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Conquer ULC handset design challenges

Posted: 16 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:ULC market? ultralow-cost handset? design challenge? 3G?

By Sanjay Noronha
NXP Semiconductors

Ultralow-cost (ULC) handsets have been a hot topic in the past three years. ULC today refers to handsets that can be built for $20 or less. They represent a unique segment of the cellular market and require a unique set of strategies for success.

The typical buyers of ULC handsets have limited average disposable income. Furthermore, many of these buyers are difficult to track due to a lack of the necessary infrastructure and systems required. It therefore does not make practical or economic sense for operators to significantly subsidize a phone in the hope that they will recover the revenue in service fees. Operators are further reluctant to adopt subsidy models since the majority of revenue they make in the ULC segment is from relatively inexpensive pure voice and SMS services rather than from higher average revenue per user (ARPU) data services.

The typical ULC handset customer then has to save up enough money to buy the phone. Despite the 'low-cost' nature of the ULC market, it is absolutely not acceptable to have a poorly performing phone. Since the user will tend not to replace his or her handset often, there is the preference to have 'as much as possible' within the acceptable price range.

Now let's look at ULC handset requirements from the operator's point of view. Revenues and profits are driven by two key factors: the number of customers and how much the customers use their phones. In a competitive market, little prevents a user from switching providers. They simply obtain a new SIM card and swap it into their phone. Therefore, it's critical for an operator to offer the best possible service and experience to the user. Better service means that the network should support as many simultaneous users as possible without adverse side effects such as dropped calls. It also means good network coverage so that the user will not be prevented from using the phone wherever needed. These aspects of better service are largely network-centric. There is much that can be done on the handset side as well to facilitate better service.

Practical choice: ULC handsets
At the core of each handset is the cellular chipset employed. The choice of cellular chipset, therefore, is key in having a winning product in the ULC market. There are four key drivers essential to creating a winning ULC product:

Radio Performance
In emerging markets, the 2.5G (GSM/GPRS/Edge) cellular infrastructure is often bought from Western nations who are migrating to next-generation technologies such as 3G. For ULC markets, this leads to an interesting deployment of infrastructure manufactured by a variety of vendors, who each implement the GSM specification slightly differently. Interoperability testing (IOT), therefore, is critical to ensure that the handset can seamlessly switch between base stations that are manufactured by different vendors. IOT requires the radio transceiver to be robust and flexible in its handling of the various infrastructure flavors. IOT also requires that the entire system including the software protocol stack is robust and tested for its ability to handle subtle timing variations between the different infrastructure implementations.

Emerging market networks suffer from problems ranging from oversubscribed networks in urban areas (thanks to limited spectrum allocated by regulatory authorities) to sparse coverage in rural areas. To combat the issue of too many users in urban areas, each handset must perform within specification on the network and not generate interference while transmitting which could affect other phones on the network that are receiving.

In rural areas, due to sparse network coverage, the sensitivity of the handset, i.e. its ability to pick up signals and function well in areas with weak network coverage is critical. Due to far from perfect network planning, the radio performance needs to be guaranteed in weak and strong field environments that are continuously varying. The sensitivity of the handset needs to be ensured across every channel that could be allocated by the network to the phone.

In addition, due to the technical challenges of designing a radio that is able to perform well on every single possible radio channel, the specification grants many exceptions to cellular chipset providers. Therefore, most chipset suppliers only guarantee robust performance on the low, middle and high ends of the frequency band. These stringent performance requirements need to be consistent even across temperature and voltage.

'Across temperature' means that in ULC markets, which tend to fall into some of the world's hotter tropical areas, even as the ambient temperature rises, the radio should perform as well as in ideal conditions (typically 25C). 'Across voltage' means that as the phone's battery discharges the radio performance should be not be compromised. At the very minimum, a complete GCF (Global Certification Forum) compliant chipset must be insisted upon where all tests including the RF tests have been run and passed.

Power consumption
Good power consumption performance (i.e. the ability to have the longest standby and talk times without recharging) is of enormous importance. We have explained the need for stringent radio performance already even as the battery discharges (i.e. available voltage falls). This need is further amplified in ULC markets, particularly in rural areas where the existence of a good and always available electricity source cannot be guaranteed. But there are other benefits of good power consumption metrics as well. A smaller battery with fewer mAh can be utilized in the phone. This allows for smaller phone form factors to drive stylish designs, and also, a smaller battery drives down the total cost of the phone.

Instead of a staid feature set comprising of just voice, SMS and a black and white screen, if a ULC phone were to add a color screen, FM tuner, music playback (MP3, etc.) and even data services like WAP and MMS, then, all of a sudden, ultralow-cost doesn't have to be ultra-boring.

From an operator point of view, additional features present the possibility of additional ARPU. Aggressive cellular service subsidy models are being experimented with, where operators lock the phone solely to their network and then provide the phone to subscribers at either a discount or even for free, and then realize revenue through the ARPU. With this business model, it is imperative that the phone cannot be hacked or the business model fails. Of course, the operator doesn't make any money if the phones are compromised, but more importantly, the ULC consumers will be unable to avail of yet another innovative way to obtain a handset and benefit from its use. So it is extremely important that the cellular chipset solution offers a robust and flexible hardware-based security mechanism so that operators have the freedom to implement innovative business models. There are challenges in adding and integrating all these features and yet maintaining the stringent cost and performance requirements of the phones in this segment. This leads us into our final pointtotal cost.

Total cost
The confluence of high mixed-signal integration with low-cost semiconductor processes has ushered in a new era of available 'single-chip' cellular chipset solutions where all the major subsystems of a phone, except for the power amplifier and non-volatile memory have been integrated on a single CMOS die. Also, with the same CMOS integration techniques, it has been possible to even integrate a large proportion of the erstwhile discrete electronic components that used to be pervasive in mobile phone designs. The result of these advances in IC technology is a reduced cost structure for the single-chip solutions and the reality of the true sub-$20 ULC handset.

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