrate on the differences between the two. A knowledge of analog call processing is
The call progression description will be centered around a dual mode mobile. This is a mobile (or portable) that is capable of operation in either the analog or digital mode. We will assume the mobile is operating in the digital mode.
The dual mode mobile is programmed almost exactly like the analog phone. The major exception
is the Station Class Mark (SCM). This is an entry into the mobile's memory that tells the switch
the unit has digital capability. When the phone makes a call origination it tells the switch what
type of phone it is.
These tests are similar to the analog phone power up tests.
After the power up tests, the mobile scans the 21 control channels. It will scan the dedicated 21
control channels, either the A side channels (313-333) or the B side channels (334-354)
depending on how the phone is programmed. It will look on the strongest control channel for the
Protocol Compatibility Indicator (PCI) bit. This bit tells the mobile whether or not the system has
digital capability. If this bit is set, the mobile proceeds with decoding the overhead message train
and going into the idle task just as in the analog system. If the PCI bit is not set the phone will
scan the secondary control channels for the strongest signal.. These are a second set of control
channels that be from 696-716 for system A or 717-737 for system B. These secondary control
channels are for digital use only. The phone then attempts to decode a possible overhead message
train. If an OMT cannot be found (this could be an analog voice channel!), the phone retunes to
the primary control channels and operates in the analog mode.
The registration process in dual mode mobiles is similar to analog phones except that the ESN in
encrypted to help prevent fraud. Registration can be when the mobile makes a call, on demand
from the switch, or when the mobile moves outside its home system.
When registering, the mobile transmits its ESN, MIN, and Station Class Mark. This will be sent on the Reverse Control Channel.
Call progression in these areas are similar to analog operation. A major difference is when the
mobile is instructed to tune to a digital traffic channel. In an analog system the mobiles is
assigned a voice channel.
If digital traffic channels are available (digital voice/data channels) the base station informs the mobile of the channel and time slot assignment. The mobile is also informed of the Digital Verification Color Code (DVCC) to expect on the traffic channel. The DVCC is an 8 bit word that is transponded on each time slot. It performs the same function as SAT in an analog call. The DVCC word has an additional 4 bits added for error correction to make it a Coded Digital Verification Color Code (CDVCC).
When the mobile receives the digital traffic channel designation on the Forward Control Channel,
it attempts to synchronize to the Forward Digital Traffic Channel. When synchronized, the
mobile transmits in its allocated time slot at its assigned power level and will transpond CDVCC.
The signal from the mobile to the base station takes a finite amount of time to travel to the cell site. A special Time Alignment procedure is used to ensure that transmission form other mobiles on the same frequency (but different time slots) do not collide. When the mobile first joins the traffic channel the distance to the base station is not known. The mobile needs to know how much to delay its transmissions it its time slot to avoid collisions, or glare.
Before the mobile starts transmitting actual data, it first transmits Shortened Bursts. This allows the base station to determine time alignment information. The base station sends the mobile timing information until it starts receiving valid data. The mobile then will begin transmitting in its normal mode.
Once the mobile is on the Digital Traffic Channel it will be operating in its assigned time slot and
with the proper time alignment. As the mobile moves through the cell's coverage area messages
are sent to adjust the mobile's power level and time alignment to maintain call quality.
During a call there are conditions that can cause the call quality to deteriorate. This results in an increasing Bit Error Rate (BER). Some causes can be:
Doppler shift causes a change in receive frequency when there is relative motion between the base
station and the mobile. These frequency (phase) changes in an analog system are relatively minor.
However, in a digital system, the Doppler shift can cause problems. A car traveling 60 mph to or
from a cell site causes a frequency shift of 80 Hz. The digital modulation scheme interprets this
as a phase change and errors can occur.
Fading occurs when the signal form a transmitter is attenuated by physical objects such as a
building or a hill. The receiver has an Automatic Gain Control (AGC) circuit to compensate for
Multipath occurs when two or more signals arrive at a receiver at different time because of
reflections. There is a time delay between the two signals because each signal traveled a different
distance. If the two signals are 180 degrees out of phase, complete cancellation occurs. This is
also a concern in analog systems.
As we have seen, there are many factors at work to degrade the digital signal which causes the
BER to increase. There are many techniques use to combat these effects such as space diversity
to minimize the effects of multipath and AGC to help control fading. But the combined effects of
all modes of interference can be to much for the radio to handle. There is one more tool in the
arsenal to fight signal degradation.....an equalizer.
In dual mode radios the equalizer compensates for the effects of distortion . The transmitted SYNC word contains a known bit sequence. The receiver takes this known bit sequence and compares it to the received signal. The equalizer can then dynamically adjust its filters to reverse the effects of distortion.
At the end of the conversation one of the parties will go on hook. If the call is on an analog
channel the process will be as discussed in the analog call processing section. If the call is on a
digital traffic channel, a Release Message will be sent either by the base station or by the mobile,
depending on which side of the call goes on-hook first. The receiving end will acknowledge the
order, the mobile turns off its transmitter and retunes to the primary control channels. The mobile
then starts the initialization procedures.
In the text above, the channels used and the purpose they serve in both analogue and digital cellular
system during call processing have been discussed
This section identifies and explains strategies that could be used to assign the needed channels.
The three main assignment strategies are :-
The common underlying theme in all fixed assignment stretegies is the permanent assignment of a set of channels
to each cell. The same set of radio frequencies is reused by another cell at some distance away.
A call attempt at a cell site can only be served by the unoccuppied channels of the predetermined set of channels at that cell
site(see diagram __); otherwise, the call is blocked. Here, when handover occurs, the MSCinforms the new BS and receive a
confirmation or rejection message from the new BS regarding the handover.
|Fixed channel assignment strategy. A-G denote different sorts of channels permanently assigned to cells.|
A very high number of users would have their call blocked(blocked traffic). The number of users serviced at any one time is
Borrowing strategies. channel a4 is borrowed and now locked to cells marked 'N'. Cells marked 'X' were already prohibited from using a4.
If all permanent channels of a cell are busy, a channel can be borrowed from a neighbouring cell, provided that this channel
does not interfere with the existing calls.
When a channel is borrowed, additional cells are prohibited from using it. The MSC supervises the borrowing procedure, following an algorithm that favors channels of cells with the most unoccuppied channels to be borrowed. The algorithm 'locks' the borrowed channels towards the cells that are one or two cell unit away from the borrower cells.
The MSC keeps record of free, serving and borrowed(therefore locked) channels and informs all the BSs about locked channels.
As for the MSC, in addition to its duties in the simple borrowing strategy, now it has to label all channels with respect to the group they belong.
Minimizes the number of calls on the relatively more 'borrowable' channels in order to reduce the locking effect of borrowed channels in additional cells. This effectively minimizes the number of blocked traffic.
In this strategy, cells have no channels themselves but refer all call attempts to the MSC, which manages all channel assignment in its region.
Each time a call attempt arrives, the BS asks the MSC for the channel with the minimum cost to be assigned. The cost function depends on the future
blocking probability, usage frequency of the candidate channel, the reuse distance of the channel, and so on. The MSC decides, on a call-by-call basis, which channel
to assign to which call attempt by searching for the available channel for which the cost function is minimum.
The MSC needs to have information regarding channel occuppancy distribution under current traffic conditions and other network-directed criteria, as well as
Flexible channel assignment strategies combine aspects of both the fixed and dynamic strategies because it
has two types channels mentioned below :-
|Permanent Channel||each cell is assigned a set of these channels and would normally suffice under light traffic loads.|
|Flexible Channel||held by the MSCand assigns these to cells whose permanent channels have become inadequate under increasing traffic loads.|
|Schedueled manner||Assumption is made that future changes in traffic distribution can be accurately predicted in time and space. The change in assignment of flexible channels is then made at the predetermined peaks of traffic change|
|Predictive manner||The traffic intensity or equivalently the blocking probability is constantly measured at every cell site so that the reallocation of the flexible channels can be carried out by the MSC at any point in time|