Design and Implementation of an Underlay Control Channel for NC-OFDM-Based Networks

Transcription

1 Design and Implementation of an Underlay Control Channel for NC-OFDM-Based Networks Ratnesh Kumbhkar, Gokul Sridharan, Narayan B. Mandayam, Ivan Seskar (, Rutgers, The State University of New Jersey) and Sastry Kompella (Naval Research Lab)

2 Outline Introduction Challenges and requirements of NC-OFDM Underlay control channel for NC-OFDM mode of operations structure of underlay channel Experiments on ORBIT testbed Results and analysis Conclusion 2

3 Introduction NC-OFDM (Non-contiguous OFDM): Transmission on only some of the subcarriers based on availability of spectrum Occupied by PU or unavailable Available spectrum Allocated to user 1 Allocated to user 2 Ideally suited for the accessing fragmented spectrum, e.g. TV white- space Enables mul>ple access system Dynamically adapt to changing spectrum condi>ons 3

4 Example Network A network with mul>ple point- to- point links 4

5 Challenges with NC-OFDM Frequency offset estimation at the receiver OFDM symbol timing estimation for each link in asynchronous environment Traditional methods used in regular OFDM can not be used due to asynchronous communication Sending channel allocation information to both transmitter and receiver of a link by the controller Exchange of control information between the transmitter and receiver of a link 5

6 Control Channel for NC-OFDM An in-band underlay control channel as unified solution to address all requirements of NC-OFDM Power in control channel is comparable to the noise floor of the system Each link is assigned a unique PN-sequence A common PN-sequence could be used for broadcasting information 6

7 Structure of Underlay Control Channel Two modes of opera>ons for control channel Mode- 1: Frequency offset es>ma>on : using short PN- sequence OFDM symbol >ming es>ma>on : using both long and short PN- sequence Mode- 2: CDMA- like transmission of control informa>on (e.g. channel alloca>on informa>on) Short PN seq. 1 2 R Long PN seq. Control information l s l l Mode 1 Mode 2 time l d 7

8 Mode-1 Operation overlay Data data underlay Control control l s l l l d N+N cp 1 R 2 stages of mode- 1 opera>on: Stage 1: Es>ma>on of frequency offset by correla>ng the incoming stream with short PN sequence Length of the PN sequence is small enough, (δf i )l s T s << 1. A threshold τ s is chosen to detect correla>on peaks First stage is successful if αr peaks cross threshold, α<=1 Correla>on peak 8

9 Mode-1 Operation OFDM symbol boundary detected overlay Data data underlay Control control l s l l l d N+N cp 1 R Stage 2: Correc>on of frequency offset for subsequent stream and correla>on with long PN sequence to iden>fy the OFDM symbol boundary A dynamic threshold is used for long PN- sequence detec>on τ l,new = βτ l,current + γν This update takes place once every P samples and ν is either a long correla>on peak detected in previous PT s seconds or a fixed value τ 0 if no peaks are detected. 9

10 Experiment Topology USRP N210 radios with SBX transceiver daughter- card are used NC- OFDM data paths and the corresponding UCC paths are implemented in C++ and Python in GNU Radio 10

11 GNURadio Model Allocation vector Transmitter ~ Data Bits Symbol Mapping S / P IFFT CP P / S DAC & RF Control Bits Parameters Underlay Encoder Allocation vector RF & ADC Receiver PS / SP FFT P / S Data bits decoder Data Bits ~ Freq. offset calc Time offset calc Underlay Decoder Control bits Parameters 11

12 MODE-1 ANALYSIS 12

13 Parameters for Mode-1 Experiment Bandwidth: 1MHz Number of subcarriers: 64 Cyclic prefix length: 16 Modulation for data channel: QPSK Modulation for control channel: BPSK Overlay signal to noise ratio (OSNR): 10 db Underlay control signal to noise ratio (UCNR): 0 db * Overlay transmission takes place over all available bandwidth (worst case scenario for underlay control channel). Short PN seq. Long PN seq. Control information 1 2 R l s l l Mode 1 Mode 2 time 13 l d

14 Mode-1 Performance Probability of false- alarm in symbol synchroniza;on 1 1 Pfa l s Pfa l s length of long PN-sequence l l length of long PN-sequence l l Repe>>on of short PN sequence, R=4 Repe>>on of short PN sequence, R=12 14

15 Mode-1 Performance Probability of missed detec;on in symbol synchroniza;on 1 1 Pmd l s Pmd l s length of long PN-sequence l l length of long PN-sequence l l Repe>>on of short PN sequence, R=4 Repe>>on of short PN sequence, R=12 15

16 Mode-1 Performance Summary Since smaller values of R result in inaccurate es>ma>on of frequency offset, the probability of missed detec>on P md is higher for smaller value of R Larger value of l l result in lower probability of false alarm P fa Combined result for P md and P fa indicate that larger values of, and provide good results We select l s = 40, l l = 320 and R = 8 for further experiments. 16

17 MODE-2 ANALYSIS 17

18 Parameters for Mode-2 Experiment Bandwidth: 1MHz Number of subcarriers: 64 Cyclic prefix length: 16 Modulation for data channel: QPSK Modulation for control channel: BPSK Underlay control signal to noise ratio (UCNR): 0 db Transmit OSNR is varied between 0 to 10 db. Spreading code length l d is chosen from (20, 40, 60) Channel allocation in two link topology: Link 1 Link 2 Guard band Short PN seq. Long PN seq. Control information 1 2 R l s l l Mode 1 Mode 2 time 18 l d

19 Mode 2 Performance For this experiment, we hold UCNR at 0 db and All other parameters are held constant except for the spreading code length l d which is chosen from the set {20, 40, 60} Single link Two-link topology Tx!Rx Tx1!Rx1 Tx2!Rx SER l d SER l d SER l d Transmit OSNR (db) Transmit OSNR (db) Transmit OSNR (db) 19

20 Mode-2 Performance Summary Overall SER performance is comparable for both topologies SER of 10 2 even when the transmit OSNR is up to 6-8 db with l d = 60. (these error rates include errors due to false alarms in the >ming recovery stage) Use of PLL can reduce the error due to false alarms Data rate of 11 kbps over 1 MHz bandwidth was obtained Control channel parameters can be changed to achieve higher SNR. 20

21 Conclusion An underlay control channel for NC-OFDM was proposed Enables transmission of control information Helps with frequency offset estimation and time synchronization in multiple p2p network Experiments were performed with USRPs Proposed control channel showed robust time and frequency offset recovery in experiments The control channel provided data rate of ~10-20 kbps for 1MHz bandwidth 21

22 Ques>on? 22