CHAPTER 3 TWO-PHASE PROPORTIONATING
3.2 D ETAILED O PERATIONS OF TPP
3.2.1 Bandwidth Translation and Slot Dispatching
A service flow in an SS issues a bandwidth request whenever necessary. After the BS receives the data traffic from the backbone network and the uplink bandwidth requests from SSs, the TPP translates them from data bytes into the OFDMA slots, which are the basic transmission unit in PHY. This can be done by dividing the data bytes by the OFDMA slot size,
Size
where the OFDMA slot size is derived by multiplying the number of bits that can be depended on the coding rate of SS encoded over a subchannel by the number of symbols in a slot, namely
slot
Notably the number of symbols in a slot is three for UL while two in DL, and the data bytes should include the size of requested bandwidth from a SS, size of the MAC headers, and PHY overhead such as the Forward Error Correction (FEC), preamble,
and guardtime.
These slots are then dispatched to the corresponding service queues comprising the five uplink classes as well as the two downlink classes with/o the latency guarantee. Each queue employs three variables, bandwidth requested slots (BRQ), Rmax, and Rmin, to accumulate the requested slots, MSTR and MRTR, which are translated from data rate to number of slots per frame duration, of the service flows.
When the total accumulated Rmin is larger than the capacity of frame, TPP denies serving the bandwidth requesting.
3.2.2 First Phase: Dividing a Frame into Downlink and Uplink Subframes
To fit the traffic data into the time frame, TPP determines the proportion of the uplink and downlink subframes according to their accumulated number of BRQs in each MAP. However, this is not trivial because of the different slot definitions of the uplink and downlink, and could result in unused symbols. For example, if the uplink is proportionally allocated 19 symbols, only 18 of them will be used to form 6 slot columns.
Fig. 4 The placement of the separator in the first phase.
This problem is solved as follows. Depicted in Fig. 4, the most appropriate placement of the separator dividing uplink and downlink subframes is assumed to be x steps from the right, in which one step is considered 6 symbols, the least common multiple of the uplink and downlink slots. This is to ensure that all symbols are used
up after the division. Two cases need to be discussed here, namely when S, the number of symbols in a frame, is odd and when S is even. If S is odd, the scheme starts with an initial condition in which a slot column exists in the uplink subframe so that the number of remained symbols, S-3, can be divided by 2 in the downlink, leaving no unused symbols. Then the separator moves x steps toward left, which is supposed to be the correct position, resulting in 1+2x slot columns for the uplink
and S x
2−3 −3
slot columns in the downlink, whose ratio should be the same as the ratio of the uplink and downlink requested slots,
where UR and DR represents the total BRQs of the uplink and downlink, respectively.
Similar concept can be applied to the case when S is even, except that in the initial condition no slot column exists in the uplink whereas S 2 slot columns are derived in the downlink,
The x can be obtained after solving the equation and notably is rounded off if it has a fraction.
3.2.2 Second Phase: Dividing Each Subframe for Queues
After properly dividing the frame into uplink and downlink subframes, we start to allocate them to service queues, respectively, in the second phase. In this phase, the
Rmin of all queues are firstly satisfied for minimum slots guarantee, followed by the proportionating of the remaining slots to queues except the UGS and ertPS whose requested slots are already served. Since higher service classes typically have higher Rmax values, we take the Rmax as the weight for proportion. However, that only referring to Rmax may cause bandwidth waste or starvation of some queues. An example for the former case is a high class queue having a BRQ very close to Rmin. The additional number of slots assigned will be more than that of other queues because of the large Rmax, leading to unnecessary bandwidth waste. Similarly, a low class queue yet having a BRQ close to Rmax may not get enough feed. We use an adjustment factor,
fix this problem so that a high class queue requiring less bandwidth (BRQ) will be reflected while a low class queue demanding much will be compensated. The remaining slots are therefore allocated according to the following proportion
min
3.2.3 Per-SS Bandwidth Grant within Each Queue
The slots allocated to each queue are further distributed to SSs in the fashion of GPSS. Similar to the second phase, the minimum number of requested slots of each SS is satisfied first. Nevertheless, the remaining slots of each queue are evenly assigned to SSs since there is no priority among them. The pseudocode for the whole procedure is described in Fig. 5 and exemplified in the next subsection.
Fig. 5 Pseudocode of TPP.
3.2.4 Example
This section elaborates an example of the TPP, in which the parameters and results of the first and second phases are depicted in Table 2(a) and 2(b), respectively.
Suppose S is 26, then the separator should be moved toward left with number of steps x=3 according to Eq. (1), indicating 6x/3=6 slot columns for uplink while(26− x6 )/2=4 slot columns for downlink. If we use direct proportion, however, the number of symbols for uplink is 16
40 60
26 60 ≅
× + , in which only 15 symbols are effective.
The uplink is adopted as an example for the second phase. The number of subchannels in a symbol is assumed to be 6 and therefore 6×6=36 slots are allocated to the uplink after the first phase. Rmin, BRQ, and Rmax of the five service classes are as in Table 2(b). The scheduler allocates the guaranteed minimum number of slots to each queue, and later proportionate the remaining slots to queues of the lower three classes according to Eq. (2) since the UGS and ertPS are already satisfied.
As we can see in the table, A-Factor to meet the amount of the requested slots for decreasing the starvation occurrence of low service class and avoiding granting to non demanding service class such as rtPS.
Table 2 Example of TPP.
2(a) Parameters and allocation results of the first phase; UR=60 and DR=40.
Algorithm Item UL DL
2(b) Parameters and allocation results of the second phase.
Item UGS ertPS rtPS nrtPS BE
Rmax 8 8 16 8 12
BRQ 8 8 6 8 12
Rmin 8 8 6 4 2
BRQ- Rmin 0 4 10
Rmax with A-Factor 0 2 6
Rmax without A-Factor 3 3 2