Previous generation cellular systems have used multiple antenna techniques such as transmit and receive diversity and beamsteering to improve the link budget. In each of these cases, a single stream of data is sent between the base station and user equipment (UE). Release 8 of the 3GPP specifications, which defines the Long Term Evolution (LTE) towards 4th generation systems, includes new requirements for spatial multiplexing — also referred to as Multiple Input Multiple Output ( MIMO) — wherein the base station and UE communicate using two or more spatial streams. The goal is to increase both the overall capacity of a cell and the data rate that a single user can expect from the system.
As a result of the increasing data rates and flexibility, the design and test of LTE systems differs in many ways from previous generations of cellular technology. In particular, LTE receiver design and test present new challenges for which test equipment and measurement methods must be adapted.
Requirements for LTE receivers
The 3GPP specifications define the LTE requirements that impact receiver design. For example, LTE must support the following changes:
- Six different channel bandwidths from 1.4 to 20 MHz;
- Frequency Division (FDD) and Time Division Duplex (TDD) modes;
- Use of the multi-gigabit DIgRF v4 standard to connect subsystems. This requires cross-domain (digital in, RF out) measurement capability. A digital test source must emulate both data traffic and the encapsulated protocol stack within the digital interface that controls RFIC functionality.
Additionally, LTE can use transmit diversity (MISO) and receive diversity (SIMO) as well as beamsteering, either alone or in combination with MIMO. LTE specifies seven different downlink transmission modes, each of which is suited to different channel and noise conditions:
- Single-antenna port; port 0 — SISO;
- Transmit diversity — MISO;
- Open-loop spatial multiplexing — MIMO (no precoding);
- Closed-loop spatial multiplexing — MIMO (precoding);
- Multi-user MIMO — MIMO (separate UEs);
- Closed-loop Rank=1 precoding; — MISO (beamsteering);
- Single-antenna port; port 5 — MISO (beamsteering).
The terms codeword, layer, precoding, and beamforming have been adopted specifically for LTE to refer to signals and their processing. The terms are used in the following ways:
- Codeword: A codeword represents user data before it is formatted for transmission. In the most common case of single-user MIMO (SU-MIMO), two codewords are sent to a single handset or UE.
- Layer (or stream): For MIMO, at least two layers must be used. Up to four are allowed. The number of layers is always less than or equal to the number of antennas.
- Precoding: Precoding modifies the layer signals before transmission. This may be done for diversity, beamsteering or spatial multiplexing. The MIMO channel conditions may favor one layer (data stream) over another. If the spatial multiplexing is closed loop, the UE provides a precoding matrix indicator (PMI) so the eNB can cross-couple the streams to counteract the imbalance in the channel.
- Eigenbeamforming (sometimes known simply as “beamforming”) modifies the transmit signals to give the best carrier-to-noise interference plus noise ratio (CINR) at the output of the channel.
Figure 1 shows how two codewords are used for a single user in the downlink. It is also possible for the codewords to be allocated to different users to create multi-user MIMO (MU-MIMO).
Fig 1: 2x2 SU-MIMO transmission.