WiMAX MIMO
WiMAX
WiMAX is the technology brand name for the implementation of the standard IEEE 802.16. One of two main areas specified by 802.16 is the air interface, or PHY (Physical layer). Aside from specifying the support of various channel bandwidths and adaptive modulation and coding, it also specifies the support for MIMO antennas to provide good NLOS characteristics.
See Also: WiMax Forum
MIMO
MIMO stands for Multiple Input and Multiple Output, and refers to the technology where there are multiple antennas at the base station and multiple antennas at the mobile device. Typical usage of multiple antenna technology includes cellular phones with two antennas, laptops with two antennas (e.g. built in the left and right side of the screen), as well as CPE devices with multiple sprouting antennas.
The predominant view in cellular technology has been to set up multiple antennas at the base station, and with each user having just one antenna on the mobile device. This allows for a low cost mobile radio with a cost weighting at the base station. As the costs for radio-frequency (RF) components in a mobile device go down, there is an attractive option in adding a second antenna to the mobile device. This trend towards multiple antennas at the (mobile) station is also expressed in Wi-Fi technology (e.g. IEEE 802.11n), where WiFi devices such as cellular phones Wi-Fi, laptops and other media devices are enabled with two or more antennas.
MIMO Technology in WiMAX
In the recent years, much attention has been put into implementing the MIMO technology with WiMAX. One benefit to having more antennas is that it benefits the reception, since the signal quality improves with the increase in number of receive antennas. Secondly, for uplink transmitting, more antennas allows for a better reach and rate of transmission. The implementation of MIMO gives WiMAX a significant increase in its sector throughput, allowing it to achieve much higher spectral frequencies than current 3G systems. [1] The MIMO configuration is negotiated dynamically between each individual base station and mobile station.
Within the 802.16 frame format there is the ability to support a mix of mobile stations with different mimo capabilities. This helps to maximize the sector throughput by leveraging the different capabilities of a diverse set of vendor mobile stations.
Space Time Code and Spatial Multiplexing
Space Time Code
A technique for transmission with multiple antennas is Transmit Diversity, which is commonly referred to Space Time Code (STC) in the IEEE 802.16e context. The concept of the method is that there are multiple antennas at the transmitter and only one antenna is needed at the receiver. However, more receive antennas can further improve the signal quality.
With a Transmit Diversity rate = 1 (aka "Matrix A" in the 802.16 standard), two different pieces of data (data bit constellations) are transfered on two different antennas at the same time (during the same symbol), and then the conjugate and/or inverse of the same two bits are transferred again on the same antennas during the next symbol. That data transfer rate with STC remains the same as the baseline case, however, the benefit to this method is the increase in robustness of the signal due to the repeated transmission of the same bits over two different antennas.
The configuration for a 2x1 Matrix A is two transmit antennas and at least one receive antenna. The result of this configuration is the same performance, within certain mathematical bounds, as if there had been two receive antennas and one transmitter antenna.
With two transmitter antennas, the two signals are transferred at the same time out of the two antennas, in which they interfere with each other. Then, the two signals are transmitted again, but as conjugates, so that the receiver can completely recover the original signals.
Spatial Multiplexing
Related to the Space Time Code technique is the use of Spatial Multiplexing (SMX), also known as Transmit Diversity rate = 2 (aka "Matrix B" in the 802.16 standard). Instead of transmitting the same bit over two antennas, this method transmits one data bit from the first antenna, and another bit from the second antenna simultaneously, per symbol. The two signals are transmitted at the same time so they interfere, but as long as the receiver has more than two antennas it will be able to separate the signals again.
This method involves a higher expense due to the increase in number of antennas, but data can be transmitted twice as fast in comparison to the previous method. The idea is to target users close to the base station, so that there is less time spent on transmitting per slot, hence the ability to service more users simultaneously within the frame.
Four Antennas
With the use of four antennas, more variations of the STC methods are possible. With rate = 1 using four antennas, data is transmitted four times per symbol, where each time the data is conjugated and/or inversed. This does not change the data rate, but does give the signal more robustness and avoids sudden increases in error rates.
With rate = 2 using four antennas, the data rate is only doubled, but increases in robustness since the same data is transmitted twice as compared to only once with using two antennas.
The third configuration that is only available using four antennas is Matrix C, where a different data bit is transmitted from the four antennas per symbol, which gives it four times the baseline data rate.
| 4 | STC (Matrix A) |
STC & SMX (Matrix B) |
SMX only (Matrix C) |
||
| Number of Transmit Antennas |
2 | STC (Matrix A) |
SMX (Matrix B) |
not possible | |
| 1 | Baseline Case | not possible | not possible | ||
| 1x | 2x | 4x | |||
| Data Rate | |||||
| 4 | STC (Matrix A) |
2xSMX (Matrix B) STC + 2xMRC (Matrix A) |
2xSMX (Matrix B) STC + 3xMRC (Matrix A) |
4xSMX (Matrix C) | |
| Tx | 2 | STC (Matrix A) |
2xSMX (Matrix B) STC + 2xMRC (Matrix A) |
2xSMX (Matrix B) STC + 3xMRC (Matrix A) |
STC + 4xMRC (Matrix A) |
| 1 | Baseline Case | Uplink: Uplink Collaborative MIMO Downlink: MRC |
MRC | MRC | |
| 1 | 2 | 3 | 4 | ||
| Rx | |||||
Note: MRC (Maximum Ratio Combining) is vendor discretionary and improves rate and range. In WiMAX, MRC at the Base Station is sometimes also referred to as Receive Beamforming.
See also: Space Time Coding and Spatial Multiplexing
Uplink Collaborative MIMO
Another technique is called Uplink Collaborative MIMO, where there is one user transmitting at the same time, in the same frequency, as another user. This type of spatial multiplexing improves the sector throughput through spatial multiplexing, without requiring multiple transmit antennas at the mobile device.
To obtain the throughput benefit, both signals overlap and interfere, at the Base Station receiver as they are intentionally not separated by time or frequency.
The common non-MIMO method for separating multiple transmissions in OFDMA is by scheduling different mobile stations at different points in an OFDMA time-frequency map.
Collaborative Spatial Multiplexing (Collaborative MIMO) is comparable to regular spatial multiplexing, where multiple (two or more) different data streams are transmitted from multiple (two or more) antennas mounted on the same device.
In the case of WiMAX, Uplink Collaborative MIMO is spatial multiplexing with two different devices, each with one antenna. This is in contrast to spatial multiplexing with two data streams from two different antennas on one device.
These transmitting devices are collaboratiing in the sense that both devices must be synchronised in time and frequency so that the intentional overlapping occurs.
The two streams of data will interfere with each other, but as long as the receiver at the base station has at least two antennas, the two data streams can be separated again.
This technique is sometimes also termed Virtual Spatial Multiplexing.
/Without 3dB power penalty
See also: Advanced MIMO communications
Adaptive Antenna Steering (AAS) AKA Beamforming
The last technique to using MIMO with WiMAX is called AAS or Beamforming. The idea is to use multiple antennas, which then basically shape the beam with the intent of improving transmission to a certain mobile station. The result is reduced interference during reception from a different mobile station, which is likely to be communicating with a different cell, sector, and base station. This improves on the strength of the signal going to the desired user, and reduces the amount of signal being transmitted to an interfering user in a different cell.
Cyclic Delay Diversity
A discretionary technique commonly employed in OFDM-based telecommunications is known as Cyclic Delay Diversity. The idea behind this technique is to delay a signal, and then transmit both the original and the delayed signals at the same time. Because the signals are coming out of two antennas, the spectrums of the two signal as seen by the receiever look different from each other. At the receiver the combine, which results in shallower spectral humps and fewer spectral notches. The closer the signal can get towards a flat channel at a certain power level, the better the throughput of the signal.
Radio Conformance Test
The WiMAX Forum has a set of standardized conformance test procedures for PHY and MAC specification compliance called the Radio Conformance Test (RCT). Any technology aspect of a particular implementation of a radio interface must first undergo the RCT. The test requirements are set by the WiMAX Forum. Generally, any aspect of the IEEE 802.16 standard that does not have a test procedure in the RCT may be assumed to not yet be implemented.
Silicon implementations
Companies that make RFICs that support WiMAX MIMO include Beceem, PMC-Sierra and Runcom.