1. Wireless Communication
Lecture 7-8

2. Circuit Switched Service:
Circuit Vs Packet Data
Circuit Switched Service:
2G system (primarily voice and data on circuit switched air interface)
Call charging based on channel holding time
Maximum number of users per TDMA channel is 8
Suitable for constant bit rate applications
Resource allocation is done such that UL and DL are paired

3. Packet Switched Service:
Section 7 – Dimensioning
Packet Switched Service:
Several users can share the same channel.
Charges based on channel usage (actual usage of byte transferred).
Resource allocation done independently on UL and DL (good for applications with asymmetrical bit rate)
Dynamic allocation of resources
Can multiplex traffic (voice, data, video).

4. GPRS System feature Variable quality of service.
Section 7 – Dimensioning
GPRS System feature
Variable quality of service.
Independent packet routing
Provides IP connectivity to mobile subscriber
Build on existing GSM infrastructure with added nodes for supporting packets
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Variable quality of service is provided to different types of services which dependents upon the coding schemes.

5. Conceptual View on GPRS
Section 7 – Dimensioning
Conceptual View on GPRS
Shared GSM and GPRS Infrastructure
Internet
Corporate Intranet
GPRS Core
BSC
BTS
GSM Voice
Access Point
GPRS Infrastructure
IP World

6. GPRS Attach / Detach Attach
Section 7 – Dimensioning
GPRS Attach / Detach
Attach
Performed when the MS indicates its presence to PLMN for the purpose of using GPRS service
Carried out between MS and SGSN
MS identifies itself with its GSM identity
GPRS subscription necessary for successful attach
Detach
Performed when the MS indicates to the PLMN that it no longer be using GPRS services

7. Section 7 – Dimensioning

8. Channel Access Utilization
Introduction (Resource Management and Utilization).
Compression and Multiplexing (Other related definitions).
The Most Important Access Utilization Schemes (Techniques):
FDMA (Frequency Division Multiple Access)
TDMA (Time Division Multiple Access)
CDMA (Coded Division Multiple Access)
In which Systems the above Schemes are used?

9. Introduction
The idea behind the Resource Management in wireless communications was how to assign channels or frequencies to the radio cells in a way that the probability of interference is sufficiently low.
The utilization of the capacity of a transmission medium can be improved through different methods that involved transmitting several connections simultaneously in a multiplex mode (Multiple Access Techniques).
By using these techniques, the shared resource (spectrum) can be divided/shared among users, ensuring Quality of Service and the required amount of interference.

10. Frequency Spectrum

11. Definitions
Multiplexing: is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end.
Compression means transmitting/storing the same amount of information using less amount of resources

12. FDM : Frequency Division Multiplexing TDM : Time Division Multiplexing
Multiplexing Methods
FDM : Frequency Division Multiplexing
FDMA: Frequency Division Multiple Access
TDM : Time Division Multiplexing
FDMA: Time Division Multiple Access
CDM : Code Division Multiplexing
CDMA: Code Division Multiple Access

13. FDMA
FDM divides the transmission frequency range (Bandwidth) into narrower bands called (subchannels).
The subchannels are smaller frequency bands and each band is capable of carrying a separate voice or data signals!
Guard bands are used to prevent interference on the receiving end of the signal
Disadvantage (FDMA): Full utilization of the available frequency band is not possible!!
Advantage: Multiple callers can share the frequency spectrum.

14. FDM Frequency Time Frequency Band Frequency Band Frequency Band
Guard Band F4
Frequency Band
Frequency F3
Guard Band F2
Frequency Band F1
Guard Band F0
Frequency Band
Time

15. FDMA-Applications
FDMA is used in a variety of applications such as: telephone systems, radio systems, cable TV at homes.
The first generation of Mobile networks. FD,TD-MA.
GSM (Global System of Mobile Telecommunication) uses FDMA in combination with TDMA.
UMTS (Universal Mobile Telecommunication Systems- 3G) in combination with other multiplexing techniques

16. TDMA : Time Division Why TDM ?
FDM sometime offer less frequency for the communication channel than the required amount
On the Other hand, in TDM the entire bandwidth of the radio channel is used but is divided into time slots that are periodically allocated to each station for the duration of the call
Pros and Cons :
It needs more synchronization between the sender and the receiver
It is more frequency-economic than FDM
This engagement of the transmission medium can cause using asynchronous time slots instead of the synchronous ones especially when the transmission pauses occurs

17. TDM Frequency Time Slot 1 Slot 3 Slot 4 Slot 2 Guard Band Guard Band

18. FDMA + TDMA
A combination of FDMA/TDMA can be used. For example, in GSM systems. The traffic is burst onto the channel at a specific periods.
With this combination, more channels can be used with less interference.
How?

19. FDMA+TDMA in GSM ETC.. 1 2 3 4 5 6 7 8 1 Voice Channels
Burst
Burst
Burst
Burst
Burst
Burst
Burst
Burst
Burst 1 2 3 4 5 6 7 8 n
0.577 ms
ETC..
200 kHz
200 kHz
200 kHz 1 2 3 4 5 6 7 8 1
Voice Channels
Signalling Channels

20. CDMA (Code Division Multiple Access)
Like TDMA, in CDMA the analog speech is coded into digital signals.
Unlike TDMA, in CDMA each conversation is assigned a unique code (a signature for each individual transmission).
The codes of different users are assigned to be different from each other (e.g orthogonal to each other).
The final signal at the receiver contains only the relevant conversation. Any other signals are picked up as a noise.

21. CDMA
Code
User 3
User 2
Frequency
User 1
Time

22. CDMA
CDMA technique is used in UMTS (Universal Mobile Telecommunication Systems) (Called also W-CDMA)
CDMA provides better signal-to-noise ratio performance than the conventional TDMA and FDMA. Which means that the required high capacity can be approved!!
Advantage: It is easy to accommodate variable user capacity as long as the user does not increase the whole energy of the multi-user signal.
Disadvantage: The power control issue which limits the max. number of users in the cell.
There are different alternatives for CDMA in UMTS:
W-CDMA
TD-CDMA
FD-CDMA

23. Shortcomings of CDMA Near-Far Problem Multipath Fading
Inter Symbol Interference (ISI)
Soft Handoff
Self Jamming

24. Near-Far Problem
The receiver must be able to receive all the required signals within the same channel bandwidth and it must be able to decode them
For the receiver to be able to decode all the signals in the channel, they should ideally all be at the same signal strength

25. Users may be received with very different powers:
Users near the base station are received with high power
Users far from the base station are received with low power

26. Solution: Power Control
This ensures that all the signals within the coverage of the base station arrive with same power at the receiver
Drawbacks
Reduced data capacity:   The power control mechanism requires data to be sent in both directions across the radio interface. This utilizes data capacity that could be otherwise used for carrying revenue earning data

27. High power handset power consumption at cell edges:  
In order to be able to maintain the required signal level at the base station when the handset is close to the edge of the cell, it will be required to transmit at a high power level.
This will reduce battery life. Other cellular systems might not require such high signal levels at the base station and may be able to conserve battery power as a result.

28. Multipath RF transmission is degraded by “multipath”
Multipath propagation occurs when there are radio reflective surfaces in the environment(cliffs, buildings, earth surface)
At the Rx antenna the total signal is the sum of
Direct rays
Rays delayed due to several reflections and a
zigzag path
Multipath can cause both fading and
inter-symbol interference (ISI)

29. Multipath Channel

30. Fading
Direct and delayed rays are out of phase at some locations

31. Solution: Rake receiver
Rake receiver is a radio receiver designed to counter the effects of multipath fading.
It does this by using several "sub-receivers" called fingers, that is, several correlators each assigned to a different multipath fading .

32. Each finger independently decodes a single multipath component; at a later stage the contribution of all fingers are combined in order to make the most use of the different transmission characteristics of each transmission path
This could very well result in higher signal-to-noise ratio (or Eb/N0) in a multipath environment

33. InterSymbol Interference (ISI)
When the multipath delay spread is greater than about 20% of the digital symbol duration, ISI can be a problem.

34. Solution: Adaptive equalizer
Receivers are equipped with an adaptive equalizer
Adaptive equalizer produces delayed copy/ies of the received signal waveform and use(s) these copy/ies to cancel the physically delayed radio signals
This equalizer examines the effect of multipath delay on the known training sequence, and then uses this information to undo that effect on the other bits in the cell using the internally delayed replicas of the signal

35. soft handoff
Higher cost and complexity of base station (particularly due to soft handoff )
Soft handoff procedure, a mobile makes a connection to two or more base stations before choosing the one with which to communicate
It is thus an example of a “make before break” operation

36. This is more complex than hard handoff used in FDMA and TDMA schemes
Self Jamming
Arises when the spreading codes used for different users are not exactly orthogonal