Quality of Service Optimization of Multimedia Traffic in Mobile Networks

ed as a single packet stream connector with a given transfer delay (20ms de- fault). TIMEOUT event occurs when retransmission timer expires before arrival of expected acknowledgement i.e. STATUS PDU from the UE RLC entity. In this state the PDUs in the retransmission buffer are retransmitted and the timer is reset. Figure B.3 RNC process model with RLC AM ARQ protocol The Node B process model is depicted in Figure B.4. It is contained in the Node B module which send data to the remote UE via the transmitter module and receives feedback via the ACK receiver module, thus modelling the functionality of the downlink channels (HS-DSCH and HS-SCCH) and the return HARQ feedback channel (uplink HS-DPCCH) respectively. The arrival state implements code for queuing arriving PDUs in the Iub frame according to the buffer management policy using the in- sert_into_BM_scheme( ) function implemented in the process model Function Block. The ACK_received state implements code for scheduling retransmissions if a NACK is received, or freeing an assigned HARQ process if ACK is received. The functions used are also implemented in the Function Block and are given in Table B.1. The scv_start state implements code to create and transmit L1 transport blocks (TB) from the queued PDUs according to the buffer management policy. Recall that the TB size is determined by the AMC scheme selected. This state contains a table to map SINR of the UE to a given AMC scheme. svc_start state also contains functions for the Iub credit allocation Appendix B HSDPA model development in OPNET 191 algorithm and functions that implement the discard timer (DT). The svc_start state is invoked every TTI by the TTI manager in order to update the aforementioned functional- ities. Figure B.4 Node B process model implementation Table B.1 list of implemented key Node B process functions Node B function name Purpose of function assign_HARQ_process( ) Assigns a free HARQ process for L1 TB frame transmission. create_HARQ_RT_packet( ) Creates a HARQ TB frame from queued RT PDUs . create_HARQ_NRT_packet( ) Creates a HARQ TB frame from queued NRT PDUs. delete_HARQ(Packet* ptr) Frees up assigned HARQ process on ACK receipt. schedule_retrans(Packet* ptr) Schedules a TB frame for retransmission on NACK receipt. retransmit(Packet* ptr ) For retransmitting a TB frame insert_into_BM_scheme ( ) Functions with particular buffer mgt. scheme (BM_scheme) used to insert an arriving PDU into position in MAC-hs queue. record_stats (void) For updating statistics of performance metrics Appendix B HSDPA model development in OPNET 192 B.3. 1 The TTI manager The TTI manager module, as mentioned earlier, provides timing to coordinate synchron- ous and regular events. It contains a process model with several functions that generate and send timing triggers (remote interrupts) to the svc_start state in the Node B process for packet transmission on HS-DSCH. Timing triggers are also sent to the L1 managers in the UEs to update the UE position and SINR which are stored in global variables that are accessed by the Node B process model for AMC mapping. Another important functionality built into the TTI manager is the packet scheduling algorithm(s) which must operate with the granularity of the HSDPA 2ms TTI. The next UE to receive transmission from the Node B module is selected from a set of eligible UEs in the HSDPA cell according to the implemented scheduling algorithm. The design allows for any type of packet scheduling algorithm to be implemented but currently the three well known HSDPA packet scheduling algorithms the Round Robin, Proportional Fair and Max C/I are incorporated. B.4 UE receiver implementation The UE receiver node model is illustrated in Figure B.5, consisting of the receiver, L1 manager, HARQ entity, MAC-hs de-assembly, RLC receiver, Packet Reassembly, TCP receiver and Application Layer. RT flow is directed to the RT playout buffer module after PDU extraction from the MAC-hs frame where they reassembled, queued and played out according to the implemented playout algorithm. NRT flow are directed through the chain leading to the Application layer, where RLC receiver sends out RLC STATUS PDUs to the RNC ARQ process while the TCP receiver sends out TCP ACK packets to the TCP sender in the NRT source module. The L1 manager contains a process model that receives remote interrupts from the TTI manager every TTI, to update the UE state. It also contains functions to calculate the current UE SINR based on received power, spreading factor, distance from base station, noise, fading and interference. SINR is updated and stored in a global variable that the Node B process model accesses, in order to map the UE SINR to a given AMC scheme. Appendix B HSDPA model development in OPNET 193 Figure B.5 UE receiver node model implementation