WiMax technology with its considerable claims is pitched against HSPA – an upgrade on existing 3G CDMA based infrastructure. This has resulted in creation of two camps, each one trying to outdo the other with claims and counterclaims. For the policy makers and regulators, not to speak of the users, such a debate creates a clouding up of the environment and unless carefully sifted, all the information floated around could lead to wrong decisions. This article attempts a dispassionate examination of the two technologies and their claims based on information available in the literature.
THE REAL KEY
The real key to the growth of the capabilities of WiMax has been the use of OFDM and MIMO technologies. Broadband wireless access (BWA) is characterized by a variety of applications, several of which are frequency hogging in nature. A key element in the implementation of such applications is the spectral efficiency of the multiplexing and multiple access techniques. Orthogonal frequency division multiplexing (OFDM) is known to be spectrally efficient under harsh propagation conditions. Sending the message at a slow rate so that the fast fading time duration is smaller than the bit duration minimizes multi-path propagation problems. To achieve high bit rate, the frequency carrier is sub-divided into a large number of sub-carriers and these are later on recombined at the receiver to recover the high bit rate signal.
Distortions or corruption of signal are of a limited nature, which can be corrected by using error correction techniques. For providing access to multiple customers, each customer can be allocated different sub-channels. The signals for each customer can be combined at the receiver to get high bit rates. This arrangement of assigning different sub-carriers to different customers and delivering the combined signal of different sub-carriers to individual customers constitutes OFDMA. These alone are however not adequate for cost effective BWA with mobility. Multi-input and multi-output (MIMO) based antenna systems have been successfully used to mitigate this problem and, in fact, their use has become an essential requirement for realizing the type of spectral efficiencies demanded of such systems.
TECH TECHNIQUES
As a consequence of the use of these techniques, WiMax achieves impressive results. However, WiMax is not the only technology which uses these techniques. Other technologies, which constitute the growth path for 3G systems towards 4G, also use or propose to use these or some of these techniques. The identified approach for long term evolution (LTE) version of the 3G technology is also based on these techniques. The difference possibly is in terms of time frame for the availability of these ultra broadband solutions. In the mean time, the results reported for CDMA based 3G upgrades through software, viz HSPA (high speed packet access) are also impressive. A major advantage that HSPA possesses despite somewhat lower speeds is that both systems and terminals are already available, ie the performance achieved has already gone through the rigors of development and commercialization while for WiMax it is still a promise. In particular, the full mobility and soft hand-off in WiMax are yet to reach levels which can be considered commercially significant. This is an important consideration for an operator who has to choose between a system available now and get a first mover advantage in the BWA arena and a system which may become available two years down the line but possibly with higher data speeds.
What is equally important is that high throughput has to be available over large distances. A variety of figures have been quoted in the literature for WiMax capabilities. The claims of 70Mbps for 70 kms by WiMax proponents are coming up for questioning. It is not that both the parameters are being achieved simultaneously, ie 70Mbps data rate and distance of 70 kms. Interference and noise do not permit this at least under the types of transmitted powers which are being quoted for WiMax systems. In a multiple user environment, individual user rates are necessarily much lower.
COST ADVANTAGES
WiMax proponents claim cost advantages due to the use of the approach of standards based equipment, unlicensed frequency bands and ability to have larger coverage. Lower equipment costs are feasible considering that WiMax standards are being adopted from the word go leading to the availability of multi vendor supplies. Off-the-shelf components are also being used. However, this advantage is of a limited nature since equipment costs constitute only 15-20% of the total installed system costs. The main cost contributor is the infrastructure. Use of unlicensed bands necessarily limits the coverage because high transmit power is not possible due to interference considerations with a large and uncontrolled number of users of other equipments such as microwave ovens, etc in such bands. These equipments could also interfere into WiMax systems. Besides, large coverage is also not demonstrated in high bit rate situations. A small range per BTS implies the need to use a larger number of base stations, which in turn implies a much greater cost of infrastructure. In locations where 3G services have already been provided, HSPA upgrade may offer cheaper BWA solution.
COST CONSIDERATION
The cost for an operator to introduce a new technology is important but its acceptability by the users is equally essential. For this reason the entry cost, which includes the cost of hand held device, is important. In the first place, the generally held view is that hand held WiMax devices are not likely to be available before 2008-09. Secondly, as a newcomer the initial numbers will be quite small and the amortization of R&D costs will make these devices expensive despite advanced chips being made available by a number of suppliers. On the other hand HSPA hand held devices are already available and therefore enjoy quantity advantage. While choosing a technology, there is another equally important consideration for the operators with regard to the customer devices particularly for the higher end and multiple service using customers.
Convergence in devices is progressing rapidly. It implies that hand held devices such as mobile phones are growing in intelligence and storage capacities to the extent that they are able to mimic laptops besides the usual complement of high quality video, high pixel camera, music, etc. There is now a clear trend to provide built-in interfaces in the personal devices while the shrinking size of the devices makes it hard to incorporate all protocols. The growth of interest in nomadic and mobile use of the Internet is evident from the fact that WiFi interface is now a near standard in laptops and most high end mobiles. It can be anticipated that this interest will spread to broadband also. The question before the operators will be whether the laptop as terminals will adequately meet the requirements of the customers or the demand will be strongly for full mobility with hand held mobile terminals. An operator's perception of the nature of demand at this point of time will determine the acceptability of WiMax or HSPA systems.
GEOGRAPHICAL SPREAD
HSPA proponents claim that since worldwide 3G systems have been installed in fairly large numbers, their infrastructure cost will be much less. This may be the case in several developed countries. It is interesting to examine what the situation in India will be with neither 3G systems installed nor the infrastructure for even 2G systems having achieved wide geographical spread in the rural areas. It has been pointed out time and again that the demand in the rural areas of our country is more data centric than voice centric even though data services alone do not produce a business case. Since a large number of future customers of mobile services have to come from non urban areas, a mobile service which can cater to both voice and data services at a reasonably low cost is required.
OPTIONS AVAILABLE
The time lead that HSPA enjoys over WiMax could be an important factor here since the provisioning of services to rural areas of India cannot wait and the operators are already going ahead with plans to provide 2G/2.5G type services in these areas, ie these operators will have the physical infrastructure in position. If they find that a business case can be built up for providing data services, they are not likely to wait and will proceed with the use of HSPA. This is of course subject to the government allowing the 3G services and making necessary spectrum available well in time. So far as the urban areas are concerned, there are several options available. These include fixed WiMax (802.16d), ADSL 2+, Optic fibre links, etc. However, if the demand in urban areas for BWA along with mobility develops in a year or so, operators will have to take a call between WiMax (802.16e) and HSPA.
PRACTICAL FACT
A practical fact of importance is that by the time these systems are introduced, a huge infrastructure of legacy network for access will already exist in most countries. The newer systems will necessarily be under pressure for providing backward compatibility otherwise the service offered by them will become somewhat less attractive because of it being an island like service forcing users to carry two sets of personal devices. Backward compatibility will be very important and for a country like India, it will have to extend right up to 2G systems. On this count it is believed that time frame wise, HSPA solution may score over WiMax.
SUBSTANTIAL IMPACT
Finally, policies and regulations could have a substantial impact on the choice of a given technology. As discussed above, for most situations (barring fixed operations) in the immediate time frame HSPA seems to be having an edge. The manner and the time frame in which LTE evolves and how WiMax developments proceed will determine the relative future standing of these two technologies. The correct policy approach would appear to be the enabling approach, ie policies and regulations should be technology agnostic providing opportunities for both to achieve their maximum potential. Harmonized frequency spectrum should be made available to the maximum extent. Thereafter, market will be able to choose the appropriate technology for a given application and geographic area.
