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reducing the transmission rate by allocating more than one time slot for HP-CCPCH, is also considered to achieve a higher link margin. However, reducing the infoimation rate results in the reduction of coherence time relative to the information rate

For packet data transfer, in W-C/TDMA a dedicated channel is assigned only if the frequency of packets for the same destination is high. Otherwise they are transferred in frame #0, Since connectionless packet data transfer does not allow power control, a high link margin has to be allocated for this purpose. Therefore, frame #0 should not be overloaded to save satel­

lite power. However, allocating a dedicated channel requires signalling overhead which also requires additional satellite energy and reduces capacity.

In SAT-CDMA, two pilot channels (index pilot channel and beam pilot channel) are transmitted at all the times from the satellite on each active DL CDMA channel, in order to achieve fast beam search. Index pilot channel pseudo noise (PN) code (whose period is shorter than that of the beam pilot period) aligns the timing of the satellite and the beam pilot channel and reduces the number of beam pilots to be searched. The beam pilot PN code (whose period is equal to I frame length) provides the frame timing information.

2.6 Differences between W-CDMA and SW-CDMA

WCDMA and SW-CDMA are the most favourable candidates amongst all the proposals sub­

mitted to the ITU. Since S-UMTS is derived from T-UMTS, SW-CDMA specifications closely follow that of UTRA. However, due to the differences between terrestrial and satellite channel characteristics, it is not possible have a common air interface for S-UMTS and T-UMTS. Some modifications to the T-UMTS standards are necessary since channel characteristics play a key role in the air interface definition. The T-UMTS channel is typically affected by lognormal long-term shadowing and by Rayleigh short term multipath fading. In contrast, due to the large free space loss and on-board power limitations mobile satellite systems are forced to operate under LOS propagation conditions. Limited RF on-board power is insufficient to overcome shadowing from hills, trees (up to 10-20 dB) and buildings. Therefore, S-UMTS operates in a Rice fading channel with a Rice factor typically varying between 7 and 15 dB [22].

The following section presents the main differences between T-UMTSAV-CDMA and S-UMTS/SW-CDMA systems [22],[29].

2.6.1 System level differences

1. In mobile-satellite CDMA systems, implementation of Rake receiver architecture is not straightforward and not very effective due to the smaller delay spreads associated with such environments [55]. This also means that the fading is non-selective, preserving the orthogonality of the spreading codes and minimising intra-beam interference.

2. In satellite systems, the dynamic range of the received power is much smaller than that of terrestrial systems (goes up to 80 dB). This is due to the reduced path loss variation within each satellite beam (in order of 3-5 dB).

3. Spot beam size is much larger in satellite systems (compared to a cell size in terrestrial systems) resulting low bps / H z/ Kvn?.

4. Contrarily to T-UMTS, the forward link represents the capacity bottleneck due to the on-board power constraints.

5. The Doppler shift complicates the signal acquisition and spectrum management proce­

dures. In IMT-2000, the large frequency separation between UL and DL (i.e. 190 MHz) induces different Doppler shifts in the UL and DL for a particular speed of a moving user. The Doppler shift experienced due to the satellite movements in GEO systems are generally limited to about 1 kHz and software in the user terminal compensate for this effect. Doppler precompensation techniques are used for the center of beam location

2.6.2 Physical layer differences

1. Permanent satellite path diversity can be exploited for mobile users. In terrestrial systems where multiple base-station simultaneous connection duration is limited in time and in terms of cell area to avoid capacity reduction. In the forward link satellite diversity must be forced by sending the same signal to different satellites through highly directive antennas. Due to the quasi-omnidirectional antenna in the MT diversity can be easily

2.6. Differences between W-CDMA and SW-CDMA 47

implemented in the return link. It was found that satellite diversity provides a practical way to reduce the blockage effect with little or no impact on capacity compared to a system based on line-of-sight operations [56].

2. Introduction of a lower chip rate option (1.92 Mcps) in addition to the basic chip rate (3.84 Mcps) to favour band sharing between different users in multi operating environ­

ments where bandwidth limitations may arise. The frame size corresponds to the lower

chip rate is 20 ms. . ■ »

3. Only FDD operation is considered. Unlike T-UMTS, the frequency allocation for S- UMTS is a paired band.

4. A dual-BPSK option is added to minimise degradation at very low data rates (less than 8 kbps) in DL. BPSK or Dual BPSK is less sensitive to frequency and phase errors at lower bit rates (i.e.2.4 kbps^. It was found [56] that use of QPSK can be envisaged to sim­

plify MT hardware implementation at the expenses of about 1 dB link loss compared to dual-BPSK. It was found that QPSK is more sensitive to non-linearity than dual-BPSK.

However, dual-BPSK requires double the number of spreading codes and potentially has inferior performance in conjunction with interference mitigation techniques.

5. Instead of UTRA’s long scrambling code (38400 chips long) when no DL mitigation techniques are adopted, an optional use of short scrambling codes (extended Gold codes of length 256 chips) is available in order to enable MMSE based interference mitigation at the user terminal receiver.

6. Synchronous Word (SW) field is available on DL pilot channel to resolve time ambiguity for satellite diversity operation.

7. DPCCH embedded pilot symbols are not required in the DL i.e. reduction of overhead due to the use of common pilot channel.

8. Addition of high penetration* paging channel (HPPCH) to complement the normal pag­

ing channel. This allows paging of users within building where LOS attenuation of up to 20 dB can be encountered and is used only when normal paging fails. Due to the power limitations, only one HPPCH per beam (and per satellite) can be active at any one moment.

9. Different scrambling codes assigned to different satellites in the DL. Different cyclic shift of the same code assigned to adjacent beams of the same satellite. Therefore, dif­

ferent beams of the same satellites are distinguished by the different offset of the same scrambling code. This is the case with single LES per satellite.

10. Reduced power control rate at one command per frame (instead of one command per slot in UTRA). Therefore a slightly different organisation of DPCCCH.

11. Unique word (UW) symbols are inserted in the UL control channel to support extended range ambiguity resolution for satellite diversity operation.

12. A longer preamble sequence with 48 symbols (instead of 16 in UTRA) for the random access channel in the return link. The preamble is spread by a binary code which is randomly selected between a limited set of codes for random access.