Q and A: MIMO

Q&A: MIMO

What advantages are offered by MIMO?

In what environments for UMTS systems will operators realize the benefit of MIMO?

Is MIMO effective in its advantages for both UMTS/HSPA and LTE?

Are there various forms of MIMO antenna technology?

How does MIMO work?

Have the improvements of MIMO been tested?

What is SDMA and how is it related to MIMO?

How much improvement do MIMO systems provide over conventional dual-antenna systems?

With which technologies will MIMO be used?

How spectrally efficient is OFDM/MIMO for different antenna configurations?

How will 3GPP, 3GPP2 and other standards bodies adopt and introduce OFDM/MIMO?

Can UMTS/WCDMA air interface benefit from the MIMO technology?

Can OFDM/MIMO be used in TDD and FDD modes?

Who are the main chipset vendors developing OFDM/MIMO technology?

What applications benefit from MIMO?

What is MIMO?

Multiple Input Multiple Output (MIMO) is an antenna technology that is used both in transmission and receiver equipment for wireless radio communication. MIMO uses multiple antennas to send multiple parallel signals (from transmitter).

MIMO has been variously defined as "two or more unique radio signals, in the same radio channel, where each signal carries different digital information" and "two or more radio signals which use beam-forming, receive combining, and spatial multiplexing".

What advantages are offered by MIMO?

MIMO holds many advantages for mobile wireless operators. Wireless systems using MIMO represent an economical way to increase user capacity, range and throughput in a variety of environments, most notably those which are enclosed and having low radio interference such as small and/or isolated cells.

The use of multiple antennas at both transmitter and receiver allows:

Substantial increase in peak data rate
Significantly higher spectrum efficiency, especially in low-interference environments
Increased system capacity (number of users)

In what environments for UMTS systems will operators realize the benefit of MIMO?

In UMTS systems, operators see a great need for MIMO in "contained environments" such as:

Hot spots similar to those serviced by today’s WiFi systems (airports, hotel lobbies, etc)
Academic campuses, in various self-contained areas (quads, auditoriums, cafeterias, etc)
Stadiums and arenas, again, which offer self-contained environments
Malls and shopping areas, favored by large numbers of younger, internet-savvy users
Mass transportation (trains, etc) with users looking for interaction and entertainment
Enclosed parks and recreation areas
Residential homes, supplanting DSL/Cable services

Tests of MIMO have proven very promising in WLANs operating in relative isolation, where interference is not a dominant factor. Spatial multiplexing MIMO should also benefit HSPA "hotspots" serving local areas such as airports, campuses, and malls, where the technology will increase capacity and peak data rates. However, in a fully loaded network with interference from adjacent cells, overall capacity gains will be more modest--in the range of 20 to 33 percent over mobile-receive diversity. Relative to a 1x1 antenna system, however, 2X2 MIMO can deliver cell throughput gains of about 80 percent. 3GPP is standardizing spatial multiplexing MIMO in Release 7 using Double Transmit Adaptive Array (D-TxAA).

Because the gains of MIMO are relatively modest for conventional networks with fully loaded cells, Rel-7 MIMO studies are now done in the context of "contained" environments where the intercell interference is limited. Examples of these environments include malls, academic campuses, and airports. In these environments, MIMO performance gains are improved, in terms of the fraction of time SM is used and overall throughput.

Is MIMO effective in its advantages for both UMTS/HSPA and LTE?

Operators believe that, notwithstanding the basic differences in the physical layers used by UMTS and LTE, the benefits envisioned from MIMO in LTE, can also be obtained from MIMO in UMTS systems, starting in Release 7.

Are there various forms of MIMO antenna technology?

The term "MIMO" is an acronym for multiple-input, multiple-output, and it is used to refer to any wireless system with multiple antennas at the transmitter and receiver. This technique can potentially increase a user's peak data rate compared to conventional single-stream transmission. Whereas multipath is an impediment for other radio systems, MIMO actually exploits multipath, relying on signals to travel across different communications paths. This results in multiple data paths effectively operating somewhat in parallel and, through appropriate decoding, in a multiplicative gain in throughput.

The most common use of the term, and in the context of HSDPA Rel-7, "MIMO" applies to spatial multiplexing (SM). SM delivers parallel streams of data to CPE by exploiting multipath. It can double (2X2 MIMO) or quadruple (4X4 MIMO) capacity and throughput.

Other types of MIMO include Space Time Transit Diversity (STTD) and Uplink Collaborative MIMO. In STTD, the same data is coded and transmitted through different antennas, which effectively doubles the power in the channel. This improves Signal Noise Ratio (SNR) for cell edge performance. Uplink Collaborative MIMO Link (also known as Uplink Mulituser MIMO) leverages conventional single Power Amplifer (PA) at device. Two devices can collaboratively transmit on the same subchannel which can also double uplink capacity.

How does MIMO work?

The most common use of the term "MIMO" applies to spatial multiplexing. The transmitter sends different data streams over each antenna. Whereas multipath is an impediment for other radio systems, MIMO—as illustrated in the Figure below—actually exploits multipath, relying on signals to travel across different communications paths. This results in multiple data paths effectively operating somewhat in parallel and, through appropriate decoding, in a multiplicative gain in throughput.

MIMO Using Multiple Paths to Boost Throughput and Capacity

Have the improvements of MIMO been tested?

Tests of MIMO have proven very promising in WLANs operating in relative isolation, where interference is not a dominant factor. Spatial multiplexing MIMO should also benefit HSPA "hotspots" serving local areas such as airports, campuses, and malls, where the technology will increase capacity and peak data rates. However, in a fully loaded network with interference from adjacent cells, overall capacity gains will be more modest--in the range of 20 to 33 percent over mobile-receive diversity. Relative to a 1x1 antenna system, however, 2X2 MIMO can deliver cell throughput gains of about 80 percent. 3GPP is standardizing spatial multiplexing MIMO in Release 7 using Double Transmit Adaptive Array (D-TxAA).

Although MIMO can significantly improve peak rates, other techniques such as Space Division Multiple Access (SDMA)—also a form of MIMO—may be even more effective than MIMO for improving capacity in high spectral efficiency systems using a reuse factor of 1.

What is SDMA and how is it related to MIMO?

The term "MIMO" is an acronym for multiple-input, multiple-output, and it is used to refer to any wireless system with multiple antennas at the transmitter and receiver. At the transmitter, multiple antennas can be used to mitigate the effects of fading via transmit diversity and to increase throughput via Space Division Multiple Access (SDMA). Beamforming and sectorization are two examples of SDMA. Beamforming can also be used for range extension. At the receiver, multiple antennas can be used for receiver combining which provides diversity and combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams can be transmitted from each antenna, with each stream carrying different information but using the same frequency resources. This technique, known as spatial multiplexing (SM), can potentially increase a user's peak data rate compared to conventional single-stream transmission.

How much improvement do MIMO systems provide over conventional dual-antenna systems?

In conventional cellular networks with fully loaded 3-sector cells and universal frequency reuse, the geometry is greater than 4dB in about 35% of the locations. Therefore SM is used about 35% of the time, and the fraction of users that achieve a peak rate equal or higher than the single-stream peak rate of 7.2 Mbps is about 25%. This is compared to the 15% that can achieve 7.2 Mbps in a conventional baseline HSDPA system with dual-antenna UEs. In terms of total average cell throughput, MIMO provides about a 33% improvement over this baseline. If cell throughput is a more important performance metric than peak data rate, it is much more efficient to use sectorization instead of SM. Using 6-sector cells each with a single antenna, the throughput improvement over the baseline is about 80%, but the tradeoff is that the peak rate remains the same since only a single antenna is used in each sector.

With which technologies will MIMO be used?

MIMO is the basis for HSPA+, LTE and other technologies such as 802.16e (WiMAX) and Ultra Mobile Broadband. It is used in combination with OFDM.

How spectrally efficient is OFDM/MIMO for different antenna configurations?

Depending on the antenna configurations at the cell site and the mobile device, OFDM with MIMO can deliver on average 2.6 bits/sec/Hz a 2X2 antenna system (2 Tx and 2 Rx antennas). To put this number in perspective, Release 6 HSDPA is expected to deliver, on average, between .7 to .8 bits/sec/Hz.

How will 3GPP, 3GPP2 and other standards bodies adopt and introduce OFDM/MIMO?

3GPP2 will adopt an OFDMA/MIMO variant for its Ultra Mobile Broadband (UMB) release. 3GPP LTE is opting for the OFDMA/MIMO in the Downlink and the more Power Amplifier (PA) friendly and battery efficient SC-FDMA/MIMO in the Uplink.

Can UMTS/WCDMA air interface benefit from the MIMO technology?

Yes, studies have shown that on average, the HSDPA aggregate cell throughput performance will increase significantly with a two parallel paths MIMO implementation. However, because of the vulnerability of a CDMA signal to inter symbol and inter code interference in a multi-paths environment, MIMO gain is limited to areas of very good radio conditions.

OFDM is highly resistant to inter-symbol interference even under severed multi-path environments along with orthogonal sub-carriers that practically eliminate interference from inter-users. Therefore, OFDM air interface is much more suited for the MIMO technology. The gain of MIMO with OFDM is expected to be increased linearly, in relation to the number of parallel streams that exist between the transmitter and the receiver.

Can OFDM/MIMO be used in TDD and FDD modes?

Yes, both implementations are possible. Early WiMAX OFDM/MIMO-based implementation will use TDD even though the pre-dominant spectrum allocation around the world is for FDD systems.

Who are the main chipset vendors developing OFDM/MIMO technology?

We expect many chip vendors to be involved in this market. OFDM/MIMO will be the baseline for all future evolutions of wireless technologies. Particularly for 3GPP LTE, OFDM/MIMO is being embraced by all the 3GPP partners and will be supported by the usual group of vendors both on the infrastructure and chipset sides. Given the popularity of OFDM/MIMO, there is the potential for leveraging an even larger economy of scale with a common OFDM/MIMO chipset across all new technologies. However, LTE OFDM/MIMO has the advantage of being the mainstream wireless technology with broad industry, governmental and user support to ensure a smooth standardized and cost effective network migration from 3G to IMT Advanced and beyond.

What applications benefit from MIMO?

MIMO can be used to advance such applications as:

Streaming Video, Music
Video Surveillance
VoIP
Video Conferencing
Interactive Gaming
Mobile TV

Return to Questions and Answers home page.

web
stats