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The frame size of all the proposals varies between 10 ms to 48 ms. In the SW-CDMA proposal, 10 ms is chosen for the frame size. This is desirable because it is also the frame duration chosen by the ETSI UTRA proposal, which has finer granularity in terms of bit rates and interleaving. For real time services, interleaving is limited to at most 20 ms. Hence interleaving cannot be spanned over many frames due to time constraints. The super frame length (600 ms) of SW-C/TDMA is different from its terrestrial counterpart, is (720 ms) however, it is a integral multiple of the GSM super frame which is 120 ms. SAT-CDMA (S .Korea) has a similar proposal to that of the ESA approach in terms of framing aspects (10 ms duration). ICO and Iridium considers only intra-burst interleaving which gives almost no additional delay due to interleaving. However, performance with interleaving is limited due to the small interleaving depths allowable. Being a GEO system. Horizons Considers a frame duration of 48 ms with 18 time slots per frame. Since it is a TDMA system, guard timing requirements are more crucial than CDMA based systems. Therefore a longer frame length is considered in order to obtain a higher efficiency in terms of overhead.

2.5. Brief comparison of IM T 2000 satellite systems proposals 41

Power control

In the W-C/TDMA proposal, only closed loop power control is considered. If a LEO constel­

lation is used, only one power control command per frame is proposed to be sent. Although, open loop power control could be useful to overcome slow shadowing caused by trees etc., it is not always suitable to mitigate multipath fading since sufficient fading correlation between DL and UL cannot be guaranteed at all times. ICO also proposes closed loop power control with two power control cycles per second to control a dynamic range of 16 dB with the step size of 1 dB. SW-CDMA utilises multilevel power control (2 bits) with the step size of 0.25-1 dB to control a dynamic range of 20 dB. The rate at which the power control commands are sent is 50 to 100 Hz. Iridium has a step size of 2 dB for TDMA and 0.5 dB for CDMA with 2 to 20 Hz for a dynamic range of more than 16 dB.

SAT-CDMA uses open loop power control for initial power setting. The signal-to-interference ratio (SIR) based closed loop power control scheme with a variable step size (0.25 & 1 dB) with 2 bit power control information is used to adapt to the propagation environment. It was found that the variable step size approach is much more effective than a single step size [54].

Hand over

In W-C/TDMA, although, soft HO is considered, fast hard HO (as soon as the signal quality falls below a threshold, HO is performed and connection to the current satellite is terminated) strategy is proposed. Fast hard HO is also advantageous in terms of channel resource require­

ments since it does not require two links simultaneously (unlike soft HO). Moreover, it is more suitable for vehicular terminals operating at higher mobile speeds since the overlapping area between beams may occur only for a short duration. However, this HO strategy may not be suitable for a MT operating solely in F/TDD mode due to its inability to provide large enough peak power to support two links simultaneously. Sharing the time resources between two links may be a good solution for such a case. Only hard inter-frequency HO is supported by SW- CDMA. This HO can be either inter-gateway or intra-gateway. Inter/Intra-gateway HO can be initiated by the FES and two carriers may either belong to the same gateway or two different gateways. For beam HO and inter-satellite HO, soft HO strategy is supported. Although ICO is not based on CDMA, a soft HO strategy is proposed with the HO decision made by the

MT. However, it has a time gap of 80 ms (2 frame duration). Implementation of soft HO for a TDMA based system is quite difficult, as it requires precise timing control. Iridium supports both soft and hard HO strategies initiated by the network with the assistance of the MT.

LEO systems such as SAT-CDMA have the drawback of increased orbital speed requiring fre­

quent inter-satellite HOs. ISLs are very useful to avoid call dropping yet they increase complex­

ity. An efficient adaptable routing algorithm, which is capable of handling the traffic without much overhead, plays an important role as far as the network is concerned. SAT-CDMA will support mobile-assisted network-dedicated HO. In addition to inter-beam and inter-satellite HO, it also supports inter-FES HO.

Synchronisation

In ESA proposals, preambles (without any dedicated channel) are used to achieve the syn­

chronisation in the UL. However in the DL, two dedicated channels (broadcast on every radio frequency (RF) channel) are used for this purpose to reduce complexity and delay.

In W-C/TDMA, the system time and frequency is assumed to be virtually located in the satel­

lite. In the feeder UL (fixed earth station (FES) to satellite) of a transparent transponder, the base station (BS) offsets its transmit times and frequencies whereas in the service UL (MT to satellite) the BS controls timing of the MT such that the signal arrives at the satellite in syn­

chronism (quasi-synchronism in service UL) with the system time and frequency. However, this requires variable delay units, clocks and oscillators in the BS in addition to a knowledge of the exact position of the satellite and the velocity of the satellite. The same randomisation code is used in all of the beams of the same satellite by offsetting the transmission by a cer­

tain number of chips among different beams. The BS controls this procedure. System wide synchronisation is assumed (in the order of ms) among all satellites of the same satellite radio access network (SRAN) either with ISLs or via a terrestrial network (for a transparent payload).

It should be noted that ISLs are considered only for non-GEO constellations

Compared to CDMA based systems, TDMA systems require more precise timing and syn­

chronisation procedures. For example, ICO being a FDMA/TDMA system, requires a timing accuracy of 1 ms.

2.5. Brief comparison o f IM T 2000 satellite systems proposals 43