SIMO vs MIMO differ in where the extra antennas sit and what they buy. A SIMO system (single-input, multiple-output) has one transmit antenna and several receive antennas, so it improves reliability and SNR on a single data stream. A MIMO system (multiple-input, multiple-output) has several antennas at both ends, so it can also carry parallel streams and raise the data rate.
This page is a direct two-way comparison for an engineer deciding whether a second transmit chain is worth adding to a SIMO link. The four-way picture is in SISO, SIMO, MISO and MIMO systems, and the theory behind the gains is in MIMO and multiple antenna systems.
SIMO vs MIMO: the core difference
The difference comes down to the rank of the channel. A SIMO link has a channel vector h with one column, so there is only one spatial direction to send along. A 2×2 MIMO link has a 2×2 channel matrix H, which can offer two usable directions when the antenna paths are different enough.
| Property | SIMO (1×2) | MIMO (2×2) |
|---|---|---|
| Transmit antennas and chains | 1 | 2 |
| Receive antennas and chains | 2 | 2 |
| Channel paths | 2 | 4 |
| Spatial streams | 1 | Up to 2 |
| Maximum diversity order | 2 | 4 |
| Array gain | 3 dB (receive) | Up to 6 dB with one stream and CSI at both ends |
| Transmit CSI needed | No | No for open-loop, yes for precoding |
| Extra hardware at the transmitter | None | Second PA, upconverter and antenna |
What is a SIMO system?
A SIMO system sends one signal and picks it up on two or more receive antennas. The receiver weights and adds the copies, normally with maximal ratio combining (MRC), so the output SNR is the sum of the branch SNRs. With mean SNR ρ per antenna and channel vector h, capacity is:
C = log2(1 + ρ ||h||2) b/s/Hz
Because ||h||2 sits inside a single logarithm, SIMO capacity grows only logarithmically with antennas. Doubling the receive antennas adds about 3 dB, or about 1 b/s/Hz at high SNR. SIMO is a form of receive diversity: it fights fading and adds gain, but it does not open a second stream.
A typical SIMO link is the uplink from a single-transmitter device to a base station or access point with several receive chains. All the complexity sits at the receiver, and the device needs no knowledge of the channel.
What is a MIMO system?
A MIMO system adds antennas at the transmitter as well. With equal power on two antennas and no channel knowledge at the transmitter, the capacity of channel H is:
C = log2 det(I + (ρ/2) H HH) b/s/Hz
The determinant expands into one logarithm per non-zero singular value of H. At high SNR, a 2×2 link therefore gains about 2 b/s/Hz for each 3 dB of SNR, where a SIMO link gains about 1. That change of slope is the multiplexing gain, and spatial multiplexing covers how the receiver separates the streams.
Worked example: SIMO vs MIMO capacity and outage
Take i.i.d. Rayleigh fading, fixed total transmit power and the same mean receive SNR per antenna for both systems. Values are averages over 300,000 random channels, and the rates assume 20 MHz of bandwidth.
| Mean SNR | SIMO 1×2 | MIMO 2×2 | MIMO/SIMO ratio | Rate in 20 MHz (SIMO, MIMO) |
|---|---|---|---|---|
| 0 dB | 1.44 b/s/Hz | 1.69 b/s/Hz | 1.17 | 29, 34 Mb/s |
| 10 dB | 4.06 b/s/Hz | 5.55 b/s/Hz | 1.37 | 81, 111 Mb/s |
| 20 dB | 7.27 b/s/Hz | 11.29 b/s/Hz | 1.55 | 145, 226 Mb/s |
| 30 dB | 10.58 b/s/Hz | 17.75 b/s/Hz | 1.68 | 212, 355 Mb/s |
The ratio tends toward 2 only as SNR becomes very high. At 0 dB the second transmit chain adds just 17%, so a cell-edge link sees little of the headline rate. Figure 2 shows the gap opening with SNR.
Reliability tells a similar story. At 15 dB mean SNR and a target of 4 b/s/Hz, the probability that the channel cannot support the target is:
| Link | Outage at 4 b/s/Hz, 15 dB | 10th percentile capacity at 15 dB |
|---|---|---|
| SISO 1×1 | 37.8% | 2.11 b/s/Hz |
| SIMO 1×2 | 8.3% | 4.16 b/s/Hz |
| MIMO 2×2 | 0.35% | 6.10 b/s/Hz |
The step from SIMO to MIMO cuts outage by a factor of about 23, because the 2×2 link has four fading paths where SIMO has two.
When more SIMO antennas beat MIMO
A second transmit chain is not the only way to spend hardware. Figure 2 includes a 1×4 SIMO link, which has five RF chains in total against four for 2×2. At 0 dB the 1×4 link delivers 2.21 b/s/Hz against 1.69 b/s/Hz, and the two curves cross near 7.7 dB, at about 4.4 b/s/Hz. Above that point 2×2 pulls away, reaching 11.29 b/s/Hz at 20 dB against 8.46 b/s/Hz. Coverage-limited links, where SNR is low, therefore favour more receive antennas, while capacity-limited links favour more streams.
Antenna correlation decides the MIMO gain
All the figures above assume independent paths. If the two transmit antennas are correlated, the gain over SIMO shrinks, and at full correlation it disappears altogether. The 2×2 link then behaves exactly like a 1×2 link, because both transmit antennas excite the same spatial direction.
| Transmit correlation | 2×2 capacity at 20 dB |
|---|---|
| 0 | 11.29 b/s/Hz |
| 0.5 | 10.93 b/s/Hz |
| 0.7 | 10.46 b/s/Hz |
| 0.9 | 9.37 b/s/Hz |
| 0.99 | 7.79 b/s/Hz |
| 1.0 | 7.27 b/s/Hz (same as SIMO 1×2) |
Moderate correlation costs little, since 0.7 still leaves most of the benefit. Near 1.0 the curve falls steeply. Close antenna spacing on a small device, or a line-of-sight path with little scattering, pushes a real link toward the right of this plot. Antenna parameters such as pattern and polarisation are the practical tools for keeping correlation down.
Advantages and disadvantages of SIMO systems
Advantages:
- Only the receiver changes, and the transmitter needs no CSI.
- Diversity and 3 dB of array gain with two antennas, with no extra transmit power.
- One PA, which means lower cost, lower DC power and no need to calibrate between transmit chains.
- No stream separation is needed, so baseband processing is simple.
Disadvantages:
- One stream, so peak rate is capped by a single logarithm.
- Each receive chain adds an LNA, downconverter and ADC, with their own power and noise figure.
- The array gain has diminishing returns: each doubling of antennas adds only 3 dB.
Advantages and disadvantages of MIMO systems
Advantages:
- Parallel streams, with no extra bandwidth or total transmit power.
- Higher diversity order, which gives far lower outage at a given SNR.
- Flexibility: the same antennas can be used for beamforming when SNR is low and for multiplexing when it is high.
Disadvantages:
- Two or more PAs and upconverters, plus more DC power and cost.
- Needs a rich channel. Correlation or a rank-deficient channel removes most of the rate gain.
- More pilots and, for closed-loop operation, channel state information feedback.
- Linearity and phase noise requirements rise, because a weak stream suffers from any error on a strong one.
Where MISO systems fit
A MISO system (multiple-input, single-output) is the mirror image of SIMO, with several transmit antennas and one receive antenna. With channel knowledge at the transmitter, a MISO link matches SIMO in both array gain and capacity. Without it, MISO gets diversity but loses the array gain, as the four-way comparison shows.
Frequently asked questions
Is SIMO better than MIMO?
Not in general, since in Rayleigh fading a MIMO link with the same receive antennas has at least the capacity of the SIMO link. SIMO is the better choice when transmitter cost or power is the limit, when SNR is low, or when a higher receive antenna count is available instead.
Does SIMO increase data rate?
It does, but only through SNR. The extra receive antennas raise the SNR, which allows a higher-order modulation or coding rate, and capacity grows logarithmically. In the example above, 1×2 reaches 4.06 b/s/Hz at 10 dB, compared with about 2.91 b/s/Hz for a single antenna.
What is the difference between SIMO and receive diversity?
SIMO describes the antenna arrangement, and receive diversity describes what is done with it. Combining signals from several receive antennas is the normal use of a SIMO link, so in practice the terms overlap.
Can a 2×2 MIMO link fall back to SIMO?
Yes. When the channel will not support two streams, systems use rank adaptation to drop to one stream. A 2×2 link in that mode still has two transmit chains, which can provide transmit diversity or beamforming on top of the receive combining.
Related topics
For the wider picture, start with MIMO and multiple antenna systems, then read spatial multiplexing and MIMO precoding. The hardware is described in MIMO, Beamforming and Phased-Array Architectures, and more background is on the antennas and RF design hubs.