1. HY539: Mobile Computing and Wireless Networks
Lecture 4: Radio Transmission

2. Roadmap Physical layer overview Problems in wireless transmissions
Access methods
physical layer (Chapter 10)

3. Shannon’s limit
For a channel without shadowing, fading, or ISI, the maximum possible data rate on a given channel of bandwidth B is
R=Blog2(1+SNR) bps,
where SNR is the received signal to noise ratio

4. Bits mapped to signal (analog signal waveform)
Adds redundancy to protect the digital information from noise and interference
Bits mapped to signal (analog signal waveform)
e.g., GFSK
e.g., TDMA,
CDMA
The information is represented as a sequence of binary bits, the binary bits are then mapped (modulated) to analog signal waveforms and transmited over a communication channel. The communication channel introduces noise and interference to corrupt the transmitted signal. At the receiver, the channel corrupted transmitted signal is mapped back to binary bits.
The received binary information is estimate of the transmitted binary information.
Channel coding is often used in digital communication systems to protect the digital information from noise and interference and reduce the number of bit errors. Channel coding is mostly accomplished by selectively introducing redundant bits into the transmitted information stream. These additional bits will allow detection and correction of bit errors in the received data stream and provide more reliable information transmission.

5. Multipath Propagation
Wall
Scattering
Receiver
Transmitter
Cabinet
Diffraction (Shadow Fading)
Reflection
Wall

6. Mobile radio channel
A single direct path between the base station and the mobile is seldom the only physical means for propagation
Hence, the free space propagation model is inaccurate when used alone
Two-ray ground reflection model considers both the direct path and a ground reflected propagation path between transmitter and receiver
Reasonably accurate for predicting the large-scale signal strength over distances of several km for mobile radio systems that use tall tower (heights which exceed 40m) or for line-of-sight micro-cell channels in urban environment

7. Hidden node problem Node 3 Node 1 Node 2
From the perspective of node 1, node 3 is hidden
If node 1 and node 3 communicate simultaneously, node 2 will be unable to make sense of anything
Node 1 and node 3 would not have any indication of the error because the collision was local to node2

8. Fading problem Node 3 Node 1 Node 2
Node 1 and 3 are placed such that their signal is not strong enough for them to detect each other’s transmissions, and yet their transmissions are strong enough to have interfered with each other at node 2

9. Carrier-Sensing Functions
Physical carrier-sensing
Expensive to build hardware for RF-based media
Transceivers can transmit and receive simultaneously only if they incorporate expensive electronics
Hidden nodes problem
Fading problem
Virtual carrier-sensing
Collision avoidance: stations delay transmission until the medium becomes idle
Reduce the probability of collisions
Undetectable collisions

10. Carrier-Sensing Functions
Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
Virtual sensing
MAC layer
control messages (RTS, CTS, ACK)
network allocation vector (NAV) to ensure that atomic operations are not interrupted
Different types of delay depending on the priority of the frame (e.g., SIFS, DIFS, backoff)

11. Question: [answers @ log]
Transceivers that transmit and receive simultaneously

12. Modulation techniques
Code Division Multiple Access (CDMA) ,
Direct Sequence Spread Spectrum (DSSS)
Frequency Hopping (FH),
Orthogonal Frequency Division Multiplexing (OFDM)

13. IEEE family
802.11b:
Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping (FH), operates at 2.4GHz, 11Mbps bitrate
802.11a: between 5GHz and 6GHz uses orthogonal frequency-division multiplexing, up to 54Mbps bitrate
802.11g: operates at 2.4GHz up to 54Mbps bitrate
All have the same architecture & use the same MAC protocol

14. Code Division Multiple Access (CDMA)
CDMA assigns a different code to each node
Codes orthogonal to each other (i.e inner-product = 0)
Each node uses its unique code to encode the data bits it sends
Nodes can transmit simultaneously
Multiple nodes per channel
Their respective receivers correctly receive a sender’s encoded data bits assuming the receiver knows the sender’s code in spite of interfering transmissions by other nodes.
Has been used extensively in military for some time due to its antijamming properties and is now beginning to find widespread civilian use, particularly for use in wireless multiple access channels.

15. CDMA Example Sender Zi,m=di*cm Data bits d0=1 d1=-1 Spread code 1 1 1
Time slot 1
Time slot 0 1 1 1 1 1 1 1 1
Channel output -1 -1 -1 -1 -1 -1 -1 -1

16. CDMA Example
When no interfering senders, the receiver would receive the encoded bits and recover the original data bit, di, by computing
di= — S Zi,m*cm
Interfering transmitted bit signals are additive M 1 M
m=1
CDMA’s assumption that interfering transmitted bit signals are additive

17. Frequency Hopping Timing the hops accurately is the challenge
slot 5
User A 4
User B 3 2 1
Time slot

18. Modulation techniques
DSSS
As CDMA except all mobile hosts and base stations use the same chipping code
Spreads the energy in a signal over a wider frequency range
Spreading ratio (i.e., number of chips) should be as low as possible to meet design requirements and avoid wasting bandwidth
FH divides the ISM band into a series of 1-MHz channels
Divides hopping sequences into non-overlapping sets
Any two members of a set are orthogonal hopping sequences
OFDM
Distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.

19. 802.11 direct-sequence Uses the Barker sequence (11-bit sequence)
It is applied to each bit in the stream by a modulo-2 adder:
when 1 is encoded, all the bits in the spreading code change;
when 0 is encoded, they stay the same
FCC imposes legal limits on the power transmission:
One watt of transmitter output power and four watss of effective radiated poewr.
Effective radiated power is transmitters power output * gain of the antenna-loss in the transmission line.

20. Media Access Protocol
Coordinates the access & use of the shared radio frequency
Carrier Sense Multiple Access protocol with collision avoidance (CSMA/CA)
Physical layer monitors the energy level on the radio frequency to determine whether another station is transmitting and provides this carrier-sensing information to the MAC protocol
If channel is sensed idle for DIFS, a station can transmit
When receiving station has correctly & completely received a frame for which it was the addressed recipient, it waits a short period of time SIFS and then sends an ACK

21. Carrier-Sensing Functions
IEEE to avoid collisions
Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
MAC layer
RTS, CTS, ACK
network allocation vector (NAV) to ensure that atomic operations are not interrupted
Different types of delay
Short Inter-frame space (SIFS):
highest priority transmissions (RTS, CTS, ACK)
DCF inter-frame space (DIFS):
minimum idle time for contention-based services
EIFS: minimum idle time in case of “erroneous” past transmission

22. RTS/CTS clearing Node 1 Node 2 Node3 Node 1 RTS Time CTS frame Node 2
(4) ACK
CTS
frame
Node 2
ACK
RTS: reserving the radio link for transmission
RTS, CTS: Silence any station that hear them

23. Positive acknowledgement of data transmission
Node 1
Node 2
Time
frame
ACK
allows stations to lock out contention during atomic operation so that atomic sequences are not interrupted by other
Hosts attempting to use the transmission medium

24. Media Access Protocol
If channel is sensed busy will defer its access until the channel is later sensed to be idle
Once the channel is sensed to be idle for time DIFS, the station computes an additional random backoff time and counts down this time as the channel is sensed idle. When the random backoff timer reaches zero, the station transmits its frame
Backoff process to avoid having multiple stations immediately begin transmission and thus collide

25. Using the NAV for virtual carrier sensing
(eg 4-8KB)
RTS
Frame
CTS
ACK
Sender
Receiver
NAV
DIFS
SIFS
NAV (RTS)
NAV(CTS)
(e.g.10ms)
Carrier-sensing functions
Contention
Window
Access to medium deferred
NAV is carried in the headers of CTS & RTS

26. Backoff with DCF
Contention window (or backoff window) follows the DIFS
Window is divided in time slots
Slot length is medium-dependent
Window length limited and medium-dependent
Hosts pick a random slot and wait for that slot before attempting to access the medium;
All slots are equally likely selections
Host that picks the first slot (earlier number) wins
Each time the retry counter increases, the contention window moves to the next greatest power of two

27. Contention window size
DIFS
Previous
Frame
31 slots
Initial
Attempt
63 slots
127 slots
1st retransmission
2nd retransmission
3rd retransmission
255 slots
Slot time:20s
The contention window is reset to its minimum size when frames are transmitted successfully, or the associated retry counter is reached and the frame is discarded