Multiple Input/Multiple Output on HSPA

Multiple Input/Multiple Output (MIMO) is a new layer one RF technique that exploits multipath propagation to increase throughput or reduce bit error rates rather than attempting to eliminate effects of multipath propagation. The data transmission is divided in multiple streams, transmitted and received via multiple antennas, polarizations or antenna modes, and recombined at the receiver end to increase overall performance.

Using multiple transmit or receive antennas is obviously not new and has been deployed since a very long time for various reasons, as Mattias Wennström points out.

• Increased antenna gain (array gain), which improves base station range/coverage
• Interference suppression using the spatial dimension, which improves system capacity (since the distance between base stations using the same time/frequency channel can be reduced)
• Reduced channel fading due to multipath propagation (spatial diversity), which improves link capacity (since data throughput can be increased or transmission power can be decreased)
• Transmitter localization in positioning services

The difference with MIMO is that it is an adaptive technique. Adaptive techniques to fight sources of interference and increase spectral efficiency exist in time, frequency, power and space domain on the physical layer of a communication channel. Adaptive techniques exist also on higher layers (e.g. scheduling). MIMO is an adaptive utilization of the layer one space domain by means of deploying multiple antennas at both sender and receiver side of a radio communication, together with algorithms that can adapt to the changing multivariate channel by selecting the optimal antenna subset based on channel state information. In other words, symbols are transmitted/received on the antenna with the best radio conditions at each particular time. Technology in Capsules explains:

The adaptive transmission is possible only if the transmitter knows the channel coefficients in advance, so that the more data can be sent through good channels. In case of time division duplex channels, this requires the channel to be stationary and hence channel details need to be fed back at the same rate as the channel characteristics are changing. In case of the frequency division duplex channels, the coefficients should be transmitted at a different frequency. To overcome the fast feedback requirement, the spatial mean of channel coefficients has been proposed; instead of the instantaneous values.

MIMO is part of the 3GPP standard for WCDMA, as well as IEEE’s 802.11n (said to offer up to eight times coverage and about six times data rates of 802.11g) and the latest draft of 802.16e (WiMax).

MIMO in WCDMA is deployed with 1, 2 or 4 antennas at UE and Node B side to improve system capacity and spectral efficiency by increasing the data throughput in the downlink within the existing 5MHz carrier.
It is being studied by 3GPP TSG WG1 (Radio Layer 1) for the 3GPP Release 7 specifications. Many proposals (for FDD/HSPA) have been assessed, including proposals from Lucent (PARC), Nortel (MPD), Mitsubishi (DSTTD), Samsung (PU2RC/CD-SIC with TPRC), Nokia (Closed Loop MIMO with 4Tx and 2Rx), LGE (Double TxAA), Ericsson (S-PARC), and more. The result of the work is documented in TR 25.876.

In RAN1#44 (Denver), a number of contributions have been discussed during the main session of the WG. There were two MIMO proposals each supported by a number of companies for the final evaluation, with no consensus yet for a single proposal to enter the evaluation phase.

1. PARC with 2 transmit antennas in combination with dynamic fall back to R99. PARC (Per Antenna Rate Control) transmits independent streams on each antenna, which are modulated with a common set of spreading codes. The data rates on each of the streams can be adjusted to account for each antenna’s channel characteristics. This is supported by Qualcomm, 3, Cingular Wireless, Ericsson, Lucent Technologies, T-Mobile, Vodafone, Orange, Mitsubishi, IPWireless (R99 fallback needs further consideration for TDD), Philips, Agere, Huawei, Telefonica and TIM.
2. Closed loop Dual stream TxAA is a MIMO scheme for sending multiple data streams with spatial multiplexing. In D-TxAA, if four transmit antennas are employed in Node B, transmit antennas are divided into two sub-groups and each sub-group transmits independent data stream with TxAA operation of a pair of transmit antennas. The data rate of each sub-group can be controlled independently. This proposal is supported by Nokia, Motorola, TI, Alcatel, Siemens, Samsung, LG, NEC, Intel, ETRI and ZTE.

After Nortel prototype demos and trial performed years ago, Ericsson has demonstrated a throughput of 28 Mbps on an HSDPA carrier during the CTIA Wireless 2006 event last month in Las Vegas. MIMO is expected to appear in commercial networks in the 2008 timeframe.