Wireless Communication Lecture 7-8

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

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).

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.

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

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

Section 7 – Dimensioning

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?

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.

Frequency Spectrum

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

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

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.

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

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

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

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

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?

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

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.

CDMA Code User 3 User 2 Frequency User 1 Time

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

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

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

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

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

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.

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)

Multipath Channel

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

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 .

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

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

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

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

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