Bandwidth with a bonus

The M5730 is part of ST-Ericsson's thin modem family. The modems are wideband CDMA (W-CDMA) mobile broadband devices that do not include multimedia processing hardware. Smartphone makers can thus pair their application processor of choice with the modem chipset. ST-Ericsson's M5730 supports the Evolved High Speed Packet Access (HSPA+) standard that delivers download data rates beyond the 7.2Mbit/s limit of HSPA. The M5730 supports a download data rate of 21Mbit/s (to the device) and 5.76 Mbit/s upstream. According to the GSM Association, there are 66 HSPA+ networks in service in 38 countries, delivering peak data rates of 21Mbit/s or more. Meanwhile, the GSA says 136 operators in 63 countries have committed to deploying HSPA+. However, market research firm Unwired Insight points out that in practice operators may roll out HSPA+ in a small part of their network only. "Operators see the need for mobile broadband is growing rapidly, and one way to increase capacity is to upgrade the network to 21Mbit/s," said Magnus Hansson, senior vice president, LTE and 3G modem solutions at ST-Ericsson. "In many cases this can be done with a software upgrade [of networking equipment] as opposed to the next step, to 42Mbit/s, where operators also need to upgrade hardware." There are three main markets for HSPA+ chipsets: USB dongles for laptops; smartphones; and embedded modules. "So far, we have seen the new access technology coming first in dongles," said Hansson. "The real boom comes when the solution can be integrated within the handset – the dongle market is much smaller than the handset market – and that is now possible with the M5730." Embedded modules, the third market, refers to integrate chipsets within laptops and consumer devices, such as netbooks, tablets and, soon, gaming devices and digital cameras. System vendor Ericsson announced in September what it claims is the first available 21Mbit/s HPSA+ embedded module, based on ST-Ericsson's M570 modem chipset, the HSPA+ predecessor to the company's M5730. The design goals for the M5730 were low power consumption and a smaller footprint to enable handset designs. ST-Ericsson claims it has already achieved unmatched low power consumption with the M570, even though this has been improved upon with the M5730. "For us, the M5730 is more about integration – we have gone from a three chip solution to a two chip one," said Hansson. The two chip M5730 chipset comprises a radio and a baseband ic. The RF5500 cmos radio implements a direct conversion rf architecture for the transmit and receive paths. "The power amplifier, antenna switch and the filters are not part of our components," said Hansson. The received rf signal is thus down converted and the analogue I and Q channels are signal processed and digitised by the RF5500 to produce an output digital bitstream that is passed to the DB5730 baseband via the 3G DigRF digital interface standard. The DB5730 baseband processor includes an ARM risc processor and a dsp, and performs the signal processing to compensate for radio channel imperfections such as fading, and the 64-QAM demodulation that is used for HSPA+. ST-Ericsson has not revealed which dsp core it has licensed and Hansson points out that future designs may be based on ST-Ericsson's own vector processor dsp. To reduce the power consumption, ST-Ericsson explored the main uses of the HSPA+ device and, in particular, the power consumed when data is being received. "There are many cases that are relevant here," said Hansson. "We have focussed on minimising the power consumption for internet browsing, streaming and gaming." For applications requiring continuous downloads, Hansson claims the device consumes 1W, half that of competing devices. "Those are the rough figures and that is a heck of a difference for small handheld devices," he said. Lowering the power consumed is important, not only to prolong the duration between battery charges, but also to minimise the heat generated. That is because other chips such as the application processor and the graphics engine also consume power and heat up the handset. "This can be problematic when watching a high quality video stream, if you sum up all those heat dissipating sources," said Hansson. ST-Ericsson has not revealed what cmos process it uses for the M5730. However, Hansson is adamant that the low power consumption achieved is due to careful architectural and system design, rather than any advanced silicon technology choice. The company analysed typical data consuming applications and these were then segmented in detail to determine how best to partition the system's functionality in hardware and software based on minimising power consumption. This has resulted in ST-Ericsson grouping functionality into what it calls power domains. Using management techniques, it switches off those domains not in use on a millisecond by millisecond basis. Clearly, a longer perspective than millisecond time windows must also be used. During internet browsing, when a data packet arrives from the basestation, parts of the chip are switched on and off in sequence every millisecond. "But there is a second domain to do with how you set up the data call between the handheld and the infrastructure," said Hansson. This requires the bringing up and putting to sleep of data links quickly using new standardised features developed by the 3rd Generation Partnership Project (3GPP). Such features include fast dormancy and Cell-Fach that enable channels to go into sleep modes, yet be awakened quickly. "This [sleep mode] is needed because you don't know when the next packet will arrive in an internet application," said Hansson. "These are not in millisecond, but hundreds of millisecond time resolutions." Such power analysis – how the system is optimised for a single packet, how that packet is processed through the different blocks, and which blocks should be implemented as hardware accelerators and which in software – and the associated development tools were all developed in house. Given the modem is around 1W, how does that influence overall handset power consumption? When sending a data stream, such as transmitting an image or a video clip, the majority of the power consumption can be due to the handset's power amplifier. "But we think the dominant case is when you download data – browsing or streaming music and video – and that is the modem and does not involve the power amplifier," said Hansson. "Then, if you have a multimedia application such as video decoding, that can consume power at the same level as the modem." In this case, the modem accounts for at most half of the total power consumed. Otherwise, it is the modem that dominates the handset's power consumption. The M5730 is already being designed into multiple customers' products, says Hansson, and several tens of thousands of chipset samples have been delivered. Meanwhile more products using the M5730 will be announced by customers before the year end.