Multiple Access Techniques for Wireless Communication Chapter 5 Multiple Access Techniques for Wireless Communication

Multi-User Techniques Multiple access techniques are used to allow a large number of mobile users to share the allocated spectrum in the most efficient manner. As the spectrum is limited, so the sharing is required to increase the capacity of cell or over a geographical area by allowing the available bandwidth to be used at the same time by different users. In wireless communication systems it is often desirable to allow the subscriber to send simultaneously information to the base station while receiving information from the base station. Radio spectrum for wireless communications is limited spectrum (300MHz-3GHz). There are several different ways to allow access to the channel. These includes mainly the following: 1) Frequency division multiple-access (FDMA) 2) Time division multiple-access (TDMA) 3) Code division multiple-access (CDMA)

Multi-User Techniques (Cont’d) MA techniques in different wireless communication systems These techniques grouped as: Narrowband Systems. Wideband Systems.

Narrowband Systems Usually FDD Narrowband FDMA Narrowband TDMA The available spectrum is divided in to a large number of narrowband channels. Usually FDD Narrowband FDMA Narrowband TDMA FDMA/FDD FDMA/TDD TDMA/FDD TDMA/TDD

Logical separation FDMA/FDD Logical separation FDMA/TDD

Logical separation TDMA/FDD Logical separation TDMA/TDD

Wideband systems Large number of transmitters on one channel. TDMA techniques CDMA techniques FDD or TDD (Multiplexing Techniques) TDMA/FDD TDMA/TDD CDMA/FDD CDMA/TDD

Logical separation CDMA/FDD Logical separation CDMA/TDD

Frequency Division Multiple Access (FDMA) FDMA: Each user is assigned a unique frequency band for the duration of the message. Applications: Analogue cellular systems (e.g., in AMPS 30KHz for each user) Two-way radio systems used by taxi companies, emergency services, etc.

Features of FDMA One phone circuit per channel. Idle time causes wasting of resources. Simultaneously and continuously transmitting. Usually implemented in narrowband systems. for example: in AMPS is a FDMA bandwidth of 30 kHz implemented. Due to narrow bandwidth for each user: Less sensitivity to multipath fading. Simple signal processing. Inflexibility to accommodate variable user data rates within a fixed bandwidth frequency slot. FDM Compared to TDM: 1) Fewer bits for synchronization. 2) Fewer bits for framing. 3) Higher cell site system costs. 3) Higher costs for duplexer used in base station and subscriber units. 4) FDMA requires RF filtering to minimize adjacent channel interference.

Number of Channels in a FDMA System Where, N … Number of channels BT … Total spectrum allocation Bguard … Guard band Bc … Channel bandwidth Example: Advanced Mobile Phone System ( AMPS, FDMA/FDD, analog cellular system 12.5 MHz per simplex band – Bt ; Bguard = 10 kHz ; Bc = 30 kHz.

Time Division Multiple Access (TDMA) Each user is assigned a unique time slot cyclically. Noncontinuous transmission (because users do not use the allotted bandwidth all the time). Digital data. Digital modulation. Used in digital cellular systems, e.g., in GSM 8 users share 200 KHz. The basic concept of TDMA.

Features of TDMA A single carrier frequency for several users. Transmission in bursts. Low battery consumption. Handoff process much simpler. Very high transmission rate. Users can be given variable data rates easily by assigning them multiple time slots. Only one power amplifier is required to support multiple users at the base station (In FDMA, each user requires a power amplifier at the base station). High synchronization overhead. Guard slots necessary. Difficulty in system timing: to ensure correct time slot arrival and framing and to cope with variable path delays in wireless systems.

Repeating Frame Structure One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time

Number of channels in a TDMA system Where, N … number of channels m … number of TDMA users per radio channel Btotal … total spectrum allocation Bguard … Guard Band Bc … channel bandwidth Example: Global System for Mobile (GSM) (TDMA/FDD, Forward link at Btotal = 25 MHz radio channels of Bc = 200 kHz, if m = 8 speech channels supported, and if no guard band is assumed) :

TDMA (Cont’d) Example Solution to Example If GSM uses a frame structure where each frame consists of 8 time slots, and each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel, find (a) the time duration of a bit, (b) the time duration of a slot, (c) the time duration of a frame, and (d) how long must a user occupying a single time slot must wait between two simultaneous transmissions. Solution to Example

Code Division Multiple Access (CDMA) Each user is assigned a unique sequence or code, so that multi-users can share the same bandwidth simultaneously. CDMA systems involve the spread spectrum (SS) technique: transmission bandwidths are much higher than that required by an individual user, with the energy of each user's signal spread with time throughout this wide channel. Two types of CDMA systems: Direct sequence (DS) CDMA Frequency hopping (FH) CDMA

Direct Sequence (DS) CDMA DS-CDMA: Each user is assigned a different spreading code which is orthogonal to other codes (Each user is given his own codeword which is orthogonal to the codes of other users and in order to detect the user, the receiver must know the codeword used by the transmitter). Used in digital cellular systems, e.g., 3G.

DS-CDMA: Processing Gain The bandwidth expansion: PG …… Processing Gain The larger the PG is, the better. It can be shown that despreading of the receiver can reduce the noise and interference power by PG times, i.e.,

Frequency Hopping (FH) CDMA FH-CDMA: Constantly changes the carrier frequencies of individual users with time. To ensure that individual users never (or rarely) hop onto the same frequency slot at the same time, the carrier frequencies are assigned according to a predetermined sequence or code.

Pros and Cons of CDMA Pros: Ability to cope with multipath channels. Immunity to interference from other users. Security. Flexibility to accommodate variable user data rates: Each user can increase their data rate without affecting other users, as long as the user does not increase the overall wideband energy of the composite multi-user signal. Cons: Complex signal processing. Requirement for large bandwidth: e.g., 5 MHz for WCDMA.