1. Physical Layer (2)

2. Goal
Physical layer design goal: send out bits as fast as possible with acceptable low error ratio
Goal of this lecture
Review. Modulation, demodulation, maximum likelihood detection
Introduction to wireless communications, CDMA, OFDM, MIMO
Introduction to error control codes

3. Review What is modulation/demodulation?
What is the waveform of a BPSK signal carrying bits “ ?”
What is the three general steps to demodulate a received signal?
What is detection? What is maximum likelihood detection?
With BPSK, -1 and 1. The noise could take 3 values, -1, 0, 1, with probability 0.1, 0.8, and 0.1, respectively. What is the error probability?

4. Cellular Phone Networks
User – base station – Telephone network
FDMA – Frequency division multiplexing
How to make sure that you are using this band, not that band?
TDMA – Time division multiplexing
CDMA – Code division multiplexing

5. GSM – Global System for Mobile Communications
Second generation cell phone system (digital, first generation analog).
GSM-900 and GSM-1800 are most widely used
GSM-900 uses MHz to send information from the mobile station to the base station (uplink) and MHz for the other direction (downlink).
FDMA + TDMA
Each user transmitting on a frequency and receiving on another frequency.
124 pairs of 200 KHz channels. Each channel divided into time slots for 8 users.
Each user is has a chance to transmit every ms. Each time he can send 114 data bits – 24.7kbps.

6. CDMA
Described in IS-95.
A good analogy in the book – You have a group of people in a room. TDMA means they talk in turn. FDMA means that those who wants to talk sit in different corners and can’t hear other pairs. CDMA means each pair talks in a different language and other people’s voices are noise to them.

7. CDMA
The whole bandwidth is used by every user. Meaning that they can send out symbols really fast.
The trick is to make what A sent appear as 0 to B.
Because we have a fast symbol rate, for each data bit, we send out, say, 8 bits, call the “small bits” chips.
Given a bit, if 1, send out, say, -1,-1,-1,1,1,-1,1,1, and if 0, 1,1,1,-1,-1,1,-1,-1
This is called the chip sequence.
The key is that each station has a unique chip sequence (language), and different languages are orthogonal. Fig

8. Wireless LAN Physical Layer
802.11b,g in the 2.4G band and a in the 5G band. People now consider as the notion of MAC layer protocol, while a, b, g, or n, are about physical layer.
802.11b. 1, 2, 5.5, 11Mbps.
1Mbps: BPSK modulation. 1 bit into 11 chips with Barker sequence −1 −1 −1 +1 −1 −1 +1 −1. Why spread spectrum? Required by FCC but was later removed
2Mbps: QPSK.
5.5M and 11M: use some bits to select chip sequence and use two bits for QPSK
802.11a. Up to 54Mbps. OFDM.
802.11g. Up to 54Mbps. OFDM.

9. OFDM (Orthogonal Frequency Division Multiplexing)
In wireless communications, in addition to the bandwidth limit and additive noise, you also have multipath fading!
The faster your symbol rate is, the more badly you will be affected by multipath fading.
In effect, OFDM is like DSL: given a wideband channel, divide it into many sub channels. Each sub-channel can be modulated/demodulated independently. Because each sub-channel is of a much smaller bandwidth, multipath fading is much less severe.
In implementation, use IFFT and FFT.