1. Advanced Wireless Communication Systems
Lecture 2: Existing Multiple Access Systems
Multiple access schemes are used to allow many users, stationary or mobile, to share simultaneously a finite amount of wireless frequency spectrum. The sharing of spectrum is required to achieve a high capacity by simultaneously allocating the available bandwidth or available amount of channels to multiple users.
We can differentiate the forward link or downlink from the base station (radio port)
and user (stationary or mobile) and reverse link or uplink from user to base station.
Administrative
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2. Introduction to Multiple Accesses
Frequency division duplexing (FDD) provides two distinct bands of frequencies for
base station and every user connected (see top scheme), that is, for the forward
and reverse links.
Time division duplexing (TDD) uses different time instead of frequency to serve
both a forward and reverse channels (see bottom scheme).
FDD can be performed by simultaneous (time t=const) transmission and reception
of information and by careful separation between frequencies for downlink and uplink.
TDD can be performed by time latency due to separate transmitting of information.
TDD allows communication on a single channel (frequency f=const).
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Adv. Wireless Comm. Sys.

3. Advanced Wireless Communication Systems Objective
These techniques can be grouped as narrowband and wideband systems.
Narrowband systems:
In such systems the transmission bandwidth of each single channel is smaller
than its expected coherence bandwidth.
Narrowband channels are usually operated using FDD called FDMA/FDD,
where subscriber is assigned a particular channel.
In narrowband TDMA systems. there are two options to serve users
by using FDD or TDD: TDMA/FDD or TDMA/TDD.
Wideband systems:
In such systems the transmission bandwidth of each single channel is much larger
than its expected coherence bandwidth. In such a system, the user transmits
information in a large part of the radio spectrum.
The most techniques are wideband TDMA and CDMA, which can use separately
either FDD and TDD or CDD (code division duplexing).
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4. Classical Wireless Communication Networks
* There are three major access techniques to share the available bandwidth in the wireless communication networks:
* Frequency Division Multiple Access (FDMA);
* Time Division Multiple Access (TDMA);
* Code Division Multiple Access (CDMA).
In addition, some other access schemes are usually used. They are:
- Packet Radio (PR).
- Space Division Multiple Access (SDMA).
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5. Wireless Network Characteristics
Classification of Multiple Accesses and Protocols is shown in Fig. 2.2.
Fig. 2.2
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6. Wireless Network Characteristics
Frequency Division Multiple Access (FDMA)
* Main principle of FDMA system is to assign individual channels to individual
users.
* As is clear seen from Fig. 2.3, each user is allocated a unique frequency band or
channel.
* These channels are assigned on demand to users who request service.
Fig. 2.3
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7. FDMA Characteristics Spectral Efficiency of FDMA:
Spectral efficiency of FDMA characterizes an efficient use of frequency spectrum and can be defined by knowledge of total number of channels in the FDMA system, , bandwidth of the system, , and a channel, :
(2.1)
Modulation Efficiency of FDMA and TDMA:
channels/MHz/km^2
(2.2)
A is the total area of service.
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8. FDMA Network Characteristics
Variant 1:
(2.3)
where area covered by one cell as hexagon in km^2:
(2.4)
N is the reuse factor of the cellular network,
M/N gives us the number of clusters in the served area,
gives the number of channels in the cluster.
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9. FDMA Network Characteristics
Variant 2:
(2.5a)
Erlangs/MHz/ km^2
(2.5b)
number of users
where the trunking efficiency factor,
Overall Efficiency of FDMA:
(2.6)
Number of Channels:
(2.7)
is the guard band at the edge of the allocated spectrum.
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10. TDMA Network
TDMA allows transmission of larger information rates than in an FDMA system.
The carrier (radio frequency) is divided to N timeslots (simply, slots) and can
be shared by N terminals (see Fig. 2.4), each of which uses a particular slot
different from slots used by the other terminals.
Fig. 2.4
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11. TDMA Network Parameters
Slots and Frames:
Guard times are utilized to allow synchronization of the receivers between
different slots and frames (see Fig. 2.5). The guard times should be minimized
and the spectrum broadens.
Address + synch
Fig. 2.5
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12. TDMA Network Characteristics
Efficiency of TDMA:
Efficiency of TDMA characterizes an efficient use of time slots/frames in the system.
It is defined by the following expression:
(2.8)
is a time of slot duration,
is the time of frame duration,
is the number of slots per frame.
For TDMA network the overall efficiency, as above for FDMA system, equals:
(2.9)
where is defined by formula (2.3) or formula (2.5)
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13. Wireless Network Characteristics
Frame Efficiency
It is a measure of the percentage of transmission data rate that contains
information, that is, it is a percentage of bits per frame, which contain
transmission data with respect to overhead data.
(2.10)
Here the number of overhead bits per frame is:
(2.11)
is the number of reference bursts per slot,
is the number of traffic bursts per slot,
is the number of bits per reference burst
is the number of bits reference burst in each slot
is the number of equivalent bits in each guard time interval
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14. TDMA Network Characteristics (cont.)
The total number of bits per frame
(2.12)
is the frame duration,
R is the channel bit rate.
Number of Channels:
It is the number of TDMA channel slots, which can be found by multiplying
the number of TDMA slots per channel by a number of channels available:
(2.13)
Example of the frame architecture of the GSM/TDMA system (see Fig. 2.6):
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15. CDMA Basic Principles Main Goal: Spreading of the information signal
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16. CDMA Basic Principles Coding with pseudo-noise (PN) sequence
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17. Code Division Multiple Access: CDMA
Spread Spectrum Multiple Access (SSMA):
- Direct Sequence Multiple Access (DS-MA).
- Frequency Hopping Multiple Access (FH-MA).
- Time Hopping Multiple Access (TH-MA)
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18. Types of SS Modulation Technique
• Direct Sequence Spread Spectrum (DS-SS) Modulation.
It is based on direct multiplication of the data-bearing signal with a high chip rate spreading PN code.
• Frequency Hopping (FH).
It based on jump in carrier frequency, at which the data-bearing signal is transmitted. This jump (change) in frequency occurs according to the corresponding spreading PN code.
• Time Hopping (TH).
Here, the data-bearing signal is not transmitted continuously; it is transmitted in form of short bursts, where the times of the bursts are decided by the spreading PN code, that is, jump in time is noted.
• Hybrid Modulation (HM).
It based on combination of the up methods to combat with their disadvantages.
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19. DS-SS-System the processing gain - pseudo-noise (PN) code bandwidth
- information data bandwidth
is the chip interval
m(t) is a data sequence
is the carrier phase angle
is the bit interval
p(t) is the PN spreading sequence
is the carrier frequency
is the carrier phase angle
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20. DS-SS Principle Two Users-Interferers
Common Case of N Users-Interferers
Original
Data Signal
Narrowband
Filter
Other
SS Users
Demodulator
Filtering
ISI
Modulated
Data
with Spreading
Interference
Receiver
Input
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21. DS-SS Basic Principle For PN code sequence: Lectures 1 & 2: Overview
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22. Frequency Hopping Spread Spectrum - FH-SS
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23. Fast-FH and Slow-FH Spread Spectrum
During one symbol of information data duration several hopes occur
Slow-FH:
During one hope duration several symbols of information data occur
Properties of FH-SS Modulation:
- Multiple Access Capability
- Multipath Interference Rejection
- Narrowband Interference Rejection
- Low Probability of Interception (LPI)
- Self jamming due to imperfect code orthogonally
- Soft handoff done by the Mobile Switching Center (MSC)
- Multipath fading is dominant due to signal spread
- The “near-far” problem makes power control crucial
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24. Time-Hoping Spread Spectrum
Properties of FH-SS Modulation:
- Multiple Access Capability.
- Multipath Interference Rejection.
- Narrowband Interference Rejection.
- Low Probability of Interception (LPI).
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25. Additional Multiple Access Technique
Space Division Multiple Access: SD-MA
Based on smart antenna:
Beam-forming or phased-array
Spot bean antennas
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