Wideband Cdma For Third Generation Mobile Communications / Edition 1 available in Hardcover
Wideband Cdma For Third Generation Mobile Communications / Edition 1
- ISBN-10:
- 089006735X
- ISBN-13:
- 9780890067352
- Pub. Date:
- 10/24/2001
- Publisher:
- Artech House, Incorporated
- ISBN-10:
- 089006735X
- ISBN-13:
- 9780890067352
- Pub. Date:
- 10/24/2001
- Publisher:
- Artech House, Incorporated
Wideband Cdma For Third Generation Mobile Communications / Edition 1
Hardcover
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$165.00Overview
Wireless system engineers responsible for designing wideband CDMA systems will appreciate the book's depth of coverage as it compares a wide array of CDMA design techniques and examines how each affects system performance. The book also describes how third generation system applications will impact radio access system design and...
• Compares and contrasts major wideband CDMA standardization proposals, including FRAMES, WCDMA in 3GPP, CDMA2000, and IS-95B
• Identifies and describes various air interface access schemes for third and fourth generation mobile communications systems
• Analyzes wideband CDMA performance in varying radio environments, and presents simulation results
• Discusses the integration of the GSM core network with wideband CDMA
1 Examines the basic principles of CDMA, including IS-95B air interference, and includes details on EDGE/136HS
Product Details
| ISBN-13: | 9780890067352 |
|---|---|
| Publisher: | Artech House, Incorporated |
| Publication date: | 10/24/2001 |
| Series: | Universal Personal Communications Series |
| Pages: | 468 |
| Product dimensions: | 6.00(w) x 9.00(h) x 1.19(d) |
About the Author
Ramjee Prasad is a professor of wireless personal and multimedia communications at Delft University of Technology, the Netherlands. He is the author of CDMA for Wireless Personal Communication, and Universal Wireless Personal Communications, both published by Artech House. He earned his Ph.D. in communications from the Birla Institute of Technology.
Tero Ojanpera is a principal engineer at the Nokia Research Center, Texas. A member of the IEEE, he earned his M.Sc. in electrical engineering from the University of Oulu.
Read an Excerpt
Chapter 9: The Division Duplex DS-CDMA
9.3.2 Asymmetric Services Services such as the Internet, multimedia applications, and file transfers often set different capacity requirements for the uplink and downlink. The utilization of a TDD frequency band is not fixed between the uplink and downlink (unlike with FDD) and this flexibility in resource allocation can be used if the air interface design is flexible enough. This is one motivation for considering the TDD extension for FDD-CDMA-based full coverage third generation mobile radio systems.9.3.3 Reciprocal Channel in Uplink and Downlink
In FDD operation, the uplink and downlink transmissions are separated by a duplex separation. Since fast fading due to multipath propagation depends on the frequency, it is uncorrelated between the uplink and downlink. The FDD transmitter cannot predict the fast fading that will affect its transmission.
In TDD operation, the same frequency is used for both the uplink and the downlink. Based on the received signal, the TDD transmitter is able to know the fast fading of the multipath channel. This assumes that the TDD frame length is shorter than the coherence time of the channel. This assumption holds if TDD mobiles are slowly moving terminals. The reciprocal channel can then be utilized for:
- Open loop power control;
- Spatio-temporal transmission diversity (adaptive antennas for transmission [3], pre-RAKE [4-6]).
With open loop power control, the need for power control signaling is reduced compared to closed loop power control. Closed loop power control signaling also introduces some delay and is subject to errors, which is not the case with open loop power control. In order to have fast enough open loop power control, the TDD frame must be short enough. According to [7], Doppler frequencies up to 80 Hz (43 km/h at 2GHz carrier frequency) can be supported with a very small degradation if the uplink part of the TDD frame length is 1.5 ms. If the TDD system is intended only for slowly moving terminals, then longer TDD frames could also be used. With open loop power control, the interference situation at the receiver is not known by the transmitter, only the signal level is known. Changes in the interference level must be signaled.
Transmission diversity can be utilized with diversity antennas (space domain diversity) or with pre-RAKE (time domain diversity). In selection diversity combining, the receiver measures the received signal from diversity antennas and selects the best antenna for reception. Antenna diversity techniques are easily applied at the base stations but those receiver techniques are not suited for small handheld terminals. In order to achieve antenna diversity in the downlink, transmission diversity is utilized at the base station. Based on uplink reception, the best antenna can be selected for downlink transmission in TDD.
In FDD-CDMA transmission, a RAKE receiver is used to collect the multipath components and to obtain multipath diversity. The optimal RAKE receiver is a matched filter to the multipath channel. If the transmitter knew the multipath channel, it could apply RAKE in the transmitter (pre-RAKE). Transmission would be such that the multipath channel would act as a matched filter to the transmitted signal. In the receiver no RAKE would be needed (i.e., multipath diversity could be obtained with a onefinger receiver). However, it should be noted that only a little multipath diversity may be available in indoor propagation environments, as shown in Chapter 7 with ITU channel models. Indoor and microcell environments are the most probable application areas for TDD communication. Therefore, antenna diversity transmission will be a more attractive diversity technique for TDD operation than the multipath diversity technique.
If such a TDD proposal, which has different solutions for the uplink and downlink (where uplink uses a single wideband carrier and downlink a multicarrier approach) is applied, channel reciprocity cannot be utilized. The effect of such a structure depends on the environment, but, as calculated in Chapter 7, the coherence bandwidth cannot be guaranteed to be so high that similar fading characteristics could be obtained for both uplink and downlink.
9.4 PROBLEMS WITH TDD-CDMA
This section introduces the problems encountered by TDD-CDMA systems and presents the disadvantages of using such a duplex method.
9.4.1 Interference From TDD Power Pulsing
If fast power control frequency with open loop is desired to support higher mobile speeds, then short TDD frames must be used. The short transmission time in each direction results in the problems listed below:
- Audible interference from pulsed transmission both internally in the terminal and to the other equipment. Generated pulsing frequency in the middle of voice band will cause problems to small size speech terminal design where audio and transmission circuits are relatively close to each other and achieving the needed isolation is costly and requires design considerations. At high power levels this may not be achievable at all.
- Base station synchronization requirements are tight and more overhead must be allocated for guard times and also for power ramps as EMC requirements limit the ramping speed.
- Fast ramping times set tighter requirements to the components (e.g., to the power amplifier).
Lower pulsing frequency, say, 100 Hz (i.e., a TDD frame of 10 ms), results in less audible pulsing but limits the maximum tolerable mobile speeds. In the TDD-CDMA in [8,9], the uplink slot and the downlink slot are both 0.625 ms, resulting in an audible interference at 800 Hz.
9.4.2 Intracell and Intercell Interference Between Uplink and Downlink
In CDMA systems, the SIR may be quite low (e.g., below - 15 dB) at carrier bandwidth. After despreading, the SIR is improved by the processing gain. In TDD systems, a transmitter located close to a receiver may block the front end of the receiver, since no RF filter can be used to separate uplink and downlink transmission as in FDD operation. This blocking may happen even if the transmitter and receiver are not operating in the same frequency channel but if they are operating in the same TDD band. In that case, the processing gain at baseband does not help since the signal is already blocked before baseband processing. These interference problems with TDD operation are considered in this section.
Within one TDD-CDMA cell, all users must be synchronized and have the same time division between uplink and downlink in order to avoid interference between uplink and downlink. This time division is based on the average uplink and downlink capacity need in that particular cell. Each user then applies multirate techniques to adapt its uplink and downlink capacity needs to the average need in that cell. The same time division must be applied to all carriers within one base station. If the base station transmits and receives at the same time as adjacent carriers, it would block its own reception.
Asymmetric usage of TDD slots will impact the radio resource in neighboring cells. This scenario is depicted in Figure 9.3 and the resulting signal to adjacent channel interference ratio is calculated in Table 9.3. Intercell interference problems occur in asymmetric TDD-CDMA if the asymmetry is different in adjacent cells even if the base stations are synchronized. MS2 is transmitting at full power at the cell border. Since MS I has different asymmetric slot allocation than MS2, its downlink slots received at the sensitivity limit are interfered with by NIS 1, causing blocking. On the other hand, since BSI can have much higher effective isotropically radiated power (EIRP) than MS2, it will interfere with BS2 receiving MS2. It is difficult to adjust the asymmetry of an individual cell in a network due to interference between adjacent cells. If TDD-CDMA cells are located adjacent to each other, offering a continuous coverage, then synchronization and asymmetry coordination between these cells is required This ensures that the near-far problems of interference between mobiles in adjacent cells can be controlled. Another scenario, where the previously described blocking effect clearly exists, is if TDD operation were also allowed in the FDD band....