1. IOT Technologies: Radio Frequency Identification (RFID)
NETW 1010
IOT Technologies:
Radio Frequency Identification (RFID)
Dr. Eng. Tallal Elshabrawy
Spring 2019

2. Radio Frequency Identification
Battery Lifetime
Years
RFID
Months
BLE
IEEE
Communication Range
Days
Few Meters
Tens-Hundreds of Meters
Kiliometers

3. Outline
Introduction
Physical Layer
Medium Access

4. Frequencies & Reading Ranges
Frequency Range
Frequencies
Passive Read Distance
Low Frequencies (LF)
KHz
10 – 20 cm
High Frequencies (HF)
13.56 MHz
< 1 meter
Ultra-High Frequencies (UHF)
MHz
<10 meters
Microwave
2.45 & 5.8 GHz
<30 meters

5. UHF RFID Systems EPCglobal Gen2 Passive Tags Backscatter Modulation
Dynamic Frame Slotted ALOHA (DFSA)
Up to 10m Read Distance

6. Backscatter Modulation
Tag Antenna
Continuous Wave from Reader
( MHz)
Modulating Signal
(Typically 40 Kbps)
Backscatter Modulated Signal
Tag Transmission States
Absorb
Reflect

7. Backscatter Communication in Action
Continuous Wave Signal
Leakage Signal
RFID
Reader
RFID Tag
Backscatter Signal
𝑃 𝑅 ∝ 𝑃 𝑇 × 𝐶 𝑑 𝑛 × 𝐶 𝑑 𝑛
Example of a Practical Backscatter Signal
Rx Power at Tag
Backscatter Rx Power at Reader

8. RFID Backscatter FM0 Modulation1
Binary 1
r,r
a,a 1
(r,r)
(a,a) 1 1
Binary 0
r,a
a,r
(r,a)
(a,r) 1
Example
r: reflect
a: absorb

9. RFID Backscatter FM0 Modelling
Received Constellation Point
when tag is in absorb state
Received Constellation Point
when tag is in reflect state
Constellation Diagram
sine axis
Envelope Detection
(r)
cosine axis r1
(a)
After removing leakage signal
(r)
(a)

10. Electronic Product Code (EPC)
Header - Tag version number
EPC Manager Manufacturer ID
Object class - Manufacturer’s product ID
Serial Number - Unit ID
With 96 bit code, 268 million companies can each categorize 16 million different products where each product category contains up to 687 billion individual units

11. Dynamic Frame Slotted ALOHA (DFSA)
Reading frame started by Query
Tag chooses random slot counter [0, 2 𝑄 −1 ]
Timeslot starts by Query or QueryRep
Each timeslot, the tag decrements its counter by one.
Tag sends an RN16 message when slot counter reaches 0
Reader replies with ACK.
Tag Sends its unique ID “EPC”.

12. Tag Random or Pseudo-Random Number Generator (RN16)
Tags shall generate 16-bit random or pseudo-random numbers (RN16)
Alias Tag ID during DFSA medium access
Shorter IDs decrease the probability of collisions during DFSA medium access
Once RN16 is Acknowledged by the reader, the tag sends its unique EPC code

13. DFSA Timeslot Types Empty Successful Collision
Reader receives no RN16 messages.
Reader can decode received RN16 message.
Reader cannot detect received RN16 messages.

14. DFSA Performance Analysis (1)
Probability that 𝑘 tags transmitting in a certain timeslot
𝑃 𝑘 = 𝑁 𝑘 ∗ 1 𝐿 𝑘 ∗ 1− 1 𝐿 𝑁−𝑘
Number of tags
Number of tags transmitting their RN16
Frame Length ( 𝐿=2 𝑄 )
Empty
Successful
Collision
𝑃 𝐸 = 𝑃 0 = 1− 1 𝐿 𝑁
𝑃 𝑆 = 𝑃 1 =𝑁∗ 1 𝐿 ∗ 1− 1 𝐿 𝑁−1
𝑃 𝐶 =1− 𝑃 0 − 𝑃 1

15. DFSA Performance Analysis (2)
Maximize Slot Throughput 𝜂 𝑠
𝜂 𝑠 = 𝑃 𝑆 = 𝑃 1 =𝑁∗ 1 𝐿 ∗ 1− 1 𝐿 𝑁−1
d𝜂 𝑠 d𝐿 =0
⇒−𝑁∗ 1 𝐿 2 ∗ 1− 1 𝐿 𝑁−1 +𝑁∗ 1 𝐿 ∗ 𝑁−1 ∗ 1− 1 𝐿 𝑁−2 ∗ 1 𝐿 2 =0
⇒− 1− 1 𝐿 𝐿 ∗ 𝑁−1 =0
⇒ 𝑳=𝑵, i.e, The reader sets the frame length to be equal to the number of competing tags

16. DFSA Performance Analysis (3)
Maximum Slot Throughput 𝜂 𝑆
𝜂 𝑆 = 1− 1 𝑁 𝑁−1
For a large Tag population 𝑁−1≈𝑁
𝜂 𝑆 = lim 𝑁→∞ 1− 1 𝑁 𝑁 = 1 𝑒 =0.368
Maximum achievable throughput is 36.8%
But How can the reader guess the Tag population to set the proper frame length?

17. Tag Population Estimation
After each inventory round, the reader identifies
𝑁 𝐸 empty slots
𝑁 𝑆 successful slots
𝑁 𝐶 collision slots
Number of remaining tags could be estimated as
𝑁≈2 ×𝑁 𝐶
𝑁≈2.39× 𝑁 𝐶
The reader then uses estimated 𝑁 in selecting the frame length for the next inventory round