Tải Radio Link Performance of 3G Technologies for Wireless Networks
Chapter 1 - Introduction 1
1.1 The Need for Third-Generation Wireless Technologies . 1
Chapter 2 - Evolution of Wireless Technologies from 2G to 3G . 3
2.1 The Path to Third Generation (3G) . 3
2.2 GSM Evolution . 5
2.3 TDMA (IS-136) Evolution . 6
2.4 CDMA (IS-95) Evolution . 6
2.5 Wideband CDMA (WCDMA) 7
2.6 PDC . 8
Chapter 3 – General Radio Packet Services (GPRS) Link Performance 9
3.1 GPRS Data Rates 9
3.2 Link Quality Control . 9
3.3 GPRS Channel Coding . 10
3.4 Simulations on GPRS Receiver Performance . 12
3.4.1 Background to the Research on GPRS Receiver Performance . 12
3.4.2 GPRS Link Performance in Noise Limited Environments . 12
3.4.3 GPRS Link Performance in Interference Limited Environments . 15
3.5 GPRS Uplink Throughput 19
3.6 Discussion . 23
Chapter 4 – Enhanced Data Rates for the GSM Evolution (EDGE) Link Performance 24
4.1 EDGE Modulations and Data Rates . 24
4.2 Link Quality Control . 25
4.3 EDGE Channel Coding . 26
4.4 Simulations on EDGE (EGPRS) Receiver Performance 33
4.4.1 Background on the Research of EDGE Receiver Performance 33
4.4.2 EDGE Bit Error Rate (BER) Link Performance . 34
4.4.2.1 EDGE Bit Error Rate (BER) Link Performance in Noise Limited
Environments 34
4.4.2.2 EDGE Bit Error Rate (BLER) Link Performance in Interference
Limited Environments 42
4.4.3 EDGE Block Error Rate (BLER) Link Performance 49
4.4.3.1 EDGE Block Error Rate (BLER) Link Performance in Noise Limited
Environments 49
4.4.3.2 EDGE Block Error Rate (BLER) Link Performance in Interference
Limited Environments 58
4.4.4 EDGE Link Performance with Receiver Impairments . 66
4.4.4.1 Error Vector Magnitude (EVM) . 66
4.4.4.2 EDGE Block Error Rate (BLER) Link Performance in Noise Limited
Environments with EVM and Frequency Offset 67
4.4.4.3 Block Error Rate (BLER) Performance in Interference-Limited
Environments with EVM and Frequency Offset 72
4.5 EDGE (EGPRS) Downlink Throughput Simulations . 76
4.5.1 Downlink Throughput in Noise Limited Environments . 77
4.5.2 Downlink Throughput in Interference Limited Environments . 82
4.6 Discussion . 86
Chapter 5 – Wideband CDMA (WCDMA) Link Performance 87
5.1 WCDMA Channel Structure . 87
5.1.1 Transport Channels 87
5.1.1.1 Dedicated Transport Channel (DCH) . 88
5.1.1.2 Common Transport Channels . 89
5.1.2 Physical Channels 90
5.1.2.1 Uplink Physical Channels . 91
5.1.2.2 Downlink Physical Channels 91
5.1.3 Mapping of Transport Channels to Physical Channels 92
5.2 Channel Coding and Modulation 93
5.2.4 Error Control Coding . 93
5.2.5 Uplink Coding, Spreading and Modulation . 95
5.2.5.1 Channel Coding and Multiplexing 95
5.2.5.2 Spreading (Channelization Codes) . 98
5.2.5.3 Uplink Scrambling 101
5.2.5.4 Uplink Dedicated Channel Structure 103
5.2.5.5 Modulation 104
5.2.6 Downlink Coding and Modulation 105
5.2.6.1 Channel Coding and Multiplexing 105
5.2.6.2 Spreading (Channelization Codes) . 107
5.2.6.3 Downlink Scrambling . 108
5.2.6.4 Downlink Dedicated Channel Structure . 109
5.2.6.5 Downlink Modulation . 110
5.3 WCDMA Power Control Mechanisms . 111
5.4 Simulations on WCDMA Link Performance 113
5.4.1 Background to the Simulation Results . 113
5.4.2 Simulation Environments and Services . 114
5.4.2.1 The Circuit Switched and Packet Switched Modes 115
5.4.3 Downlink Performance 117
5.4.3.1 Speech, Indoor Office A, 3 Km/h . 118
5.4.3.2 Speech, Outdoor to Indoor and Pedestrian A, 3 Km/h . 120
5.4.3.3 Speech, Vehicular A, 120 Km/h . 122
5.4.3.4 Speech, Vehicular B, 120 Km/h . 124
5.4.3.5 Speech, Vehicular B, 250 Km/h . 126
5.4.3.6 Circuit Switched, Long Constrained Data Delay – LCD, Multiple
Channel Types 128
5.4.3.7 Unconstrained Data Delay - UDD 144, Vehicular A . 130
5.4.3.8 Unconstrained Data Delay - UDD 384, Outdoor to Indoor 132
5.4.3.9 Unconstrained Data Delay - UDD 2048, Multiple Channel Types 134
5.4.4 Downlink Performance in the Presence of Interference 136
5.5 Discussion . 138
Chapter 6 - Conclusions 139
Appendix A - Abbreviations and Acronyms 142
References and Bibliography 145
VITA . 149
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2560 chips
Pilot TFCI FBI TPC
0 1 2 14...........
10 ms
DPDCH
DPCCH
Uplink
DCH
Figure 5-12 Uplink dedicated channel structure. [ET97, HOL00]
DPDCH spreading factor DPDCH channel bit rate (kbps) Maximum user data rate with
R=1/2 coding (kbps)
256 15 7.5
128 30 15
64 60 30
32 120 60
16 240 120
8 480 240
4 960 480
4,with 6 parallel channels 5760 2300
Table 5-4 – WCDMA Uplink Dedicated Physical Data Channel (DPDCH) data rates with and
without coding. [Hol00]
5.2.5.5 Modulation
The complex-valued uplink signal produced by the diagram shown in Figure 5-8 is fed to
the modulator illustrated in Figure 5-12. Square-Root Raised Cosine pulse shaping with
roll-off factor equal to 0.22 is employed.
105
Pulse
Shaping
Pulse
Shaping
Split real &
imaginary
parts
X
X
+
Re{S}
Im{S}
S (from Figure 5-9)
-sin (ω t)
cos (ω t)
Figure 5-13 – WCDMA Uplink Modulator. [3GP01i]
5.2.6 Downlink Coding and Modulation
5.2.6.1 Channel Coding and Multiplexing
The downlink coding and multiplexing chain is very similar to the uplink one, consisting
of the same steps, as illustrated in Figure 5-14. The main difference is in the order in
which the rate matching and the interleaving functions are performed.
106
CRC Attachment
Transport block
concatenation/ Code block
segmentation
Channel Coding
Rate matching
First Insertion of
DTX indication
First Interleaving
Radio frame segmentation
Transport Channel
multiplexing
Second Insertion of DTX
indication
Second interleaving (10ms)
Physical Channel mapping
DPDCH1 DPDCH2 DPDCHn
Radio frame segmentation
Physical Channel
segmentation
CRC Attachment
Transport block
concatenation/ Code block
segmentation
Channel Coding
Rate matching
First Insertion of
DTX indication
First Interleaving
Other Transport Channels
Figure 5-14 - Downlink Coding and Multiplexing chain. [3GP01h]
107
5.2.6.2 Spreading (Channelization Codes)
The downlink channelization codes are the same as used in the downlink. Each cell site
sector utilizes one code tree (one scrambling code) and all links established in the sector
share the assigned code tree. Common channels and dedicated channels share the same
code tree with the exception of the SCH, which is not under a scrambling code [Hol00].
Unlike in the uplink, the downlink spreading factor does not vary on a frame-by-frame
basis. Data rate variation is accomplished either by rate matching or by discontinuous
transmission. When parallel code channels are used for a single user all codes have the
same spreading factor and are under the same code tree [Hol00]. Figure 5-15 shows the
block diagram of the downlink multiplexer, while Figure 5-16 illustrates how different
channels are combined.
Serial-to-
parallel
converter
X
X
+
donlink physical channels
(except SCH)
I
-sin (ω t)
cos (ω t)
I+jQ
X
Q
j
X
S
Sdl,n
Figure 5-15 – Downlink I-Q code multiplexer. [3GP01i]
108
Σ
X
X
Σ
X
P-SCH
X
S-SCH
Gp
Gs
T
G1
G2
Downlink physical channels
(S from Figure 5-15)
Figure 5-16 -Combining of the downlink physical channels. [3GP01i]
5.2.6.3 Downlink Scrambling
The downlink scrambling codes are derived from the same family of long codes used in
the uplink - Gold codes. The short codes are not used in the downlink. A total of 218-1 =
262,143 scrambling codes can be generated, but not all of them are used. A primary and a
secondary group have been defined. The set of primary scrambling codes is limited to
512 sequences, in order to facilitate the cell search procedure. There is, therefore, the
need for code planning during the network design phase. The secondary group contains
15 sequences.
Figure 5-17 shows the block diagram of the downlink scrambling code generator.
109
+17 16 15 14 13 12 11 10 9 7 6 5 4 3 2 1 0
+
+
17 16 15 14 13 12 11 10 9 7 6 5 4 3 2 1 0
+
+
I
Q
+
Figure 5-17 - Downlink scrambling code generator. [3GP01i]
5.2.6.4 Downlink Dedicated Channel Structure
The downlink DPDCH also has a frame slot structure composed of 15 slots per 10ms
radio frame, with the slot duration totaling 2560 chips (666.67µs). The spreading factors
range from 4 to 512. Restrictions in the time adjustment step of 256 chips during soft
handoff limit the use of the spreading factor 512. Such spreading factor is generally used
when there is low downlink activity and in these cases soft handoffs are rarely required.
Figure 5-18 shows the slot and frame structures for the downlink DPCH and Table 5-5
lists the maximum data rates for the various supported spreading factors with and without
coding.
110
2560 chips
TFCI TPC Data pilot
0 1 2 14...........
10 ms
Slot
Downlink
DCH
Data
Figure 5-18 - Downlink dedicated channel structure. [ET97, HOL00]
Spreading
Factor
Channel Symbol
Rate (kbps)
Channel Bit
Rate (kbps)
DPDCH channel
bit rate range
(kbps)
Maximum user data
rate with ½ rate
coding (kbps)
512 7.5 15 3~6 1~3
256 15 30 12~24 6~12
128 30 60 42~51 20~24
64 60 120 90 45
32 120 240 210 105
16 240 480 432 215
8 480 960 912 456
4 960 1920 1872 936
4, with 3
parallel codes
2880 5760 5616 2,300
Table 5-5 – WCDMA Downlink Dedicated Physical Data Channel (DPDCH) data rates with and
without coding. [Hol00]
5.2.6.5 Downlink Modulation
The downlink utilizes conventional QPSK modulation with time-multiplexed control and
data streams. The effect of DTX is not present in the downlink, since the common
111
channels are continuously transmitted. Figure 5-19 illustrates the block diagram for the
downlink modulator. Square-Root Raised Cosine pulse shaping with roll-off factor equal
to 0.22 is employed.
Pulse
Shaping
Pulse
Shaping
Split real &
imaginary
parts
X
X
+
Re{T}
Im{T}
T (from Figure 5-16)
-sin (ω t)
cos (ω t)
Figure 5-19 - Downlink Quadrature Phase shift Keying (QPSK) modulator. [3GP01i]
5.3 WCDMA Power Control Mechanisms
Power control is a fundamental part of CDMA-based systems, particularly in the uplink.
The fundamental reason for the uplink power control is to prevent any overpowered
mobile stations from blocking access to the whole cell by unnecessarily raising the noise
level.
WCDMA employs power control in both the downlink and uplinks. The solution for both
links is based on a dual-loop technique. The outer loop utilizes an open-loop to provide a
coarse initial power setting at the beginning of the connection. The inner-loop is a fast
closed-loop control acting at a rate of 1.5 kHz, i.e., 1,500 corrections per second.
In the uplink the base station performs the estimates of the received Signal –to-
Interference Ratio (SIR) and compares it to a target value. If the received SIR is higher
than the target it commands the mobile station to power down; if it is lower it commands
the mobile station to power up.
112
The downlink uses power control to improve link performance as the mobile moves away
from the serving cell, suffering increased other-cell interference. Also, it provides
additional power margin for low-speed mobiles affected by Rayleigh fading. At low
speeds the interleaving and error correcting codes do not work effectively, because the
duration of the fading nulls may cause more bits to be in error than those mechanisms can
correct. In these cases the fast downlink power control helps compensate for the
diminished signal-to-noise due to fading.
Figure 5-20 exemplifies the power control reaction to a fading channel and Figure 5-21
shows the resulting received power for the same link.
Figure 5-20 - Reaction of the WCDMA closed-loop fast power control to the fading channel. [Hol00]
Figure 5-21 - Effect of the WCDMA closed-loop fast power control on the received power. [Hol00]
113
When compared to a link with slow power control, fast power control reduces the
necessary Eb/No for the same quality requirement. Table 5-6 shows the gain obtained
with power control for three propagation environments.
Eb/No (dB) - Slow
Power Control (dB)
Eb/No (dB) - Fast Power
Control – 1.5 kHz
Gain from fast power
control (dB)
ITU Pedestrian A 3 Km/h 11.3 5.5 5.8
ITU Vehucular A 3 Km/h 8.5 6.7 1.8
ITU Vehucular A 50 Km/h 6.8 7.3 -0.5
Table 5-6 – Required Eb/No values for WCDMA with slow power control and fast power control for
different propagation environments. [Hol00]
5.4 Simulations on WCDMA Link Performance
5.4.1 Background to the Simulation Results
WCDMA link level performance has been extensively researched and simulated. The
numerous test conditions arising from the complexity of the technology have produced a
vast array of simulation results. This work focuses on those for the test scenarios defined
by the standardization committees, namely t...
Download miễn phí Radio Link Performance of 3G Technologies for Wireless Networks
Chapter 1 - Introduction 1
1.1 The Need for Third-Generation Wireless Technologies . 1
Chapter 2 - Evolution of Wireless Technologies from 2G to 3G . 3
2.1 The Path to Third Generation (3G) . 3
2.2 GSM Evolution . 5
2.3 TDMA (IS-136) Evolution . 6
2.4 CDMA (IS-95) Evolution . 6
2.5 Wideband CDMA (WCDMA) 7
2.6 PDC . 8
Chapter 3 – General Radio Packet Services (GPRS) Link Performance 9
3.1 GPRS Data Rates 9
3.2 Link Quality Control . 9
3.3 GPRS Channel Coding . 10
3.4 Simulations on GPRS Receiver Performance . 12
3.4.1 Background to the Research on GPRS Receiver Performance . 12
3.4.2 GPRS Link Performance in Noise Limited Environments . 12
3.4.3 GPRS Link Performance in Interference Limited Environments . 15
3.5 GPRS Uplink Throughput 19
3.6 Discussion . 23
Chapter 4 – Enhanced Data Rates for the GSM Evolution (EDGE) Link Performance 24
4.1 EDGE Modulations and Data Rates . 24
4.2 Link Quality Control . 25
4.3 EDGE Channel Coding . 26
4.4 Simulations on EDGE (EGPRS) Receiver Performance 33
4.4.1 Background on the Research of EDGE Receiver Performance 33
4.4.2 EDGE Bit Error Rate (BER) Link Performance . 34
4.4.2.1 EDGE Bit Error Rate (BER) Link Performance in Noise Limited
Environments 34
4.4.2.2 EDGE Bit Error Rate (BLER) Link Performance in Interference
Limited Environments 42
4.4.3 EDGE Block Error Rate (BLER) Link Performance 49
4.4.3.1 EDGE Block Error Rate (BLER) Link Performance in Noise Limited
Environments 49
4.4.3.2 EDGE Block Error Rate (BLER) Link Performance in Interference
Limited Environments 58
4.4.4 EDGE Link Performance with Receiver Impairments . 66
4.4.4.1 Error Vector Magnitude (EVM) . 66
4.4.4.2 EDGE Block Error Rate (BLER) Link Performance in Noise Limited
Environments with EVM and Frequency Offset 67
4.4.4.3 Block Error Rate (BLER) Performance in Interference-Limited
Environments with EVM and Frequency Offset 72
4.5 EDGE (EGPRS) Downlink Throughput Simulations . 76
4.5.1 Downlink Throughput in Noise Limited Environments . 77
4.5.2 Downlink Throughput in Interference Limited Environments . 82
4.6 Discussion . 86
Chapter 5 – Wideband CDMA (WCDMA) Link Performance 87
5.1 WCDMA Channel Structure . 87
5.1.1 Transport Channels 87
5.1.1.1 Dedicated Transport Channel (DCH) . 88
5.1.1.2 Common Transport Channels . 89
5.1.2 Physical Channels 90
5.1.2.1 Uplink Physical Channels . 91
5.1.2.2 Downlink Physical Channels 91
5.1.3 Mapping of Transport Channels to Physical Channels 92
5.2 Channel Coding and Modulation 93
5.2.4 Error Control Coding . 93
5.2.5 Uplink Coding, Spreading and Modulation . 95
5.2.5.1 Channel Coding and Multiplexing 95
5.2.5.2 Spreading (Channelization Codes) . 98
5.2.5.3 Uplink Scrambling 101
5.2.5.4 Uplink Dedicated Channel Structure 103
5.2.5.5 Modulation 104
5.2.6 Downlink Coding and Modulation 105
5.2.6.1 Channel Coding and Multiplexing 105
5.2.6.2 Spreading (Channelization Codes) . 107
5.2.6.3 Downlink Scrambling . 108
5.2.6.4 Downlink Dedicated Channel Structure . 109
5.2.6.5 Downlink Modulation . 110
5.3 WCDMA Power Control Mechanisms . 111
5.4 Simulations on WCDMA Link Performance 113
5.4.1 Background to the Simulation Results . 113
5.4.2 Simulation Environments and Services . 114
5.4.2.1 The Circuit Switched and Packet Switched Modes 115
5.4.3 Downlink Performance 117
5.4.3.1 Speech, Indoor Office A, 3 Km/h . 118
5.4.3.2 Speech, Outdoor to Indoor and Pedestrian A, 3 Km/h . 120
5.4.3.3 Speech, Vehicular A, 120 Km/h . 122
5.4.3.4 Speech, Vehicular B, 120 Km/h . 124
5.4.3.5 Speech, Vehicular B, 250 Km/h . 126
5.4.3.6 Circuit Switched, Long Constrained Data Delay – LCD, Multiple
Channel Types 128
5.4.3.7 Unconstrained Data Delay - UDD 144, Vehicular A . 130
5.4.3.8 Unconstrained Data Delay - UDD 384, Outdoor to Indoor 132
5.4.3.9 Unconstrained Data Delay - UDD 2048, Multiple Channel Types 134
5.4.4 Downlink Performance in the Presence of Interference 136
5.5 Discussion . 138
Chapter 6 - Conclusions 139
Appendix A - Abbreviations and Acronyms 142
References and Bibliography 145
VITA . 149
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Tóm tắt nội dung tài liệu:
ata2560 chips
Pilot TFCI FBI TPC
0 1 2 14...........
10 ms
DPDCH
DPCCH
Uplink
DCH
Figure 5-12 Uplink dedicated channel structure. [ET97, HOL00]
DPDCH spreading factor DPDCH channel bit rate (kbps) Maximum user data rate with
R=1/2 coding (kbps)
256 15 7.5
128 30 15
64 60 30
32 120 60
16 240 120
8 480 240
4 960 480
4,with 6 parallel channels 5760 2300
Table 5-4 – WCDMA Uplink Dedicated Physical Data Channel (DPDCH) data rates with and
without coding. [Hol00]
5.2.5.5 Modulation
The complex-valued uplink signal produced by the diagram shown in Figure 5-8 is fed to
the modulator illustrated in Figure 5-12. Square-Root Raised Cosine pulse shaping with
roll-off factor equal to 0.22 is employed.
105
Pulse
Shaping
Pulse
Shaping
Split real &
imaginary
parts
X
X
+
Re{S}
Im{S}
S (from Figure 5-9)
-sin (ω t)
cos (ω t)
Figure 5-13 – WCDMA Uplink Modulator. [3GP01i]
5.2.6 Downlink Coding and Modulation
5.2.6.1 Channel Coding and Multiplexing
The downlink coding and multiplexing chain is very similar to the uplink one, consisting
of the same steps, as illustrated in Figure 5-14. The main difference is in the order in
which the rate matching and the interleaving functions are performed.
106
CRC Attachment
Transport block
concatenation/ Code block
segmentation
Channel Coding
Rate matching
First Insertion of
DTX indication
First Interleaving
Radio frame segmentation
Transport Channel
multiplexing
Second Insertion of DTX
indication
Second interleaving (10ms)
Physical Channel mapping
DPDCH1 DPDCH2 DPDCHn
Radio frame segmentation
Physical Channel
segmentation
CRC Attachment
Transport block
concatenation/ Code block
segmentation
Channel Coding
Rate matching
First Insertion of
DTX indication
First Interleaving
Other Transport Channels
Figure 5-14 - Downlink Coding and Multiplexing chain. [3GP01h]
107
5.2.6.2 Spreading (Channelization Codes)
The downlink channelization codes are the same as used in the downlink. Each cell site
sector utilizes one code tree (one scrambling code) and all links established in the sector
share the assigned code tree. Common channels and dedicated channels share the same
code tree with the exception of the SCH, which is not under a scrambling code [Hol00].
Unlike in the uplink, the downlink spreading factor does not vary on a frame-by-frame
basis. Data rate variation is accomplished either by rate matching or by discontinuous
transmission. When parallel code channels are used for a single user all codes have the
same spreading factor and are under the same code tree [Hol00]. Figure 5-15 shows the
block diagram of the downlink multiplexer, while Figure 5-16 illustrates how different
channels are combined.
Serial-to-
parallel
converter
X
X
+
donlink physical channels
(except SCH)
I
-sin (ω t)
cos (ω t)
I+jQ
X
Q
j
X
S
Sdl,n
Figure 5-15 – Downlink I-Q code multiplexer. [3GP01i]
108
Σ
X
X
Σ
X
P-SCH
X
S-SCH
Gp
Gs
T
G1
G2
Downlink physical channels
(S from Figure 5-15)
Figure 5-16 -Combining of the downlink physical channels. [3GP01i]
5.2.6.3 Downlink Scrambling
The downlink scrambling codes are derived from the same family of long codes used in
the uplink - Gold codes. The short codes are not used in the downlink. A total of 218-1 =
262,143 scrambling codes can be generated, but not all of them are used. A primary and a
secondary group have been defined. The set of primary scrambling codes is limited to
512 sequences, in order to facilitate the cell search procedure. There is, therefore, the
need for code planning during the network design phase. The secondary group contains
15 sequences.
Figure 5-17 shows the block diagram of the downlink scrambling code generator.
109
+17 16 15 14 13 12 11 10 9 7 6 5 4 3 2 1 0
+
+
17 16 15 14 13 12 11 10 9 7 6 5 4 3 2 1 0
+
+
I
Q
+
Figure 5-17 - Downlink scrambling code generator. [3GP01i]
5.2.6.4 Downlink Dedicated Channel Structure
The downlink DPDCH also has a frame slot structure composed of 15 slots per 10ms
radio frame, with the slot duration totaling 2560 chips (666.67µs). The spreading factors
range from 4 to 512. Restrictions in the time adjustment step of 256 chips during soft
handoff limit the use of the spreading factor 512. Such spreading factor is generally used
when there is low downlink activity and in these cases soft handoffs are rarely required.
Figure 5-18 shows the slot and frame structures for the downlink DPCH and Table 5-5
lists the maximum data rates for the various supported spreading factors with and without
coding.
110
2560 chips
TFCI TPC Data pilot
0 1 2 14...........
10 ms
Slot
Downlink
DCH
Data
Figure 5-18 - Downlink dedicated channel structure. [ET97, HOL00]
Spreading
Factor
Channel Symbol
Rate (kbps)
Channel Bit
Rate (kbps)
DPDCH channel
bit rate range
(kbps)
Maximum user data
rate with ½ rate
coding (kbps)
512 7.5 15 3~6 1~3
256 15 30 12~24 6~12
128 30 60 42~51 20~24
64 60 120 90 45
32 120 240 210 105
16 240 480 432 215
8 480 960 912 456
4 960 1920 1872 936
4, with 3
parallel codes
2880 5760 5616 2,300
Table 5-5 – WCDMA Downlink Dedicated Physical Data Channel (DPDCH) data rates with and
without coding. [Hol00]
5.2.6.5 Downlink Modulation
The downlink utilizes conventional QPSK modulation with time-multiplexed control and
data streams. The effect of DTX is not present in the downlink, since the common
111
channels are continuously transmitted. Figure 5-19 illustrates the block diagram for the
downlink modulator. Square-Root Raised Cosine pulse shaping with roll-off factor equal
to 0.22 is employed.
Pulse
Shaping
Pulse
Shaping
Split real &
imaginary
parts
X
X
+
Re{T}
Im{T}
T (from Figure 5-16)
-sin (ω t)
cos (ω t)
Figure 5-19 - Downlink Quadrature Phase shift Keying (QPSK) modulator. [3GP01i]
5.3 WCDMA Power Control Mechanisms
Power control is a fundamental part of CDMA-based systems, particularly in the uplink.
The fundamental reason for the uplink power control is to prevent any overpowered
mobile stations from blocking access to the whole cell by unnecessarily raising the noise
level.
WCDMA employs power control in both the downlink and uplinks. The solution for both
links is based on a dual-loop technique. The outer loop utilizes an open-loop to provide a
coarse initial power setting at the beginning of the connection. The inner-loop is a fast
closed-loop control acting at a rate of 1.5 kHz, i.e., 1,500 corrections per second.
In the uplink the base station performs the estimates of the received Signal –to-
Interference Ratio (SIR) and compares it to a target value. If the received SIR is higher
than the target it commands the mobile station to power down; if it is lower it commands
the mobile station to power up.
112
The downlink uses power control to improve link performance as the mobile moves away
from the serving cell, suffering increased other-cell interference. Also, it provides
additional power margin for low-speed mobiles affected by Rayleigh fading. At low
speeds the interleaving and error correcting codes do not work effectively, because the
duration of the fading nulls may cause more bits to be in error than those mechanisms can
correct. In these cases the fast downlink power control helps compensate for the
diminished signal-to-noise due to fading.
Figure 5-20 exemplifies the power control reaction to a fading channel and Figure 5-21
shows the resulting received power for the same link.
Figure 5-20 - Reaction of the WCDMA closed-loop fast power control to the fading channel. [Hol00]
Figure 5-21 - Effect of the WCDMA closed-loop fast power control on the received power. [Hol00]
113
When compared to a link with slow power control, fast power control reduces the
necessary Eb/No for the same quality requirement. Table 5-6 shows the gain obtained
with power control for three propagation environments.
Eb/No (dB) - Slow
Power Control (dB)
Eb/No (dB) - Fast Power
Control – 1.5 kHz
Gain from fast power
control (dB)
ITU Pedestrian A 3 Km/h 11.3 5.5 5.8
ITU Vehucular A 3 Km/h 8.5 6.7 1.8
ITU Vehucular A 50 Km/h 6.8 7.3 -0.5
Table 5-6 – Required Eb/No values for WCDMA with slow power control and fast power control for
different propagation environments. [Hol00]
5.4 Simulations on WCDMA Link Performance
5.4.1 Background to the Simulation Results
WCDMA link level performance has been extensively researched and simulated. The
numerous test conditions arising from the complexity of the technology have produced a
vast array of simulation results. This work focuses on those for the test scenarios defined
by the standardization committees, namely t...