Introduction to CDMA ALOHA. 3. Access Control Techniques for CDMA ALOHA

Transcription

1 Introduction to CDMA ALOHA 3. Access Control Techniques for CDMA ALOHA Takaya Yamazato Center for Information Media Studies, Nagoya University Nagoya , Japan

2 CDMA ALOHA CDMA ALOHA Soft Capacity High spectral efficiency Simultaneous transmission Robustness against interference and distortion Flexibility for multimedia CDMA ALOHA Preserving good features of CDMA and ALOHA + High throughput Fundamental protocol Random access Simplicity Random access Simultaneous packet transmission High throughput performance Flexible transmission of multimedia signal

3 Why Access Control? Control Signal Hubstation Normalized throughput S/N CDMA U-ALOHA Pure ALOHA N= Eb/No= [db] L=500 [bits] CDMA ALOHA Packet transmission accoding to the information broadcasted from the hubstation Normalized offered load G/N Improvement of maximum throughput No degradation of throughput in high offered traffic load

4 Channel load Control Signal t Power spectral density Hubstation How? f X Difficulty of carrier sensing by each user due to the low peak power transmission. X Fluctuation of channel load during a packet transmission CDMA ALOHA Low peak power transmission of CDMA signal D A little throughput improvement by slotted system

5 Access Control for CDMA ALOHA Channel load Control Signal CDMA ALOHA t Hubstation Fluctuation of channel load 1 Access control protocol should be based on the channel load status observed by hub-station 2 Packet access should be accomplished in accordance with control signal broadcast from hub-station 3 CDMA Unslotted ALOHA (CDMA U-ALOHA) is an appropriate candidate

6 Access Timing Delay Control Signal Satellite CDMA ALOHA Time difference between channel load sensing and associated packet access timing Channel load Hub sensing station User station Control signal Packet transmission Propagation Propagation delay delay Process delay Remarkable in satellite communication system GEO : 0.50 [sec] LEO : 0.02 [sec]

7 Access Control for CDMA ALOHA Transmission access control protocol 1. Channel load sensing protocol (CLSP) 2. Modified CLSP Retransmission control protocol 3. Packet retransmission control (PRC) Transmission and retransmission control protocol 4. Optimum access control protocol (OACP) CDMA Unslotted ALOHA systems with buffers

8 System Model X X X Contol signal PN PN 1 1 CDMA ALOHA PN 1 X CDMA ALOHA PN 1 Hubstation CDMA ALOHA + AWGN Centralized single-hop network DS/SS modulated packet Poisson generation of packet Equal power reception G : Offered load L : Fixed packet length [bit] BER -- see (2.1)

9 Transmission Control Protocol Control Signal CDMA ALOHA Hubstation Packet transmission accoding to the information broadcasted from the hubstation Packet transmission is controlled by Hubstation 1. Channel load sensing protocol (CLSP) 2. Modified CLSP

10 Channel Load Sensing Protocol (CLSP) Channel load CDMA ALOHA Permission or Rejection CDMA ALOHA CDMA ALOHA CDMA ALOHA Fluctuation of channel load threshold a t Hubstation Hub-station observes the channel load, actual number of on-going packets According to the channel load, the information of permission or prohibition is broadcasted. Users transmit according to such information. Channel load is always kept less than or equal to the threshold, a.

11 Throughput of CDMA ALOHA with CLSP Throughput S without CLSP N=60 L=500 [bits] Eb/No=10 [db] Offered load G with CLSP a = Significant improvement in throughput No degradation in high offered load Throughput depends on threshold, a

12 Access Timing Delay Packet Transmission Packet Transmission Hub Station User Station Propagation Delay Process Delay Propagation Delay Process Delay Access Timing Delay Time difference between channel load sensing and associated packet access timing t D : Access timing delay normalized by packet duration GEO : t D = 9.20 LEO : t D = 0.26 L=500 [bit] R=9,600 [bps]

13 Throughput Degradation of CLSP Throughput S N=60 L=500 [bits] Eb/No=10 [db] a=9 td=0 without CLSP (no access timing delay) td =0.1 td =0.26 (LEO) Severe degradation in the presence of access timing delay 2 1 td =9.2 (GEO) td= Offered load G

14 Throughput S N=60 L=500 [bits] Eb/No=10 [db] a=9 Throughput Degradation of CLSP Packet Transmission td=9.2 (GEO) td=0 without CLSP Offered load G Propagation Delay Process Delay Propagation Delay Process Delay Packet Transmission Hub Station User Station (no access timing delay) td=1 Access Timing Delay td=0.1 td=0.26 (LEO) In the presence of access timing delay, since the packet access control is done by the past information, the throughput would degrade Access control based not on instantaneous channel load but on average channel load

15 Estimation of G Channel load CDMA ALOHA Ptr CDMA ALOHA CDMA ALOHA CDMA ALOHA Fluctuation of channel load Calculation of Ptr Hubstation User transmit his packet with Ptr, or stops transmitting its packet with 1-Ptr Modified CLSP t Broadcast Ptr Hub-station estimate the offered traffic load, G. According to the estimated G, the hub-station broadcast the probability, Ptr. Ptr is obtained so that actual offered load is set to the value which gains the maximum throughput. User transmits his packets according to Ptr

16 Throughput of CDMA ALOHA with MCLSP Throughput S N=60 L=500 [bits] Eb/No=10 [db] Analysis Simulation td =0.26 (LEO) td =9.2 (GEO) ts =10 ts =1 No access control Maximum throughput is the same as one without access control. Robustness against access timing delay Gmax Offered load G

17 Retransmission Control Protocol Transmission Hub station User station X X Packet error Packet transmission Hubstation Control signal Delay Retransmission control signal Delay Retransmission Packet retransmission If packet error occurs, a user schedules the packet at a later time according to a delay distribution This distribution is calculated and broadcast by a hub-station 3. Packet retransmission control (PRC)

18 Packet Retransmission Control (PRC) G 0 originating mode success fail success fail retransmission mode Transition of users between the originating mode and retransmission mode Transmission X Hubstation Control signal Delay G r Retransmission Appropriate distribution of delay is calculated and broadcast The distribution is obtained by a observation of channel load PRC is equivalent to a control of retransmission offered load, Gr time

19 Throughput of CDMA ALOHA with PRC Throughput S Gr=25 Gr=Go N=60 L=500 [bits] Eb/No=10 [db] Offered load of new packet Go Gr=15 PRC Gr=5 Maximum throughput is the same as one without access control. Throughput is almost same as MCLSP Robustness against access timing delay

20 What is the optimum access control? Channel Load Sensing Protocol (CLSP) Higher throughput Weakness to access timing delay Modified CLSP (MCLSP) Robustness against access timing delay No gain in maximum throughput Packet Retransmission Control (PRC) Robustness against access timing delay No gain in maximum throughput?

21 Transmission control signal Optimal Access Control Protocol (OACP) Transmission OACP CLSP+PRC Hubstation If packet error occurred Retransmission control signal Retransmission CLSP : O Improvement in maximum throughput X Weakness against access timing delay PRC : O Robust against access timing delay X No improvement in maximum throughput CLSP + PRC = Optimum

22 Optimal Access Control Protocol (OACP) New Packets Offered Load Go Is the channel load less than a? Yes CDMA Unslotted ALOHA Channel Retransmitted Packets Transmitted with Pre Retransmission Control Successful Packets No Tramsmission Control Random Retransmission Delay Unsuccessful Packets Hub-station estimate the offered traffic load, G. If the channel load is below a, then packet transmission is allowed. Otherwise, the users move into retransmission mode (CLSP). Backlogged packet is controlled according to the retransmission probability broadcast from the hubstation (PRC).

23 Throughput of CDMA ALOHA with PRC Throughput S N=60 L=500 [bits] Eb/No=10 [db] OACP CLSP w/o Access Control td =0.1 td =0.26 (LEO) td= Offered load of new packet Go Maximum throughput is the same as CLSP if access timing delay is negligible Robustness against access timing delay

24 Delay performance of CDMA ALOHA with OACP 160 Delay D N=60 L=500 [bits] Eb/No=10 [db] K=1000 OACP td=0 =0.1 =0.26 =1 = Offered load of new packet Go

25 CDMA Unslotted ALOHA with Buffers Each user is equipped with a certain size of queueing.... CDMA ALOHA with Buffer CDMA ALOHA Channel Hubstation buffers. Retransmission packet can be managed by each of users. Autonomous control of packet transmission may be possible.

26 CDMA Unslotted ALOHA with Buffers Each user is equipped with a finite buffer l Full buffer? Yes Rejection No Yes Busy mode Busy? No Idle mode p Transmission to the channel capacity of B packets. Packet arriving at an idle status is transmitted Correct reception Incorrect reception immediately. Removal Retransmission p Packets are served in a first-in-first-out (FIFO) Schematic of packet flow at each user station discipline. Busy user attempt to transmit packet with rate p.

27 Throughput of CDMA U-ALOHA with Buffers 6 The larger buffer 5 size a user has, the more rapidly Throughput S line:analysis dot:simulation B= N=60 Eb/No=10dB K=100 L=500 p=0.1 the throughput is increasing. Throughput is almost same as MCLSP or PRC Offered load G

28 Rejection probability Q R line:analysis dot:simulation Rejection Probability B= N=60 Eb/No=10dB K=100 L=500 p= Offered load of new packet Go X If a packet arrives at a user with a full buffer, this packet is rejected The larger buffer size a user has, the less rejection probability is

29 Delay Performance Average delay D N=60 Eb/No=10dB K=100 L=500 p=0.1 line:analysis dot:simulation B=5 B=4 B=3 B=2 B= Offered load of new packet Go The number of busy user increases by increasing the buffer size. Average delay increases in compensation for reduction of the rejection of packet transmission.

30 CDMA ALOHA for Multimedia signals voice data + voice data Hubstation voice data control signal signal data + voice User-Station Different media which have different characteristics are handled simultaneously CDMA is suitable for handling multimedia signal Multi-rate CDMA Multi-code CDMA Access control for multimedia signals 1 Integrated voice and data system 2 High and low bit rate data transmission

31 1. Integrated Voice and Data System Voice : Real time delivery Circuit switch mode (Reservation mode) Voice users have to reserve the channel before they transmit their signals by sending a reservation packet. Once they get the reservation, they continue to transmit their signal until voice call ends. If the number of simultaneous established users reaches to a certain threshold, voice users cannot access to get the reservation. Data : Some tolerance to transmission delay Packet switch mode Data users transmit thier packets on the CDMA Unslotted ALOHA.

32 Integrated CDMA Voice Signal & CDMA Data Packet (CDMA unslotted ALOHA)... Number of server = a time The channels reserved for voice users The channels ocupied by data pakets The available channels for additional voice and data users Channel load status seen at hubstation Interference from both media Priority of voice medium (continuous voice call until call ends) Necessity of traffic control

33 System Model user 1 (voice) user 2 (data) user.. (data) PN sequence 2 user.. (voice) user K (voice) Random signature CLSP Control Signal Hub Station Data packet : CDMA Unslotted ALOHA Voice signal : Circuit Switch Total bandwidth W = 20MHz Band expansion factor N = 312 (=W/R) Voice signal : Poisson generation Bit rate : R = 32k [bps] Exponential signal length Length 60.0 [sec] Silence period 1.7 [sec] Talk spurt 1.0 [sec] Data packet : Poisson generation Bit rate : R = 32k [bps] Fixed packet length L = 500 [bit] (0.01 [sec])

34 Traffic Control for Voice and Data Media The number of simultaneous voice and data signals, k a : Threshold for data packet b : Threshold of voice signal b a voice user can access to the channel Data packet users can access to the channel 0 Why b> a? The required signal quality of voice < that of data packet The signal quality must be guaranteed for voice signals

35 Mean Number of Voice Call The mean number of voice calls Threshold b =98 (Pb(b)<10-3 ) N =312 Eb/N0= V.A rate = 0.4 No data packet Gd=0 without V.A with V.A Voice activity (V.A) improves the number of simultaneous call. Access control by counting the number of talk spurt of voice signal Voice Offered Load, Gv

36 Data Throughput Throughput for Data Gv=20 Gv=10 with V.A no V.A N =312 Eb/N0= V.A rate = Data Offered Load, Gd Without access control Data throughput S Consideration of V.A No consideration of V.A N =312 Eb/N0= V.A rate = 0.4 Erlang capacity for voice Er = 40 (= Maxinum Gv) Data offered load Gd With access control

37 2. High and Low Bit Rate Data Transmission Class I : High-bit-rate packet Class II: Low-bit-rate packet Class I Control Signal Hubstation Multi-rate CDMA system O Multi-code Multi-processing gain Multi-modulation Class II Multi-code CDMA + CDMA ALOHA Low bit rate High bit rate MC-CDMA ALOHA

38 System Model High bit rate Source M.R b Low bit rate Transmitter of Class I Sub-stream (Sub-packet) L [bits] M Sub-packets transmitted in one sub-packet duration M Transmitter of Class II: Conventional CDMA Slotted ALOHA Transmitter b Class I : Poisson generation Bit rate MRb [bit/sec] Fixed packet length MLb [bits] Class II: Poisson generation Bit rate Rb [bit/sec] Fixed packet length Lb [bits] Class I has priority over Class II

39 Channel Status at Hubstation 1 2 M One sub-packet One packet from class II (CDMA) 1 2 M One packet from class I (MC-CDMA) Slot Slot Slot Time

40 Throughput performances I Throughput, S I S I When M=1 (Single bit rate) When M=8 Eb/N0 = N=128 L b =500 bits G 2 =10 S I When M=16 S I Total throughput, S tot = S 1 + S S tot When M=1 (SIngle bit rate ) Eb/N0 = N=128 L b =500 bits G 2 =10 S tot When M=8 with MCLSP S tot When M=8 (without MCLSP) Offered load of class I, G1 MC-CDMA Slotted ALOHA (Only class I user) Total offered load, G tot = G 1 + G 2 MC-CDMA Slotted ALOHA with MCLSP (class I and II)

41 Conclusions Fundamentals of CDMA ALOHA Throughput analysis of CDMA ALOHA CDMA ALOHA v.s. Narrow Band ALOHA Access control techniques for CDMA ALOHA CLSP, MCLSP, PRC, OACP CDMA ALOHA with Buffers Multimedia signal transmission using CDMA ALOHA Integrated voice and data system Multi-rate transmission using MC-CDMA