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GPRS Multi Slot Capability(Type1 MS)
As we know that in GSM one timeslot is allocated to a particular mobile however unlike GSM in GPRS each timeslot is shared between different Mobiles dynamically (Dynamic Allocation).
The idea behind Dynamic Allocation is that each Mobile is assigned a unique value per Uplink TBF called the USF. Each Mobile looks for USF value coming in the downlink messages in the MAC header and if USF value is matched with the USF value contained in the downlink message then the Mobile transmits data in the subsequent block in the Uplink.
Thus USF acts like a sharing resource where in the Mobile finding the USF value would transmit in the Uplink. Moreover the point to be noted here is that even if a Mobile is allocated resources in Uplink in case of Dynamic allocation it need to monitor corresponding time slots in Downlink.
Now let us look at how actually this would look like. First of all we should know that the Downlink and Uplink in GSM/GPRS is separated by 45 MHZ in Frequency Domain and
Three timeslots in Time Domain. The idea behind this three time slot separation is to make sure that Mobile need not receive and transmit at the same time. So if a Mobile is allocated one time slot in GPRS the allocation would look something like this
The above diagram the timeslot Zero is allocated to the Mobile. Thus Mobile first needs to receive the Downlink burst(assume it to be the last burst which will eventually form a block).After detecting that the Downlink contained the USF value which was assigned to the Mobile the Mobile will do the following
1. Switch frequency from Downlink to Uplink
2.Transmit a burst in Uplink
3.Do measurements in timeslot 1,2 and 3 if required
4.Switch frequency from Uplink to Downlink
As we can see the Mobile needs to switch the operating frequency when it transitions from Uplink to Downlink or from Downlink to Uplink.
Now Let us look at few definitions given in 5.02
Ttb: relates to the time needed for the MS to get ready to transmit. This minimum requirement will only be used when adjacent cell power measurements are not required by the service selected. For type 1 MS it is the minimum number of timeslots that will be allowed between the end of the last previous receive TS and the first next transmit TS or between the previous transmit TS and the next transmit TS when the frequency is changed in between. It should be noted that, in practice, the minimum time allowed may be reduced by the amount of the timing advance.
Tra: Relates to the time needed for the MS to perform adjacent cell signal level measurement and get ready to receive.For type 1 MS it is the minimum number of timeslots that will be allowed between the previous transmit or receive TS and the next receive TS when measurement is to be performed between.
Rx: describes the maximum number of receive timeslots that the MS can use per TDMA frame. The MS must be able to support all integer values of receive TS from 0 to Rx (depending on the services supported by the MS). The receive TS need not be contiguous. For type 1 MS, the receive TS shall be allocated within window of size Rx, and no transmit TS shall occur between receive TS within a TDMA frame.
Tx: describes the maximum number of transmit timeslots that the MS can use per TDMA frame. The MS must be able to support all integer values of transmit TS from 0 to Tx (depending on the services supported by the MS). The transmit TS need not be contiguous. For type 1 MS, the transmit TS shall be allocated within window of size Tx, and no receive TS shall occur between transmit TS within a TDMA frame.
Thus from the above definitions we can see that a Mobile has to obey Ttb Constraints while switching frequency from Downlink to Uplink and Tra constraints when switching from Uplink to Downlink. The minimum value of Ttb is one timeslot and Tra is two timeslots. Moreover as per the definition of Rx and Tx there cannot be a sequence like RX-TX-RX within a single frame and there cannot be a sequence like TX-RX-TX with in a single frame.
Keeping the above four rules in mind let us draw some conclusions
1. There can be maximum two time slots in dynamic Allocation that too continuous
2. There can be maximum three time slots in Extended Dynamic allocation. that too continuous
Let us look at cases to decide about this claim
In the above case there are two time slots allocated to the Mobile in the Uplink namely 0 and 2 here the TTb constraint is getting violated so this allocation is not valid. On the same grounds we can say that time slots 0 and 3 cannot be allocated.
In the above case there are two time slots allocated namely 0 and 4. This allocation violates Tra rule and hence this allocation is also not valid. Moreover this also violating another rule that we have a combination of RX-TX-RX combination within a single frame. On similar grounds we can see that combination 0 and 5 , 0 and 6 , 0 and 7 are also violating these rules. Hence we can see that no other combination except a two timeslot continuous combination can be allocated otherwise one or the other rule would get violated.
Let us se the case of Extended Dynamic allocation. In extended Dynamic allocation the user need to receive the information in corresponding downlink time slots and transmit in all the uplink time slots which have a value greater than equal the timeslot containing the USF. For example if a Mobile is allocated timeslots 0 and 1 then if it receives a USF in timeslot 0 then it need not monitor timeslot 1 and it can transmit in time slots 0 and 1.However if it gets USf in time slot 1 then it can only transmit in timeslot 1.
In the above diagram a Mobile is allocated three timeslots in extended time slot allocation. If Mobile get USF in timeslot 0 then it need not monitor timeslot 1 and 2 and can straight away switch frequency thus Ttb rule is not violated. If we take the case in which a Mobile get the USF in timeslot 1 then it need not monitor timeslot 2 and can straight away switch frequency and thus again no rule is violated. On similar lines we can say that Mobile getting a USF allocation in time slot 2 would also do justification.
Categories: Wireless
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