SISO vs MIMO is the choice between one antenna at each end of a radio link and several. SISO, SIMO, MISO and MIMO are the four antenna configurations of wireless communication, named by how many antennas feed the channel (input) and how many collect from it (output). Only MIMO can carry parallel data streams, while SIMO and MISO use their extra antennas to raise SNR and reduce fading.
This page compares all four on the same terms, with the same transmit power and the same channel statistics, to help with choosing one. If the choice is only between two of them, SIMO vs MIMO goes further into that pair. The underlying theory is in MIMO and multiple antenna systems.
SISO, SIMO, MISO and MIMO at a glance
The “input” is the input to the channel, which is the transmitter, and the “output” is the receiver. A 2×1 link therefore has two transmit antennas and one receive antenna.
| System | Antennas (Tx × Rx) | What the extra antennas do | Who needs CSI | Typical use |
|---|---|---|---|---|
| SISO | 1×1 | Nothing, there are none | Receiver only | Low-cost IoT radios, FM and many broadcast links |
| SIMO | 1×Nr | Receive diversity and array gain | Receiver | Uplink from a single-transmitter device |
| MISO | Nt×1 | Transmit diversity, or beamforming with CSI | Transmitter, for beamforming | Downlink to a single-antenna device |
| MIMO | Nt×Nr | Parallel streams, diversity and beamforming | Receiver, plus transmitter if precoded | Wi-Fi, LTE and 5G |
What is a SISO antenna system?
A SISO antenna system has one antenna at the transmitter and one at the receiver, with a single RF chain at each end. The channel is one complex number h, so capacity is:
C = log2(1 + ρ |h|2) b/s/Hz
SISO (single-input, single-output) is the simplest and cheapest radio. It has no array gain and no spatial diversity, so its only defences against a multipath fade are frequency diversity, time diversity through coding, and link budget margin. A SISO antenna can still be directive: a dish on a point-to-point link is a single antenna port, and it counts as SISO however high its gain.
SIMO: several antennas at the receiver
SIMO (single-input, multiple-output) keeps one transmitter and adds receive chains. Combining the branches gives an array gain of 10 log10(Nr) dB and a diversity order of Nr. No channel knowledge is needed at the transmitter, which makes SIMO a good fit for small, battery-powered transmitters talking to a capable receiver.
MISO: several antennas at the transmitter
MISO (multiple-input, single-output) is where channel knowledge matters most. Two cases need separating:
- Without transmit CSI, the transmitter splits its power and uses a space-time code such as Alamouti. It gains transmit diversity but no array gain.
- With transmit CSI, it pre-rotates each antenna’s phase so that the signals add in phase at the receiver. This gives both array gain and diversity, and matches SIMO exactly. Beamforming in MIMO systems explains how.
MIMO: several antennas at both ends
MIMO (multiple-input, multiple-output) has a channel matrix instead of a vector, so it can support up to min(Nt, Nr) streams. It is the only one of the four whose capacity grows linearly with antenna count at high SNR, provided the paths are sufficiently different.
SISO vs MIMO: worked comparison at 10 dB SNR
The table compares the smallest examples of each type in i.i.d. Rayleigh fading. Mean receive SNR per antenna is 10 dB, total transmit power is the same in every row, and rates use 20 MHz of bandwidth. Results are from one million random channels. Ergodic capacity is the long-term average, and 1% outage capacity is the rate the channel can support 99% of the time.
| System | Ergodic capacity | Rate in 20 MHz | 1% outage capacity | Rate in 20 MHz |
|---|---|---|---|---|
| SISO 1×1 | 2.91 b/s/Hz | 58 Mb/s | 0.14 b/s/Hz | 2.8 Mb/s |
| MISO 2×1, no CSI | 3.16 b/s/Hz | 63 Mb/s | 0.80 b/s/Hz | 16 Mb/s |
| SIMO 1×2 | 4.06 b/s/Hz | 81 Mb/s | 1.31 b/s/Hz | 26 Mb/s |
| MISO 2×1, with CSI | 4.06 b/s/Hz | 81 Mb/s | 1.31 b/s/Hz | 26 Mb/s |
| MIMO 2×2, no CSI | 5.55 b/s/Hz | 111 Mb/s | 2.67 b/s/Hz | 53 Mb/s |
Two points stand out. Average capacity understates what extra antennas do, since going from SISO to 2×2 raises the average by a factor of 1.9 but raises the 1% outage rate by a factor of about 19. Also, the gap between the two MISO rows is the value of transmit CSI: 0.9 b/s/Hz on average, equal to about 3 dB of array gain at this SNR.
What each configuration does for range
Designers often want range, not rate. Suppose a link needs 2 b/s/Hz with 1% outage. The table gives the mean SNR required, the saving against SISO, and the range multiple if that saving is spent on distance with a path loss exponent of 3.5.
| System | Mean SNR for 1% outage at 2 b/s/Hz | Saving against SISO | Range multiple |
|---|---|---|---|
| SISO 1×1 | 24.8 dB | None | 1.00 |
| MISO 2×1, no CSI | 16.1 dB | 8.7 dB | 1.77 |
| SIMO 1×2 | 13.1 dB | 11.7 dB | 2.16 |
| MISO 2×1, with CSI | 13.1 dB | 11.7 dB | 2.16 |
| MIMO 2×2, no CSI | 7.8 dB | 16.9 dB | 3.05 |
The SISO figure can be checked by hand: x = (22 – 1)/ρ and outage is 1 – e-x, so 1% outage needs ρ = 3/0.01005, which is 24.75 dB. These savings are large because a 1% outage target is dominated by deep fades. On a link with a strong line-of-sight path there are fewer deep fades, and the savings shrink toward the array gain alone. They also assume uncorrelated antennas, and the correlation penalty is shown in SIMO vs MIMO.
Hardware cost of each configuration
Each row of the comparison has a hardware price, and the cost is not symmetrical.
| System | RF chains (Tx + Rx) | Main added hardware | Added digital work |
|---|---|---|---|
| SISO 1×1 | 1 + 1 | None | Equalisation only |
| SIMO 1×2 | 1 + 2 | LNA, downconverter, ADC | Channel estimate per antenna, combining |
| MISO 2×1 | 2 + 1 | PA, upconverter, DAC | Space-time coding or precoding |
| MIMO 2×2 | 2 + 2 | Both of the above | Stream separation, rank adaptation |
The transmit chain is usually the expensive one, because a PA often dominates DC power. That is why battery-powered devices commonly transmit on fewer antennas than they receive on. Receive chains cost less power, but each one adds its own noise figure, and a MIMO receiver needs matched LO distribution to its chains.
How to choose between SISO, SIMO, MISO and MIMO
- Cost and power come first, and the channel is benign: SISO. A line-of-sight link or a low-rate IoT link may not need more.
- The transmitter is small and the receiver is capable: SIMO. This suits uplinks.
- The receiver is small and the transmitter is capable: MISO, with CSI if it can be obtained. This suits downlinks to simple devices.
- Rate is the goal and the channel is rich in multipath: MIMO, with spatial multiplexing at high SNR and precoding when CSI is available.
Frequently asked questions
What is the difference between SISO and MIMO?
SISO uses one antenna at each end and one stream. MIMO uses several at each end and can send up to min(Nt, Nr) streams. In the 10 dB example, 2×2 MIMO reaches 5.55 b/s/Hz against 2.91 b/s/Hz for SISO, with far fewer deep fades.
Is MISO the same as SIMO?
They mirror each other. With CSI at the transmitter, a MISO link has the same capacity as the reversed SIMO link. Without it, MISO loses the array gain, which is why it scored 3.16 b/s/Hz in the table against 4.06 b/s/Hz for SIMO.
Is Wi-Fi SISO or MIMO?
Most modern Wi-Fi access points and laptops are MIMO, commonly 2×2 or larger. Low-cost IoT Wi-Fi chips are often 1×1, which is SISO. Wi-Fi rate tables are therefore commonly quoted by the number of spatial streams.
Do SISO and MIMO mean the same in control systems?
The terms are used in the same sense but apply to a different subject. In control theory, SISO and MIMO describe how many inputs and outputs a plant has, such as actuators and sensors. The matrix mathematics is similar, but antennas and radio channels are not involved.
Related topics
Continue with MIMO and multiple antenna systems for the theory, SIMO vs MIMO for the two-way comparison and channel state information for how transmitters learn the channel. MIMO, Beamforming and Phased-Array Architectures covers the hardware, and the antennas hub has more on antenna design.