Mother Nature still in charge.
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With GPRS now proven to be only slightly better than HSCDS, a path was cleared for a third combatant to step into the 2.5G ring, EDGE (Enhanced Data Rates for GSM Evolution). EDGE is a higher data rate, logical extension of GPRS whose claim to fame is its supposed ability to deliver maximum user data rates in excess of 384 kbps.
This feat is accomplished by changing the modulation format to pack more bits of information into each slice of frequency spectrum; theoretically retaining full system capacity while increasing achievable data rates. If by now you are somewhat wary about these much higher numbers, then good, you are learning fast, because the fly in the EDGE ointment is error rate correction, or maintaining acceptable quality of service (QoS).
As with the other competing systems, to get the fastest possible EDGE connection you have to live on top of the cell tower and be the only one on the system. The higher modulation levels of EDGE (a TDMA system) require increased signal strength at the receiver than GPRS. Move away from the base of the cell tower, and EDGE's link quality control protocol will reduce the data rate to maintain a continually acceptable QoS, with co-channel interference being the primary rate-killing culprit. Long range, overcrowded cells, and/or lots of co-channel interference will quickly put paid to those 384 kbps you are paying a premium for.
However, EDGE's link-quality control is significantly better than that offered by GPRS, as indicated by Ericsson simulations. According to Ericsson, even when measured at a comparable GPRS signal strength, EDGE delivers twice the data rate performance per slot. But as EDGE uses higher signal strengths than GPRS, data rates will keep climbing after the weaker GPRS signal poops out. Using the real world BT Cellnet GPRS experience as a guide, this probably means an average single slot EDGE data user will see about 45-60 kbps performance, and maybe a little higher in some networks. In comparison to GPRS, EDGE does seem to enjoy an edge. However, even with this improvement, performance is just about comparable to analog dial-up, so the broadband beef is still out there somewhere roaming the high frequency range. In GSM-based systems, an upgrade to EDGE will involve mainly software upgrades and estimates for upgrading to the system are in the range of $12,000 per base station controller, which are still reasonable.
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vxm.com
Interesting paper but they still are quoting the original chip rate, the one that was cut since Mother Nature would not let it meet the bandwidth limits by uning infinitely steep roll off.
An old interesting Ericsson paper, good for a giggle but no longer on the web site. Seems to be the source of many of the CDMA comments on this thread.
Comparison of
D-AMPS 1900 & IS-95 CDMA
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Conclusion
In summary, four key factors become evident when evaluating and comparing D-AMPS 1900 and IS-95 CDMA systems:
Coverage: D-AMPS offers substantially better coverage than IS-95 CDMA.
Cost: The D-AMPS infrastructure costs up to 50% less than IS-95 CDMA.
Capacity: Using an 8 kbps full-rate speech coder, the basic D-AMPS technology offers more than 6 times the capacity of AMPS.
And with hierarchical cell structures capacity is virtually unlimited.
Quality: The D-AMPS speech quality outperforms IS-95 CDMA under all conditions.
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Another interesting article giving some of the limitations of GPRS and EDGE. What is this Abis stuff?
mobileapplicationsinitiative.com
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2.4 TECHNICAL LIMITATIONS TO THE THEORETICAL CAPACITY
Although the system is awaited with high expectations from manufacturers and operators, the actual take-up of GPRS usage among subscribers is still an open issue. It is still somewhat unclear what the total costs for the customer will add up to in terms of terminals and network usage. Furthermore, it has been difficult to estimate the capacity that will be available to the users, due to the inherent limitations in networks and terminals. For this reason, much of the current press-coverage on the coming GPRS-system is presented inaccurately [15], leaving the potential users with expectations that will not be fulfilled. For instance, the bandwidth that will be available to the user will actually remain modest in size and vary significantly, depending on the time of day, the total number of users and the current geographical location.
As explained in the previous section, the maximum theoretical data rates of 171.2kbps require an optimal coding scheme (CS-4). As such, the maximum speeds must be checked against the actual constraints in the network and terminals. The reality is that mobile networks are always likely to have lower transmissions speeds than fixed networks.
The increased data rates of GPRS are achieved as a result of two major aspects of the GPRS-system: improved coding schemes and the support of multiple timeslots. However, three main aspects prevent a user from ever achieving the maximum theoretical speed, namely the allocation of timeslots, restrictions in the terminals, as well as the actual
availability of coding schemes.
2.4.1 ALLOCATION OF TIMESLOTS
Because GPRS and GSM use the same radio resource, it is unlikely that a network operator would ever assign all eight timeslots to GPRS-traffic, since voice still will be a dominant service. In fact, how to allocate the timeslots to GPRS and GSM is supposedly an open issue among the operators. It seems clear, however, that GSM-traffic in all cases will have precedence over GPRS-traffic. One possible scenario is that one or maybe two times lots will be statically assigned to GPRS as illustrated in Figure 3, whereas the remaining timeslots will be assigned dynamically between GPRS and GSM.
However, the most likely and simple solution is to avoid dedication of timeslots altogether. Since GSM-traffic has precedence, GPRS-traffic will be offered a varying amount of capacity. The available timeslots will in turn be divided between all the GPRS-users on the carrier at the given time. It should also be noted that among the carriers of one basestation there will always be at least one signalling channel (mapped to the same amount of timeslots ). The number of signalling channels depend on the number of carriers as well as the particular network environment.
2.4.2 RESTRICTIONS IN TERMINALS
To take advantage of higher data transmission speeds the GPRS-terminals will have to support several multiple timeslots simultaneously. In fact, in able to send and receive the theoretical maximum of 171.2 kbps the terminal must incorporate transmission and reception of 8 timeslots (in both the downlink and the uplink). This requires considerable amounts of processing and transceiver power in the terminal, adding great complexity to such a small device.
In reality, terminal manufacturers are indicating that they will support a limited number of multislot classes, at least in the first stage of GPRS-terminal evolution. According to representatives from the manufacturers, the terminals will initially support 1 timeslot in the uplink and 3 timeslots in the downlink. Quite soon the terminals will increase their capabilities to 2 timeslots in the uplink and 4 times lots in the downlink. Whether the evolution continues to improve further is not clear, but it is supposedly difficult to produce terminals that incorporate more than 4 timeslots in either direction.
2.4.3 A v AILABILITY OF CODING SCHEMES
All coding schemes are mandatory in the GPRS-terminals, whereas only one out the four coding schemes is mandatory in the network itself. As network implementation of coding schemes is currently in progress among infrastructure manufacturers, it is evident that it will become very expensive to incorporate CS-3 and CS-4 in the networks. This is mainly due to the necessary upgrades on the Abis-link between the Base Transceiver Stations (BTS) and the Base Station Controllers (BSC).
In essence, there exists an economical trade-off for the operators between the immediate expenditure of upgrading these links, compared to the cost of introducing a significantly improved modulation technique for Enhanced Data for GSM Evolution (EDGE) at a later stage. In other words, the decision to be made is whether to enable CS-3 and CS-4 capabilities at the first stage, or waiting for EDGE and then improving the link to cope with both cases. Another aspect related to this is that some operators may choose to focus entirely on UMTS and thus leave out the EDGE-upgrade. Although it is not absolutely certain, it seems likely that GPRS coding schemes 3 and 4 actually will be left out of the implementation due to the manufacturing complexity and the immense initial cost for the operators to cater for the possible throughput on the Abis-link. This means that the maximum available capacity will limit itself to multiples of 13.4 kbps (CS-2 on the radio-layer). The Abis-matter is discussed in further detail in a later section.
So, if there are enough consecutive timeslots available on one carrier, the maximum data transmission rate in practice restricts itself to 4 timeslots at 13.4 kbps, or 53.6 kbps. However, this rate exists at the radio-level and therefore includes overhead from the higher protocol-layers. The actual rates at application-level are approximated in the model at the end of the thesis. Also, it must be perfectly clear that the available data-rates will be further restricted due to other factors that will be explained in later sections. Such factors include the amount of retransmissions, the Quality of Service (QoS), the level of compression, the applications being used and most importantly, the number of active GPRS connections at the given time. This thesis will not estimate an average transmission rate, but as a reference, figures from a reliable source in the telecommunication industry indicate a typical rate of around 30kbps at radio level [ 17]. It should further be noted that even if this thesis reflects the current status among the manufacturers, there is always the possibility that CS-3 and CS-4, as well as terminals with improved multislot capabilities, can be available in the unforeseeable future.
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3.2.4 EDGE
Enhanced data rates for GSM and TDMA Evolution (EDGE) is an enhancement of the air interface of both GSM- and the TDMA-systems that will enable data rates of up to theoretically 59kbps per timeslot. This is accomplished by the use of higher level modulation in combination with improved link control mechanisms. As with GPRS (and UMTS), the bitrates will depend on the radio conditions. Some EDGE-users will only experience GPRS-like bitrates but the majority will have a quite significant improvement. In fact, bitrates like 48kbps (i.e. 384kbps per carrier) will be possible in selected areas, also in practice. However, the actual throughput offered to the end-users will depend on the possible coding scheme (MSC1 to MSC9) as well as on the capabilities of the EDGE-terminals. Although it is difficult to speculate on any actual data transmission speeds for EDGE at this point, it is clear that EDGE will make it possible to extend the advantages of GPRS, i.e. fast setup, higher data rates and sharing of radio-resources. All in all, this will improve the utilization and the spectrum efficiency of the network.
While GSM uses a conventional modulation scheme called Gaussian Minimum Shift Keying (GMSK), EDGE will use a technique labelled eight-phase-shift-keying (8 PSK) in combination with GMSK. Although an EDGE introduction is very straightforward from an architectural point of view, it will be more expensive for the mobile network operators than the upgrade of both HSCSD and GPRS since it requires a completely new modulation scheme and improved link control mechanisms [19]. A major hardware upgrade is required to the radio portion of the network since each base station needs an EGDE transceiver. And as mentioned previously, EDGE also requires a costly upgrade of the Abis-link between the Base Transceiver Station (BTS) and the Base Station Controller (BSC).
Although compatibility with GSM is one of the attractions of the EDGE-technology, it requires high quality radio conditions to reach maximum speeds. Some GSM-networks may not be suitable without additional infrastructure buildout to the cellstructure, which in turn makes a transition to EDGE less economically viable to an operator. It is quite obvious that EDGE will be implemented by those operators that are unsuccessful in bidding for 3G-licences (UMTS), but wish to offer similar services, perhaps at lower costs. However, at this point it seems like all incumbent operators will receive a 3G-license. EDGE may also be used as a migration path towards 3G but this is seen as less likely since both standards will be available in roughly the same time-frame. GPRS-operators with 3G-licenses are expected to go straight to UMTS-standard depending on license terms and conditions. A third possibility for EDGE is to use it to fill the service gap between the gradual deployment of UMTS and the available GPRS-standard. It is expected that many European operators will choose to implement EDGE as a complement to UMTS for this purpose.
Understanding what EDGE is all about is however complicated by the fact that EDGE is applicable to two standards (both GSM and TDMA) which are dominant in multiple markets. Specifically, since North America has a different spectrum availability situation, EDGE will actually function as 3G in this market. This is because operators in North America do not have any available spectrum to use for 3G but have to resort to spectrum already used by the 2G-standards.
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