WLAN a Algorithm Packet Detection Carrier Frequency Offset, and Symbol Timing. Hung-Yi Lu

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1 WLAN a Algorithm Packet Detection Carrier Frequency Offset, and Symbol Timing Hung-Yi Lu

2 Outline Packet Dection Carrier Frequency Offset Cordic Symbol Timing

3 WLAN a Rx Flow Chart

4 802.11a Preamble Structure

5 Timing Estimation Timing estimation consists of two main tasks Packet synchronization Symbol synchronization The first task of the receiver is to detect the start of an incoming packet Broadcast system naturally do not require packet detection. However, for a packet oriented network, find the packet is obviously of central importance for high network performance

6 Packet Detection

7 Packet Detection(1/3) Packet detection is the task of finding an approximate estimate of the preamble of an incoming data packet. Generally packet detection cam be describe as a binary hypothesis. Two statement: Null Hypothesis:H0 Alternative Hypothesis:H1

8 Packet Detection(2/3) The actual test is usually of the form that test whether a decision variable mn exceeds a predefine threshold Th. H H 0 1 : m : m n n < Th Packet not present Th Packet present The performance of the packet detection algorithm can be summarized with two probabilities: PD: The probability of detecting a packet when it is truly present. PFA: The probability that the test incorrectly decides that a packet is present, when actually there is none.

9 Packet Detection(3/3) A little higher PFA can be tolerate to guarantee good PD Method Received Signal Energy Detection Double Sliding Window Packet Detection Using Preamble for Packet Detection Main Method for WLAN : Preamble for Packet Detection

10 Received Signal Energy Detection(1/4) The simplest algorithm for finding the start edge of the incoming packet is to measure the received signal energy When there is no packet being received, the received signal rn = wn.when the packet starts, the received energy is increased by the signal component rn= sn+ wn m n = L 1 k = 0 r * n krn k = L 1 k= 0 r n k 2 The decision variable mn

11 Received Signal Energy Detection(2/4) This type of sum is called a sliding window. m 2 n+ 1 = mn + rn + 1 rn L+ 1 2 Data in r15 r14 LL r1 r0 mn LL mn+1

12 Received Signal Energy Detection(4/4)

13 Received Signal Energy Detection(3/4) The level of the noise power is generally unknown and can change when the receiver adjusts its RF amplifier setting. When a wanted packet is incoming, its received signal strength depends on the power setting of the transmitter and on the total path loss from the transmitter to the receiver.

14 Double Sliding Window Packet Detection(1/5) The double sliding window packet detection algorithm calculates two consecutive sliding windows of the received energy. The basic principle is to form the decision variable mn as a ratio of the total energy contained inside the two windows.

15 Double Sliding Window Packet Detection(2/5) When only noise is received the response is flat. ~ noise ~ A B Th mn

16 Double Sliding Window Packet Detection(3/5) When the packet edge start to cover the A window, the energy in the A window gets higher until the point where A is totally contained inside the start of the packet After this point B window starts to also collect signal energy, and when it is also completely inside the received packet, the response of mn is flat again. packet A B Th mn

17 Double Sliding Window Packet Detection(4/5) Equation shows the calculation of the A window value (an) and the calculation for B window (bn). Then the decision variable is formed by dividing the value of the an by bn. Using the double sliding window algorithm is a good approach, if the receiver does not have additional information about the received data n n n L l l n L l l n l n n M m m n M m m n m n n b a m r r r b r r r a = = = = = = + = + + = = * *

18 Double Sliding Window Packet Detection(5/5)

19 Using The Structure of The Preamble(1/3) Overall the method resembles the double sliding window algorithm, but it takes advantage of the periodicity of the short training symbols at the start of the preamble This approach is called delay and correlate algorithm

20 Structure for packet Detection(2/3) The C window is a crosscorrelation between the received signal and a delay version of the received signal The P window calculates the received signal energy during the crosscorrelation window. The value of the P window is used to normalize the decision statistic, so that it is not dependent on absolute received power level.

21 Structure for packet Detection(3/3)

22 Carrier Frequency Offset

23 Introduction(1/2) One of the drawbacks of OFDM is its sensitivity to carrier frequency offset This degradation is caused by two main phenomena Reduction of amplitude of the desired subcarrier ICI caused by neighboring carriers.

24 Introduction(2/2) The various algorithm that have been developed to estimate carrier frequency offsets in OFDM systems are divided into three types Data-aided Cyclic prefix Training symbol Non-data-aided For WLAN application,data-aided is the most important.

25 Carrier Frequency Offset(1/6) We first derive a data-aided maximum likelihood estimator that operates on the received time domain signal. The training information required is at least two consequently repeated symbols.

26 Carrier Frequency Offset(2/6) sn :training signal yn :passband signal

27 Carrier Frequency Offset(3/6) D: The delay between the identical samples of the two repeated symbol

28 Carrier Frequency Offset(4/6) 1 2πDT s fˆδ

29 Carrier Frequency Offset(5/6) The angle of z is unambiguously defined only in the range[-π, π). Thus if the absolute value of the frequency error is larger than the following limit the estimate will be incorrect For a,the sample time is 50ns,and the delay D=16

30 Carrier Frequency Offset(6/6) The carrier frequency in a is approximately 5.3GHz,and the standard specifies a maximum oscillator error of 20 part per million (ppm). This amounts to a 6 9 frequency error of f = = 212kHz Now consider the long training symbols. Δ f Δ max 1 = 4 2DTs = kHz

31 Carrier Frequency Offset -CORDIC Introduction

32 CFO Block Diagram 1 2πDT s fˆδ CORDIC

33 Introduct to CORDIC y (x,y) φ θ (x,y ) x (x,y ) (x,y )

34 Two Types in CORDIC Vectoring Force y-component to zero Rotation Force z-component to zero

35 Vectoring Mode

36 Rotation Mode

37 Symbol Timing

38 Symbol Timing(1/7) Symbol timing refers to the task of finding the precise moment of which individual OFDM symbols start and end. A WLAN receiver cannot afford to spend any time beyond the preamble to find the symbol timing. WLAN receivers have knowledge of the preamble available to them, which enables the receiver to use simple crosscorrelation based symbol timing algorithm.

39 Symbol Timing(2/7) After the packet detector has provided an estimate of the start edge of the packet, the symbol timing algorithm refines the estimate to sample level precision. The refinement is performed by calculating the crosscorrelation of the received signal rn and a known reference tk L 1 ˆ t = arg max s n k = 0 r n + k t * k 2

40 Symbol Timing(3/7) rn tk

41 Symbol Timing(4/7) rn Parallel Load tk

42 Symbol Timing(5/7) Using this correlator to find the start point of the Data field θ where L = 16 samples where N = 64 samples ^ D θ + L = arg max Re{ rkr ( ) *( k N)} θ k = θ copy copy samples

43 Symbol Timing(6/7)

44 Symbol Timing(7/7)

Chapter 4 Investigation of OFDM Synchronization Techniques

Chapter 4 Investigation of OFDM Synchronization Techniques In this chapter, basic function blocs of OFDM-based synchronous receiver such as: integral and fractional frequency offset detection, symbol timing