Cellular network operators are investing heavily in next-generation infrastructures with the goal of enticing subscribers into a mobile multimedia environment. To capitalize on their investment, operators are counting on a significant boost in average revenue per user (ARPU) from increased access of audio-visual content through high-speed cellular networks. However, carriers know that they can't rely solely on their own high-speed infrastructures to ensure a satisfying wireless multimedia user experience. That's why they're driving OEMs to develop high-speed feature-rich handsets, with a push towards small form-factor designs.

Addressing the demand for higher throughput, the 3rd Generation Partnership Project (3GPP) introduced high-speed downlink packet access (HSDPA) in Release 5. Otherwise known as 3.5G cellular technology, HSDPA is an enhancement of wideband code division multiple access (WCDMA). This new feature is backwards compatible with WCDMA, requires no new spectrum to roll out in the network, and promises as much as a sevenfold increase in peak data rates in the downlink direction.

As most experienced wireless users know, data throughput achieved in the field usually falls far short of the advertised peak rates. This is even the case with HSDPA-enabled terminals. To realize HSDPA peak rates under actual radio conditions, these cellular devices must utilize more advanced radio solutions. In particular, the application of receive diversity can more than double the effective HSDPA throughput across the entire cell and significantly improve network efficiency and capacity. This performance improvement results in a more robust end-user experience while web browsing, downloading files, and streaming media.

Enabling wireless broadband
HSDPA applies a shared-channel transmission concept, which makes more efficient use of available channelization codes and power resources than a dedicated channel—as used in standard WCDMA networks. This shared channel is defined in WCDMA 3GPP Release 5 as the high-speed downlink shared channel (HS-DSCH). The increased efficiency of code and power use in the HS-DSCH boosts cell capacity by more than twice that of a dedicated channel, thus paving the way for higher data rates.

HSDPA networks further increase system capacity by implementing short transmission time intervals (TTI) with fast scheduling techniques. The basestation uses these efficient scheduling methods to monitor the link quality to each mobile user on the network and prioritizes data traffic accordingly. As a result, network latency is reduced by a factor of up to 20.

HSDPA networks also dynamically and quickly adapt channel-coding and modulation depending on radio conditions. In clean radio environments, HSPDA systems use 16-bit quadrature amplitude modulation (16-QAM) to increase throughput. Under adverse radio conditions, HSDPA networks dynamically switch to quadrature phase shift keying (QPSK) modulation. Standard WCDMA transmission, on the other hand, is limited to QPSK modulation only. WCDMA networks must therefore employ power-control techniques to compensate for changes in radio conditions. Because HSDPA networks don't rely solely on power control to optimize gain, they're more power efficient than WCDMA networks. This improvement in spectral efficiency can yield an increase in system capacity of up to 300%.

These performance enhancements enable HSDPA networks to achieve a significant throughput advantage over WCDMA networks. In Category 10 operation, HSDPA systems can theoretically reach throughput levels up to 14.4 Mbits/s. HSDPA data rates are characteristic of those achieved in IEEE 802.11 networks, which have proven sufficient to support such services as Web browsing, file server access, and even streaming audio and video (Fig. 1).

1. Each new generation of technology brings higher throughputs and a new set of applications.

There's a price to be paid for higher data rates. The typical constellation for 16-QAM has a smaller symbol decision area compared to QPSK. As a result, the received 16-QAM signal is more susceptible to spectral impairments, which translates into a tighter error vector magnitude (EVM) requirement for HSDPA receivers.

The EVM performance of the RF transceiver is adversely affected by four primary factors: phase and magnitude response of the receive filter; phase error due to the frequency synthesizer; I/Q mismatch; and dc offset (in direct conversion receivers).

The application of programmable digital filtering in the transceiver can considerably improve the receiver's phase and magnitude response. Implementing digital equalization techniques in the transceiver can equalize the phase error. Digital calibration methods can also be applied to correct I/Q mismatch and dc offsets. Ensuring that the dynamic range of the analog-to-digital converters (ADCs) is optimized will also help mitigate EVM degradation. These digital-centric design enhancements are best implemented using nanometer CMOS technology, which provides the greatest integration, performance, and cost benefits.

Making such improvements to RF transceiver design is a must for better EVM performance. However, a more robust RF subsystem design is required to mitigate the effects of multi-path interference and fading conditions.

Receive diversity
In high-density mobile environments, such as in cities and other urban areas, the mobile terminal is often subject to multi-path interference. In such cases, the received signal contains multiple noisy time-delayed copies of the desired signal (Fig. 2). This type of interference can cause deep fading and even nulls at the receiver. Under such adverse radio conditions, data throughput and network efficiency can be greatly compromised.

2. Time-delayed copies of a signal are often received in noisy environments.

To reduce the degradation of signal integrity resulting from multi-path interference, receive diversity must be incorporated into the mobile device's RF subsystem. Diversity operation mitigates deep fades by enabling receivers to concurrently receive and process independent RF signals from two distinct antennas to maximize signal quality and reception. Receive diversity results in fewer fades in the combined signal, thus allowing the decoder in the baseband processor to perform better. The result is improved quality of service (QoS) throughout the entire cell and a boost in data rates of more than twice that of single-antenna designs.

Receive diversity reduces basestation power requirements because less power needs to be transmitted to maintain a high-quality link between the basestation and the handset. With receive diversity, the mobile device can "see" and process two signals instead of one, reducing the likelihood of the basestation having to transmit more power to contend with poor signal quality. This means that the cell coverage for existing subscribers can be extended and the saved system resources can be allocated to new subscribers. Simulation results demonstrate the advantage of receive diversity throughout the cell for both QPSK and 16-QAM in a Category 6 HSDPA network (Fig. 3). At the middle of the cell, receive diversity improves throughput by more than twice that of non-diversity receivers.

3. The advantage of receive diversity becomes obvious through simulation.