Wireless Communication
Lecture 1 – Introduction
Homework:
See Website “Lectures” for each day’s portfolio question, homework problems, and suggested review problems. Additional references are also available for many sections.
Topics:
- Overview of Class policies
- Syllabus and UofU cheating policies
- Schedule
- Overview of point-to-point links
- Overview of Cellular systems
1. A representative basic wireless system:
Tx
voice/data -> vocoder/compression -> forward error correction (FEC)/interleaving -> mapping -> pulse shaping -> modulation -> IF/RF filter and AGC -> antenna
channel noise and distortion
Rx
antenna -> RF/IF filter and AGC -> sampling -> matched filter -> demap -> equalize -> detect -> FEC -> uncompress -> voice data
A simplified example: unshaped, un-coded, uncompressed BPSK (Sec. 6.8.1), with no IF/RF processing in white Gaussian noise:
bits (0,1) -> mapped (1, -1) -> modulate -> add Gaussian noise -> demodulate -> match filter -> detect
BPSK: Binary Phase Shift Keying
where E = energy per bit, T = period of the bit, fc is the carrier frequency
Constellation Diagrams:
Plot magnitude and phase of each possible symbol
Reminder of Gaussian noise, thermal noise, and
Bit error rate, Pe = Q( sqrt( 2Eb/N0 )) (Eq. 6.74)
Draw out this function logarithmic in BER vs. Eb/N0 in dB.
Reminder of dB.
Why modulate?
1 Giant antennas would be needed without modulation.
2 Without modulation, multiple users would interfere with each other.
2. What is a
cellular system?
Cell systems address the idea of frequency
re-use.
A system that divides a geographic region into segments (cells) where a localized wireless communication system can be “replicated” and reused on each cell.
What systems should you watch
out for during this class?
Table 1.1
AMPS = Advanced Mobile Phone System (1983)
Analog system
Modulation is FM = Frequency Modulation (3 kHz voice modulated onto carrier)
Frequency 824-894 MHz
Bandwidth for each channel = 30 kHz
Access FDMA = Frequency Division Multiple Access (Each user uses a different 30 kHz channel)
IS-95 (1993)
Digital System
Frequency 824-894 MHz
Modulation (QPSK, BPSK)
Multiple Access: CDMA DSSS = Code Division Multiple Access Direct Sequence Spread Spectrum
Channel Bandwidth 1.25 MHz
DCS-1900 (nearly equivalent to
European GSM = Global System for
Modulation (GMSK)
Frequency 1.85-1.99 GHz
Channel bandwidth = 200 kHz
Multiple access = TDMA
Types of
Table 1.4
Simplex = can send data in only one direction (and can’t get any confirmation that data was received) (like a one-way pager system)
Half-Duplex = talk in only one direction at a time (old fashioned walkie-talkie, push button to talk). These systems have only one communication channel and can use it only to send OR receive, but not both simultaneously.
Full-Duplex = talk in both directions simultaneously (like a cell phone)
FDD = Frequency Division Duplex (2 separate frequency channels are used, one to transmit and the other to receive)
TDD = Time Division Duplex (one frequency channel is used, with separate time slots for send and receive… voice and ear are slow enough that the user cannot tell that there is a delay)
3. Where is the Cellular Telephone System? (Section 1.4.3.1)
PSTN = Public switched telephone network
Standard wire-line network, fixed entry/exit points.
MSC =
Connected by wireline to the PSTN
Connected (usually wireless, but sometimes through PSTN) to cell phone base stations
Responsible for connecting all of the cell phone calls to the PSTN
Base station = individual cell tower
Connected (usually wireless) to the MSC
Connected (always wireless) to many mobile units simultaneously, changes dynamically.
Has many receivers and transmitters, usually many antennas.
FCC = Forward control channel (control data about the call sent from base station to mobile) (control channels are about 5% of total channels)
RCC = Reverse control channel (control data about the call sent from mobile to base station)
FVC = Forward voice channel (voice sent from base station to mobile)
RVC = Reverse voice channel (voice sent from mobile to base station)
Information needed for a call:
Who am I ? (MIN = mobile identification number = telephone number)
Whom can we bill? (ESN = electronic serial number , information used to find the
HLR = home location register for each roaming or non-roaming mobile)
Where am I?
Station Class mark (how much power can you transmit?)
How a Call is made from PSTN to Mobile:
Joe to PSTN:
Hello, this is Joe. I want to talk to Lynn.
PSTN to MSC:
Hello, I have a call for Lynn.
MSC to all base stations:
Calling all base stations. Where is Lynn?
Base station #1: Base Station #2: Base Station #3:
Is Lynn here? Is Lynn here? Is Lynn here?
Lynn to station #1:
(loudly) HERE
My ESN is 1123445
My Station Mark is 100 mW
Lynn to station #2:
(softly) Here
My ESN is 1123445
My Station Mark is 100 mW
Base station #1 to MSC:
Lynn is HERE and she is THIS loud.
Her ESN is 1123445
Her Station Mark is 100 mW
Base station #2 to MSC:
Lynn is here and she is this loud.
Her ESN is 1123445
Her Station Mark is 10 mW
MSC (thinking):
Hmm… does
Yes, it does! From her ESN, I can find her HLR (home location register) and decide how much to bill her and if she has any special calling plans or features.
I can bill her 70 cents per minute!
Which base station shall we use? #1 was louder than #2, I can tell that from her station mark.
MSC to base station #1:
Base station #1, set up a voice channel for Lynn.
Base Station #1 (thinking):
Hmm.. we need a new voice channel
for
Base Station #1 to Lynn:
Lynn, please set your frequency to 824 MHz. (Lynn’s phone rings.)
Lynn to Base Station #1:
(Lynn picks up the phone) Ok, frequency set to 824 MHz. “Hello?”
Base station #1 to MSC:
Ok, Lynn is ready to receive voice. Here is her message, “Hello?”
MSC to PSTN:
Ok, Lynn is ready to receive voice. Here is her message, “Hello?”
PSTN to Joe:
“Hello?”
Joe to PSTN:
“Hello, Lynn, this is Joe.”
PSTN to MSC:
Message to Lynn: “Hello, Lynn, this is Joe.”
MSC to Base station #1:
Message to Lynn: “Hello, Lynn, this is Joe.”
Base station #1 to Lynn:
Message to Lynn: “Hello, Lynn, this is Joe.”
Questions:
- What will happen when Lynn moves out of range of base station #1?
- What would happen if Lynn and Joe were both on mobile phones?
- Who gets the money?
Alphabet Soup:
AMPS = Advanced Mobile Phone System
BPSK = Binary phase shift keying
CDMA = Code Division Multiple Access
DSSS = Direct Sequence Spread Spectrum
DQPSK = Differential Quadrature Phase Shift Keying
ESN = electronic serial number
FCC = Forward control channel
FDD = Frequency Division Duplex
FDMA = Frequency Division Multiple Access
FM = Frequency Modulation
FVC = Forward voice channel
GSM = Global System for Mobile
HLR = home location register
MIN = mobile identification number
MSC = Mobile Switching Center
MTSO = Mobile telephone switching office
PSTN = Public switched telephone network
QPSK = Quadrature phase shift keying
RCC = Reverse control channel
RVC = Reverse voice channel
TDD = Time Division Duplex
TDMA = Time Division Multiple Access
USDC = US Digital Cellular
4. “Cell” and frequency reuse
Hexagonal Design…FIGURE 3.1
Assume: omni at center, 1-5 mi radius, 100 W max BS, MS 600 mW, 20-30 users per channel.
System-level analysis of number of required subscribers in an area, how many frequencies will be needed (when and where) and how to meet that need with the most cost-effective solution.
S = # of available duplex (2-way) channels in a cluster
K = # of radio channels available
B = total bandwidth (70 MHz GSM)
CB = channel bandwidth (200 kHz simplex, 400 kHz duplex)
K = B/CB = 70e6/200e3=350
N = cluster size = # of disjoint channel groups (# of cells before you encounter freq reuse) 4,7,12 are typical
S = kN = 350*4 = 1400 channels available in a cluster of 4 cells.
FIGURE 3.2
Cluster: group of all cells that have different frequencies (N cells in a cluster)
N = I2 + IJ + J2
(1) Move I cells to right
(2) Turn 60 degrees counter-clockwise
(3) Move J cells.
C = # of channels in a system = S*M
M = # of clusters in the system (number of times the disjoint groups are replicated).